Controlled Crystallization of CsRb‐Based Multi‐Cation Perovskite Using a Blended Sequential Process for High‐Performance Solar Cells

High‐quality perovskite films with low defect densities and high carrier‐diffusion lengths are essential for high‐performance perovskite devices. Herein, a modified two‐step sequential method is proposed, in which equal amounts of Cs + and Rb + ions (Cs + Rb + ) are added to a PbI 2 solution in the first step of perovskite deposition. The incorporation of Cs + Rb + results in trace amounts of δ‐CsPbI 3 , leading to a reduction in the grain size in the PbI 2 layer that facilitates better penetration of the organic salts in the second step of the precursor deposition. Meanwhile, the low density of the δ‐CsPbI 3 crystals act as seeds to regulate the nucleation and growth of the perovskite crystals. A single‐layered columnar large‐grain perovskite film is obtained. The addition of Cs + Rb + promotes crystal growth along the (100) orientation, which is beneficial for the reduction of surface defects. The incorporated Rb + ions can regulate the remaining PbI 2 in the perovskite. The improved quality of the perovskite film results in a significantly prolonged carrier lifetime of ≈7.01 and an enhanced electron diffusion length of ≈2.84 . As a result, the photovoltaic parameters are all improved in the established devices and a power conversion efficiency of 21.49% is reached.