Suppressing Halide Segregation of Wide Bandgap Perovskite by Interface Molecular Coordination for High‐Performance All‐Perovskite Tandem Solar Cells

Abstract Hole transporting layers made by self‐assembled molecules (SAMs) are emerging as promising hole transporting materials (HTMs) for perovskite‐based tandem solar cells, owing to their reduced parasitic absorption and effective carrier extraction. However, perovskite films grown on HTM substrates typically exhibit a high defect density, which adversely affects device performance. In this study, we investigated the film growth kinetics of wide‐bandgap perovskite on monolayer material substrates and uncovered a halide phase segregation in the initial nucleation stage during crystal growth kinetics at the interface, which brings small grain sizes and significant lattice strain within the perovskite film. To address this issue, we introduced a biphosphate‐substituted molecule on the HTM surface to coordinate with PbBr 2 that suppresses halide phase segregation, leading to improved crystallographic orientation and a reduction in defect density. As a result, the wide‐bandgap (1.77 eV) perovskite solar cells (PSCs) achieved a power conversion efficiency (PCE) of 19.5% with an open‐circuit voltage of 1.35 V, while tandem devices reached an impressive efficiency of 28.65%.