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

Cesium lead triiodide (CsPbI₃) 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 CsPbI₃ 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 CsPbI₃. 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 PPA₂(Cs_xFA_1-x)_n−1Pb_nI_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 (DMAPbI₃) 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_0.7FA_0.3PbI₃ 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.