Halide segregation and structural dynamics of wide-bandgap perovskites in solar cells

Wide bandgap (WBG) mixed-halide perovskites are promising candidates for tandem and semitransparent solar cells owing to the increased V_OC and the controllable optical properties arising from engineered mixed halide compositions. However, their stability is hindered by photoinduced halide segregation, causing phase instability and long-term degradation. In this work, we compare two structural modification strategies: a surface post-treatment to form a 2D/3D bilayer and an additive approach to induce a quasi-2D layer using a 2D spacer ligand (butylammonium iodide (BAI)). The bilayer method passivates interfacial defects and halide vacancies, promoting crystallinity, suppressing ion migration, and enhancing stability. In contrast, quasi-2D domains not only exhibit quantum well-like features and passivation effects, but also cause vertical heterogeneity and hinder charge transport. Photoluminescence (PL) analysis reveals differences in ion redistribution and phase segregation mechanisms. As a result, 2D/3D bilayer devices demonstrate superior phase stability, reduced hysteresis, and enhanced performance (PCE 19.03%) with long-term operational stability (>1700 h at >80% efficiency) and high indoor efficiency (PCE 27% at 1000 lux). This study underscores the critical role of 2D spacer processing routes in tailoring perovskite film structure and performance, offering valuable insights into material-device interplay for stable and efficient WBG perovskite solar cells.