Prominent reverse saturable absorption of lead-free two-dimensional organic-inorganic hybrid double perovskites promoted by defects

Two-dimensional (2D) organic-inorganic hybrid perovskites have exhibited prominent reverse saturable absorption (RSA), yet their practical application is hindered by the lack of a defect-tolerance performance-enhancing strategy and lead-related toxicity. This study demonstrates that the realization of RSA via excited-state absorption is an effective strategy to achieve defect-promoted RSA in a series of lead-free 2D hybrid double perovskites (A₄AgBiBr₈), where A represents propylammonium (PA), butylammonium (BA), and hexylammonium (HA). Among these, the (PA)₄AgBiBr₈ shows the best performance. The nonlinear absorption coefficient of the solution-phase deposited thin film is as high as (2.0 ± 0.3) × 10⁴ cm GW⁻¹, a value that is competitive with those of most mechanically-exfoliated 2D hybrid lead halide perovskites and inorganic monolayer/few-layer 2D materials. Spectral characterization and density functional theory calculation demonstrate that Br vacancies in A₄AgBiBr₈ introduce abundant defect states, which act as intermediate states and enable efficient excited-state absorption. Consequently, a larger number of defect states results in a stronger RSA response.