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 VOC 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.