Optimized Crystallization of CsPbI ₂ Br Films through the Incorporation of H ₂ O for High‐Efficiency All‐Inorganic Perovskite/Organic Tandem Solar Cell

Abstract Among all‐inorganic perovskites, CsPbI 2 Br possesses excellent photovoltaic performance, intrinsic thermal stability and suitable bandgap and superior phase‐stability, demonstrating large application potential for single‐junction perovskite solar cells (PSCs) and perovskite/organic tandem solar cells (TSCs). However, a critical challenge persists in the effective control of the crystallization process of the CsPbI 2 Br film, as inadequate crystallization can result in a substantial decrease in the photoelectric efficiency of solar cells. Herein, a simple approach utilizing a small amount of H 2 O additive is introduced to optimize the crystallization process of CsPbI 2 Br film. Specifically, the introduction of water facilitates the formation of numerous CsPbI 2 Br seed crystals within the precursor solution, promoting subsequent Ostwald ripening process of the perovskite film. Additionally, the presence of water enhances the desorption of DMSO from CsPbI 2 Br, further improving overall crystallization. As a result, a CsPbI 2 Br‐basedPSC exhibits a high power conversion efficiency (PCE) of 16.7% and an open‐circuit voltage ( V OC ) of 1.36 V. By integrating these CsPbI 2 Br‐based front sub‐cells with narrow‐bandgap PM6:Y6‐based rear sub‐cells, a high PCE of 23.21% and V OC of 2.17 V is achieved, illustrating the potential of TSCs for high‐efficiency tandem solar cells. Furthermore, the unencapsulated TSCs display superior stability under ambient air conditions and operation.