Structural-disorder-induced enhancement of second-harmonic generation in bismuth-based perovskite-inspired materials

Bismuth-based perovskite-inspired materials (Bi-PIMs) offer a sustainable alternative to lead halide perovskites (LHPs), yet their optoelectronic performance is limited by sub-optimal thin-film morphology and high defect density. Consequently, Bi-PIM solar cells underperform compared to their LHP-based counterparts. Could the defect-driven structural disorder of Bi-PIMs expand their application beyond photovoltaics? We investigate the nonlinear optical (NLO) response of Bi-PIMs from the (Cu-)Ag-Bi-I family, which crystallize in centrosymmetric space groups with significant local distortions due to numerous cation vacancies. Structural calculations reveal that lattice distortions lead to local symmetry breaking. Using NLO microscopy, we discover that centrosymmetric Bi-PIMs exhibit NLO characteristics, specifically second- and third-harmonic generation, at room temperature. Notably, strong second-harmonic generation (SHG) emission is observed from Bi-PIM polycrystalline films with many cation vacancies. These findings suggest that the inherent labile and disordered structures of polycrystalline Bi-PIMs offer an advantage for SHG, paving the way for future photonic applications.