The Bandgap as a Moving Target: Reversible Bandgap Instabilities in Multiple-Cation Mixed-Halide Perovskite Solar Cells

Multiple-cation mixed-halide perovskites show high power-conversion efficiencies and recently improved stability. But, even most advanced absorber materials still suffer from instabilities of the bandgap under illumination and applied bias. Here, we employ modulation spectroscopy as a highly sensitive electro-optical measurement technique to directly reveal such instabilities. We find a reversible decrease of the absorber bandgap of up to 70 meV in complete solar cells. In situ X-ray diffraction measurements under illumination and bias confirm structural changes of the perovskite and their reversibility, which are attributed to a segregation of the halides. These processes are most strongly activated when illumination and bias are combined, which leads to a 5 times increased shift compared to that with illumination only. Since this scenario is intrinsic to the solar cell's operation conditions, it can never be avoided completely. Furthermore, the bandgap decrease is strongly enhanced, but still reversible, by high relative humidity and oxygen content supporting the strict requirement of efficient encapsulation.