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

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‐FAPbI 3 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 FAPbI 3 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 FA 0.95 Cs 0.05 PbI 3 perovskite thin film with significantly reduced nonradiative recombination is confirmed. An area of 25 mm 2 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.