Current-Induced Phase Segregation in Mixed Halide Hybrid Perovskites and its Impact on Two-Terminal Tandem Solar Cell Design

Mixed halide hybrid perovskites are of significant interest because their bandgap can be tuned as a current-matched top-cell in tandem photovoltaics. However, several mixed halide perovskites phase segregate under illumination, exhibit large voltage deficits, and produce unstable photocurrents. We investigate the origin of phase segregation and implication for tandems with mixed halide large-bandgap (∼1.75 eV) perovskites. We show explicitly that MAPb­(I<sub>0.6</sub>Br<sub>0.4</sub>)<sub>3</sub> and (MA<sub>0.9</sub>,Cs<sub>0.1</sub>)­Pb­(I<sub>0.6</sub>,Br<sub>0.4</sub>)<sub>3</sub>, termed “MA” and “MACs”, respectively, rapidly phase segregate in the dark upon 1 sun equivalent current injection. This is direct experimental evidence that conduction band electrons or valence band holes are the culprit behind phase segregation. In contrast, (FA<sub>0.83</sub>,Cs<sub>0.17</sub>)­Pb­(I<sub>0.66</sub>,Br<sub>0.34</sub>)<sub>3</sub>, or “FACs,” prepared at only 75 °C resists phase segregation below 4 sun injection. FACs prepared at 165 °C yields larger grains and withstands higher injected carrier concentrations before phase segregation. The FACs and MACs devices sustain near constant power output at 1 sun and do not affect the current output of a CIGS bottom cell when used as an incident light filter.