Controlling Nucleation and Crystal Growth Orientation via Intermediate Ion-Complex Formation for Efficient -FA_0.95Cs_0.05PbI₃ 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 alpha-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 alpha-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.