Interfacial energy-level engineering with bilayer hole transport layers for suppressing phase separation in mixed-halide perovskites

Mixed-halide perovskites suffer from light-induced ion migration, causing phase separation and reduced stability. Current strategies, such as compositional engineering or additive passivation, partially mitigate these issues but often overlook critical factors like carrier extraction efficiency and interfacial energy-level alignment. Here, we introduce a PEDOT:PSS/NiOx bilayer hole transport layer (HTL) that optimizes energy-level alignment, achieving a threefold enhanced hole extraction rate (kH = 1.17 × 108 s−1). Comprehensive microscopic analyses reveal that the optimized bilayer HTL effectively reduces localized charge accumulation, kinetically suppressing ion migration and phase separation. Consequently, the perovskite film retains compositional homogeneity under prolonged illumination, substantially enhancing both photovoltaic performance and operational stability. This work provides valuable insights into interfacial energy-level engineering for mixed-halide perovskites and offers practical strategies for designing robust perovskite solar cells.