Understanding the effect of light and temperature on the optical properties and stability of mixed-ion halide perovskites†
The stability of organic–inorganic halide perovskite films plays an important role for their successful incorporation as absorber materials in solar cells under realistic operation conditions. While light-induced effects have been observed and traced to phase segregation, the impact of different stressors simultaneously is mostly unexplored. In this work, we investigate the combined influence of light and elevated temperature on the performance of mixed-cation mixed-halide perovskites. In particular, we compare the effect of different A-site cations on the photoluminescence (PL) properties and film stability when both stressors are used simultaneously. We find two pathways underlying the PL peak reduction and PL shift in the optical properties. For perovskite films composed of formamidinium and methylammonium as A-site cations, we can correlate the decrease in film performance to the formation of Pb(I,Br)2 and an increase in electron–phonon interactions. Similarly, Rb doping in the perovskite film exhibits comparable results. Contrary, using Cs as an additional A-site cation greatly enhances the overall performance and results in more stable film structures which indicates that Cs is effective in stiffening the perovskite lattice, which can be attributed to a better size match for the Pb(I,Br)3 sublattice as predicted by the Goldschmidt tolerance factor. These findings suggest that it is of importance to carefully select stressors when assessing performance related parameters of perovskite solar cells.