Controlling Nucleation and Crystal Growth Orientation via Intermediate Ion‐Complex Formation for Efficient α‐FA0.95Cs0.05PbI3 Perovskite Solar Cells

Abstract

The formamidinium–cesium (FA–Cs) alloyed metal halide perovskites demonstrate superior photovoltaics device stability; however, alloying at lower Cs concentrations (≤5%) undergoes complex intermediate phase transitions that disrupt morphological and structural properties. Herein, an intermediate ion‐complex method in a bid is attempted to achieve a near‐optimal energy bandgap of 1.52 eV for α‐FAPbI3 perovskite (alloyed with 5% Cs+ cation). Upon incorporation of judicious amount of smaller and more volatile ions (such as Cl−/Br− and methylammonium [MA]+/Cs+) into the host FAPbI3 system, these ions are observed to persist in the “transitional‐phase” and “stimulate” the formation of α‐phase. Consequently, using X‐ray and photoluminescence quantum efficiency measurements, the formation of a (110)‐plane‐oriented high‐quality FA0.95Cs0.05PbI3 perovskite thin film with significantly reduced nonradiative recombination is confirmed. An area of 25 mm2 yields a power conversion efficiency (PCE) of 20.21% and a stabilized power output of 19.82%, leveraging open‐circuit voltage of 90.4% of that Shockley–Queisser limit. The devices sustain over 1000 h without any PCE degradation.