Air-Processed n -Butylpyridinium Bromide-Doped CsPbI ₂ Br Perovskite Solar Cells: Stress Relief, Energy Level Modulation, and 14.51% Efficiency

CsPbI 2 Br perovskite solar cells (PSCs) have emerged as a research focus in third-generation photovoltaics due to their optimal optical bandgap (1.8–1.9 eV) and excellent thermal stability derived from lattice compatibility of the cesium ion (Cs + ). However, solution-processed CsPbI 2 Br films are plagued by intrinsic defects arising from nonequilibrium crystallization and lattice distortion caused by ionic radius differences. These issues synergistically induce carrier nonradiative recombination and ion migration, severely restricting device efficiency and stability. To tackle these challenges, we employed the ionic liquid N-butylpyridinium bromide (N-BuPyBr), which comprises a pyridine ring, butyl chain, and Br –, to optimize the crystal structure and modulate energy levels. Specifically, Br – forms strong coordination bonds with Pb 2+ to passivate halogen vacancies, while the cation interacts with I – and Pb 2+ to suppress defect-mediated ion migration. Additionally, the hydrophobic structure of the cations retards moisture intrusion, thereby enhancing the environmental stability. The synergistic effect of N-BuPyBr alleviates structural stress, reduces lattice distortion, downshifts the conduction band minimum, and enhances energy level alignment with the electron transport layer (ETL). Consequently, the treated PSCs achieved a power conversion efficiency (PCE) of 14.51% and exhibited superior stability under 25% relative humidity (RH) in ambient conditions, maintaining high performance over an extended period.