Nanostructured Solar Cells
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LaNaSC – Laboratory for Nanostructured Solar Cells
The Laboratory for Nanostructured Solar Cells (LaNaSC) develops nano- and micro-structures of chalcopyrite-type semiconductors (Cu(In,Ga)Se2) for application in photovoltaic energy conversion.
We currently follow four research lines:
- Development of advanced thin-film solar cells by the implementation of micro- and nanostructures.
- Development and application of scanning probe microscopy techniques for the characterization of solar cell materials and light-induced phenomena at the nanometer scale.
- Development of 2D materials for optoelectronis applications.
- Development of growth methods for chalcopyrite nanostructures i.e. quantum dots and nanowires. The goal is to combine the excellent light absorbing properties of chalcopyrite-type materials with the quantum properties of nanostructured materials, and thereby provide a pathway for the enhancement of power conversion efficiencies of photovoltaic devices beyond the Shockley-Queisser limit.
The laboratory is equipped with various materials preparation facilities consisting in a molecular beam epitaxy (MBE) setup for the growth of nanostructured chalcopyrite-type semiconductors, a hybrid sputter system for Cu(In,Ga)Se2, Mo, and ZnO, and an evaporation system for Cu(In,Ga)Se2 thin films.
The lab also operates an ultra-high vacuum scanning probe microscope, combining STM, AFM, and KPFM facilities with surface photovoltage methods to study light-induced phenomena at the nanometer scale.
The LaNaSC group: (standing) Diego Colombara, Marcel Claro, Nicoleta Nicoara, Pedro Anacleto, Sascha Sadewasser, Deepanjan Sharma, João Gonçalo. (sitting) Bernhard Baumgartner, Daniel Brito, José Virtuoso, Marina Alves
We are always open for new team members. If you are interested in joining us and applying for external funding (i.e. MSCA, FCT, etc.), please contact Sascha by email.
RESEARCH LINES
Chalcopyrite, Cu(In,Ga)Se2 (CIGSe), materials have excellent light absorbing properties and are used in the thin-film solar cell technology with the highest power conversion efficiency. We are aiming at incorporating novel concepts to improve thin film solar cells using nano- and micrometer structures into the device structure.
Currently, we focus our research efforts on two approaches:
- We develop micro solar cells for micro-concentrator solar cell applications. The goal is to develop highly efficient solar cells with a significant reduction in usage of absorber materials. By concentrating the sunlight onto micrometer sized CIGSe solar cells, the materials consumption of the solar cell material can be significantly reduced, leading to cost improvements. We combine cleanroom technology with the growth of CIGSe materials to obtain the micro solar cells.
- Development of nanostructures for chalcopyrite thin-film solar cells. The goal is to use passivation and light management techniques to improve solar cell performance. We use cleanroom technologies to introduce a passivation layer with contact holes in between the back contact and the absorber layer. This reduces back contact recombination and allows for thinner absorber layers, leading to cost savings for solar cell devices.
Project leader: Sascha Sadewasser
Team members: Pedro Anacleto, Ana Pérez Rodríguez, José Virtuoso, Marina Alves
We use ultrahigh vacuum scanning probe microscopy (UHV-SPM) methods to characterize the physical properties of chalcopyrite nanostructures and solar cell materials at the nanoscale.
Scanning probe methods include regular atomic force microscopy, Kelvin probe force microscopy, surface photovoltage measurements and scanning tunneling microscopy. We are especially interested in the interaction of light with solar cell materials at the nanoscale.
Scanning probe microscopy system
Project leader: Sascha Sadewasser
Team members: Nicoleta Nicoara, Deepanjan Sharma
We use vapor depositions and molecular beam epitaxy to grow 2D materials with a focus on semiconducting chalcogenide materials, e.g. MoS2, MoSe2, In2Se3, etc. Our goal is to extensively characterize these materials and implement them in optoelectronic devices, e.g. photodetectors.
Project leader: Sascha Sadewasser
Team members: Marcel Claro, Francisco de Matos
Chalcopyrite, Cu(In,Ga)Se2 (CIGSe), materials have excellent light absorbing properties and are used in the thin-film solar cell technology with the highest power conversion efficiency. We are working with these materials at the nanometer length scale with the goal to increase power conversion efficiencies.
We aim to develop growth methods for chalcopyrite nanostructures, i.e. quantum dots and nanowires. The goal is to combine the excellent light absorbing properties of chalcopyrite-type materials with the quantum properties of nanostructured materials, and thereby provide a pathway for the enhancement of power conversion efficiencies of photovoltaic devices beyond the Shockley-Queisser limit. We use a molecular beam epitaxy (MBE) system to evaporate the constituent elements (Cu, In, Ga, Se) onto epitaxial substrates, where at low evaporation rates and thin coverage the formation of nano-sized crystallites occurs.
Molecular Beam Epitaxy system for Cu(In,Ga)Se2 nanostructure growth
Project leader: Sascha Sadewasser
Team members: Marcel Claro
ON GOING RESEARCH PROJECTS
Semi-transparent solar cells for building-integrated photovoltaics
Semi-transparent photovoltaic windows will be developed based on narrow stripes of solar cells below the eye’s resolution, leading to a natural daylight spectrum on the inside and the ability to use solar cells with high power conversion efficiency.
This project will be carried out in close collaboration with the group of Prof. Phillip Dale at the University of Luxembourg.
Micro-concentrator thin film solar cells
The innovative idea of the MiconCell project is to combine highly-efficient CIGSe thin-film technology with the concentrator PV approach and shrink the size scale to the micrometer range. The benefits of this novel concept are materials savings of the critical raw materials In and Ga by a factor of about 100, and an efficiency increase by 2-6% due to the use of concentrated sunlight. The main goal of the project is to develop CIGSe micro-concentrator solar cells that are monolithically integrated with concentrator optics.
Correlated Analysis of Inorganic Solar Cells in and outside an Electron Microscope
The CASOLEM project aims to fabricate in-situ TEM cartridges for carrying out electrical measurements of CIGS solar cells. These chips will be developed in the Nanostructured Materials Group and will be universally adaptable, provide flexibility and significant advantages to the existing ones and will enable simultaneous
experimentation both inside and outside the TEM. Our role in this project is to provide CIGSe solar cell samples specially designed to test the developed cartridges.
Advanced architectures for ultra-thin high-efficiency CIGS solar cells with high manufacturability
PREVIOUS RESEARCH PROJECTS
Super high efficiency Cu(In,Ga)Se2 thin-film solar cells approaching 25%
Large-scale printing of novel photovolaics based on Cu(In,Ga)Se2 chalcopyrite.
Large area two dimensional heterostructures for photodetectors.
This project is carried out in collaboration with the 2D Materials and Devices Group and the groups of Prof. Ricardo Ribeiro and Prof. Nuno Peres from the University of Minho.
Nanotechnology-based functional solutions.
PUBLICATIONS
-
2021
Ishwor Khatri, Pedro Santos, Pedro Anacleto, Deepanjan Sharma, Mohamed Belmoubarik, Nicoleta Nicoara, Mutsumi Sugiyama, Sascha Sadewasser
Effect of Se-Free Annealing on Cesium Fluoride-Treated Cu(In,Ga)Se2 Thin Films and Corresponding Solar Cells Journal Article
PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, (2100462), 2021.
@article{Khatri2021,
title = {Effect of Se-Free Annealing on Cesium Fluoride-Treated Cu(In,Ga)Se2 Thin Films and Corresponding Solar Cells},
author = {Ishwor Khatri, Pedro Santos, Pedro Anacleto, Deepanjan Sharma, Mohamed Belmoubarik, Nicoleta Nicoara, Mutsumi Sugiyama, Sascha Sadewasser},
url = {https://doi.org/10.1002/pssr.202100462},
doi = {10.1002/pssr.202100462},
year = {2021},
date = {2021-11-06},
journal = {PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS},
number = {2100462},
abstract = {The effect of selenium-free annealing on cesium fluoride (CsF)-treated Cu(In,Ga)Se2 (CIGS) thin films is investigated and their solar cell performance is evaluated. Annealing of CsF-treated CIGS thin films changes the surface morphology and the chemical composition, and reduces the Urbach energy. However, the full benefit of the reduced Urbach energies of the annealed samples is not obtained, because of an increased buffer/CIGS interface recombination as a consequence of re-evaporation of the alkali-containing layer from the surface region upon annealing. On the other hand, CsF-treated CIGS thin films without any annealing after the post-deposition treatment (PDT) preserve the alkali-containing layer at the surface, thereby leading to an improved buffer/CIGS interface. Consequently, the open-circuit voltage (V OC) and fill factor improve significantly in these devices. The Urbach energies of both annealed and nonannealed solar cells are calculated from external quantum efficiency measurement to understand their impact on V OC and V OC deficit. No correlation between the V OC deficit and the Urbach energies is observed.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The effect of selenium-free annealing on cesium fluoride (CsF)-treated Cu(In,Ga)Se2 (CIGS) thin films is investigated and their solar cell performance is evaluated. Annealing of CsF-treated CIGS thin films changes the surface morphology and the chemical composition, and reduces the Urbach energy. However, the full benefit of the reduced Urbach energies of the annealed samples is not obtained, because of an increased buffer/CIGS interface recombination as a consequence of re-evaporation of the alkali-containing layer from the surface region upon annealing. On the other hand, CsF-treated CIGS thin films without any annealing after the post-deposition treatment (PDT) preserve the alkali-containing layer at the surface, thereby leading to an improved buffer/CIGS interface. Consequently, the open-circuit voltage (V OC) and fill factor improve significantly in these devices. The Urbach energies of both annealed and nonannealed solar cells are calculated from external quantum efficiency measurement to understand their impact on V OC and V OC deficit. No correlation between the V OC deficit and the Urbach energies is observed. -
2020
Diego Colombara and Hossam Elanzeery and Nicoleta Nicoara and Deepanjan Sharma andMarcel Claro and Torsten Schwarz and Anna Koprek and Max Hilaire Wolter and Michele Melchiorre and Mohit Sood and Nathalie Valle and Oleksandr Bondarchuk and Finn Babbe and Conrad Spindler and Oana Cojocaru-Miredin and Dierk Raabe and Phillip J Dale and Sascha Sadewasser and Susanne Siebentritt
Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface Journal Article
Nature Communications, 11 , pp. 3634, 2020.
@article{Colombara2020,
title = {Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface},
author = {Diego Colombara and Hossam Elanzeery and Nicoleta Nicoara and Deepanjan Sharma andMarcel Claro and Torsten Schwarz and Anna Koprek and Max Hilaire Wolter and Michele Melchiorre and Mohit Sood and Nathalie Valle and Oleksandr Bondarchuk and Finn Babbe and Conrad Spindler and Oana Cojocaru-Miredin and Dierk Raabe and Phillip J Dale and Sascha Sadewasser and Susanne Siebentritt},
url = {https://www.nature.com/articles/s41467-020-17434-8},
doi = {10.1038/s41467-020-17434-8},
year = {2020},
date = {2020-07-20},
journal = {Nature Communications},
volume = {11},
pages = {3634},
abstract = {The electrical and optoelectronic properties of materials are determined by the chemical potentials of their constituents. The relative density of point defects is thus controlled, allowing to craft microstructure, trap densities and doping levels. Here, we show that the chemical potentials of chalcogenide materials near the edge of their existence region are not only determined during growth but also at room temperature by post-processing. In particular, we study the generation of anion vacancies, which are critical defects in chalcogenide semiconductors and topological insulators. The example of CuInSe2 photovoltaic semiconductor reveals that single phase material crosses the phase boundary and forms surface secondary phases upon oxidation, thereby creating anion vacancies. The arising metastable point defect population explains a common root cause of performance losses. This study shows how selective defect annihilation is attained with tailored chemical treatments that mitigate anion vacancy formation and improve the performance of CuInSe2 solar cells.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The electrical and optoelectronic properties of materials are determined by the chemical potentials of their constituents. The relative density of point defects is thus controlled, allowing to craft microstructure, trap densities and doping levels. Here, we show that the chemical potentials of chalcogenide materials near the edge of their existence region are not only determined during growth but also at room temperature by post-processing. In particular, we study the generation of anion vacancies, which are critical defects in chalcogenide semiconductors and topological insulators. The example of CuInSe2 photovoltaic semiconductor reveals that single phase material crosses the phase boundary and forms surface secondary phases upon oxidation, thereby creating anion vacancies. The arising metastable point defect population explains a common root cause of performance losses. This study shows how selective defect annihilation is attained with tailored chemical treatments that mitigate anion vacancy formation and improve the performance of CuInSe2 solar cells.Viviana Sousa and Bruna F Gonçalves and Yitzchak S Rosen and José Virtuoso and Pedro Anacleto and Fátima M Cerqueira and Evgeny Modin and Pedro Alpuim and Oleg I Lebedev and Shlomo Magdassi and Sascha Sadewasser and Yury V Kolen’ko
Over 6% Efficient Cu(In,Ga)Se2 Solar Cell Screen-Printed from Oxides on Fluorine-Doped Tin Oxide Journal Article
ACS Applied Energy Materials, 6 , pp. 3120-3126, 2020.
@article{Sousa2020,
title = {Over 6% Efficient Cu(In,Ga)Se2 Solar Cell Screen-Printed from Oxides on Fluorine-Doped Tin Oxide},
author = {Viviana Sousa and Bruna F Gonçalves and Yitzchak S Rosen and José Virtuoso and Pedro Anacleto and Fátima M Cerqueira and Evgeny Modin and Pedro Alpuim and Oleg I Lebedev and Shlomo Magdassi and Sascha Sadewasser and Yury V Kolen’ko},
url = {https://doi.org/10.1021/acsaem.9b01999},
doi = {10.1021/acsaem.9b01999},
year = {2020},
date = {2020-03-31},
journal = {ACS Applied Energy Materials},
volume = {6},
pages = {3120-3126},
abstract = {A new approach to fabricate copper, indium, gallium diselenide (CIGSe) solar cells on conductive fluorine-doped tin oxide (FTO) reached an efficiency of over 6% for a champion photovoltaic device. Commercial oxide nanoparticles are formulated into high-quality screen-printable ink based on ethyl cellulose solution in terpineol. The high homogeneity and good adhesion properties of the oxide ink play an important role in obtaining dense and highly crystalline photoabsorber layers. This finding reveals that solution-based screen-printing from readily available oxide precursors provides an interesting cost-effective alternative to current vacuum- and energy-demanding processes of the CIGSe solar cell fabrication.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}A new approach to fabricate copper, indium, gallium diselenide (CIGSe) solar cells on conductive fluorine-doped tin oxide (FTO) reached an efficiency of over 6% for a champion photovoltaic device. Commercial oxide nanoparticles are formulated into high-quality screen-printable ink based on ethyl cellulose solution in terpineol. The high homogeneity and good adhesion properties of the oxide ink play an important role in obtaining dense and highly crystalline photoabsorber layers. This finding reveals that solution-based screen-printing from readily available oxide precursors provides an interesting cost-effective alternative to current vacuum- and energy-demanding processes of the CIGSe solar cell fabrication.David Fuster and Pedro Anacleto and José Virtuoso and Marco Zutter and Daniel Brito and Marina Alves and Luis Aparicio and David Fuertes Marrón and Fernando Briones and Sascha Sadewasser and Jorge M García
System for manufacturing complete Cu(In,Ga)Se2 solar cells in situ under vacuum Journal Article
Solar Energy, 198 , pp. 490-498, 2020.
@article{Fuster2020,
title = {System for manufacturing complete Cu(In,Ga)Se2 solar cells in situ under vacuum},
author = {David Fuster and Pedro Anacleto and José Virtuoso and Marco Zutter and Daniel Brito and Marina Alves and Luis Aparicio and David Fuertes Marrón and Fernando Briones and Sascha Sadewasser and Jorge M García},
url = {https://doi.org/10.1016/j.solener.2020.01.073},
doi = {10.1016/j.solener.2020.01.073},
year = {2020},
date = {2020-03-01},
journal = {Solar Energy},
volume = {198},
pages = {490-498},
abstract = {We present the development of a small foot-print physical vapor deposition (PVD) system for in-situ deposition of all layers required in a complete Cu(In,Ga)Se2 (CIGS) solar cell. Seven sputtering magnetrons and one valved-cracker source have been custom designed and manufactured for this system, named SpuTtering for Advanced Research (STAR). The purpose of STAR is to develop a technique to fabricate a complete CIGS solar cell, including contacts, absorber, buffer, and window layers, under high vacuum with the aim to transfer this technology to a future industrial production line. The system’s capabilities and its relatively high throughput place it somewhere in between research and industrial development levels. It is possible to work on the deposition of the back contact, the CIGS absorber, and the window layer of three solar cells simultaneously. Calibration data, selection of parameters for the deposition of the individual layers, and initial results of a complete CIGS solar cell developed with STAR are reported.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}We present the development of a small foot-print physical vapor deposition (PVD) system for in-situ deposition of all layers required in a complete Cu(In,Ga)Se2 (CIGS) solar cell. Seven sputtering magnetrons and one valved-cracker source have been custom designed and manufactured for this system, named SpuTtering for Advanced Research (STAR). The purpose of STAR is to develop a technique to fabricate a complete CIGS solar cell, including contacts, absorber, buffer, and window layers, under high vacuum with the aim to transfer this technology to a future industrial production line. The system’s capabilities and its relatively high throughput place it somewhere in between research and industrial development levels. It is possible to work on the deposition of the back contact, the CIGS absorber, and the window layer of three solar cells simultaneously. Calibration data, selection of parameters for the deposition of the individual layers, and initial results of a complete CIGS solar cell developed with STAR are reported.Susanne Siebentritt and Enrico Avancini and Marcus Bär and Jakob Bombsch and Emilie Bourgeois and Stephan Buecheler and Romain Carron and Celia Castro and Sebastien Duguay and Roberto Félix and Evelyn Handick and Dimitrios Hariskos and Ville Havu and Philip Jackson and Hannu-Pekka Komsa and Thomas Kunze and Maria Malitckaya and Roberto Menozzi and Milos Nesladek and Nicoleta Nicoara and Martti Puska and Mohit Raghuwanshi and Philippe Pareige and Sascha Sadewasser and Giovanna Sozzi and Ayodhya Nath Tiwari and Shigenori Ueda and Arantxa Vilalta-Clemente and Thomas Paul Weiss and Florian Werner and Regan G Wilks and Wolfram Witte and Max Hilaire Wolter
Heavy alkali treatment of Cu(In,Ga)Se2 solar cells: surface versus bulk effects Journal Article
Advanced Energy Materials, 10 , pp. 1903752, 2020.
@article{Siebentritt2020,
title = {Heavy alkali treatment of Cu(In,Ga)Se2 solar cells: surface versus bulk effects},
author = {Susanne Siebentritt and Enrico Avancini and Marcus Bär and Jakob Bombsch and Emilie Bourgeois and Stephan Buecheler and Romain Carron and Celia Castro and Sebastien Duguay and Roberto Félix and Evelyn Handick and Dimitrios Hariskos and Ville Havu and Philip Jackson and Hannu-Pekka Komsa and Thomas Kunze and Maria Malitckaya and Roberto Menozzi and Milos Nesladek and Nicoleta Nicoara and Martti Puska and Mohit Raghuwanshi and Philippe Pareige and Sascha Sadewasser and Giovanna Sozzi and Ayodhya Nath Tiwari and Shigenori Ueda and Arantxa Vilalta-Clemente and Thomas Paul Weiss and Florian Werner and Regan G Wilks and Wolfram Witte and Max Hilaire Wolter},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/aenm.201903752},
doi = {10.1002/aenm.201903752},
year = {2020},
date = {2020-02-25},
journal = {Advanced Energy Materials},
volume = {10},
pages = {1903752},
abstract = {Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post-deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open-circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single-phase Cu-alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post-deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open-circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single-phase Cu-alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries. -
2019
Marina Alves and Ana Pérez-Rodr í and Phillip J Dale and César Dom í and Sascha Sadewasser
Thin-film micro-concentrator solar cells Journal Article
Journal of Physics: Energy, 2 , pp. 012001, 2019.
@article{Alves2019b,
title = {Thin-film micro-concentrator solar cells},
author = {Marina Alves and Ana Pérez-Rodr í and Phillip J Dale and César Dom í and Sascha Sadewasser},
url = {https://doi.org/10.1088%2F2515-7655%2Fab4289},
doi = {10.1088/2515-7655/ab4289},
year = {2019},
date = {2019-11-26},
journal = {Journal of Physics: Energy},
volume = {2},
pages = {012001},
abstract = {Photovoltaic (PV) energy conversion of sunlight into electricity is now a well-established technology and a strong further expansion of PV will be seen in the future to answer the increasing demand for clean and renewable energy. Concentrator PV (CPV) employs optical elements to concentrate sunlight onto small solar cells, offering the possibility of replacing expensive solar cells with more economic optical elements, and higher device power conversion efficiencies. While CPV has mainly been explored for highly efficient single-crystalline and multi-junction solar cells, the combination of thin-film solar cells with the concentration approach opens up new horizons in CPV. Typical fabrication of thin-film solar cells can be modified for efficient, high-throughput and parallel production of organized arrays of micro solar cells. Their combination with microlens arrays promises to deliver micro-concentrator solar modules with a similar form factor to present day flat-panel PV. Such thin-film micro-concentrator PV modules would use significantly less semiconductor solar cell material (reducing the use of critical raw materials) and lead to a higher energy production (by means of concentrated sunlight), with the potential to lead to a lower levelized cost of electricity. This review article gives an overview of the present state-of-the-art in the fabrication of thin-film micro solar cells based on Cu(In,Ga)Se2 absorber materials and introduces optical concentration systems that can be combined to build the future thin-film micro-concentrator PV technology.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Photovoltaic (PV) energy conversion of sunlight into electricity is now a well-established technology and a strong further expansion of PV will be seen in the future to answer the increasing demand for clean and renewable energy. Concentrator PV (CPV) employs optical elements to concentrate sunlight onto small solar cells, offering the possibility of replacing expensive solar cells with more economic optical elements, and higher device power conversion efficiencies. While CPV has mainly been explored for highly efficient single-crystalline and multi-junction solar cells, the combination of thin-film solar cells with the concentration approach opens up new horizons in CPV. Typical fabrication of thin-film solar cells can be modified for efficient, high-throughput and parallel production of organized arrays of micro solar cells. Their combination with microlens arrays promises to deliver micro-concentrator solar modules with a similar form factor to present day flat-panel PV. Such thin-film micro-concentrator PV modules would use significantly less semiconductor solar cell material (reducing the use of critical raw materials) and lead to a higher energy production (by means of concentrated sunlight), with the potential to lead to a lower levelized cost of electricity. This review article gives an overview of the present state-of-the-art in the fabrication of thin-film micro solar cells based on Cu(In,Ga)Se2 absorber materials and introduces optical concentration systems that can be combined to build the future thin-film micro-concentrator PV technology.Nicoleta Nicoara and Roby Manaligod and Philip Jackson and Dimitrios Hariskos and Wolfram Witte and Giovanna Sozzi and Roberto Menozzi and Sascha Sadewasser
Direct evidence for grain boundary passivation in Cu(In,Ga)Se2 solar cells through alkali-fluoride post deposition treatments Journal Article
Nature Communications, 10 , pp. 3980, 2019.
@article{Nicoara2019,
title = {Direct evidence for grain boundary passivation in Cu(In,Ga)Se2 solar cells through alkali-fluoride post deposition treatments},
author = {Nicoleta Nicoara and Roby Manaligod and Philip Jackson and Dimitrios Hariskos and Wolfram Witte and Giovanna Sozzi and Roberto Menozzi and Sascha Sadewasser},
url = {https://doi.org/10.1038/s41467-019-11996-y},
doi = {10.1038/s41467-019-11996-y},
year = {2019},
date = {2019-09-04},
journal = {Nature Communications},
volume = {10},
pages = {3980},
abstract = {The properties and performance of polycrystalline materials depend critically on the properties of their grain boundaries. Polycrystalline photovoltaic materials – e.g. hybrid halide perovskites, copper indium gallium diselenide (CIGSe) and cadmium telluride – have already demonstrated high efficiencies and promise cost-effective electricity supply. For CIGSe-based solar cells, an efficiency above 23% has recently been achieved using an alkali-fluoride post-deposition treatment; however, its full impact and functional principle are not yet fully understood. Here, we show direct evidence for the passivation of grain boundaries in CIGSe treated with three different alkali-fluorides through a detailed study of the nanoscale optoelectronic properties. We determine a correlation of the surface potential change at grain boundaries with the open-circuit voltage, which is supported by numerical simulations. Our results suggest that heavier alkali elements might lead to better passivation by reducing the density of charged defects and increasing the formation of secondary phases at grain boundaries.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The properties and performance of polycrystalline materials depend critically on the properties of their grain boundaries. Polycrystalline photovoltaic materials – e.g. hybrid halide perovskites, copper indium gallium diselenide (CIGSe) and cadmium telluride – have already demonstrated high efficiencies and promise cost-effective electricity supply. For CIGSe-based solar cells, an efficiency above 23% has recently been achieved using an alkali-fluoride post-deposition treatment; however, its full impact and functional principle are not yet fully understood. Here, we show direct evidence for the passivation of grain boundaries in CIGSe treated with three different alkali-fluorides through a detailed study of the nanoscale optoelectronic properties. We determine a correlation of the surface potential change at grain boundaries with the open-circuit voltage, which is supported by numerical simulations. Our results suggest that heavier alkali elements might lead to better passivation by reducing the density of charged defects and increasing the formation of secondary phases at grain boundaries.Marek Piotrowski and Jérôme Borme and Enrique Carbó-Argibay and Deepanjan Sharma and Nicoleta Nicoara and Sascha Sadewasser and Dmitri Y Petrovykh and Carlos Rodríguez-Abreu and Yury V Kolen{'}ko
Template-directed self-organization of colloidal PbTe nanocrystals into pillars, conformal coatings, and self-supported membranes Journal Article
Nanoscale Adv., 1 , pp. 3049-3055, 2019.
@article{Piotrowski2019,
title = {Template-directed self-organization of colloidal PbTe nanocrystals into pillars, conformal coatings, and self-supported membranes},
author = {Marek Piotrowski and Jérôme Borme and Enrique Carbó-Argibay and Deepanjan Sharma and Nicoleta Nicoara and Sascha Sadewasser and Dmitri Y Petrovykh and Carlos Rodríguez-Abreu and Yury V Kolen{'}ko},
url = {http://dx.doi.org/10.1039/C9NA00370C},
doi = {10.1039/C9NA00370C},
year = {2019},
date = {2019-06-17},
journal = {Nanoscale Adv.},
volume = {1},
pages = {3049-3055},
abstract = {We demonstrate the formation of three morphologies relevant for integration with miniaturized devices—microscale pillars, conformal coatings, and self-supported membranes—via template-directed self-organization of lead telluride (PbTe) colloidal nanocrystals (NCs). Optimizing the self-organization process towards producing one of these morphologies typically involves adjusting the surface chemistry of the particles, as a means of controlling the particle–particle and particle–template interactions. In contrast, we have produced each of the three morphologies of close-packed NCs by adjusting only the solvent and concentration of NCs, to ensure that the high quality of the ca. 10 nm PbTe NCs produced by hot-injection colloidal synthesis, which we used as model “building blocks,” remains consistent across all three configurations. For the first two morphologies, the NCs were deposited as colloidal suspensions onto micropatterned silicon substrates. The microscale cuboid pillars (1 μm × 1 μm × 0.6 μm) were formed by depositing NC dispersions in toluene onto templates patterned with resist grid motifs, followed by the resist removal after the slow evaporation of toluene and formation of the micropillars. Conformal coatings were produced by switching the solvent from toluene to a faster drying hexane and pouring NC dispersions onto silicon templates with topographically patterned microstructures. In a similar process, self-supported NC membranes were formed from NC dispersions in hexane on the surface of diethylene glycol and transferred onto the micropatterned templates. The demonstrated combination of bottom-up self-organization with top-down micropatterned templates provides a scalable route for design and fabrication of NC ensembles in morphologies and form-factors that are compatible with their integration into miniaturized devices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}We demonstrate the formation of three morphologies relevant for integration with miniaturized devices—microscale pillars, conformal coatings, and self-supported membranes—via template-directed self-organization of lead telluride (PbTe) colloidal nanocrystals (NCs). Optimizing the self-organization process towards producing one of these morphologies typically involves adjusting the surface chemistry of the particles, as a means of controlling the particle–particle and particle–template interactions. In contrast, we have produced each of the three morphologies of close-packed NCs by adjusting only the solvent and concentration of NCs, to ensure that the high quality of the ca. 10 nm PbTe NCs produced by hot-injection colloidal synthesis, which we used as model “building blocks,” remains consistent across all three configurations. For the first two morphologies, the NCs were deposited as colloidal suspensions onto micropatterned silicon substrates. The microscale cuboid pillars (1 μm × 1 μm × 0.6 μm) were formed by depositing NC dispersions in toluene onto templates patterned with resist grid motifs, followed by the resist removal after the slow evaporation of toluene and formation of the micropillars. Conformal coatings were produced by switching the solvent from toluene to a faster drying hexane and pouring NC dispersions onto silicon templates with topographically patterned microstructures. In a similar process, self-supported NC membranes were formed from NC dispersions in hexane on the surface of diethylene glycol and transferred onto the micropatterned templates. The demonstrated combination of bottom-up self-organization with top-down micropatterned templates provides a scalable route for design and fabrication of NC ensembles in morphologies and form-factors that are compatible with their integration into miniaturized devices.Marco Zutter and Jose Virtuoso and Pedro Anacleto and Liam Yasin and Marina Alves and Miguel Madeira and Oleksandr Bondarchuk and Saibal Mitra and David Fuster Signes and Jorge M Garcia and Fernando Briones and Rolf Waechter and Oliver Kiowski and Dimitrios Hariskos and Diego Colombara and Sascha Sadewasser
Giant Voc Boost of Low-Temperature Annealed Cu(In,Ga)Se2 with Sputtered Zn(O,S) Buffers Journal Article
physica status solidi (RRL) – Rapid Research Letters, 13 , pp. 1900145, 2019.
@article{Zutter2019,
title = {Giant Voc Boost of Low-Temperature Annealed Cu(In,Ga)Se2 with Sputtered Zn(O,S) Buffers},
author = {Marco Zutter and Jose Virtuoso and Pedro Anacleto and Liam Yasin and Marina Alves and Miguel Madeira and Oleksandr Bondarchuk and Saibal Mitra and David Fuster Signes and Jorge M Garcia and Fernando Briones and Rolf Waechter and Oliver Kiowski and Dimitrios Hariskos and Diego Colombara and Sascha Sadewasser},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/pssr.201900145},
doi = {10.1002/pssr.201900145},
year = {2019},
date = {2019-05-30},
journal = {physica status solidi (RRL) – Rapid Research Letters},
volume = {13},
pages = {1900145},
abstract = {Large-scale industrial fabrication of Cu(In,Ga)Se2 (CIGS) photovoltaic panels would benefit significantly if the buffer layer chemical bath deposition could be replaced by a cadmium-free dry vacuum process suitable for in-line production. This Letter reports on the development of a Zn(O,S) buffer layer deposited by vacuum-based magnetron sputtering from a single target onto commercial CIGS absorbers cut from a module-size glass/Mo/CIGS stack. The buffer-window stack consisting of Zn(O0.75S0.25)/i-ZnO/ZnO:Al is optimized for layer thickness and optical and electronic properties, leading to an average device efficiency of 4.7%, which can be improved by annealing at 200 °C to a maximum of 10.5%, mainly due to a considerable increase in the open-circuit voltage (Voc). Temperature-dependent current density versus voltage (J–V) characteristics show a reduced interface recombination upon annealing, explaining the observed Voc boost. Quantum efficiency shows improvements in the long and short wavelength region, setting in at different annealing temperatures, and photoemission depth profiling indicates interdiffusion of all atomic species at the CIGS/Zn(O,S) interface. Electrical device simulations explain the observed effects by a modification of the band offset at the interface and defects passivation. Both effects are attributed to the observed interdiffusion during annealing.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Large-scale industrial fabrication of Cu(In,Ga)Se2 (CIGS) photovoltaic panels would benefit significantly if the buffer layer chemical bath deposition could be replaced by a cadmium-free dry vacuum process suitable for in-line production. This Letter reports on the development of a Zn(O,S) buffer layer deposited by vacuum-based magnetron sputtering from a single target onto commercial CIGS absorbers cut from a module-size glass/Mo/CIGS stack. The buffer-window stack consisting of Zn(O0.75S0.25)/i-ZnO/ZnO:Al is optimized for layer thickness and optical and electronic properties, leading to an average device efficiency of 4.7%, which can be improved by annealing at 200 °C to a maximum of 10.5%, mainly due to a considerable increase in the open-circuit voltage (Voc). Temperature-dependent current density versus voltage (J–V) characteristics show a reduced interface recombination upon annealing, explaining the observed Voc boost. Quantum efficiency shows improvements in the long and short wavelength region, setting in at different annealing temperatures, and photoemission depth profiling indicates interdiffusion of all atomic species at the CIGS/Zn(O,S) interface. Electrical device simulations explain the observed effects by a modification of the band offset at the interface and defects passivation. Both effects are attributed to the observed interdiffusion during annealing.H Limborço and P M Salomé and R Ribeiro-Andrade and J P Teixeira and N Nicoara and K Abderrafi and J P Leitão and J C Gonzalez and S Sadewasser
CuInSe2 quantum dots grown by molecular beam epitaxy on amorphous SiO2 surfaces Journal Article
Beilstein J. Nanotechnol., 10 , pp. 1103–1111, 2019.
@article{Limborço2019,
title = {CuInSe2 quantum dots grown by molecular beam epitaxy on amorphous SiO2 surfaces},
author = {H Limborço and P M Salomé and R Ribeiro-Andrade and J P Teixeira and N Nicoara and K Abderrafi and J P Leitão and J C Gonzalez and S Sadewasser},
url = {https://www.beilstein-journals.org/bjnano/articles/10/110},
doi = {10.3762/bjnano.10.110},
year = {2019},
date = {2019-05-22},
journal = {Beilstein J. Nanotechnol.},
volume = {10},
pages = {1103–1111},
abstract = {The currently most efficient polycrystalline solar cells are based on the Cu(In,Ga)Se2 compound as a light absorption layer. However, in view of new concepts of nanostructured solar cells, CuInSe2 nanostructures are of high interest. In this work, we report CuInSe2 nanodots grown through a vacuum-compatible co-evaporation growth process on an amorphous surface. The density, mean size, and peak optical emission energy of the nanodots can be controlled by changing the growth temperature. Scanning transmission electron microscopy measurements confirmed the crystallinity of the nanodots as well as chemical composition and structure compatible with tetragonal CuInSe2. Photoluminescence measurements of CdS-passivated nanodots showed that the nanodots are optoelectronically active with a broad emission extending to energies above the CuInSe2 bulk bandgap and in agreement with the distribution of sizes. A blue-shift of the luminescence is observed as the average size of the nanodots gets smaller, evidencing quantum confinement in all samples. By using simple quantum confinement calculations, we correlate the photoluminescence peak emission energy with the average size of the nanodots.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The currently most efficient polycrystalline solar cells are based on the Cu(In,Ga)Se2 compound as a light absorption layer. However, in view of new concepts of nanostructured solar cells, CuInSe2 nanostructures are of high interest. In this work, we report CuInSe2 nanodots grown through a vacuum-compatible co-evaporation growth process on an amorphous surface. The density, mean size, and peak optical emission energy of the nanodots can be controlled by changing the growth temperature. Scanning transmission electron microscopy measurements confirmed the crystallinity of the nanodots as well as chemical composition and structure compatible with tetragonal CuInSe2. Photoluminescence measurements of CdS-passivated nanodots showed that the nanodots are optoelectronically active with a broad emission extending to energies above the CuInSe2 bulk bandgap and in agreement with the distribution of sizes. A blue-shift of the luminescence is observed as the average size of the nanodots gets smaller, evidencing quantum confinement in all samples. By using simple quantum confinement calculations, we correlate the photoluminescence peak emission energy with the average size of the nanodots.David Correia and Daniel Siopa and Diego Colombara and Sara Tombolato and Pedro M P Salomé and Kamal Abderrafi and Pedro Anacleto and Phillip J Dale and Sascha Sadewasser
Area-selective electrodeposition of micro islands for CuInSe2-based photovoltaics Journal Article
Results in Physics, 12 , pp. 2136-2140, 2019.
@article{Correia2019,
title = {Area-selective electrodeposition of micro islands for CuInSe2-based photovoltaics},
author = {David Correia and Daniel Siopa and Diego Colombara and Sara Tombolato and Pedro M P Salomé and Kamal Abderrafi and Pedro Anacleto and Phillip J Dale and Sascha Sadewasser},
url = {http://www.sciencedirect.com/science/article/pii/S2211379719305339},
doi = {10.1016/j.rinp.2019.02.047},
year = {2019},
date = {2019-02-20},
journal = {Results in Physics},
volume = {12},
pages = {2136-2140},
abstract = {For mass fabrication of highly-efficient photovoltaic modules based on Cu(In,Ga)Se2 (CIGSe) absorber layers the availability and cost of the critical raw materials In and Ga present a potential bottleneck. The micro-concentrator solar cell concept provides a solution by using micro lenses to concentrate incoming sun light on an array of micro-sized CIGSe solar cells. The challenge is to fabricate CIGSe micro islands in exactly the desired positions using only the required material. Here, we analyze the area-selective electrodeposition of CuInSe2 into holes in an insulating SiO2 template layer as a material-efficient fabrication approach. We observe that the deposition process shows a strong dependence on the hole size, with a faster deposition around the hole perimeter. Based on a model developed for electrochemical reactions at ultra-micro electrodes, we develop numerical simulations for the electrochemical deposition process. The simulations consider the changing micro-electrode geometry throughout the deposition process, and provide a reasonable fit to the experimental data. Finally, it is shown that CuInSe2 micro solar cells fabricated by electrodeposition reach efficiencies of 4.8% under 1 sun, providing a proof-of-concept demonstration meriting further development.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}For mass fabrication of highly-efficient photovoltaic modules based on Cu(In,Ga)Se2 (CIGSe) absorber layers the availability and cost of the critical raw materials In and Ga present a potential bottleneck. The micro-concentrator solar cell concept provides a solution by using micro lenses to concentrate incoming sun light on an array of micro-sized CIGSe solar cells. The challenge is to fabricate CIGSe micro islands in exactly the desired positions using only the required material. Here, we analyze the area-selective electrodeposition of CuInSe2 into holes in an insulating SiO2 template layer as a material-efficient fabrication approach. We observe that the deposition process shows a strong dependence on the hole size, with a faster deposition around the hole perimeter. Based on a model developed for electrochemical reactions at ultra-micro electrodes, we develop numerical simulations for the electrochemical deposition process. The simulations consider the changing micro-electrode geometry throughout the deposition process, and provide a reasonable fit to the experimental data. Finally, it is shown that CuInSe2 micro solar cells fabricated by electrodeposition reach efficiencies of 4.8% under 1 sun, providing a proof-of-concept demonstration meriting further development.R Ribeiro-Andrade and S Sahayaraj and B Vermang and M R Correia and S Sadewasser and J C González and P A Fernandes and P M P Salomé
Voids in Kesterites and the Influence of Lamellae Preparation by Focused Ion Beam for Transmission Electron Microscopy Analyses Journal Article
IEEE Journal of Photovoltaics, 9 , pp. 565-570, 2019.
@article{Ribeiro-Andrade2019,
title = {Voids in Kesterites and the Influence of Lamellae Preparation by Focused Ion Beam for Transmission Electron Microscopy Analyses},
author = {R Ribeiro-Andrade and S Sahayaraj and B Vermang and M R Correia and S Sadewasser and J C González and P A Fernandes and P M P Salomé},
url = {https://ieeexplore.ieee.org/document/8610192},
doi = {10.1109/JPHOTOV.2018.2889602},
year = {2019},
date = {2019-01-11},
journal = {IEEE Journal of Photovoltaics},
volume = {9},
pages = {565-570},
abstract = {Kesterite solar cells based on Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTSe) are potential future candidates to be used in thin-film solar cells. The technology still has to be developed to a great extent and for this to happen, high levels of confidence in the characterization methods are required, so that improvements can be made on solid interpretations. In this study, we show that the interpretations of one of the most used characterization techniques in kesterites, scanning transmission electron microscopy (STEM), might be affected by its specimen preparation when using focused ion beam (FIB). Using complementary measurements based on scanning electron microscopy and Raman scattering spectroscopy, compelling evidence shows that secondary phases of ZnSe mixed in the bulk of CZTSe are the likely cause of the appearance of voids in STEM lamellae. Sputtering simulations support this interpretation by showing that Zn in a ZnSe matrix is preferentially sputtered compared with any metal atom in a CZTSe matrix.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Kesterite solar cells based on Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTSe) are potential future candidates to be used in thin-film solar cells. The technology still has to be developed to a great extent and for this to happen, high levels of confidence in the characterization methods are required, so that improvements can be made on solid interpretations. In this study, we show that the interpretations of one of the most used characterization techniques in kesterites, scanning transmission electron microscopy (STEM), might be affected by its specimen preparation when using focused ion beam (FIB). Using complementary measurements based on scanning electron microscopy and Raman scattering spectroscopy, compelling evidence shows that secondary phases of ZnSe mixed in the bulk of CZTSe are the likely cause of the appearance of voids in STEM lamellae. Sputtering simulations support this interpretation by showing that Zn in a ZnSe matrix is preferentially sputtered compared with any metal atom in a CZTSe matrix. -
2018
Nicoleta Nicoara and Sylvie Harel and Thomas Lepetit and Ludovic Arzel and Nicolas Barreau and Sascha Sadewasser
Impact of KF post-deposition treatment on ageing of the Cu(In,Ga)Se2 surface and its interface with CdS Journal Article
ACS Applied Energy Materials, 1 , pp. 2681-2688, 2018.
@article{Nicoara2018,
title = {Impact of KF post-deposition treatment on ageing of the Cu(In,Ga)Se2 surface and its interface with CdS},
author = {Nicoleta Nicoara and Sylvie Harel and Thomas Lepetit and Ludovic Arzel and Nicolas Barreau and Sascha Sadewasser},
url = {https://doi.org/10.1021/acsaem.8b00365},
doi = {10.1021/acsaem.8b00365},
year = {2018},
date = {2018-05-10},
journal = {ACS Applied Energy Materials},
volume = {1},
pages = {2681-2688},
abstract = {Recent world record efficiencies for thin film solar cells based on Cu(In,Ga)Se2 (CIGS) have been realized with absorbers subject to alkali fluoride post deposition treatments (PDT). We investigated the effect of ambient air exposure on the electronic properties of CIGS with KF-PDT in a combined time-dependent Kelvin probe force microscopy and X-ray photoelectron spectroscopy study. We also studied the early stage formation of the absorber/buffer interface after the initial deposition of CdS in the chemical bath. Our study shows that the KF-PDT, as well as the CdS deposition process induce an increase in the overall surface work function, as compared to bare CIGS. A K–In–Se compound forms after the KF-PDT, accompanied by a stable In oxide which explains the remarkable stability of the contact potential difference to air exposure, confirming phenomenological observations in many laboratories. In clear contrast to the untreated CIGS, the KF-treated CIGS/CdS interface shows a significant variation in the surface potential (∼360 mV) over approximately 7 h air-exposure. We attribute this variation to a Cd–In intermixing at interface, whose chemical stability is susceptible to air-exposure. Our results contribute to the understanding of the electronic properties of the KF-treated and untreated CIGS/CdS junction during the early stages of the interface formation, which impact the overall device properties.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Recent world record efficiencies for thin film solar cells based on Cu(In,Ga)Se2 (CIGS) have been realized with absorbers subject to alkali fluoride post deposition treatments (PDT). We investigated the effect of ambient air exposure on the electronic properties of CIGS with KF-PDT in a combined time-dependent Kelvin probe force microscopy and X-ray photoelectron spectroscopy study. We also studied the early stage formation of the absorber/buffer interface after the initial deposition of CdS in the chemical bath. Our study shows that the KF-PDT, as well as the CdS deposition process induce an increase in the overall surface work function, as compared to bare CIGS. A K–In–Se compound forms after the KF-PDT, accompanied by a stable In oxide which explains the remarkable stability of the contact potential difference to air exposure, confirming phenomenological observations in many laboratories. In clear contrast to the untreated CIGS, the KF-treated CIGS/CdS interface shows a significant variation in the surface potential (∼360 mV) over approximately 7 h air-exposure. We attribute this variation to a Cd–In intermixing at interface, whose chemical stability is susceptible to air-exposure. Our results contribute to the understanding of the electronic properties of the KF-treated and untreated CIGS/CdS junction during the early stages of the interface formation, which impact the overall device properties.Sascha Sadewasser and Nicoleta Nicoara and Santiago D Solares
Artifacts in time-resolved Kelvin probe force microscopy Journal Article
Beilstein J. Nanotechnol., 9 , pp. 1272–1281, 2018.
@article{Sadewasser2018,
title = {Artifacts in time-resolved Kelvin probe force microscopy},
author = {Sascha Sadewasser and Nicoleta Nicoara and Santiago D Solares},
url = {https://www.beilstein-journals.org/bjnano/articles/9/119},
doi = {10.3762/bjnano.9.119},
year = {2018},
date = {2018-04-24},
journal = {Beilstein J. Nanotechnol.},
volume = {9},
pages = {1272–1281},
abstract = {Kelvin probe force microscopy (KPFM) has been used for the characterization of metals, insulators, and semiconducting materials on the nanometer scale. Especially in semiconductors, the charge dynamics are of high interest. Recently, several techniques for time-resolved measurements with time resolution down to picoseconds have been developed, many times using a modulated excitation signal, e.g., light modulation or bias modulation that induces changes in the charge carrier distribution. For fast modulation frequencies, the KPFM controller measures an average surface potential, which contains information about the involved charge carrier dynamics. Here, we show that such measurements are prone to artifacts due to frequency mixing, by performing numerical dynamics simulations of the cantilever oscillation in KPFM subjected to a bias-modulated signal. For square bias pulses, the resulting time-dependent electrostatic forces are very complex and result in intricate mixing of frequencies that may, in some cases, have a component at the detection frequency, leading to falsified KPFM measurements. Additionally, we performed fast Fourier transform (FFT) analyses that match the results of the numerical dynamics simulations. Small differences are observed that can be attributed to transients and higher-order Fourier components, as a consequence of the intricate nature of the cantilever driving forces. These results are corroborated by experimental measurements on a model system. In the experimental case, additional artifacts are observed due to constructive or destructive interference of the bias modulation with the cantilever oscillation. Also, in the case of light modulation, we demonstrate artifacts due to unwanted illumination of the photodetector of the beam deflection detection system. Finally, guidelines for avoiding such artifacts are given.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Kelvin probe force microscopy (KPFM) has been used for the characterization of metals, insulators, and semiconducting materials on the nanometer scale. Especially in semiconductors, the charge dynamics are of high interest. Recently, several techniques for time-resolved measurements with time resolution down to picoseconds have been developed, many times using a modulated excitation signal, e.g., light modulation or bias modulation that induces changes in the charge carrier distribution. For fast modulation frequencies, the KPFM controller measures an average surface potential, which contains information about the involved charge carrier dynamics. Here, we show that such measurements are prone to artifacts due to frequency mixing, by performing numerical dynamics simulations of the cantilever oscillation in KPFM subjected to a bias-modulated signal. For square bias pulses, the resulting time-dependent electrostatic forces are very complex and result in intricate mixing of frequencies that may, in some cases, have a component at the detection frequency, leading to falsified KPFM measurements. Additionally, we performed fast Fourier transform (FFT) analyses that match the results of the numerical dynamics simulations. Small differences are observed that can be attributed to transients and higher-order Fourier components, as a consequence of the intricate nature of the cantilever driving forces. These results are corroborated by experimental measurements on a model system. In the experimental case, additional artifacts are observed due to constructive or destructive interference of the bias modulation with the cantilever oscillation. Also, in the case of light modulation, we demonstrate artifacts due to unwanted illumination of the photodetector of the beam deflection detection system. Finally, guidelines for avoiding such artifacts are given.Pedro M P Salomé and Bart Vermang and Rodrigo Ribeiro-Andrade and Jennifer P Teixeira and José M V Cunha and Manuel J Mendes and Sirazul Haque and Jêrome Borme and Hugo Águas and Elvira Fortunato and Rodrigo Martins and Juan C González and Joaquim P Leitão and Paulo A Fernandes and Marika Edoff and Sascha Sadewasser
Passivation of Interfaces in Thin Film Solar Cells: Understanding the Effects of a Nanostructured Rear Point Contact Layer Journal Article
Advanced Materials Interfaces, 5 , pp. 1701101, 2018.
@article{Salomé2018,
title = {Passivation of Interfaces in Thin Film Solar Cells: Understanding the Effects of a Nanostructured Rear Point Contact Layer},
author = {Pedro M P Salomé and Bart Vermang and Rodrigo Ribeiro-Andrade and Jennifer P Teixeira and José M V Cunha and Manuel J Mendes and Sirazul Haque and Jêrome Borme and Hugo Águas and Elvira Fortunato and Rodrigo Martins and Juan C González and Joaquim P Leitão and Paulo A Fernandes and Marika Edoff and Sascha Sadewasser},
doi = {10.1002/admi.201701101},
year = {2018},
date = {2018-01-23},
journal = {Advanced Materials Interfaces},
volume = {5},
pages = {1701101},
abstract = {Thin film solar cells based in Cu(In,Ga)Se2 (CIGS) are among the most efficient polycrystalline solar cells, surpassing CdTe and even polycrystalline silicon solar cells. For further developments, the CIGS technology has to start incorporating different solar cell architectures and strategies that allow for very low interface recombination. In this work, ultrathin 350 nm CIGS solar cells with a rear interface passivation strategy are studied and characterized. The rear passivation is achieved using an Al2O3 nanopatterned point structure. Using the cell results, photoluminescence measurements, and detailed optical simulations based on the experimental results, it is shown that by including the nanopatterned point contact structure, the interface defect concentration lowers, which ultimately leads to an increase of solar cell electrical performance mostly by increase of the open circuit voltage. Gains to the short circuit current are distributed between an increased rear optical reflection and also due to electrical effects. The approach of mixing several techniques allows us to make a discussion considering the different passivation gains, which has not been done in detail in previous works. A solar cell with a nanopatterned rear contact and a 350 nm thick CIGS absorber provides an average power conversion efficiency close to 10%.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Thin film solar cells based in Cu(In,Ga)Se2 (CIGS) are among the most efficient polycrystalline solar cells, surpassing CdTe and even polycrystalline silicon solar cells. For further developments, the CIGS technology has to start incorporating different solar cell architectures and strategies that allow for very low interface recombination. In this work, ultrathin 350 nm CIGS solar cells with a rear interface passivation strategy are studied and characterized. The rear passivation is achieved using an Al2O3 nanopatterned point structure. Using the cell results, photoluminescence measurements, and detailed optical simulations based on the experimental results, it is shown that by including the nanopatterned point contact structure, the interface defect concentration lowers, which ultimately leads to an increase of solar cell electrical performance mostly by increase of the open circuit voltage. Gains to the short circuit current are distributed between an increased rear optical reflection and also due to electrical effects. The approach of mixing several techniques allows us to make a discussion considering the different passivation gains, which has not been done in detail in previous works. A solar cell with a nanopatterned rear contact and a 350 nm thick CIGS absorber provides an average power conversion efficiency close to 10%. -
2017
Nicoleta Nicoara and Thomas Kunze and Philip Jackson and Dimitrios Hariskos and Roberto Félix Duarte and Regan G Wilks and Wolfram Witte and Marcus Bär and Sascha Sadewasser
Evidence for chemical and electronic non-uniformities in the formation of the interface of RbF-treated Cu(In,Ga)Se2 with CdS Journal Article
ACS Appl. Mater. Interfaces, 9 , pp. 44173−44180, 2017.
@article{Nicoara2017b,
title = {Evidence for chemical and electronic non-uniformities in the formation of the interface of RbF-treated Cu(In,Ga)Se2 with CdS},
author = {Nicoleta Nicoara and Thomas Kunze and Philip Jackson and Dimitrios Hariskos and Roberto Félix Duarte and Regan G Wilks and Wolfram Witte and Marcus Bär and Sascha Sadewasser},
url = {https://pubs.acs.org/doi/10.1021/acsami.7b12448},
doi = {10.1021/acsami.7b12448},
year = {2017},
date = {2017-11-27},
journal = {ACS Appl. Mater. Interfaces},
volume = {9},
pages = {44173−44180},
abstract = {We report on the initial stages of CdS buffer layer formation on Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers subjected to rubidium fluoride (RbF) postdeposition treatment (PDT). A detailed characterization of the CIGSe/CdS interface for different chemical bath deposition (CBD) times of the CdS layer is obtained from spatially resolved atomic and Kelvin probe force microscopy and laterally integrating X-ray spectroscopies. The observed spatial inhomogeneity in the interface’s structural, chemical, and electronic properties of samples undergoing up to 3 min of CBD treatments is indicative of a complex interface formation including an incomplete coverage and/or nonuniform composition of the buffer layer. It is expected that this result impacts solar cell performance, in particular when reducing the CdS layer thickness (e.g., in an attempt to increase the collection in the ultraviolet wavelength region). Our work provides important findings on the absorber/buffer interface formation and reveals the underlying mechanism for limitations in the reduction of the CdS thickness, even when an alkali PDT is applied to the CIGSe absorber.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}We report on the initial stages of CdS buffer layer formation on Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers subjected to rubidium fluoride (RbF) postdeposition treatment (PDT). A detailed characterization of the CIGSe/CdS interface for different chemical bath deposition (CBD) times of the CdS layer is obtained from spatially resolved atomic and Kelvin probe force microscopy and laterally integrating X-ray spectroscopies. The observed spatial inhomogeneity in the interface’s structural, chemical, and electronic properties of samples undergoing up to 3 min of CBD treatments is indicative of a complex interface formation including an incomplete coverage and/or nonuniform composition of the buffer layer. It is expected that this result impacts solar cell performance, in particular when reducing the CdS layer thickness (e.g., in an attempt to increase the collection in the ultraviolet wavelength region). Our work provides important findings on the absorber/buffer interface formation and reveals the underlying mechanism for limitations in the reduction of the CdS thickness, even when an alkali PDT is applied to the CIGSe absorber.Nicolas Barreau and Agathe Frelon and Thomas Lepetit and Eric Gautron and Nicolas Gautier and Rodrigo Ribeiro-Andrade and Nicoleta Nicoara and Sascha Sadewasser and Pawel Zabierowski and Ludovic Arzel and Léo Choubrac and Sylvie Harel and Catherine Deudon and Camille Latouche and Stéphane Jobic and Maite Caldes and Lionel Assmann and Polyxeni Tsoulka and Emmanuel V Péan and Justine Lorthioir and Frédéric Geschier and Isabelle Braems and Matthieu Moret and Olivier Briot and Guy Ouvrard
High efficiency solar cell based on full PVD processed Cu(In,Ga)Se2/CdIn2S4 heterojunction Journal Article
Solar RRL, 1 , pp. 1700140, 2017.
@article{Barreau2017,
title = {High efficiency solar cell based on full PVD processed Cu(In,Ga)Se2/CdIn2S4 heterojunction},
author = {Nicolas Barreau and Agathe Frelon and Thomas Lepetit and Eric Gautron and Nicolas Gautier and Rodrigo Ribeiro-Andrade and Nicoleta Nicoara and Sascha Sadewasser and Pawel Zabierowski and Ludovic Arzel and Léo Choubrac and Sylvie Harel and Catherine Deudon and Camille Latouche and Stéphane Jobic and Maite Caldes and Lionel Assmann and Polyxeni Tsoulka and Emmanuel V Péan and Justine Lorthioir and Frédéric Geschier and Isabelle Braems and Matthieu Moret and Olivier Briot and Guy Ouvrard},
url = {https://doi.org/10.1002/solr.201700140},
doi = {10.1002/solr.201700140},
year = {2017},
date = {2017-10-10},
journal = {Solar RRL},
volume = {1},
pages = {1700140},
abstract = {The original goal of our study is to synthesize by co‐evaporation the phase that could be formed at the interface between polycrystalline p‐Cu(In,Ga)Se2 treated with KF and n‐CdS. Hence, a new buffer layer, CdIn2S4 (C24), deposited by co‐evaporation is presented for the use in thin film solar cells, exhibiting device efficiencies as high as 16.2%, comparable to that obtained on a reference standard CdS‐buffered device. The physico‐chemical and optical properties of close to stoichiometry 400 nm‐thick films of C24 show similar properties to what has been reported in the literature for single crystals. The layer stack used for solar cells is investigated by transmission electron microscopy, showing the formation of an ultrathin Cd‐deficient C24 layer at the CIGSe/C24 interface, while a clear lattice match is observed at the C24/ZnO interface. Advanced electrical characterizations of the devices suggest that the output voltage and fill factor of the solar cells based on Cu(In,Ga)Se2/(PVD)C24 are limited by tunneling‐enhanced recombination through extended band tail states. These results open new routes to explain the superiority of wet processes used for the junction formation compared to vacuum‐based approaches.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The original goal of our study is to synthesize by co‐evaporation the phase that could be formed at the interface between polycrystalline p‐Cu(In,Ga)Se2 treated with KF and n‐CdS. Hence, a new buffer layer, CdIn2S4 (C24), deposited by co‐evaporation is presented for the use in thin film solar cells, exhibiting device efficiencies as high as 16.2%, comparable to that obtained on a reference standard CdS‐buffered device. The physico‐chemical and optical properties of close to stoichiometry 400 nm‐thick films of C24 show similar properties to what has been reported in the literature for single crystals. The layer stack used for solar cells is investigated by transmission electron microscopy, showing the formation of an ultrathin Cd‐deficient C24 layer at the CIGSe/C24 interface, while a clear lattice match is observed at the C24/ZnO interface. Advanced electrical characterizations of the devices suggest that the output voltage and fill factor of the solar cells based on Cu(In,Ga)Se2/(PVD)C24 are limited by tunneling‐enhanced recombination through extended band tail states. These results open new routes to explain the superiority of wet processes used for the junction formation compared to vacuum‐based approaches.K Abderrafi and R.-Ribeiro Andrade and N Nicoara and M F Cerqueira and Gonzalez M Debs and H Limborço and P M P Salomé and J C Gonzalez and F Briones and J M Garcia and S Sadewasser
Epitaxial CuInSe2 thin films grown by molecular beam epitaxy and migration enhanced epitaxy Journal Article
J. Crystal Growth, 475 , pp. 300-306, 2017.
@article{Abderrafi2017,
title = {Epitaxial CuInSe2 thin films grown by molecular beam epitaxy and migration enhanced epitaxy},
author = {K Abderrafi and R.-Ribeiro Andrade and N Nicoara and M F Cerqueira and Gonzalez M Debs and H Limborço and P M P Salomé and J C Gonzalez and F Briones and J M Garcia and S Sadewasser},
url = {https://doi.org/10.1016/j.jcrysgro.2017.07.010},
year = {2017},
date = {2017-10-01},
journal = {J. Crystal Growth},
volume = {475},
pages = {300-306},
abstract = {While CuInSe2 chalcopyrite materials are mainly used in their polycrystalline form to prepare thin film solar cells, epitaxial layers have been used for the characterization of defects. Typically, epitaxial layers are grown by metal-organic vapor phase epitaxy or molecular beam epitaxy (MBE). Here we present epitaxial layers grown by migration enhanced epitaxy (MEE) and compare the materials quality to MBE grown layers. CuInSe2 layers were grown on GaAs (0 0 1) substrates by co-evaporation of Cu, In, and Se using substrate temperatures of 450 °C, 530 °C, and 620 °C. The layers were characterized by high resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and atomic force microscopy (AFM). HR-XRD and HR-TEM show a better crystalline quality of the MEE grown layers, and Raman scattering measurements confirm single phase CuInSe2. AFM shows the previously observed faceting of the (0 0 1) surface into 1 1 2 facets with trenches formed along the [1 1 0] direction. The surface of MEE-grown samples appears smoother compared to MBE-grown samples, a similar trend is observed with increasing growth temperature.Keywords: A3: Migration enhanced epitaxy; A3: Molecular beam epitaxy; B2: Semiconducting ternary compounds; A1: Crystal structure; A1: Surface structure},
keywords = {},
pubstate = {published},
tppubtype = {article}
}While CuInSe2 chalcopyrite materials are mainly used in their polycrystalline form to prepare thin film solar cells, epitaxial layers have been used for the characterization of defects. Typically, epitaxial layers are grown by metal-organic vapor phase epitaxy or molecular beam epitaxy (MBE). Here we present epitaxial layers grown by migration enhanced epitaxy (MEE) and compare the materials quality to MBE grown layers. CuInSe2 layers were grown on GaAs (0 0 1) substrates by co-evaporation of Cu, In, and Se using substrate temperatures of 450 °C, 530 °C, and 620 °C. The layers were characterized by high resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and atomic force microscopy (AFM). HR-XRD and HR-TEM show a better crystalline quality of the MEE grown layers, and Raman scattering measurements confirm single phase CuInSe2. AFM shows the previously observed faceting of the (0 0 1) surface into 1 1 2 facets with trenches formed along the [1 1 0] direction. The surface of MEE-grown samples appears smoother compared to MBE-grown samples, a similar trend is observed with increasing growth temperature.Keywords: A3: Migration enhanced epitaxy; A3: Molecular beam epitaxy; B2: Semiconducting ternary compounds; A1: Crystal structure; A1: Surface structure
S Sadewasser
Geometry and materials considerations for thin film micro-concentrator solar cells Journal Article
Solar Energy, 159 , pp. 186-191, 2017.
@article{Sadewasser2017b,
title = {Geometry and materials considerations for thin film micro-concentrator solar cells},
author = {S Sadewasser},
url = {http://dx.doi.org/10.1016/j.solener.2017.09.035},
doi = {10.1016/j.solener.2017.09.035},
year = {2017},
date = {2017-09-15},
journal = {Solar Energy},
volume = {159},
pages = {186-191},
abstract = {Using concentrated sunlight for photovoltaic energy conversion has long been identified as a way to make cost-intensive solar cell materials and devices more cost effective. The recently proposed micro-concentrator approach for Cu(In,Ga)Se2 brings concentrator photovoltaics from the centimeter and millimeter scale down to the micrometer scale, with the goal of reducing the amount of critical raw materials. We show here that the micro-concentrator approach has large benefits in terms of the heat dissipation and that heat management on the micro-scale allows the use of concentration factors as high as 1000× without any special cooling efforts. Specifically, we show that line-shaped micro-concentrator solar cells need to be either smaller than 50 μm wide, or be used at low concentration factors below 100×. In contrast, island-shaped micro-concentrator solar cells show improved efficiencies at cell sizes of (100 μm)2 and concentration factors up to 1000×. Additionally, we consider different materials for the substrate and the lens array for light concentration in view of their heat management capacities. The results presented here provide design guidelines for the further development of thin film micro-concentrator solar cells, applicable to a variety of materials systems, e.g. Cu(In,Ga)Se2, CdTe, or metal halide perovskite solar cells.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Using concentrated sunlight for photovoltaic energy conversion has long been identified as a way to make cost-intensive solar cell materials and devices more cost effective. The recently proposed micro-concentrator approach for Cu(In,Ga)Se2 brings concentrator photovoltaics from the centimeter and millimeter scale down to the micrometer scale, with the goal of reducing the amount of critical raw materials. We show here that the micro-concentrator approach has large benefits in terms of the heat dissipation and that heat management on the micro-scale allows the use of concentration factors as high as 1000× without any special cooling efforts. Specifically, we show that line-shaped micro-concentrator solar cells need to be either smaller than 50 μm wide, or be used at low concentration factors below 100×. In contrast, island-shaped micro-concentrator solar cells show improved efficiencies at cell sizes of (100 μm)2 and concentration factors up to 1000×. Additionally, we consider different materials for the substrate and the lens array for light concentration in view of their heat management capacities. The results presented here provide design guidelines for the further development of thin film micro-concentrator solar cells, applicable to a variety of materials systems, e.g. Cu(In,Ga)Se2, CdTe, or metal halide perovskite solar cells.P M P Salomé and R Ribeiro-Andrade and J P Teixeira and J Keller and T Törndahl and N Nicoara and M Edoff and J C González and J P Leitão and S Sadewasser
Cd and Cu inter-diffusion in Cu(In,Ga)Se2/CdS hetero-interfaces Journal Article
IEEE Journal of Photovoltaics, Vol. 7 (no 3), pp. 858-863 p., 2017, ISSN: ISSN 2156-3381.
@article{Andrade2017,
title = {Cd and Cu inter-diffusion in Cu(In,Ga)Se2/CdS hetero-interfaces},
author = {P M P Salomé and R Ribeiro-Andrade and J P Teixeira and J Keller and T Törndahl and N Nicoara and M Edoff and J C González and J P Leitão and S Sadewasser},
url = {http://ieeexplore.ieee.org/document/7865928/},
doi = {10.1109/JPHOTOV.2017.2666550},
issn = {ISSN 2156-3381},
year = {2017},
date = {2017-05-29},
journal = {IEEE Journal of Photovoltaics},
volume = {Vol. 7},
number = {no 3},
pages = {858-863 p.},
abstract = {We report a detailed characterization of an industrylike prepared Cu(In, Ga) Se-2 (CIGS)/CdS heterojunction by scanning transmission electron microscopy and photoluminescence (PL). Energy dispersive X-ray spectroscopy shows the presence of several regions in the CIGS layer that are Cu deprived and Cd enriched, suggesting the segregation of Cd-Se. Concurrently, the CdS layer shows Cd-deprived regions with the presence of Cu, suggesting a segregation of Cu-S. The two types of segregations are always found together, which, to the best of our knowledge, is observed for the first time. The results indicate that there is a diffusion process that replaces Cu with Cd in the CIGS layer and Cd with Cu in the CdS layer. Using a combinatorial approach, we identified that this effect is independent of focused-ion beam sample preparation and of the transmission electron microscopy grid. Furthermore, PL measurements before and after an HCl etch indicate a lower degree of defects in the postetch sample, compatible with the segregates removal. We hypothesize that Cu2-x Se nanodomains react during the chemical bath process to form these segregates since the chemical reaction that dominates this process is thermodynamically favorable. These results provide important additional information about the formation of the CIGS/CdS interface.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}We report a detailed characterization of an industrylike prepared Cu(In, Ga) Se-2 (CIGS)/CdS heterojunction by scanning transmission electron microscopy and photoluminescence (PL). Energy dispersive X-ray spectroscopy shows the presence of several regions in the CIGS layer that are Cu deprived and Cd enriched, suggesting the segregation of Cd-Se. Concurrently, the CdS layer shows Cd-deprived regions with the presence of Cu, suggesting a segregation of Cu-S. The two types of segregations are always found together, which, to the best of our knowledge, is observed for the first time. The results indicate that there is a diffusion process that replaces Cu with Cd in the CIGS layer and Cd with Cu in the CdS layer. Using a combinatorial approach, we identified that this effect is independent of focused-ion beam sample preparation and of the transmission electron microscopy grid. Furthermore, PL measurements before and after an HCl etch indicate a lower degree of defects in the postetch sample, compatible with the segregates removal. We hypothesize that Cu2-x Se nanodomains react during the chemical bath process to form these segregates since the chemical reaction that dominates this process is thermodynamically favorable. These results provide important additional information about the formation of the CIGS/CdS interface.N Nicoara and Th. Lepetit and L Arzel and S Harel and N Barreau and S Sadewasser
Effect of the KF post-deposition treatment on grain boundary properties in Cu(In, Ga)Se2 thin films Journal Article
Scientific Reports 7, (Article number: 41361), 2017.
@article{Nicoara2017b,
title = {Effect of the KF post-deposition treatment on grain boundary properties in Cu(In, Ga)Se2 thin films},
author = {N Nicoara and Th. Lepetit and L Arzel and S Harel and N Barreau and S Sadewasser},
url = {https://www.nature.com/articles/srep41361},
doi = {doi:10.1038/srep41361},
year = {2017},
date = {2017-01-27},
journal = {Scientific Reports 7},
number = {Article number: 41361},
abstract = {Significant power conversion efficiency improvements have recently been achieved for thin-film solar cells based on a variety of polycrystalline absorbers, including perovskites, CdTe, and Cu(In,Ga)Se2 (CIGS). The passivation of grain boundaries (GBs) through (post-deposition) treatments is a crucial step for this success. For the case of CIGS, the introduction of a potassium fluoride post-deposition treatment (KF-PDT) has boosted their power conversion efficiency to the best performance of all polycrystalline solar cells. Direct and indirect effects of potassium at the interface and interface-near region in the CIGS layer are thought to be responsible for this improvement. Here, we show that also the electronic properties of the GBs are beneficially modified by the KF-PDT. We used Kelvin probe force microscopy to study the effect of the KF-PDT on the CIGS surface by spatially resolved imaging of the surface potential. We find a clear difference for the GB electronic properties: the KF-PDT increases the band bending at GBs by about 70% and results in a narrower distribution of work function values at the GBs. This effect of the KF-PDT on the GB electronic properties is expected to contribute to the improved efficiency values observed for CIGS thin-film solar cells with KF-PDT.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Significant power conversion efficiency improvements have recently been achieved for thin-film solar cells based on a variety of polycrystalline absorbers, including perovskites, CdTe, and Cu(In,Ga)Se2 (CIGS). The passivation of grain boundaries (GBs) through (post-deposition) treatments is a crucial step for this success. For the case of CIGS, the introduction of a potassium fluoride post-deposition treatment (KF-PDT) has boosted their power conversion efficiency to the best performance of all polycrystalline solar cells. Direct and indirect effects of potassium at the interface and interface-near region in the CIGS layer are thought to be responsible for this improvement. Here, we show that also the electronic properties of the GBs are beneficially modified by the KF-PDT. We used Kelvin probe force microscopy to study the effect of the KF-PDT on the CIGS surface by spatially resolved imaging of the surface potential. We find a clear difference for the GB electronic properties: the KF-PDT increases the band bending at GBs by about 70% and results in a narrower distribution of work function values at the GBs. This effect of the KF-PDT on the GB electronic properties is expected to contribute to the improved efficiency values observed for CIGS thin-film solar cells with KF-PDT.Keller Törndahl Sadewasser J T S P.M.P. Salomé J.P. Teixeira and J P Leitão
Influence of CdS and ZnSnO Buffer Layers on the Photoluminescence of Cu(In,Ga)Se2 Thin Films Journal Article
IEEE Journal of Photovoltaics, Volume: 7 (Issue: 2), pp. 670 - 675, 2017.
@article{Salomé2017b,
title = {Influence of CdS and ZnSnO Buffer Layers on the Photoluminescence of Cu(In,Ga)Se2 Thin Films},
author = {Keller Törndahl Sadewasser J T S P.M.P. Salomé J.P. Teixeira and J P Leitão},
url = {http://ieeexplore.ieee.org/document/7822954/},
doi = {10.1109/JPHOTOV.2016.2639347},
year = {2017},
date = {2017-01-18},
journal = {IEEE Journal of Photovoltaics},
volume = {Volume: 7},
number = {Issue: 2},
pages = {670 - 675},
abstract = {The search for alternatives to the CdS buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells has turned out to be quite promising in terms of power conversion efficiency. In this paper, the typically used chemical-bath-deposited CdS layer is compared with an atomic-layer-deposited Zn1-xSnxOy (ZnSnO). An optical study by external quantum efficiency and photoluminescence on the influence of different buffer layers on the defect properties of CIGS is presented. For both buffer layers, the CIGS bulk and CIGS/buffer interface are strongly influenced by electrostatic fluctuating potentials, which are less pronounced for the sample with the ZnSnO buffer layer. This is associated with a lower concentration of donor defects at the CIGS near-interface layer. A change in the bandgap of the CIGS as a consequence of the buffer layer deposition is observed. This study expands the knowledge of defects in the complex quaternary semiconductor CIGS, which, as discussed, can be affected even by the choice of buffer layer and its deposition process.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The search for alternatives to the CdS buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells has turned out to be quite promising in terms of power conversion efficiency. In this paper, the typically used chemical-bath-deposited CdS layer is compared with an atomic-layer-deposited Zn1-xSnxOy (ZnSnO). An optical study by external quantum efficiency and photoluminescence on the influence of different buffer layers on the defect properties of CIGS is presented. For both buffer layers, the CIGS bulk and CIGS/buffer interface are strongly influenced by electrostatic fluctuating potentials, which are less pronounced for the sample with the ZnSnO buffer layer. This is associated with a lower concentration of donor defects at the CIGS near-interface layer. A change in the bandgap of the CIGS as a consequence of the buffer layer deposition is observed. This study expands the knowledge of defects in the complex quaternary semiconductor CIGS, which, as discussed, can be affected even by the choice of buffer layer and its deposition process.S. Sadewasser P. Salomé and H Rodriguez-Alvarez
Materials efficient deposition and heat management of CuInSe2 micro-concentrator solar cells Journal Article
Solar Energy Materials and Solar Cells, Volume 159 , pp. Pages 496-502, 2017.
@article{Sadewasser2017b,
title = {Materials efficient deposition and heat management of CuInSe2 micro-concentrator solar cells},
author = {S. Sadewasser P. Salomé and H Rodriguez-Alvarez},
url = {https://doi.org/10.1016/j.solmat.2016.09.041},
year = {2017},
date = {2017-01-01},
journal = {Solar Energy Materials and Solar Cells},
volume = {Volume 159},
pages = {Pages 496-502},
abstract = {Thin-film solar cells based on Cu(In, Ga)Se2 chalcopyrite materials have achieved record power conversion efficiencies (PCE) above 22%, higher than any other thin film technology. Here, a proof-of-principle for Cu(In, Ga)Se2 solar cells with a reduced materials consumption by a factor of ~100 is demonstrated for which simulations indicate an even higher PCE potential. The concept is based on using micrometer-sized solar cells onto which sunlight is concentrated using a lens array. Thereby, all incoming sunlight is absorbed in solar cells that only cover a fraction (on the order of 1/100) of the surface, creating thus significant material savings. A fabrication process based on selective growth of CuInSe2 micro solar cells is presented, using electrodeposition into holes of an insulating SiO2 layer on a Mo back electrical contact. Proof-of-principle micro solar cells using this materials-savings approach are realized and characterized. Additionally, the heat management of micro-concentrator solar cells is studied using finite element simulations. For square micro solar cells up to 100 µm side length the temperature increase due to the concentrated sunlight can be kept below 4 °C above that of a flat panel solar cell. An increase of the PCE by 4.9% is estimated, considering the combined effect of the open circuit voltage increase due to concentrated sunlight, and the PCE decrease due to the temperature increase.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Thin-film solar cells based on Cu(In, Ga)Se2 chalcopyrite materials have achieved record power conversion efficiencies (PCE) above 22%, higher than any other thin film technology. Here, a proof-of-principle for Cu(In, Ga)Se2 solar cells with a reduced materials consumption by a factor of ~100 is demonstrated for which simulations indicate an even higher PCE potential. The concept is based on using micrometer-sized solar cells onto which sunlight is concentrated using a lens array. Thereby, all incoming sunlight is absorbed in solar cells that only cover a fraction (on the order of 1/100) of the surface, creating thus significant material savings. A fabrication process based on selective growth of CuInSe2 micro solar cells is presented, using electrodeposition into holes of an insulating SiO2 layer on a Mo back electrical contact. Proof-of-principle micro solar cells using this materials-savings approach are realized and characterized. Additionally, the heat management of micro-concentrator solar cells is studied using finite element simulations. For square micro solar cells up to 100 µm side length the temperature increase due to the concentrated sunlight can be kept below 4 °C above that of a flat panel solar cell. An increase of the PCE by 4.9% is estimated, considering the combined effect of the open circuit voltage increase due to concentrated sunlight, and the PCE decrease due to the temperature increase.Törndahl Teixeira Nicoara R-Ribeiro Andrade Stroppa González Edoff Leitão Sadewasser T J P N D G J C M J P S P.M.P Salomé J. Keller
CdS and Zn1−xSnxOy buffer layers for CIGS solar cells Journal Article
Solar Energy Materials and Solar Cells, Volume 159 , pp. Pages 272-281, 2017.
@article{Salomé2017b,
title = {CdS and Zn1−xSnxOy buffer layers for CIGS solar cells},
author = {Törndahl Teixeira Nicoara R-Ribeiro Andrade Stroppa González Edoff Leitão Sadewasser T J P N D G J C M J P S P.M.P Salomé J. Keller},
url = {https://doi.org/10.1016/j.solmat.2016.09.023},
year = {2017},
date = {2017-01-01},
journal = {Solar Energy Materials and Solar Cells},
volume = {Volume 159},
pages = {Pages 272-281},
abstract = {Thin film solar cells based on Cu(In,Ga)Se2 (CIGS), where just the buffer layer is changed, were fabricated and studied. The effects of two different buffer layers, CdS and ZnxSn1−xOy (ZnSnO), are compared using several characterization techniques. We compared both devices and observe that the ZnSnO-based solar cells have similar values of power conversion efficiency as compared to the cells with CdS buffer layers. The ZnSnO-based devices have higher values in the short-circuit current (Jsc) that compensate for lower values in fill factor (FF) and open circuit voltage (Voc) than CdS based devices. Kelvin probe force microscopy (KPFM) results indicate that CdS provides junctions with slightly higher surface photovoltage (SPV) than ZnSnO, thus explaining the lower Voc potential for the ZnSnO sample. The TEM analysis shows a poly-crystalline ZnSnO layer and we have not detected any strong evidence of diffusion of Zn or Sn into the CIGS. From the photoluminescence measurements, we concluded that both samples are being affected by fluctuating potentials, although this effect is higher for the CdS sample.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Thin film solar cells based on Cu(In,Ga)Se2 (CIGS), where just the buffer layer is changed, were fabricated and studied. The effects of two different buffer layers, CdS and ZnxSn1−xOy (ZnSnO), are compared using several characterization techniques. We compared both devices and observe that the ZnSnO-based solar cells have similar values of power conversion efficiency as compared to the cells with CdS buffer layers. The ZnSnO-based devices have higher values in the short-circuit current (Jsc) that compensate for lower values in fill factor (FF) and open circuit voltage (Voc) than CdS based devices. Kelvin probe force microscopy (KPFM) results indicate that CdS provides junctions with slightly higher surface photovoltage (SPV) than ZnSnO, thus explaining the lower Voc potential for the ZnSnO sample. The TEM analysis shows a poly-crystalline ZnSnO layer and we have not detected any strong evidence of diffusion of Zn or Sn into the CIGS. From the photoluminescence measurements, we concluded that both samples are being affected by fluctuating potentials, although this effect is higher for the CdS sample. -
2016
Teixeira Stroppa R.-Ribeiro Andrade Nicoara Abderrafi Leitão González J P D N K J J C H. Limborço P.M.P. Salomé and S Sadewasser
Synthesis and formation mechanism of CuInSe2 nanowires by one-step self-catalysed evaporation growth Journal Article
CrystEngComm, Vol. 18 (Issue 37), pp. 7147-7153, 2016.
@article{Limborço2016,
title = {Synthesis and formation mechanism of CuInSe2 nanowires by one-step self-catalysed evaporation growth},
author = {Teixeira Stroppa R.-Ribeiro Andrade Nicoara Abderrafi Leitão González J P D N K J J C H. Limborço P.M.P. Salomé and S Sadewasser},
url = {http://pubs.rsc.org/en/content/articlelanding/2016/ce/c6ce01317a#!divAbstract},
doi = {10.1039/C6CE01317A},
year = {2016},
date = {2016-08-17},
journal = {CrystEngComm},
volume = {Vol. 18},
number = {Issue 37},
pages = {7147-7153},
abstract = {High-quality CuInSe2 (CISe) nanowires have been prepared by a one-step evaporation process. The presented growth process results in a composite material consisting of CISe NWs on top of a polycrystalline CISe base layer. The nanowires were extensively characterized by transmission electron microscopy, confirming their composition and atomic-scale crystal structure with a very low number of structural defects. From these analyses, we infer that the growth axis is along the [111] direction. The polycrystalline base layer has a tetragonal chalcopyrite structure and is optically active as confirmed by X-ray diffraction and photoluminescence (PL) analysis, respectively. Potential applications of this composite CISe NW/base-layer material for photovoltaic energy conversion are supported by the reduced reflectivity of the material and its strong PL intensity. The presented growth method is based on elemental evaporation under vacuum conditions, which makes the process compatible with the fabrication of photovoltaic devices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}High-quality CuInSe2 (CISe) nanowires have been prepared by a one-step evaporation process. The presented growth process results in a composite material consisting of CISe NWs on top of a polycrystalline CISe base layer. The nanowires were extensively characterized by transmission electron microscopy, confirming their composition and atomic-scale crystal structure with a very low number of structural defects. From these analyses, we infer that the growth axis is along the [111] direction. The polycrystalline base layer has a tetragonal chalcopyrite structure and is optically active as confirmed by X-ray diffraction and photoluminescence (PL) analysis, respectively. Potential applications of this composite CISe NW/base-layer material for photovoltaic energy conversion are supported by the reduced reflectivity of the material and its strong PL intensity. The presented growth method is based on elemental evaporation under vacuum conditions, which makes the process compatible with the fabrication of photovoltaic devices.Sousa Fernandes Sadewasser Cunha M G P A S A F da J.P. Teixeira P.M.P. Salomé and J P Leitão
Optical and structural investigation of inline Cu2ZnSnS4 based solar cells Journal Article
Phys. Status Solidi B, Volume 253 (Issue 11), pp. Pages 2129–2135, 2016.
@article{Teixeira2016,
title = {Optical and structural investigation of inline Cu2ZnSnS4 based solar cells},
author = {Sousa Fernandes Sadewasser Cunha M G P A S A F da J.P. Teixeira P.M.P. Salomé and J P Leitão},
url = {http://onlinelibrary.wiley.com/doi/10.1002/pssb.201600453/full},
doi = {10.1002/pssb.201600453},
year = {2016},
date = {2016-08-01},
journal = {Phys. Status Solidi B},
volume = {Volume 253},
number = {Issue 11},
pages = {Pages 2129–2135},
abstract = {The structural and optical properties of two solar cells in which the Cumath formulaZnSnSmath formula absorber layer was sulphurized by two different methods (S flux and graphite box), were studied. The grain sizes are dependent on the sulphurization method, the larger ones being obtained for the sulphurization in a S flux. The optical properties were investigated by photoluminescence (PL). A broad and asymmetric band was observed for the sample with the larger grains, whereas for the other one a very broad emission was obtained, mostly influenced by the CdS buffer layer. The dependence on the excitation power revealed the influence of fluctuating potentials created by strong doping and high compensation of the absorber layer. Radiative recombination channels are quite different from the ones typical of semiconductor materials with flat bands. A relationship between the PL intensity from the absorber layer measured at low temperatures, and the final PV performance is established. Thus, we propose that PL can be used as an evaluation experimental technique in order to decide if a certain absorber should be processed into a full solar cell or not.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The structural and optical properties of two solar cells in which the Cumath formulaZnSnSmath formula absorber layer was sulphurized by two different methods (S flux and graphite box), were studied. The grain sizes are dependent on the sulphurization method, the larger ones being obtained for the sulphurization in a S flux. The optical properties were investigated by photoluminescence (PL). A broad and asymmetric band was observed for the sample with the larger grains, whereas for the other one a very broad emission was obtained, mostly influenced by the CdS buffer layer. The dependence on the excitation power revealed the influence of fluctuating potentials created by strong doping and high compensation of the absorber layer. Radiative recombination channels are quite different from the ones typical of semiconductor materials with flat bands. A relationship between the PL intensity from the absorber layer measured at low temperatures, and the final PV performance is established. Thus, we propose that PL can be used as an evaluation experimental technique in order to decide if a certain absorber should be processed into a full solar cell or not.Donzel-Gargand Ch. Frisk Joel Salomé Borme Sadewasser Ch. Platzer-Björkman O J P J S B. Vermang Y. Ren and M Edoff
Rear surface optimization of CZTS solar cells by use of a passivation layer with nano-sized point openings Journal Article
IEEE Journal of Photovoltaics, Volume: 6 (Issue: 1), pp. 332 - 336, 2016, ISSN: 2156-3381.
@article{Vermang2016,
title = {Rear surface optimization of CZTS solar cells by use of a passivation layer with nano-sized point openings},
author = {Donzel-Gargand Ch. Frisk Joel Salomé Borme Sadewasser Ch. Platzer-Björkman O J P J S B. Vermang Y. Ren and M Edoff},
url = {http://ieeexplore.ieee.org/document/7329914/},
doi = {10.1109/JPHOTOV.2015.2496864},
issn = {2156-3381},
year = {2016},
date = {2016-01-01},
journal = {IEEE Journal of Photovoltaics},
volume = {Volume: 6},
number = {Issue: 1},
pages = {332 - 336},
abstract = {Previously, an innovative way to reduce rear interface recombination in Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu2 (Zn,Sn)(S,Se)4 (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al2 O3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage (VOC ; +17%rel.), short-circuit current (JSC ; +5%rel.), and fill factor (FF; +9%rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32%rel. is obtained for the rear passivated cells.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Previously, an innovative way to reduce rear interface recombination in Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu2 (Zn,Sn)(S,Se)4 (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al2 O3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage (VOC ; +17%rel.), short-circuit current (JSC ; +5%rel.), and fill factor (FF; +9%rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32%rel. is obtained for the rear passivated cells. -
2015
Sadewasser S P.M.P. Salomé H. Rodriguez-Alvarez
Incorporation of alkali metals in chalcogenide solar cells Journal Article
Solar Energy Materials & Solar Cells, 143 , pp. 9, 2015.
@article{Salomé2015,
title = {Incorporation of alkali metals in chalcogenide solar cells},
author = {Sadewasser S P.M.P. Salomé H. Rodriguez-Alvarez},
url = {http://www.sciencedirect.com/science/article/pii/S0927024815002809},
year = {2015},
date = {2015-12-03},
journal = {Solar Energy Materials & Solar Cells},
volume = {143},
pages = {9},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Viktor Fjällström Tobias Törndaht Uwe Zimmermann Marika Edoff Piotr Szaniawski Pedro Salomé
Influence of varying Cu content on growth and performance of Ga-graded Cu(In,Ga)Se2 solar Cells Journal Article
IEEE J. PV, 5 , pp. 1775, 2015.
@article{Szaniawski2015,
title = {Influence of varying Cu content on growth and performance of Ga-graded Cu(In,Ga)Se2 solar Cells},
author = {Viktor Fjällström Tobias Törndaht Uwe Zimmermann Marika Edoff Piotr Szaniawski Pedro Salomé},
url = {http://ieeexplore.ieee.org/xpl/articleDetails.jsp?reload=true&arnumber=7296602},
year = {2015},
date = {2015-10-12},
journal = {IEEE J. PV},
volume = {5},
pages = {1775},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Ribeiro Fernandes Salomé Cunha Leitão Silva González G M P A P M P A F J P M I N J C da da A. Abelenda M. Sánchez
Anomalous persistent photoconductivity in Cu2ZnSnS4 thin films and solar cells Journal Article
Solar Energy Materials & Solar Cells, 137 , pp. 164, 2015.
@article{Abelenda2015,
title = {Anomalous persistent photoconductivity in Cu2ZnSnS4 thin films and solar cells},
author = {Ribeiro Fernandes Salomé Cunha Leitão Silva González G M P A P M P A F J P M I N J C da da A. Abelenda M. Sánchez},
url = {http://www.sciencedirect.com/science/article/pii/S0927024815000598},
year = {2015},
date = {2015-06-01},
journal = {Solar Energy Materials & Solar Cells},
volume = {137},
pages = {164},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Vermang Salomé Rostvall Zimmermann B P M P F U V. Fjällström P. Szaniawski and M.Edoff
Recovery after potential induced degradation of Cu(In,Ga)Se2 solar cells with CdS and Zn(O,S) buffer layers Journal Article
IEEE J. Photovoltaics, 5 , pp. 664, 2015.
@article{Fjällström2015,
title = {Recovery after potential induced degradation of Cu(In,Ga)Se2 solar cells with CdS and Zn(O,S) buffer layers},
author = {Vermang Salomé Rostvall Zimmermann B P M P F U V. Fjällström P. Szaniawski and M.Edoff},
year = {2015},
date = {2015-01-12},
journal = {IEEE J. Photovoltaics},
volume = {5},
pages = {664},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
GROUP LEADER
THE TEAM
Nicoleta Nicoara
Staff Researcher
Pedro Anacleto
Research Engineer
Ishwor Khatri
Research Fellow
Diego Garzón
Research Fellow
Carlos Rosário
Research Fellow
Pedro Sousa
Research Fellow
José Virtuoso
Bolseiro de Investigação
Marina Alves
Bolseiro de Investigação
Daniel Brito
PhD Fellow (FCT-QPI)
Diego Colombara
Associated group member
Former Group Members
Alessandro Cavalli
Research Fellow (MSCA-IF) (2019-2022)
Francisco de Matos
Bolseiro de Investigação (2019)
Diego Colombara
Research Fellow (2017 – 2019)
Daniel Brito
MSc student (2019)
Marina Alves
MSc student (2018)
Marco Zutter
MSc student (2018)
José Romero
Bolseiro de Investigação (2017)
Umesh Gomes
Research Fellow (2017)
Paulo Salvador
MSc student (2016)
David Correia
MSc student (2016)
Rodrigo Ribeiro-Andrade
Visiting Scientist (2015-2016)
Vanessa Iglesias
Cofund Fellow (2014-2016)
Henrique Limborço
Visiting Scientist (2014-2015)
Humberto Rodriguez-Alvarez
Research Fellow (2012-2014)
David Fuertes Marrón
Visiting Scientist (2013)
Erasmus Experience at INL
LaNaSC – Laboratory for Nanostructured Solar Cells
The Laboratory for Nanostructured Solar Cells (LaNaSC) develops nano- and micro-structures of chalcopyrite-type semiconductors (Cu(In,Ga)Se2) for application in photovoltaic energy conversion.
We currently follow four research lines:
- Development of advanced thin-film solar cells by the implementation of micro- and nanostructures.
- Development and application of scanning probe microscopy techniques for the characterization of solar cell materials and light-induced phenomena at the nanometer scale.
- Development of 2D materials for optoelectronis applications.
- Development of growth methods for chalcopyrite nanostructures i.e. quantum dots and nanowires. The goal is to combine the excellent light absorbing properties of chalcopyrite-type materials with the quantum properties of nanostructured materials, and thereby provide a pathway for the enhancement of power conversion efficiencies of photovoltaic devices beyond the Shockley-Queisser limit.
The laboratory is equipped with various materials preparation facilities consisting in a molecular beam epitaxy (MBE) setup for the growth of nanostructured chalcopyrite-type semiconductors, a hybrid sputter system for Cu(In,Ga)Se2, Mo, and ZnO, and an evaporation system for Cu(In,Ga)Se2 thin films.
The lab also operates an ultra-high vacuum scanning probe microscope, combining STM, AFM, and KPFM facilities with surface photovoltage methods to study light-induced phenomena at the nanometer scale.
The LaNaSC group: (standing) Diego Colombara, Marcel Claro, Nicoleta Nicoara, Pedro Anacleto, Sascha Sadewasser, Deepanjan Sharma, João Gonçalo. (sitting) Bernhard Baumgartner, Daniel Brito, José Virtuoso, Marina Alves
We are always open for new team members. If you are interested in joining us and applying for external funding (i.e. MSCA, FCT, etc.), please contact Sascha by email.
RESEARCH LINES
Chalcopyrite, Cu(In,Ga)Se2 (CIGSe), materials have excellent light absorbing properties and are used in the thin-film solar cell technology with the highest power conversion efficiency. We are aiming at incorporating novel concepts to improve thin film solar cells using nano- and micrometer structures into the device structure.
Currently, we focus our research efforts on two approaches:
- We develop micro solar cells for micro-concentrator solar cell applications. The goal is to develop highly efficient solar cells with a significant reduction in usage of absorber materials. By concentrating the sunlight onto micrometer sized CIGSe solar cells, the materials consumption of the solar cell material can be significantly reduced, leading to cost improvements. We combine cleanroom technology with the growth of CIGSe materials to obtain the micro solar cells.
- Development of nanostructures for chalcopyrite thin-film solar cells. The goal is to use passivation and light management techniques to improve solar cell performance. We use cleanroom technologies to introduce a passivation layer with contact holes in between the back contact and the absorber layer. This reduces back contact recombination and allows for thinner absorber layers, leading to cost savings for solar cell devices.
Project leader: Sascha Sadewasser
Team members: Pedro Anacleto, Ana Pérez Rodríguez, José Virtuoso, Marina Alves
We use ultrahigh vacuum scanning probe microscopy (UHV-SPM) methods to characterize the physical properties of chalcopyrite nanostructures and solar cell materials at the nanoscale.
Scanning probe methods include regular atomic force microscopy, Kelvin probe force microscopy, surface photovoltage measurements and scanning tunneling microscopy. We are especially interested in the interaction of light with solar cell materials at the nanoscale.
Scanning probe microscopy system
Project leader: Sascha Sadewasser
Team members: Nicoleta Nicoara, Deepanjan Sharma
We use vapor depositions and molecular beam epitaxy to grow 2D materials with a focus on semiconducting chalcogenide materials, e.g. MoS2, MoSe2, In2Se3, etc. Our goal is to extensively characterize these materials and implement them in optoelectronic devices, e.g. photodetectors.
Project leader: Sascha Sadewasser
Team members: Marcel Claro, Francisco de Matos
Chalcopyrite, Cu(In,Ga)Se2 (CIGSe), materials have excellent light absorbing properties and are used in the thin-film solar cell technology with the highest power conversion efficiency. We are working with these materials at the nanometer length scale with the goal to increase power conversion efficiencies.
We aim to develop growth methods for chalcopyrite nanostructures, i.e. quantum dots and nanowires. The goal is to combine the excellent light absorbing properties of chalcopyrite-type materials with the quantum properties of nanostructured materials, and thereby provide a pathway for the enhancement of power conversion efficiencies of photovoltaic devices beyond the Shockley-Queisser limit. We use a molecular beam epitaxy (MBE) system to evaporate the constituent elements (Cu, In, Ga, Se) onto epitaxial substrates, where at low evaporation rates and thin coverage the formation of nano-sized crystallites occurs.
Molecular Beam Epitaxy system for Cu(In,Ga)Se2 nanostructure growth
Project leader: Sascha Sadewasser
Team members: Marcel Claro
ON GOING RESEARCH PROJECTS
Semi-transparent solar cells for building-integrated photovoltaics
Semi-transparent photovoltaic windows will be developed based on narrow stripes of solar cells below the eye’s resolution, leading to a natural daylight spectrum on the inside and the ability to use solar cells with high power conversion efficiency.
This project will be carried out in close collaboration with the group of Prof. Phillip Dale at the University of Luxembourg.
Micro-concentrator thin film solar cells
The innovative idea of the MiconCell project is to combine highly-efficient CIGSe thin-film technology with the concentrator PV approach and shrink the size scale to the micrometer range. The benefits of this novel concept are materials savings of the critical raw materials In and Ga by a factor of about 100, and an efficiency increase by 2-6% due to the use of concentrated sunlight. The main goal of the project is to develop CIGSe micro-concentrator solar cells that are monolithically integrated with concentrator optics.
Correlated Analysis of Inorganic Solar Cells in and outside an Electron Microscope
The CASOLEM project aims to fabricate in-situ TEM cartridges for carrying out electrical measurements of CIGS solar cells. These chips will be developed in the Nanostructured Materials Group and will be universally adaptable, provide flexibility and significant advantages to the existing ones and will enable simultaneous
experimentation both inside and outside the TEM. Our role in this project is to provide CIGSe solar cell samples specially designed to test the developed cartridges.
Advanced architectures for ultra-thin high-efficiency CIGS solar cells with high manufacturability
Large area two dimensional heterostructures for photodetectors.
This project is carried out in collaboration with the 2D Materials and Devices Group and the groups of Prof. Ricardo Ribeiro and Prof. Nuno Peres from the University of Minho.
Nanotechnology-based functional solutions.
PREVIOUS RESEARCH PROJECTS
Super high efficiency Cu(In,Ga)Se2 thin-film solar cells approaching 25%
Large-scale printing of novel photovolaics based on Cu(In,Ga)Se2 chalcopyrite.
Large area two dimensional heterostructures for photodetectors.
This project is carried out in collaboration with the 2D Materials and Devices Group and the groups of Prof. Ricardo Ribeiro and Prof. Nuno Peres from the University of Minho.
Nanotechnology-based functional solutions.
PUBLICATIONS
-
2020
Diego Colombara and Hossam Elanzeery and Nicoleta Nicoara and Deepanjan Sharma andMarcel Claro and Torsten Schwarz and Anna Koprek and Max Hilaire Wolter and Michele Melchiorre and Mohit Sood and Nathalie Valle and Oleksandr Bondarchuk and Finn Babbe and Conrad Spindler and Oana Cojocaru-Miredin and Dierk Raabe and Phillip J Dale and Sascha Sadewasser and Susanne Siebentritt
Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface Journal Article
Nature Communications, 11 , pp. 3634, 2020.
@article{Colombara2020,
title = {Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface},
author = {Diego Colombara and Hossam Elanzeery and Nicoleta Nicoara and Deepanjan Sharma andMarcel Claro and Torsten Schwarz and Anna Koprek and Max Hilaire Wolter and Michele Melchiorre and Mohit Sood and Nathalie Valle and Oleksandr Bondarchuk and Finn Babbe and Conrad Spindler and Oana Cojocaru-Miredin and Dierk Raabe and Phillip J Dale and Sascha Sadewasser and Susanne Siebentritt},
url = {https://www.nature.com/articles/s41467-020-17434-8},
doi = {10.1038/s41467-020-17434-8},
year = {2020},
date = {2020-07-20},
journal = {Nature Communications},
volume = {11},
pages = {3634},
abstract = {The electrical and optoelectronic properties of materials are determined by the chemical potentials of their constituents. The relative density of point defects is thus controlled, allowing to craft microstructure, trap densities and doping levels. Here, we show that the chemical potentials of chalcogenide materials near the edge of their existence region are not only determined during growth but also at room temperature by post-processing. In particular, we study the generation of anion vacancies, which are critical defects in chalcogenide semiconductors and topological insulators. The example of CuInSe2 photovoltaic semiconductor reveals that single phase material crosses the phase boundary and forms surface secondary phases upon oxidation, thereby creating anion vacancies. The arising metastable point defect population explains a common root cause of performance losses. This study shows how selective defect annihilation is attained with tailored chemical treatments that mitigate anion vacancy formation and improve the performance of CuInSe2 solar cells.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The electrical and optoelectronic properties of materials are determined by the chemical potentials of their constituents. The relative density of point defects is thus controlled, allowing to craft microstructure, trap densities and doping levels. Here, we show that the chemical potentials of chalcogenide materials near the edge of their existence region are not only determined during growth but also at room temperature by post-processing. In particular, we study the generation of anion vacancies, which are critical defects in chalcogenide semiconductors and topological insulators. The example of CuInSe2 photovoltaic semiconductor reveals that single phase material crosses the phase boundary and forms surface secondary phases upon oxidation, thereby creating anion vacancies. The arising metastable point defect population explains a common root cause of performance losses. This study shows how selective defect annihilation is attained with tailored chemical treatments that mitigate anion vacancy formation and improve the performance of CuInSe2 solar cells.Viviana Sousa and Bruna F Gonçalves and Yitzchak S Rosen and José Virtuoso and Pedro Anacleto and Fátima M Cerqueira and Evgeny Modin and Pedro Alpuim and Oleg I Lebedev and Shlomo Magdassi and Sascha Sadewasser and Yury V Kolen’ko
Over 6% Efficient Cu(In,Ga)Se2 Solar Cell Screen-Printed from Oxides on Fluorine-Doped Tin Oxide Journal Article
ACS Applied Energy Materials, 6 , pp. 3120-3126, 2020.
@article{Sousa2020,
title = {Over 6% Efficient Cu(In,Ga)Se2 Solar Cell Screen-Printed from Oxides on Fluorine-Doped Tin Oxide},
author = {Viviana Sousa and Bruna F Gonçalves and Yitzchak S Rosen and José Virtuoso and Pedro Anacleto and Fátima M Cerqueira and Evgeny Modin and Pedro Alpuim and Oleg I Lebedev and Shlomo Magdassi and Sascha Sadewasser and Yury V Kolen’ko},
url = {https://doi.org/10.1021/acsaem.9b01999},
doi = {10.1021/acsaem.9b01999},
year = {2020},
date = {2020-03-31},
journal = {ACS Applied Energy Materials},
volume = {6},
pages = {3120-3126},
abstract = {A new approach to fabricate copper, indium, gallium diselenide (CIGSe) solar cells on conductive fluorine-doped tin oxide (FTO) reached an efficiency of over 6% for a champion photovoltaic device. Commercial oxide nanoparticles are formulated into high-quality screen-printable ink based on ethyl cellulose solution in terpineol. The high homogeneity and good adhesion properties of the oxide ink play an important role in obtaining dense and highly crystalline photoabsorber layers. This finding reveals that solution-based screen-printing from readily available oxide precursors provides an interesting cost-effective alternative to current vacuum- and energy-demanding processes of the CIGSe solar cell fabrication.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}A new approach to fabricate copper, indium, gallium diselenide (CIGSe) solar cells on conductive fluorine-doped tin oxide (FTO) reached an efficiency of over 6% for a champion photovoltaic device. Commercial oxide nanoparticles are formulated into high-quality screen-printable ink based on ethyl cellulose solution in terpineol. The high homogeneity and good adhesion properties of the oxide ink play an important role in obtaining dense and highly crystalline photoabsorber layers. This finding reveals that solution-based screen-printing from readily available oxide precursors provides an interesting cost-effective alternative to current vacuum- and energy-demanding processes of the CIGSe solar cell fabrication.David Fuster and Pedro Anacleto and José Virtuoso and Marco Zutter and Daniel Brito and Marina Alves and Luis Aparicio and David Fuertes Marrón and Fernando Briones and Sascha Sadewasser and Jorge M García
System for manufacturing complete Cu(In,Ga)Se2 solar cells in situ under vacuum Journal Article
Solar Energy, 198 , pp. 490-498, 2020.
@article{Fuster2020,
title = {System for manufacturing complete Cu(In,Ga)Se2 solar cells in situ under vacuum},
author = {David Fuster and Pedro Anacleto and José Virtuoso and Marco Zutter and Daniel Brito and Marina Alves and Luis Aparicio and David Fuertes Marrón and Fernando Briones and Sascha Sadewasser and Jorge M García},
url = {https://doi.org/10.1016/j.solener.2020.01.073},
doi = {10.1016/j.solener.2020.01.073},
year = {2020},
date = {2020-03-01},
journal = {Solar Energy},
volume = {198},
pages = {490-498},
abstract = {We present the development of a small foot-print physical vapor deposition (PVD) system for in-situ deposition of all layers required in a complete Cu(In,Ga)Se2 (CIGS) solar cell. Seven sputtering magnetrons and one valved-cracker source have been custom designed and manufactured for this system, named SpuTtering for Advanced Research (STAR). The purpose of STAR is to develop a technique to fabricate a complete CIGS solar cell, including contacts, absorber, buffer, and window layers, under high vacuum with the aim to transfer this technology to a future industrial production line. The system’s capabilities and its relatively high throughput place it somewhere in between research and industrial development levels. It is possible to work on the deposition of the back contact, the CIGS absorber, and the window layer of three solar cells simultaneously. Calibration data, selection of parameters for the deposition of the individual layers, and initial results of a complete CIGS solar cell developed with STAR are reported.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}We present the development of a small foot-print physical vapor deposition (PVD) system for in-situ deposition of all layers required in a complete Cu(In,Ga)Se2 (CIGS) solar cell. Seven sputtering magnetrons and one valved-cracker source have been custom designed and manufactured for this system, named SpuTtering for Advanced Research (STAR). The purpose of STAR is to develop a technique to fabricate a complete CIGS solar cell, including contacts, absorber, buffer, and window layers, under high vacuum with the aim to transfer this technology to a future industrial production line. The system’s capabilities and its relatively high throughput place it somewhere in between research and industrial development levels. It is possible to work on the deposition of the back contact, the CIGS absorber, and the window layer of three solar cells simultaneously. Calibration data, selection of parameters for the deposition of the individual layers, and initial results of a complete CIGS solar cell developed with STAR are reported.Susanne Siebentritt and Enrico Avancini and Marcus Bär and Jakob Bombsch and Emilie Bourgeois and Stephan Buecheler and Romain Carron and Celia Castro and Sebastien Duguay and Roberto Félix and Evelyn Handick and Dimitrios Hariskos and Ville Havu and Philip Jackson and Hannu-Pekka Komsa and Thomas Kunze and Maria Malitckaya and Roberto Menozzi and Milos Nesladek and Nicoleta Nicoara and Martti Puska and Mohit Raghuwanshi and Philippe Pareige and Sascha Sadewasser and Giovanna Sozzi and Ayodhya Nath Tiwari and Shigenori Ueda and Arantxa Vilalta-Clemente and Thomas Paul Weiss and Florian Werner and Regan G Wilks and Wolfram Witte and Max Hilaire Wolter
Heavy alkali treatment of Cu(In,Ga)Se2 solar cells: surface versus bulk effects Journal Article
Advanced Energy Materials, 10 , pp. 1903752, 2020.
@article{Siebentritt2020,
title = {Heavy alkali treatment of Cu(In,Ga)Se2 solar cells: surface versus bulk effects},
author = {Susanne Siebentritt and Enrico Avancini and Marcus Bär and Jakob Bombsch and Emilie Bourgeois and Stephan Buecheler and Romain Carron and Celia Castro and Sebastien Duguay and Roberto Félix and Evelyn Handick and Dimitrios Hariskos and Ville Havu and Philip Jackson and Hannu-Pekka Komsa and Thomas Kunze and Maria Malitckaya and Roberto Menozzi and Milos Nesladek and Nicoleta Nicoara and Martti Puska and Mohit Raghuwanshi and Philippe Pareige and Sascha Sadewasser and Giovanna Sozzi and Ayodhya Nath Tiwari and Shigenori Ueda and Arantxa Vilalta-Clemente and Thomas Paul Weiss and Florian Werner and Regan G Wilks and Wolfram Witte and Max Hilaire Wolter},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/aenm.201903752},
doi = {10.1002/aenm.201903752},
year = {2020},
date = {2020-02-25},
journal = {Advanced Energy Materials},
volume = {10},
pages = {1903752},
abstract = {Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post-deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open-circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single-phase Cu-alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Chalcopyrite solar cells achieve efficiencies above 23%. The latest improvements are due to post-deposition treatments (PDT) with heavy alkalis. This study provides a comprehensive description of the effect of PDT on the chemical and electronic structure of surface and bulk of Cu(In,Ga)Se2. Chemical changes at the surface appear similar, independent of absorber or alkali. However, the effect on the surface electronic structure differs with absorber or type of treatment, although the improvement of the solar cell efficiency is the same. Thus, changes at the surface cannot be the only effect of the PDT treatment. The main effect of PDT with heavy alkalis concerns bulk recombination. The reduction in bulk recombination goes along with a reduced density of electronic tail states. Improvements in open-circuit voltage appear together with reduced band bending at grain boundaries. Heavy alkalis accumulate at grain boundaries and are not detected in the grains. This behavior is understood by the energetics of the formation of single-phase Cu-alkali compounds. Thus, the efficiency improvement with heavy alkali PDT can be attributed to reduced band bending at grain boundaries, which reduces tail states and nonradiative recombination and is caused by accumulation of heavy alkalis at grain boundaries. -
2019
Marina Alves and Ana Pérez-Rodr í and Phillip J Dale and César Dom í and Sascha Sadewasser
Thin-film micro-concentrator solar cells Journal Article
Journal of Physics: Energy, 2 , pp. 012001, 2019.
@article{Alves2019b,
title = {Thin-film micro-concentrator solar cells},
author = {Marina Alves and Ana Pérez-Rodr í and Phillip J Dale and César Dom í and Sascha Sadewasser},
url = {https://doi.org/10.1088%2F2515-7655%2Fab4289},
doi = {10.1088/2515-7655/ab4289},
year = {2019},
date = {2019-11-26},
journal = {Journal of Physics: Energy},
volume = {2},
pages = {012001},
abstract = {Photovoltaic (PV) energy conversion of sunlight into electricity is now a well-established technology and a strong further expansion of PV will be seen in the future to answer the increasing demand for clean and renewable energy. Concentrator PV (CPV) employs optical elements to concentrate sunlight onto small solar cells, offering the possibility of replacing expensive solar cells with more economic optical elements, and higher device power conversion efficiencies. While CPV has mainly been explored for highly efficient single-crystalline and multi-junction solar cells, the combination of thin-film solar cells with the concentration approach opens up new horizons in CPV. Typical fabrication of thin-film solar cells can be modified for efficient, high-throughput and parallel production of organized arrays of micro solar cells. Their combination with microlens arrays promises to deliver micro-concentrator solar modules with a similar form factor to present day flat-panel PV. Such thin-film micro-concentrator PV modules would use significantly less semiconductor solar cell material (reducing the use of critical raw materials) and lead to a higher energy production (by means of concentrated sunlight), with the potential to lead to a lower levelized cost of electricity. This review article gives an overview of the present state-of-the-art in the fabrication of thin-film micro solar cells based on Cu(In,Ga)Se2 absorber materials and introduces optical concentration systems that can be combined to build the future thin-film micro-concentrator PV technology.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Photovoltaic (PV) energy conversion of sunlight into electricity is now a well-established technology and a strong further expansion of PV will be seen in the future to answer the increasing demand for clean and renewable energy. Concentrator PV (CPV) employs optical elements to concentrate sunlight onto small solar cells, offering the possibility of replacing expensive solar cells with more economic optical elements, and higher device power conversion efficiencies. While CPV has mainly been explored for highly efficient single-crystalline and multi-junction solar cells, the combination of thin-film solar cells with the concentration approach opens up new horizons in CPV. Typical fabrication of thin-film solar cells can be modified for efficient, high-throughput and parallel production of organized arrays of micro solar cells. Their combination with microlens arrays promises to deliver micro-concentrator solar modules with a similar form factor to present day flat-panel PV. Such thin-film micro-concentrator PV modules would use significantly less semiconductor solar cell material (reducing the use of critical raw materials) and lead to a higher energy production (by means of concentrated sunlight), with the potential to lead to a lower levelized cost of electricity. This review article gives an overview of the present state-of-the-art in the fabrication of thin-film micro solar cells based on Cu(In,Ga)Se2 absorber materials and introduces optical concentration systems that can be combined to build the future thin-film micro-concentrator PV technology.Nicoleta Nicoara and Roby Manaligod and Philip Jackson and Dimitrios Hariskos and Wolfram Witte and Giovanna Sozzi and Roberto Menozzi and Sascha Sadewasser
Direct evidence for grain boundary passivation in Cu(In,Ga)Se2 solar cells through alkali-fluoride post deposition treatments Journal Article
Nature Communications, 10 , pp. 3980, 2019.
@article{Nicoara2019,
title = {Direct evidence for grain boundary passivation in Cu(In,Ga)Se2 solar cells through alkali-fluoride post deposition treatments},
author = {Nicoleta Nicoara and Roby Manaligod and Philip Jackson and Dimitrios Hariskos and Wolfram Witte and Giovanna Sozzi and Roberto Menozzi and Sascha Sadewasser},
url = {https://doi.org/10.1038/s41467-019-11996-y},
doi = {10.1038/s41467-019-11996-y},
year = {2019},
date = {2019-09-04},
journal = {Nature Communications},
volume = {10},
pages = {3980},
abstract = {The properties and performance of polycrystalline materials depend critically on the properties of their grain boundaries. Polycrystalline photovoltaic materials – e.g. hybrid halide perovskites, copper indium gallium diselenide (CIGSe) and cadmium telluride – have already demonstrated high efficiencies and promise cost-effective electricity supply. For CIGSe-based solar cells, an efficiency above 23% has recently been achieved using an alkali-fluoride post-deposition treatment; however, its full impact and functional principle are not yet fully understood. Here, we show direct evidence for the passivation of grain boundaries in CIGSe treated with three different alkali-fluorides through a detailed study of the nanoscale optoelectronic properties. We determine a correlation of the surface potential change at grain boundaries with the open-circuit voltage, which is supported by numerical simulations. Our results suggest that heavier alkali elements might lead to better passivation by reducing the density of charged defects and increasing the formation of secondary phases at grain boundaries.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The properties and performance of polycrystalline materials depend critically on the properties of their grain boundaries. Polycrystalline photovoltaic materials – e.g. hybrid halide perovskites, copper indium gallium diselenide (CIGSe) and cadmium telluride – have already demonstrated high efficiencies and promise cost-effective electricity supply. For CIGSe-based solar cells, an efficiency above 23% has recently been achieved using an alkali-fluoride post-deposition treatment; however, its full impact and functional principle are not yet fully understood. Here, we show direct evidence for the passivation of grain boundaries in CIGSe treated with three different alkali-fluorides through a detailed study of the nanoscale optoelectronic properties. We determine a correlation of the surface potential change at grain boundaries with the open-circuit voltage, which is supported by numerical simulations. Our results suggest that heavier alkali elements might lead to better passivation by reducing the density of charged defects and increasing the formation of secondary phases at grain boundaries.Marek Piotrowski and Jérôme Borme and Enrique Carbó-Argibay and Deepanjan Sharma and Nicoleta Nicoara and Sascha Sadewasser and Dmitri Y Petrovykh and Carlos Rodríguez-Abreu and Yury V Kolen{'}ko
Template-directed self-organization of colloidal PbTe nanocrystals into pillars, conformal coatings, and self-supported membranes Journal Article
Nanoscale Adv., 1 , pp. 3049-3055, 2019.
@article{Piotrowski2019,
title = {Template-directed self-organization of colloidal PbTe nanocrystals into pillars, conformal coatings, and self-supported membranes},
author = {Marek Piotrowski and Jérôme Borme and Enrique Carbó-Argibay and Deepanjan Sharma and Nicoleta Nicoara and Sascha Sadewasser and Dmitri Y Petrovykh and Carlos Rodríguez-Abreu and Yury V Kolen{'}ko},
url = {http://dx.doi.org/10.1039/C9NA00370C},
doi = {10.1039/C9NA00370C},
year = {2019},
date = {2019-06-17},
journal = {Nanoscale Adv.},
volume = {1},
pages = {3049-3055},
abstract = {We demonstrate the formation of three morphologies relevant for integration with miniaturized devices—microscale pillars, conformal coatings, and self-supported membranes—via template-directed self-organization of lead telluride (PbTe) colloidal nanocrystals (NCs). Optimizing the self-organization process towards producing one of these morphologies typically involves adjusting the surface chemistry of the particles, as a means of controlling the particle–particle and particle–template interactions. In contrast, we have produced each of the three morphologies of close-packed NCs by adjusting only the solvent and concentration of NCs, to ensure that the high quality of the ca. 10 nm PbTe NCs produced by hot-injection colloidal synthesis, which we used as model “building blocks,” remains consistent across all three configurations. For the first two morphologies, the NCs were deposited as colloidal suspensions onto micropatterned silicon substrates. The microscale cuboid pillars (1 μm × 1 μm × 0.6 μm) were formed by depositing NC dispersions in toluene onto templates patterned with resist grid motifs, followed by the resist removal after the slow evaporation of toluene and formation of the micropillars. Conformal coatings were produced by switching the solvent from toluene to a faster drying hexane and pouring NC dispersions onto silicon templates with topographically patterned microstructures. In a similar process, self-supported NC membranes were formed from NC dispersions in hexane on the surface of diethylene glycol and transferred onto the micropatterned templates. The demonstrated combination of bottom-up self-organization with top-down micropatterned templates provides a scalable route for design and fabrication of NC ensembles in morphologies and form-factors that are compatible with their integration into miniaturized devices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}We demonstrate the formation of three morphologies relevant for integration with miniaturized devices—microscale pillars, conformal coatings, and self-supported membranes—via template-directed self-organization of lead telluride (PbTe) colloidal nanocrystals (NCs). Optimizing the self-organization process towards producing one of these morphologies typically involves adjusting the surface chemistry of the particles, as a means of controlling the particle–particle and particle–template interactions. In contrast, we have produced each of the three morphologies of close-packed NCs by adjusting only the solvent and concentration of NCs, to ensure that the high quality of the ca. 10 nm PbTe NCs produced by hot-injection colloidal synthesis, which we used as model “building blocks,” remains consistent across all three configurations. For the first two morphologies, the NCs were deposited as colloidal suspensions onto micropatterned silicon substrates. The microscale cuboid pillars (1 μm × 1 μm × 0.6 μm) were formed by depositing NC dispersions in toluene onto templates patterned with resist grid motifs, followed by the resist removal after the slow evaporation of toluene and formation of the micropillars. Conformal coatings were produced by switching the solvent from toluene to a faster drying hexane and pouring NC dispersions onto silicon templates with topographically patterned microstructures. In a similar process, self-supported NC membranes were formed from NC dispersions in hexane on the surface of diethylene glycol and transferred onto the micropatterned templates. The demonstrated combination of bottom-up self-organization with top-down micropatterned templates provides a scalable route for design and fabrication of NC ensembles in morphologies and form-factors that are compatible with their integration into miniaturized devices.Marco Zutter and Jose Virtuoso and Pedro Anacleto and Liam Yasin and Marina Alves and Miguel Madeira and Oleksandr Bondarchuk and Saibal Mitra and David Fuster Signes and Jorge M Garcia and Fernando Briones and Rolf Waechter and Oliver Kiowski and Dimitrios Hariskos and Diego Colombara and Sascha Sadewasser
Giant Voc Boost of Low-Temperature Annealed Cu(In,Ga)Se2 with Sputtered Zn(O,S) Buffers Journal Article
physica status solidi (RRL) – Rapid Research Letters, 13 , pp. 1900145, 2019.
@article{Zutter2019,
title = {Giant Voc Boost of Low-Temperature Annealed Cu(In,Ga)Se2 with Sputtered Zn(O,S) Buffers},
author = {Marco Zutter and Jose Virtuoso and Pedro Anacleto and Liam Yasin and Marina Alves and Miguel Madeira and Oleksandr Bondarchuk and Saibal Mitra and David Fuster Signes and Jorge M Garcia and Fernando Briones and Rolf Waechter and Oliver Kiowski and Dimitrios Hariskos and Diego Colombara and Sascha Sadewasser},
url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/pssr.201900145},
doi = {10.1002/pssr.201900145},
year = {2019},
date = {2019-05-30},
journal = {physica status solidi (RRL) – Rapid Research Letters},
volume = {13},
pages = {1900145},
abstract = {Large-scale industrial fabrication of Cu(In,Ga)Se2 (CIGS) photovoltaic panels would benefit significantly if the buffer layer chemical bath deposition could be replaced by a cadmium-free dry vacuum process suitable for in-line production. This Letter reports on the development of a Zn(O,S) buffer layer deposited by vacuum-based magnetron sputtering from a single target onto commercial CIGS absorbers cut from a module-size glass/Mo/CIGS stack. The buffer-window stack consisting of Zn(O0.75S0.25)/i-ZnO/ZnO:Al is optimized for layer thickness and optical and electronic properties, leading to an average device efficiency of 4.7%, which can be improved by annealing at 200 °C to a maximum of 10.5%, mainly due to a considerable increase in the open-circuit voltage (Voc). Temperature-dependent current density versus voltage (J–V) characteristics show a reduced interface recombination upon annealing, explaining the observed Voc boost. Quantum efficiency shows improvements in the long and short wavelength region, setting in at different annealing temperatures, and photoemission depth profiling indicates interdiffusion of all atomic species at the CIGS/Zn(O,S) interface. Electrical device simulations explain the observed effects by a modification of the band offset at the interface and defects passivation. Both effects are attributed to the observed interdiffusion during annealing.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Large-scale industrial fabrication of Cu(In,Ga)Se2 (CIGS) photovoltaic panels would benefit significantly if the buffer layer chemical bath deposition could be replaced by a cadmium-free dry vacuum process suitable for in-line production. This Letter reports on the development of a Zn(O,S) buffer layer deposited by vacuum-based magnetron sputtering from a single target onto commercial CIGS absorbers cut from a module-size glass/Mo/CIGS stack. The buffer-window stack consisting of Zn(O0.75S0.25)/i-ZnO/ZnO:Al is optimized for layer thickness and optical and electronic properties, leading to an average device efficiency of 4.7%, which can be improved by annealing at 200 °C to a maximum of 10.5%, mainly due to a considerable increase in the open-circuit voltage (Voc). Temperature-dependent current density versus voltage (J–V) characteristics show a reduced interface recombination upon annealing, explaining the observed Voc boost. Quantum efficiency shows improvements in the long and short wavelength region, setting in at different annealing temperatures, and photoemission depth profiling indicates interdiffusion of all atomic species at the CIGS/Zn(O,S) interface. Electrical device simulations explain the observed effects by a modification of the band offset at the interface and defects passivation. Both effects are attributed to the observed interdiffusion during annealing.H Limborço and P M Salomé and R Ribeiro-Andrade and J P Teixeira and N Nicoara and K Abderrafi and J P Leitão and J C Gonzalez and S Sadewasser
CuInSe2 quantum dots grown by molecular beam epitaxy on amorphous SiO2 surfaces Journal Article
Beilstein J. Nanotechnol., 10 , pp. 1103–1111, 2019.
@article{Limborço2019,
title = {CuInSe2 quantum dots grown by molecular beam epitaxy on amorphous SiO2 surfaces},
author = {H Limborço and P M Salomé and R Ribeiro-Andrade and J P Teixeira and N Nicoara and K Abderrafi and J P Leitão and J C Gonzalez and S Sadewasser},
url = {https://www.beilstein-journals.org/bjnano/articles/10/110},
doi = {10.3762/bjnano.10.110},
year = {2019},
date = {2019-05-22},
journal = {Beilstein J. Nanotechnol.},
volume = {10},
pages = {1103–1111},
abstract = {The currently most efficient polycrystalline solar cells are based on the Cu(In,Ga)Se2 compound as a light absorption layer. However, in view of new concepts of nanostructured solar cells, CuInSe2 nanostructures are of high interest. In this work, we report CuInSe2 nanodots grown through a vacuum-compatible co-evaporation growth process on an amorphous surface. The density, mean size, and peak optical emission energy of the nanodots can be controlled by changing the growth temperature. Scanning transmission electron microscopy measurements confirmed the crystallinity of the nanodots as well as chemical composition and structure compatible with tetragonal CuInSe2. Photoluminescence measurements of CdS-passivated nanodots showed that the nanodots are optoelectronically active with a broad emission extending to energies above the CuInSe2 bulk bandgap and in agreement with the distribution of sizes. A blue-shift of the luminescence is observed as the average size of the nanodots gets smaller, evidencing quantum confinement in all samples. By using simple quantum confinement calculations, we correlate the photoluminescence peak emission energy with the average size of the nanodots.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The currently most efficient polycrystalline solar cells are based on the Cu(In,Ga)Se2 compound as a light absorption layer. However, in view of new concepts of nanostructured solar cells, CuInSe2 nanostructures are of high interest. In this work, we report CuInSe2 nanodots grown through a vacuum-compatible co-evaporation growth process on an amorphous surface. The density, mean size, and peak optical emission energy of the nanodots can be controlled by changing the growth temperature. Scanning transmission electron microscopy measurements confirmed the crystallinity of the nanodots as well as chemical composition and structure compatible with tetragonal CuInSe2. Photoluminescence measurements of CdS-passivated nanodots showed that the nanodots are optoelectronically active with a broad emission extending to energies above the CuInSe2 bulk bandgap and in agreement with the distribution of sizes. A blue-shift of the luminescence is observed as the average size of the nanodots gets smaller, evidencing quantum confinement in all samples. By using simple quantum confinement calculations, we correlate the photoluminescence peak emission energy with the average size of the nanodots.David Correia and Daniel Siopa and Diego Colombara and Sara Tombolato and Pedro M P Salomé and Kamal Abderrafi and Pedro Anacleto and Phillip J Dale and Sascha Sadewasser
Area-selective electrodeposition of micro islands for CuInSe2-based photovoltaics Journal Article
Results in Physics, 12 , pp. 2136-2140, 2019.
@article{Correia2019,
title = {Area-selective electrodeposition of micro islands for CuInSe2-based photovoltaics},
author = {David Correia and Daniel Siopa and Diego Colombara and Sara Tombolato and Pedro M P Salomé and Kamal Abderrafi and Pedro Anacleto and Phillip J Dale and Sascha Sadewasser},
url = {http://www.sciencedirect.com/science/article/pii/S2211379719305339},
doi = {10.1016/j.rinp.2019.02.047},
year = {2019},
date = {2019-02-20},
journal = {Results in Physics},
volume = {12},
pages = {2136-2140},
abstract = {For mass fabrication of highly-efficient photovoltaic modules based on Cu(In,Ga)Se2 (CIGSe) absorber layers the availability and cost of the critical raw materials In and Ga present a potential bottleneck. The micro-concentrator solar cell concept provides a solution by using micro lenses to concentrate incoming sun light on an array of micro-sized CIGSe solar cells. The challenge is to fabricate CIGSe micro islands in exactly the desired positions using only the required material. Here, we analyze the area-selective electrodeposition of CuInSe2 into holes in an insulating SiO2 template layer as a material-efficient fabrication approach. We observe that the deposition process shows a strong dependence on the hole size, with a faster deposition around the hole perimeter. Based on a model developed for electrochemical reactions at ultra-micro electrodes, we develop numerical simulations for the electrochemical deposition process. The simulations consider the changing micro-electrode geometry throughout the deposition process, and provide a reasonable fit to the experimental data. Finally, it is shown that CuInSe2 micro solar cells fabricated by electrodeposition reach efficiencies of 4.8% under 1 sun, providing a proof-of-concept demonstration meriting further development.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}For mass fabrication of highly-efficient photovoltaic modules based on Cu(In,Ga)Se2 (CIGSe) absorber layers the availability and cost of the critical raw materials In and Ga present a potential bottleneck. The micro-concentrator solar cell concept provides a solution by using micro lenses to concentrate incoming sun light on an array of micro-sized CIGSe solar cells. The challenge is to fabricate CIGSe micro islands in exactly the desired positions using only the required material. Here, we analyze the area-selective electrodeposition of CuInSe2 into holes in an insulating SiO2 template layer as a material-efficient fabrication approach. We observe that the deposition process shows a strong dependence on the hole size, with a faster deposition around the hole perimeter. Based on a model developed for electrochemical reactions at ultra-micro electrodes, we develop numerical simulations for the electrochemical deposition process. The simulations consider the changing micro-electrode geometry throughout the deposition process, and provide a reasonable fit to the experimental data. Finally, it is shown that CuInSe2 micro solar cells fabricated by electrodeposition reach efficiencies of 4.8% under 1 sun, providing a proof-of-concept demonstration meriting further development.R Ribeiro-Andrade and S Sahayaraj and B Vermang and M R Correia and S Sadewasser and J C González and P A Fernandes and P M P Salomé
Voids in Kesterites and the Influence of Lamellae Preparation by Focused Ion Beam for Transmission Electron Microscopy Analyses Journal Article
IEEE Journal of Photovoltaics, 9 , pp. 565-570, 2019.
@article{Ribeiro-Andrade2019,
title = {Voids in Kesterites and the Influence of Lamellae Preparation by Focused Ion Beam for Transmission Electron Microscopy Analyses},
author = {R Ribeiro-Andrade and S Sahayaraj and B Vermang and M R Correia and S Sadewasser and J C González and P A Fernandes and P M P Salomé},
url = {https://ieeexplore.ieee.org/document/8610192},
doi = {10.1109/JPHOTOV.2018.2889602},
year = {2019},
date = {2019-01-11},
journal = {IEEE Journal of Photovoltaics},
volume = {9},
pages = {565-570},
abstract = {Kesterite solar cells based on Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTSe) are potential future candidates to be used in thin-film solar cells. The technology still has to be developed to a great extent and for this to happen, high levels of confidence in the characterization methods are required, so that improvements can be made on solid interpretations. In this study, we show that the interpretations of one of the most used characterization techniques in kesterites, scanning transmission electron microscopy (STEM), might be affected by its specimen preparation when using focused ion beam (FIB). Using complementary measurements based on scanning electron microscopy and Raman scattering spectroscopy, compelling evidence shows that secondary phases of ZnSe mixed in the bulk of CZTSe are the likely cause of the appearance of voids in STEM lamellae. Sputtering simulations support this interpretation by showing that Zn in a ZnSe matrix is preferentially sputtered compared with any metal atom in a CZTSe matrix.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Kesterite solar cells based on Cu2ZnSnS4 and Cu2ZnSnSe4 (CZTSe) are potential future candidates to be used in thin-film solar cells. The technology still has to be developed to a great extent and for this to happen, high levels of confidence in the characterization methods are required, so that improvements can be made on solid interpretations. In this study, we show that the interpretations of one of the most used characterization techniques in kesterites, scanning transmission electron microscopy (STEM), might be affected by its specimen preparation when using focused ion beam (FIB). Using complementary measurements based on scanning electron microscopy and Raman scattering spectroscopy, compelling evidence shows that secondary phases of ZnSe mixed in the bulk of CZTSe are the likely cause of the appearance of voids in STEM lamellae. Sputtering simulations support this interpretation by showing that Zn in a ZnSe matrix is preferentially sputtered compared with any metal atom in a CZTSe matrix. -
2018
Nicoleta Nicoara and Sylvie Harel and Thomas Lepetit and Ludovic Arzel and Nicolas Barreau and Sascha Sadewasser
Impact of KF post-deposition treatment on ageing of the Cu(In,Ga)Se2 surface and its interface with CdS Journal Article
ACS Applied Energy Materials, 1 , pp. 2681-2688, 2018.
@article{Nicoara2018,
title = {Impact of KF post-deposition treatment on ageing of the Cu(In,Ga)Se2 surface and its interface with CdS},
author = {Nicoleta Nicoara and Sylvie Harel and Thomas Lepetit and Ludovic Arzel and Nicolas Barreau and Sascha Sadewasser},
url = {https://doi.org/10.1021/acsaem.8b00365},
doi = {10.1021/acsaem.8b00365},
year = {2018},
date = {2018-05-10},
journal = {ACS Applied Energy Materials},
volume = {1},
pages = {2681-2688},
abstract = {Recent world record efficiencies for thin film solar cells based on Cu(In,Ga)Se2 (CIGS) have been realized with absorbers subject to alkali fluoride post deposition treatments (PDT). We investigated the effect of ambient air exposure on the electronic properties of CIGS with KF-PDT in a combined time-dependent Kelvin probe force microscopy and X-ray photoelectron spectroscopy study. We also studied the early stage formation of the absorber/buffer interface after the initial deposition of CdS in the chemical bath. Our study shows that the KF-PDT, as well as the CdS deposition process induce an increase in the overall surface work function, as compared to bare CIGS. A K–In–Se compound forms after the KF-PDT, accompanied by a stable In oxide which explains the remarkable stability of the contact potential difference to air exposure, confirming phenomenological observations in many laboratories. In clear contrast to the untreated CIGS, the KF-treated CIGS/CdS interface shows a significant variation in the surface potential (∼360 mV) over approximately 7 h air-exposure. We attribute this variation to a Cd–In intermixing at interface, whose chemical stability is susceptible to air-exposure. Our results contribute to the understanding of the electronic properties of the KF-treated and untreated CIGS/CdS junction during the early stages of the interface formation, which impact the overall device properties.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Recent world record efficiencies for thin film solar cells based on Cu(In,Ga)Se2 (CIGS) have been realized with absorbers subject to alkali fluoride post deposition treatments (PDT). We investigated the effect of ambient air exposure on the electronic properties of CIGS with KF-PDT in a combined time-dependent Kelvin probe force microscopy and X-ray photoelectron spectroscopy study. We also studied the early stage formation of the absorber/buffer interface after the initial deposition of CdS in the chemical bath. Our study shows that the KF-PDT, as well as the CdS deposition process induce an increase in the overall surface work function, as compared to bare CIGS. A K–In–Se compound forms after the KF-PDT, accompanied by a stable In oxide which explains the remarkable stability of the contact potential difference to air exposure, confirming phenomenological observations in many laboratories. In clear contrast to the untreated CIGS, the KF-treated CIGS/CdS interface shows a significant variation in the surface potential (∼360 mV) over approximately 7 h air-exposure. We attribute this variation to a Cd–In intermixing at interface, whose chemical stability is susceptible to air-exposure. Our results contribute to the understanding of the electronic properties of the KF-treated and untreated CIGS/CdS junction during the early stages of the interface formation, which impact the overall device properties.Sascha Sadewasser and Nicoleta Nicoara and Santiago D Solares
Artifacts in time-resolved Kelvin probe force microscopy Journal Article
Beilstein J. Nanotechnol., 9 , pp. 1272–1281, 2018.
@article{Sadewasser2018,
title = {Artifacts in time-resolved Kelvin probe force microscopy},
author = {Sascha Sadewasser and Nicoleta Nicoara and Santiago D Solares},
url = {https://www.beilstein-journals.org/bjnano/articles/9/119},
doi = {10.3762/bjnano.9.119},
year = {2018},
date = {2018-04-24},
journal = {Beilstein J. Nanotechnol.},
volume = {9},
pages = {1272–1281},
abstract = {Kelvin probe force microscopy (KPFM) has been used for the characterization of metals, insulators, and semiconducting materials on the nanometer scale. Especially in semiconductors, the charge dynamics are of high interest. Recently, several techniques for time-resolved measurements with time resolution down to picoseconds have been developed, many times using a modulated excitation signal, e.g., light modulation or bias modulation that induces changes in the charge carrier distribution. For fast modulation frequencies, the KPFM controller measures an average surface potential, which contains information about the involved charge carrier dynamics. Here, we show that such measurements are prone to artifacts due to frequency mixing, by performing numerical dynamics simulations of the cantilever oscillation in KPFM subjected to a bias-modulated signal. For square bias pulses, the resulting time-dependent electrostatic forces are very complex and result in intricate mixing of frequencies that may, in some cases, have a component at the detection frequency, leading to falsified KPFM measurements. Additionally, we performed fast Fourier transform (FFT) analyses that match the results of the numerical dynamics simulations. Small differences are observed that can be attributed to transients and higher-order Fourier components, as a consequence of the intricate nature of the cantilever driving forces. These results are corroborated by experimental measurements on a model system. In the experimental case, additional artifacts are observed due to constructive or destructive interference of the bias modulation with the cantilever oscillation. Also, in the case of light modulation, we demonstrate artifacts due to unwanted illumination of the photodetector of the beam deflection detection system. Finally, guidelines for avoiding such artifacts are given.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Kelvin probe force microscopy (KPFM) has been used for the characterization of metals, insulators, and semiconducting materials on the nanometer scale. Especially in semiconductors, the charge dynamics are of high interest. Recently, several techniques for time-resolved measurements with time resolution down to picoseconds have been developed, many times using a modulated excitation signal, e.g., light modulation or bias modulation that induces changes in the charge carrier distribution. For fast modulation frequencies, the KPFM controller measures an average surface potential, which contains information about the involved charge carrier dynamics. Here, we show that such measurements are prone to artifacts due to frequency mixing, by performing numerical dynamics simulations of the cantilever oscillation in KPFM subjected to a bias-modulated signal. For square bias pulses, the resulting time-dependent electrostatic forces are very complex and result in intricate mixing of frequencies that may, in some cases, have a component at the detection frequency, leading to falsified KPFM measurements. Additionally, we performed fast Fourier transform (FFT) analyses that match the results of the numerical dynamics simulations. Small differences are observed that can be attributed to transients and higher-order Fourier components, as a consequence of the intricate nature of the cantilever driving forces. These results are corroborated by experimental measurements on a model system. In the experimental case, additional artifacts are observed due to constructive or destructive interference of the bias modulation with the cantilever oscillation. Also, in the case of light modulation, we demonstrate artifacts due to unwanted illumination of the photodetector of the beam deflection detection system. Finally, guidelines for avoiding such artifacts are given.Pedro M P Salomé and Bart Vermang and Rodrigo Ribeiro-Andrade and Jennifer P Teixeira and José M V Cunha and Manuel J Mendes and Sirazul Haque and Jêrome Borme and Hugo Águas and Elvira Fortunato and Rodrigo Martins and Juan C González and Joaquim P Leitão and Paulo A Fernandes and Marika Edoff and Sascha Sadewasser
Passivation of Interfaces in Thin Film Solar Cells: Understanding the Effects of a Nanostructured Rear Point Contact Layer Journal Article
Advanced Materials Interfaces, 5 , pp. 1701101, 2018.
@article{Salomé2018,
title = {Passivation of Interfaces in Thin Film Solar Cells: Understanding the Effects of a Nanostructured Rear Point Contact Layer},
author = {Pedro M P Salomé and Bart Vermang and Rodrigo Ribeiro-Andrade and Jennifer P Teixeira and José M V Cunha and Manuel J Mendes and Sirazul Haque and Jêrome Borme and Hugo Águas and Elvira Fortunato and Rodrigo Martins and Juan C González and Joaquim P Leitão and Paulo A Fernandes and Marika Edoff and Sascha Sadewasser},
doi = {10.1002/admi.201701101},
year = {2018},
date = {2018-01-23},
journal = {Advanced Materials Interfaces},
volume = {5},
pages = {1701101},
abstract = {Thin film solar cells based in Cu(In,Ga)Se2 (CIGS) are among the most efficient polycrystalline solar cells, surpassing CdTe and even polycrystalline silicon solar cells. For further developments, the CIGS technology has to start incorporating different solar cell architectures and strategies that allow for very low interface recombination. In this work, ultrathin 350 nm CIGS solar cells with a rear interface passivation strategy are studied and characterized. The rear passivation is achieved using an Al2O3 nanopatterned point structure. Using the cell results, photoluminescence measurements, and detailed optical simulations based on the experimental results, it is shown that by including the nanopatterned point contact structure, the interface defect concentration lowers, which ultimately leads to an increase of solar cell electrical performance mostly by increase of the open circuit voltage. Gains to the short circuit current are distributed between an increased rear optical reflection and also due to electrical effects. The approach of mixing several techniques allows us to make a discussion considering the different passivation gains, which has not been done in detail in previous works. A solar cell with a nanopatterned rear contact and a 350 nm thick CIGS absorber provides an average power conversion efficiency close to 10%.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Thin film solar cells based in Cu(In,Ga)Se2 (CIGS) are among the most efficient polycrystalline solar cells, surpassing CdTe and even polycrystalline silicon solar cells. For further developments, the CIGS technology has to start incorporating different solar cell architectures and strategies that allow for very low interface recombination. In this work, ultrathin 350 nm CIGS solar cells with a rear interface passivation strategy are studied and characterized. The rear passivation is achieved using an Al2O3 nanopatterned point structure. Using the cell results, photoluminescence measurements, and detailed optical simulations based on the experimental results, it is shown that by including the nanopatterned point contact structure, the interface defect concentration lowers, which ultimately leads to an increase of solar cell electrical performance mostly by increase of the open circuit voltage. Gains to the short circuit current are distributed between an increased rear optical reflection and also due to electrical effects. The approach of mixing several techniques allows us to make a discussion considering the different passivation gains, which has not been done in detail in previous works. A solar cell with a nanopatterned rear contact and a 350 nm thick CIGS absorber provides an average power conversion efficiency close to 10%. -
2017
Nicoleta Nicoara and Thomas Kunze and Philip Jackson and Dimitrios Hariskos and Roberto Félix Duarte and Regan G Wilks and Wolfram Witte and Marcus Bär and Sascha Sadewasser
Evidence for chemical and electronic non-uniformities in the formation of the interface of RbF-treated Cu(In,Ga)Se2 with CdS Journal Article
ACS Appl. Mater. Interfaces, 9 , pp. 44173−44180, 2017.
@article{Nicoara2017b,
title = {Evidence for chemical and electronic non-uniformities in the formation of the interface of RbF-treated Cu(In,Ga)Se2 with CdS},
author = {Nicoleta Nicoara and Thomas Kunze and Philip Jackson and Dimitrios Hariskos and Roberto Félix Duarte and Regan G Wilks and Wolfram Witte and Marcus Bär and Sascha Sadewasser},
url = {https://pubs.acs.org/doi/10.1021/acsami.7b12448},
doi = {10.1021/acsami.7b12448},
year = {2017},
date = {2017-11-27},
journal = {ACS Appl. Mater. Interfaces},
volume = {9},
pages = {44173−44180},
abstract = {We report on the initial stages of CdS buffer layer formation on Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers subjected to rubidium fluoride (RbF) postdeposition treatment (PDT). A detailed characterization of the CIGSe/CdS interface for different chemical bath deposition (CBD) times of the CdS layer is obtained from spatially resolved atomic and Kelvin probe force microscopy and laterally integrating X-ray spectroscopies. The observed spatial inhomogeneity in the interface’s structural, chemical, and electronic properties of samples undergoing up to 3 min of CBD treatments is indicative of a complex interface formation including an incomplete coverage and/or nonuniform composition of the buffer layer. It is expected that this result impacts solar cell performance, in particular when reducing the CdS layer thickness (e.g., in an attempt to increase the collection in the ultraviolet wavelength region). Our work provides important findings on the absorber/buffer interface formation and reveals the underlying mechanism for limitations in the reduction of the CdS thickness, even when an alkali PDT is applied to the CIGSe absorber.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}We report on the initial stages of CdS buffer layer formation on Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorbers subjected to rubidium fluoride (RbF) postdeposition treatment (PDT). A detailed characterization of the CIGSe/CdS interface for different chemical bath deposition (CBD) times of the CdS layer is obtained from spatially resolved atomic and Kelvin probe force microscopy and laterally integrating X-ray spectroscopies. The observed spatial inhomogeneity in the interface’s structural, chemical, and electronic properties of samples undergoing up to 3 min of CBD treatments is indicative of a complex interface formation including an incomplete coverage and/or nonuniform composition of the buffer layer. It is expected that this result impacts solar cell performance, in particular when reducing the CdS layer thickness (e.g., in an attempt to increase the collection in the ultraviolet wavelength region). Our work provides important findings on the absorber/buffer interface formation and reveals the underlying mechanism for limitations in the reduction of the CdS thickness, even when an alkali PDT is applied to the CIGSe absorber.Nicolas Barreau and Agathe Frelon and Thomas Lepetit and Eric Gautron and Nicolas Gautier and Rodrigo Ribeiro-Andrade and Nicoleta Nicoara and Sascha Sadewasser and Pawel Zabierowski and Ludovic Arzel and Léo Choubrac and Sylvie Harel and Catherine Deudon and Camille Latouche and Stéphane Jobic and Maite Caldes and Lionel Assmann and Polyxeni Tsoulka and Emmanuel V Péan and Justine Lorthioir and Frédéric Geschier and Isabelle Braems and Matthieu Moret and Olivier Briot and Guy Ouvrard
High efficiency solar cell based on full PVD processed Cu(In,Ga)Se2/CdIn2S4 heterojunction Journal Article
Solar RRL, 1 , pp. 1700140, 2017.
@article{Barreau2017,
title = {High efficiency solar cell based on full PVD processed Cu(In,Ga)Se2/CdIn2S4 heterojunction},
author = {Nicolas Barreau and Agathe Frelon and Thomas Lepetit and Eric Gautron and Nicolas Gautier and Rodrigo Ribeiro-Andrade and Nicoleta Nicoara and Sascha Sadewasser and Pawel Zabierowski and Ludovic Arzel and Léo Choubrac and Sylvie Harel and Catherine Deudon and Camille Latouche and Stéphane Jobic and Maite Caldes and Lionel Assmann and Polyxeni Tsoulka and Emmanuel V Péan and Justine Lorthioir and Frédéric Geschier and Isabelle Braems and Matthieu Moret and Olivier Briot and Guy Ouvrard},
url = {https://doi.org/10.1002/solr.201700140},
doi = {10.1002/solr.201700140},
year = {2017},
date = {2017-10-10},
journal = {Solar RRL},
volume = {1},
pages = {1700140},
abstract = {The original goal of our study is to synthesize by co‐evaporation the phase that could be formed at the interface between polycrystalline p‐Cu(In,Ga)Se2 treated with KF and n‐CdS. Hence, a new buffer layer, CdIn2S4 (C24), deposited by co‐evaporation is presented for the use in thin film solar cells, exhibiting device efficiencies as high as 16.2%, comparable to that obtained on a reference standard CdS‐buffered device. The physico‐chemical and optical properties of close to stoichiometry 400 nm‐thick films of C24 show similar properties to what has been reported in the literature for single crystals. The layer stack used for solar cells is investigated by transmission electron microscopy, showing the formation of an ultrathin Cd‐deficient C24 layer at the CIGSe/C24 interface, while a clear lattice match is observed at the C24/ZnO interface. Advanced electrical characterizations of the devices suggest that the output voltage and fill factor of the solar cells based on Cu(In,Ga)Se2/(PVD)C24 are limited by tunneling‐enhanced recombination through extended band tail states. These results open new routes to explain the superiority of wet processes used for the junction formation compared to vacuum‐based approaches.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The original goal of our study is to synthesize by co‐evaporation the phase that could be formed at the interface between polycrystalline p‐Cu(In,Ga)Se2 treated with KF and n‐CdS. Hence, a new buffer layer, CdIn2S4 (C24), deposited by co‐evaporation is presented for the use in thin film solar cells, exhibiting device efficiencies as high as 16.2%, comparable to that obtained on a reference standard CdS‐buffered device. The physico‐chemical and optical properties of close to stoichiometry 400 nm‐thick films of C24 show similar properties to what has been reported in the literature for single crystals. The layer stack used for solar cells is investigated by transmission electron microscopy, showing the formation of an ultrathin Cd‐deficient C24 layer at the CIGSe/C24 interface, while a clear lattice match is observed at the C24/ZnO interface. Advanced electrical characterizations of the devices suggest that the output voltage and fill factor of the solar cells based on Cu(In,Ga)Se2/(PVD)C24 are limited by tunneling‐enhanced recombination through extended band tail states. These results open new routes to explain the superiority of wet processes used for the junction formation compared to vacuum‐based approaches.K Abderrafi and R.-Ribeiro Andrade and N Nicoara and M F Cerqueira and Gonzalez M Debs and H Limborço and P M P Salomé and J C Gonzalez and F Briones and J M Garcia and S Sadewasser
Epitaxial CuInSe2 thin films grown by molecular beam epitaxy and migration enhanced epitaxy Journal Article
J. Crystal Growth, 475 , pp. 300-306, 2017.
@article{Abderrafi2017,
title = {Epitaxial CuInSe2 thin films grown by molecular beam epitaxy and migration enhanced epitaxy},
author = {K Abderrafi and R.-Ribeiro Andrade and N Nicoara and M F Cerqueira and Gonzalez M Debs and H Limborço and P M P Salomé and J C Gonzalez and F Briones and J M Garcia and S Sadewasser},
url = {https://doi.org/10.1016/j.jcrysgro.2017.07.010},
year = {2017},
date = {2017-10-01},
journal = {J. Crystal Growth},
volume = {475},
pages = {300-306},
abstract = {While CuInSe2 chalcopyrite materials are mainly used in their polycrystalline form to prepare thin film solar cells, epitaxial layers have been used for the characterization of defects. Typically, epitaxial layers are grown by metal-organic vapor phase epitaxy or molecular beam epitaxy (MBE). Here we present epitaxial layers grown by migration enhanced epitaxy (MEE) and compare the materials quality to MBE grown layers. CuInSe2 layers were grown on GaAs (0 0 1) substrates by co-evaporation of Cu, In, and Se using substrate temperatures of 450 °C, 530 °C, and 620 °C. The layers were characterized by high resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and atomic force microscopy (AFM). HR-XRD and HR-TEM show a better crystalline quality of the MEE grown layers, and Raman scattering measurements confirm single phase CuInSe2. AFM shows the previously observed faceting of the (0 0 1) surface into 1 1 2 facets with trenches formed along the [1 1 0] direction. The surface of MEE-grown samples appears smoother compared to MBE-grown samples, a similar trend is observed with increasing growth temperature.Keywords: A3: Migration enhanced epitaxy; A3: Molecular beam epitaxy; B2: Semiconducting ternary compounds; A1: Crystal structure; A1: Surface structure},
keywords = {},
pubstate = {published},
tppubtype = {article}
}While CuInSe2 chalcopyrite materials are mainly used in their polycrystalline form to prepare thin film solar cells, epitaxial layers have been used for the characterization of defects. Typically, epitaxial layers are grown by metal-organic vapor phase epitaxy or molecular beam epitaxy (MBE). Here we present epitaxial layers grown by migration enhanced epitaxy (MEE) and compare the materials quality to MBE grown layers. CuInSe2 layers were grown on GaAs (0 0 1) substrates by co-evaporation of Cu, In, and Se using substrate temperatures of 450 °C, 530 °C, and 620 °C. The layers were characterized by high resolution X-ray diffraction (HR-XRD), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and atomic force microscopy (AFM). HR-XRD and HR-TEM show a better crystalline quality of the MEE grown layers, and Raman scattering measurements confirm single phase CuInSe2. AFM shows the previously observed faceting of the (0 0 1) surface into 1 1 2 facets with trenches formed along the [1 1 0] direction. The surface of MEE-grown samples appears smoother compared to MBE-grown samples, a similar trend is observed with increasing growth temperature.Keywords: A3: Migration enhanced epitaxy; A3: Molecular beam epitaxy; B2: Semiconducting ternary compounds; A1: Crystal structure; A1: Surface structure
S Sadewasser
Geometry and materials considerations for thin film micro-concentrator solar cells Journal Article
Solar Energy, 159 , pp. 186-191, 2017.
@article{Sadewasser2017b,
title = {Geometry and materials considerations for thin film micro-concentrator solar cells},
author = {S Sadewasser},
url = {http://dx.doi.org/10.1016/j.solener.2017.09.035},
doi = {10.1016/j.solener.2017.09.035},
year = {2017},
date = {2017-09-15},
journal = {Solar Energy},
volume = {159},
pages = {186-191},
abstract = {Using concentrated sunlight for photovoltaic energy conversion has long been identified as a way to make cost-intensive solar cell materials and devices more cost effective. The recently proposed micro-concentrator approach for Cu(In,Ga)Se2 brings concentrator photovoltaics from the centimeter and millimeter scale down to the micrometer scale, with the goal of reducing the amount of critical raw materials. We show here that the micro-concentrator approach has large benefits in terms of the heat dissipation and that heat management on the micro-scale allows the use of concentration factors as high as 1000× without any special cooling efforts. Specifically, we show that line-shaped micro-concentrator solar cells need to be either smaller than 50 μm wide, or be used at low concentration factors below 100×. In contrast, island-shaped micro-concentrator solar cells show improved efficiencies at cell sizes of (100 μm)2 and concentration factors up to 1000×. Additionally, we consider different materials for the substrate and the lens array for light concentration in view of their heat management capacities. The results presented here provide design guidelines for the further development of thin film micro-concentrator solar cells, applicable to a variety of materials systems, e.g. Cu(In,Ga)Se2, CdTe, or metal halide perovskite solar cells.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Using concentrated sunlight for photovoltaic energy conversion has long been identified as a way to make cost-intensive solar cell materials and devices more cost effective. The recently proposed micro-concentrator approach for Cu(In,Ga)Se2 brings concentrator photovoltaics from the centimeter and millimeter scale down to the micrometer scale, with the goal of reducing the amount of critical raw materials. We show here that the micro-concentrator approach has large benefits in terms of the heat dissipation and that heat management on the micro-scale allows the use of concentration factors as high as 1000× without any special cooling efforts. Specifically, we show that line-shaped micro-concentrator solar cells need to be either smaller than 50 μm wide, or be used at low concentration factors below 100×. In contrast, island-shaped micro-concentrator solar cells show improved efficiencies at cell sizes of (100 μm)2 and concentration factors up to 1000×. Additionally, we consider different materials for the substrate and the lens array for light concentration in view of their heat management capacities. The results presented here provide design guidelines for the further development of thin film micro-concentrator solar cells, applicable to a variety of materials systems, e.g. Cu(In,Ga)Se2, CdTe, or metal halide perovskite solar cells.P M P Salomé and R Ribeiro-Andrade and J P Teixeira and J Keller and T Törndahl and N Nicoara and M Edoff and J C González and J P Leitão and S Sadewasser
Cd and Cu inter-diffusion in Cu(In,Ga)Se2/CdS hetero-interfaces Journal Article
IEEE Journal of Photovoltaics, Vol. 7 (no 3), pp. 858-863 p., 2017, ISSN: ISSN 2156-3381.
@article{Andrade2017,
title = {Cd and Cu inter-diffusion in Cu(In,Ga)Se2/CdS hetero-interfaces},
author = {P M P Salomé and R Ribeiro-Andrade and J P Teixeira and J Keller and T Törndahl and N Nicoara and M Edoff and J C González and J P Leitão and S Sadewasser},
url = {http://ieeexplore.ieee.org/document/7865928/},
doi = {10.1109/JPHOTOV.2017.2666550},
issn = {ISSN 2156-3381},
year = {2017},
date = {2017-05-29},
journal = {IEEE Journal of Photovoltaics},
volume = {Vol. 7},
number = {no 3},
pages = {858-863 p.},
abstract = {We report a detailed characterization of an industrylike prepared Cu(In, Ga) Se-2 (CIGS)/CdS heterojunction by scanning transmission electron microscopy and photoluminescence (PL). Energy dispersive X-ray spectroscopy shows the presence of several regions in the CIGS layer that are Cu deprived and Cd enriched, suggesting the segregation of Cd-Se. Concurrently, the CdS layer shows Cd-deprived regions with the presence of Cu, suggesting a segregation of Cu-S. The two types of segregations are always found together, which, to the best of our knowledge, is observed for the first time. The results indicate that there is a diffusion process that replaces Cu with Cd in the CIGS layer and Cd with Cu in the CdS layer. Using a combinatorial approach, we identified that this effect is independent of focused-ion beam sample preparation and of the transmission electron microscopy grid. Furthermore, PL measurements before and after an HCl etch indicate a lower degree of defects in the postetch sample, compatible with the segregates removal. We hypothesize that Cu2-x Se nanodomains react during the chemical bath process to form these segregates since the chemical reaction that dominates this process is thermodynamically favorable. These results provide important additional information about the formation of the CIGS/CdS interface.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}We report a detailed characterization of an industrylike prepared Cu(In, Ga) Se-2 (CIGS)/CdS heterojunction by scanning transmission electron microscopy and photoluminescence (PL). Energy dispersive X-ray spectroscopy shows the presence of several regions in the CIGS layer that are Cu deprived and Cd enriched, suggesting the segregation of Cd-Se. Concurrently, the CdS layer shows Cd-deprived regions with the presence of Cu, suggesting a segregation of Cu-S. The two types of segregations are always found together, which, to the best of our knowledge, is observed for the first time. The results indicate that there is a diffusion process that replaces Cu with Cd in the CIGS layer and Cd with Cu in the CdS layer. Using a combinatorial approach, we identified that this effect is independent of focused-ion beam sample preparation and of the transmission electron microscopy grid. Furthermore, PL measurements before and after an HCl etch indicate a lower degree of defects in the postetch sample, compatible with the segregates removal. We hypothesize that Cu2-x Se nanodomains react during the chemical bath process to form these segregates since the chemical reaction that dominates this process is thermodynamically favorable. These results provide important additional information about the formation of the CIGS/CdS interface.N Nicoara and Th. Lepetit and L Arzel and S Harel and N Barreau and S Sadewasser
Effect of the KF post-deposition treatment on grain boundary properties in Cu(In, Ga)Se2 thin films Journal Article
Scientific Reports 7, (Article number: 41361), 2017.
@article{Nicoara2017b,
title = {Effect of the KF post-deposition treatment on grain boundary properties in Cu(In, Ga)Se2 thin films},
author = {N Nicoara and Th. Lepetit and L Arzel and S Harel and N Barreau and S Sadewasser},
url = {https://www.nature.com/articles/srep41361},
doi = {doi:10.1038/srep41361},
year = {2017},
date = {2017-01-27},
journal = {Scientific Reports 7},
number = {Article number: 41361},
abstract = {Significant power conversion efficiency improvements have recently been achieved for thin-film solar cells based on a variety of polycrystalline absorbers, including perovskites, CdTe, and Cu(In,Ga)Se2 (CIGS). The passivation of grain boundaries (GBs) through (post-deposition) treatments is a crucial step for this success. For the case of CIGS, the introduction of a potassium fluoride post-deposition treatment (KF-PDT) has boosted their power conversion efficiency to the best performance of all polycrystalline solar cells. Direct and indirect effects of potassium at the interface and interface-near region in the CIGS layer are thought to be responsible for this improvement. Here, we show that also the electronic properties of the GBs are beneficially modified by the KF-PDT. We used Kelvin probe force microscopy to study the effect of the KF-PDT on the CIGS surface by spatially resolved imaging of the surface potential. We find a clear difference for the GB electronic properties: the KF-PDT increases the band bending at GBs by about 70% and results in a narrower distribution of work function values at the GBs. This effect of the KF-PDT on the GB electronic properties is expected to contribute to the improved efficiency values observed for CIGS thin-film solar cells with KF-PDT.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Significant power conversion efficiency improvements have recently been achieved for thin-film solar cells based on a variety of polycrystalline absorbers, including perovskites, CdTe, and Cu(In,Ga)Se2 (CIGS). The passivation of grain boundaries (GBs) through (post-deposition) treatments is a crucial step for this success. For the case of CIGS, the introduction of a potassium fluoride post-deposition treatment (KF-PDT) has boosted their power conversion efficiency to the best performance of all polycrystalline solar cells. Direct and indirect effects of potassium at the interface and interface-near region in the CIGS layer are thought to be responsible for this improvement. Here, we show that also the electronic properties of the GBs are beneficially modified by the KF-PDT. We used Kelvin probe force microscopy to study the effect of the KF-PDT on the CIGS surface by spatially resolved imaging of the surface potential. We find a clear difference for the GB electronic properties: the KF-PDT increases the band bending at GBs by about 70% and results in a narrower distribution of work function values at the GBs. This effect of the KF-PDT on the GB electronic properties is expected to contribute to the improved efficiency values observed for CIGS thin-film solar cells with KF-PDT.Keller Törndahl Sadewasser J T S P.M.P. Salomé J.P. Teixeira and J P Leitão
Influence of CdS and ZnSnO Buffer Layers on the Photoluminescence of Cu(In,Ga)Se2 Thin Films Journal Article
IEEE Journal of Photovoltaics, Volume: 7 (Issue: 2), pp. 670 - 675, 2017.
@article{Salomé2017b,
title = {Influence of CdS and ZnSnO Buffer Layers on the Photoluminescence of Cu(In,Ga)Se2 Thin Films},
author = {Keller Törndahl Sadewasser J T S P.M.P. Salomé J.P. Teixeira and J P Leitão},
url = {http://ieeexplore.ieee.org/document/7822954/},
doi = {10.1109/JPHOTOV.2016.2639347},
year = {2017},
date = {2017-01-18},
journal = {IEEE Journal of Photovoltaics},
volume = {Volume: 7},
number = {Issue: 2},
pages = {670 - 675},
abstract = {The search for alternatives to the CdS buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells has turned out to be quite promising in terms of power conversion efficiency. In this paper, the typically used chemical-bath-deposited CdS layer is compared with an atomic-layer-deposited Zn1-xSnxOy (ZnSnO). An optical study by external quantum efficiency and photoluminescence on the influence of different buffer layers on the defect properties of CIGS is presented. For both buffer layers, the CIGS bulk and CIGS/buffer interface are strongly influenced by electrostatic fluctuating potentials, which are less pronounced for the sample with the ZnSnO buffer layer. This is associated with a lower concentration of donor defects at the CIGS near-interface layer. A change in the bandgap of the CIGS as a consequence of the buffer layer deposition is observed. This study expands the knowledge of defects in the complex quaternary semiconductor CIGS, which, as discussed, can be affected even by the choice of buffer layer and its deposition process.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The search for alternatives to the CdS buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells has turned out to be quite promising in terms of power conversion efficiency. In this paper, the typically used chemical-bath-deposited CdS layer is compared with an atomic-layer-deposited Zn1-xSnxOy (ZnSnO). An optical study by external quantum efficiency and photoluminescence on the influence of different buffer layers on the defect properties of CIGS is presented. For both buffer layers, the CIGS bulk and CIGS/buffer interface are strongly influenced by electrostatic fluctuating potentials, which are less pronounced for the sample with the ZnSnO buffer layer. This is associated with a lower concentration of donor defects at the CIGS near-interface layer. A change in the bandgap of the CIGS as a consequence of the buffer layer deposition is observed. This study expands the knowledge of defects in the complex quaternary semiconductor CIGS, which, as discussed, can be affected even by the choice of buffer layer and its deposition process.S. Sadewasser P. Salomé and H Rodriguez-Alvarez
Materials efficient deposition and heat management of CuInSe2 micro-concentrator solar cells Journal Article
Solar Energy Materials and Solar Cells, Volume 159 , pp. Pages 496-502, 2017.
@article{Sadewasser2017b,
title = {Materials efficient deposition and heat management of CuInSe2 micro-concentrator solar cells},
author = {S. Sadewasser P. Salomé and H Rodriguez-Alvarez},
url = {https://doi.org/10.1016/j.solmat.2016.09.041},
year = {2017},
date = {2017-01-01},
journal = {Solar Energy Materials and Solar Cells},
volume = {Volume 159},
pages = {Pages 496-502},
abstract = {Thin-film solar cells based on Cu(In, Ga)Se2 chalcopyrite materials have achieved record power conversion efficiencies (PCE) above 22%, higher than any other thin film technology. Here, a proof-of-principle for Cu(In, Ga)Se2 solar cells with a reduced materials consumption by a factor of ~100 is demonstrated for which simulations indicate an even higher PCE potential. The concept is based on using micrometer-sized solar cells onto which sunlight is concentrated using a lens array. Thereby, all incoming sunlight is absorbed in solar cells that only cover a fraction (on the order of 1/100) of the surface, creating thus significant material savings. A fabrication process based on selective growth of CuInSe2 micro solar cells is presented, using electrodeposition into holes of an insulating SiO2 layer on a Mo back electrical contact. Proof-of-principle micro solar cells using this materials-savings approach are realized and characterized. Additionally, the heat management of micro-concentrator solar cells is studied using finite element simulations. For square micro solar cells up to 100 µm side length the temperature increase due to the concentrated sunlight can be kept below 4 °C above that of a flat panel solar cell. An increase of the PCE by 4.9% is estimated, considering the combined effect of the open circuit voltage increase due to concentrated sunlight, and the PCE decrease due to the temperature increase.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Thin-film solar cells based on Cu(In, Ga)Se2 chalcopyrite materials have achieved record power conversion efficiencies (PCE) above 22%, higher than any other thin film technology. Here, a proof-of-principle for Cu(In, Ga)Se2 solar cells with a reduced materials consumption by a factor of ~100 is demonstrated for which simulations indicate an even higher PCE potential. The concept is based on using micrometer-sized solar cells onto which sunlight is concentrated using a lens array. Thereby, all incoming sunlight is absorbed in solar cells that only cover a fraction (on the order of 1/100) of the surface, creating thus significant material savings. A fabrication process based on selective growth of CuInSe2 micro solar cells is presented, using electrodeposition into holes of an insulating SiO2 layer on a Mo back electrical contact. Proof-of-principle micro solar cells using this materials-savings approach are realized and characterized. Additionally, the heat management of micro-concentrator solar cells is studied using finite element simulations. For square micro solar cells up to 100 µm side length the temperature increase due to the concentrated sunlight can be kept below 4 °C above that of a flat panel solar cell. An increase of the PCE by 4.9% is estimated, considering the combined effect of the open circuit voltage increase due to concentrated sunlight, and the PCE decrease due to the temperature increase.Törndahl Teixeira Nicoara R-Ribeiro Andrade Stroppa González Edoff Leitão Sadewasser T J P N D G J C M J P S P.M.P Salomé J. Keller
CdS and Zn1−xSnxOy buffer layers for CIGS solar cells Journal Article
Solar Energy Materials and Solar Cells, Volume 159 , pp. Pages 272-281, 2017.
@article{Salomé2017b,
title = {CdS and Zn1−xSnxOy buffer layers for CIGS solar cells},
author = {Törndahl Teixeira Nicoara R-Ribeiro Andrade Stroppa González Edoff Leitão Sadewasser T J P N D G J C M J P S P.M.P Salomé J. Keller},
url = {https://doi.org/10.1016/j.solmat.2016.09.023},
year = {2017},
date = {2017-01-01},
journal = {Solar Energy Materials and Solar Cells},
volume = {Volume 159},
pages = {Pages 272-281},
abstract = {Thin film solar cells based on Cu(In,Ga)Se2 (CIGS), where just the buffer layer is changed, were fabricated and studied. The effects of two different buffer layers, CdS and ZnxSn1−xOy (ZnSnO), are compared using several characterization techniques. We compared both devices and observe that the ZnSnO-based solar cells have similar values of power conversion efficiency as compared to the cells with CdS buffer layers. The ZnSnO-based devices have higher values in the short-circuit current (Jsc) that compensate for lower values in fill factor (FF) and open circuit voltage (Voc) than CdS based devices. Kelvin probe force microscopy (KPFM) results indicate that CdS provides junctions with slightly higher surface photovoltage (SPV) than ZnSnO, thus explaining the lower Voc potential for the ZnSnO sample. The TEM analysis shows a poly-crystalline ZnSnO layer and we have not detected any strong evidence of diffusion of Zn or Sn into the CIGS. From the photoluminescence measurements, we concluded that both samples are being affected by fluctuating potentials, although this effect is higher for the CdS sample.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Thin film solar cells based on Cu(In,Ga)Se2 (CIGS), where just the buffer layer is changed, were fabricated and studied. The effects of two different buffer layers, CdS and ZnxSn1−xOy (ZnSnO), are compared using several characterization techniques. We compared both devices and observe that the ZnSnO-based solar cells have similar values of power conversion efficiency as compared to the cells with CdS buffer layers. The ZnSnO-based devices have higher values in the short-circuit current (Jsc) that compensate for lower values in fill factor (FF) and open circuit voltage (Voc) than CdS based devices. Kelvin probe force microscopy (KPFM) results indicate that CdS provides junctions with slightly higher surface photovoltage (SPV) than ZnSnO, thus explaining the lower Voc potential for the ZnSnO sample. The TEM analysis shows a poly-crystalline ZnSnO layer and we have not detected any strong evidence of diffusion of Zn or Sn into the CIGS. From the photoluminescence measurements, we concluded that both samples are being affected by fluctuating potentials, although this effect is higher for the CdS sample. -
2016
Teixeira Stroppa R.-Ribeiro Andrade Nicoara Abderrafi Leitão González J P D N K J J C H. Limborço P.M.P. Salomé and S Sadewasser
Synthesis and formation mechanism of CuInSe2 nanowires by one-step self-catalysed evaporation growth Journal Article
CrystEngComm, Vol. 18 (Issue 37), pp. 7147-7153, 2016.
@article{Limborço2016,
title = {Synthesis and formation mechanism of CuInSe2 nanowires by one-step self-catalysed evaporation growth},
author = {Teixeira Stroppa R.-Ribeiro Andrade Nicoara Abderrafi Leitão González J P D N K J J C H. Limborço P.M.P. Salomé and S Sadewasser},
url = {http://pubs.rsc.org/en/content/articlelanding/2016/ce/c6ce01317a#!divAbstract},
doi = {10.1039/C6CE01317A},
year = {2016},
date = {2016-08-17},
journal = {CrystEngComm},
volume = {Vol. 18},
number = {Issue 37},
pages = {7147-7153},
abstract = {High-quality CuInSe2 (CISe) nanowires have been prepared by a one-step evaporation process. The presented growth process results in a composite material consisting of CISe NWs on top of a polycrystalline CISe base layer. The nanowires were extensively characterized by transmission electron microscopy, confirming their composition and atomic-scale crystal structure with a very low number of structural defects. From these analyses, we infer that the growth axis is along the [111] direction. The polycrystalline base layer has a tetragonal chalcopyrite structure and is optically active as confirmed by X-ray diffraction and photoluminescence (PL) analysis, respectively. Potential applications of this composite CISe NW/base-layer material for photovoltaic energy conversion are supported by the reduced reflectivity of the material and its strong PL intensity. The presented growth method is based on elemental evaporation under vacuum conditions, which makes the process compatible with the fabrication of photovoltaic devices.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}High-quality CuInSe2 (CISe) nanowires have been prepared by a one-step evaporation process. The presented growth process results in a composite material consisting of CISe NWs on top of a polycrystalline CISe base layer. The nanowires were extensively characterized by transmission electron microscopy, confirming their composition and atomic-scale crystal structure with a very low number of structural defects. From these analyses, we infer that the growth axis is along the [111] direction. The polycrystalline base layer has a tetragonal chalcopyrite structure and is optically active as confirmed by X-ray diffraction and photoluminescence (PL) analysis, respectively. Potential applications of this composite CISe NW/base-layer material for photovoltaic energy conversion are supported by the reduced reflectivity of the material and its strong PL intensity. The presented growth method is based on elemental evaporation under vacuum conditions, which makes the process compatible with the fabrication of photovoltaic devices.Sousa Fernandes Sadewasser Cunha M G P A S A F da J.P. Teixeira P.M.P. Salomé and J P Leitão
Optical and structural investigation of inline Cu2ZnSnS4 based solar cells Journal Article
Phys. Status Solidi B, Volume 253 (Issue 11), pp. Pages 2129–2135, 2016.
@article{Teixeira2016,
title = {Optical and structural investigation of inline Cu2ZnSnS4 based solar cells},
author = {Sousa Fernandes Sadewasser Cunha M G P A S A F da J.P. Teixeira P.M.P. Salomé and J P Leitão},
url = {http://onlinelibrary.wiley.com/doi/10.1002/pssb.201600453/full},
doi = {10.1002/pssb.201600453},
year = {2016},
date = {2016-08-01},
journal = {Phys. Status Solidi B},
volume = {Volume 253},
number = {Issue 11},
pages = {Pages 2129–2135},
abstract = {The structural and optical properties of two solar cells in which the Cumath formulaZnSnSmath formula absorber layer was sulphurized by two different methods (S flux and graphite box), were studied. The grain sizes are dependent on the sulphurization method, the larger ones being obtained for the sulphurization in a S flux. The optical properties were investigated by photoluminescence (PL). A broad and asymmetric band was observed for the sample with the larger grains, whereas for the other one a very broad emission was obtained, mostly influenced by the CdS buffer layer. The dependence on the excitation power revealed the influence of fluctuating potentials created by strong doping and high compensation of the absorber layer. Radiative recombination channels are quite different from the ones typical of semiconductor materials with flat bands. A relationship between the PL intensity from the absorber layer measured at low temperatures, and the final PV performance is established. Thus, we propose that PL can be used as an evaluation experimental technique in order to decide if a certain absorber should be processed into a full solar cell or not.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}The structural and optical properties of two solar cells in which the Cumath formulaZnSnSmath formula absorber layer was sulphurized by two different methods (S flux and graphite box), were studied. The grain sizes are dependent on the sulphurization method, the larger ones being obtained for the sulphurization in a S flux. The optical properties were investigated by photoluminescence (PL). A broad and asymmetric band was observed for the sample with the larger grains, whereas for the other one a very broad emission was obtained, mostly influenced by the CdS buffer layer. The dependence on the excitation power revealed the influence of fluctuating potentials created by strong doping and high compensation of the absorber layer. Radiative recombination channels are quite different from the ones typical of semiconductor materials with flat bands. A relationship between the PL intensity from the absorber layer measured at low temperatures, and the final PV performance is established. Thus, we propose that PL can be used as an evaluation experimental technique in order to decide if a certain absorber should be processed into a full solar cell or not.Donzel-Gargand Ch. Frisk Joel Salomé Borme Sadewasser Ch. Platzer-Björkman O J P J S B. Vermang Y. Ren and M Edoff
Rear surface optimization of CZTS solar cells by use of a passivation layer with nano-sized point openings Journal Article
IEEE Journal of Photovoltaics, Volume: 6 (Issue: 1), pp. 332 - 336, 2016, ISSN: 2156-3381.
@article{Vermang2016,
title = {Rear surface optimization of CZTS solar cells by use of a passivation layer with nano-sized point openings},
author = {Donzel-Gargand Ch. Frisk Joel Salomé Borme Sadewasser Ch. Platzer-Björkman O J P J S B. Vermang Y. Ren and M Edoff},
url = {http://ieeexplore.ieee.org/document/7329914/},
doi = {10.1109/JPHOTOV.2015.2496864},
issn = {2156-3381},
year = {2016},
date = {2016-01-01},
journal = {IEEE Journal of Photovoltaics},
volume = {Volume: 6},
number = {Issue: 1},
pages = {332 - 336},
abstract = {Previously, an innovative way to reduce rear interface recombination in Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu2 (Zn,Sn)(S,Se)4 (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al2 O3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage (VOC ; +17%rel.), short-circuit current (JSC ; +5%rel.), and fill factor (FF; +9%rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32%rel. is obtained for the rear passivated cells.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Previously, an innovative way to reduce rear interface recombination in Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells has been successfully developed. In this work, this concept is established in Cu2 (Zn,Sn)(S,Se)4 (CZTSSe) cells to demonstrate its potential for other thin-film technologies. Therefore, ultrathin CZTS cells with an Al2 O3 rear surface passivation layer having nanosized point openings are fabricated. The results indicate that introducing such a passivation layer can have a positive impact on open-circuit voltage (VOC ; +17%rel.), short-circuit current (JSC ; +5%rel.), and fill factor (FF; +9%rel.), compared with corresponding unpassivated cells. Hence, a promising efficiency improvement of 32%rel. is obtained for the rear passivated cells. -
2015
Sadewasser S P.M.P. Salomé H. Rodriguez-Alvarez
Incorporation of alkali metals in chalcogenide solar cells Journal Article
Solar Energy Materials & Solar Cells, 143 , pp. 9, 2015.
@article{Salomé2015,
title = {Incorporation of alkali metals in chalcogenide solar cells},
author = {Sadewasser S P.M.P. Salomé H. Rodriguez-Alvarez},
url = {http://www.sciencedirect.com/science/article/pii/S0927024815002809},
year = {2015},
date = {2015-12-03},
journal = {Solar Energy Materials & Solar Cells},
volume = {143},
pages = {9},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Viktor Fjällström Tobias Törndaht Uwe Zimmermann Marika Edoff Piotr Szaniawski Pedro Salomé
Influence of varying Cu content on growth and performance of Ga-graded Cu(In,Ga)Se2 solar Cells Journal Article
IEEE J. PV, 5 , pp. 1775, 2015.
@article{Szaniawski2015,
title = {Influence of varying Cu content on growth and performance of Ga-graded Cu(In,Ga)Se2 solar Cells},
author = {Viktor Fjällström Tobias Törndaht Uwe Zimmermann Marika Edoff Piotr Szaniawski Pedro Salomé},
url = {http://ieeexplore.ieee.org/xpl/articleDetails.jsp?reload=true&arnumber=7296602},
year = {2015},
date = {2015-10-12},
journal = {IEEE J. PV},
volume = {5},
pages = {1775},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Ribeiro Fernandes Salomé Cunha Leitão Silva González G M P A P M P A F J P M I N J C da da A. Abelenda M. Sánchez
Anomalous persistent photoconductivity in Cu2ZnSnS4 thin films and solar cells Journal Article
Solar Energy Materials & Solar Cells, 137 , pp. 164, 2015.
@article{Abelenda2015,
title = {Anomalous persistent photoconductivity in Cu2ZnSnS4 thin films and solar cells},
author = {Ribeiro Fernandes Salomé Cunha Leitão Silva González G M P A P M P A F J P M I N J C da da A. Abelenda M. Sánchez},
url = {http://www.sciencedirect.com/science/article/pii/S0927024815000598},
year = {2015},
date = {2015-06-01},
journal = {Solar Energy Materials & Solar Cells},
volume = {137},
pages = {164},
keywords = {},
pubstate = {published},
tppubtype = {article}
}Vermang Salomé Rostvall Zimmermann B P M P F U V. Fjällström P. Szaniawski and M.Edoff
Recovery after potential induced degradation of Cu(In,Ga)Se2 solar cells with CdS and Zn(O,S) buffer layers Journal Article
IEEE J. Photovoltaics, 5 , pp. 664, 2015.
@article{Fjällström2015,
title = {Recovery after potential induced degradation of Cu(In,Ga)Se2 solar cells with CdS and Zn(O,S) buffer layers},
author = {Vermang Salomé Rostvall Zimmermann B P M P F U V. Fjällström P. Szaniawski and M.Edoff},
year = {2015},
date = {2015-01-12},
journal = {IEEE J. Photovoltaics},
volume = {5},
pages = {664},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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Daniel Brito
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Vanessa Iglesias
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Henrique Limborço
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Humberto Rodriguez-Alvarez
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David Fuertes Marrón
Visiting Scientist (2013)