Quasi‐2D Scaffolding for Enhanced Stability and Efficiency in 1.67 eV Cs‐Rich Pure‐Iodide Perovskite Solar Cells

Abstract Cesium lead triiodide (CsPbI 3 ) perovskitesare promising candidates for top cells in tandem solar cells owing to their superior thermal and photostability. However, their practical application is hindered by poor phase stability, as CsPbI 3 readily converts from the perovskite phase to the non‐perovskite phase. To improve both phase stability and efficiency without significantly altering the bandgap, some fraction of formamidinium (FA + ) is introduced into CsPbI 3 . This study demonstrates that a quasi‐2D perovskite intermediate effectively modulates the crystallization process and improves the film quality of Cs‐rich, pure‐iodide wide‐bandgap perovskites, leading to a significant enhancement in open‐circuit voltage (V OC ). Propylphenylammonium chloride (PPACl) facilitates the formation of a quasi‐2D PPA 2 (Cs x FA 1‐x ) n−1 Pb n I 3n+1 phase, which acts as a scaffold to promote the oriented crystallization of 3D perovskites. This quasi‐2D intermediate can mitigate structural distortion in the perovskite lattice by alleviating lattice mismatch, typically associated with the dimethylammonium lead triiodide (DMAPbI 3 ) to final α‐phase transition. Thus, the approach enhances crystallinity and morphology, reducing defect density and V OC loss in the 3D perovskite. Consequently, the optimized Cs 0.7 FA 0.3 PbI 3 perovskite solar cells (PSCs) achieve a power conversion efficiency of 21.42%, marking one of the highest efficiencies reported for Cs‐rich wide‐bandgap PSCs under standard AM 1.5 G illumination.