Formamidinium-cesium lead iodide (FACsPbI3), a halide compound that has become increasingly popular for use in perovskite solar cells as it combines high efficiency and reasonable heat resistance. However, solar cells made of this particular compound have a rather ambivalent relationship with sunlight, a problem that is well-known in the field, but barely understood. While the light of the sun feeds it with the much-wanted energy to convert into electricity, it also impairs the stability of the cells. Over time this affects their performance.

To understand why this is the case, our group together with Peking University and University of California San Diego did both practical experiments – monitoring the photovoltaic performance of the panels over 600 hours of exposure and characterizing the degraded perovskites – and theoretical analysis by Zehua Chen.

We conclude that sunlight generates free carriers in the perovskite, which tend to funnel to regions where the band gap is lowest, in this case the formamidinium perovskite. The resulting energy differences make the mixed compounds that worked together so well to make the cell efficient, fall apart into separate clusters. It appears that especially the cesium-heavy clusters are photoinactive and current-blocking, limiting the performance of the device

The findings are published on Joule and highlighted as homepage news item “understanding the love-hate relationship of halide perovskite with the sun” at our university TU/e.

The atomic structure of mixed FACsPbI3 perovskite, where it separates into two CsPbI3 (green region) and FAPbI3 clusters under light excitation.