Looking inside a solar cell

January 8, 2024

In a ground-breaking study published in Nature Energy, researchers from the Nanostructured Solar Cells group have unveiled critical insights into the optimisation of thin-film solar cells, shedding light on methods to enhance their efficiency and pave the way for more cost-effective electricity generation.

Photovoltaic power conversion utilising polycrystalline light-absorbing semiconductors has long been recognised for its potential to revolutionise solar energy. Among these technologies, polycrystalline CIGS (copper indium gallium selenide) stands out as a high-performing option, and recent advances have propelled its efficiency even further through an alkali-fluoride post-deposition treatment, which elevates the charge-carrier concentration.

However, the team led by Sascha Sadewasser discovered a previously overlooked challenge in the application of this treatment – inhomogeneities in the conductivity of individual material grains, which they trace back to the charge-carrier concentration. This revelation has significant implications for the efficiency of solar cells and their overall performance.

Using an emerging conductive atomic force microscopy tomography technique, the research team at INL literally scratched away material layer by layer, generating three-dimensional conductivity maps. A detailed analysis of these maps provided for a sub-micrometre scale visualisation of the carrier concentration grain by grain, allowing for a detailed look into the inside of the solar cell.

This artwork was created by Sascha Sadewasser with the assistance of AI-generated algorithms.

In collaboration with the Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) in Germany, the team has tested the alkali-fluoride post-deposition treatment with different elements. The study found that a lower efficiency solar cells treated with potassium-fluoride correlate with more pronounced inhomogeneity of charge-carrier concentration. In contrast, better performing solar cells treated with rubidium- and caesium-fluoride showcased narrower distributions at higher charge-carrier concentrations. Deepanjan Sharma, the PhD candidate conducting this study adds “the charge-carrier concentration of the p-type CIGS absorber layer was identified as a key factor directly influencing the open-circuit voltage of solar cell devices, thereby impacting overall device performance”.

“These findings have immediate implications for the development of higher efficiency thin-film photovoltaics,” said Sascha Sadewasser. “By optimising the alkali-fluoride post-deposition treatment we can unlock the full potential of polycrystalline semiconductor materials and propel the advancement of clean energy technologies”.

The researchers’ pioneering atomic force microscopy tomography and data analysis methods, showcased in this study, are not only applicable to CIGS thin-film solar cells but hold promise for a wide range of polycrystalline semiconductor and energy materials. This breakthrough opens new avenues for research and development across the renewable energy landscape.

As the world seeks sustainable solutions to address the growing demand for clean energy, this research marks a significant step forward in harnessing the power of thin-film solar cells, bringing us one step closer to a greener and more sustainable future.

Text & Photography by: Catarina Moura, Science Communication Officer