Defect-Dependent Nonlinear Absorption in the Lead-Free Double-Perovskite Cs2AgBiBr6

Lead-free perovskites have attracted increasing attention because they can address the toxicity and instability problems inherent to lead-halide perovskites. Furthermore, the nonlinear optical (NLO) properties of lead-free perovskites are rarely explored. Herein, we report significant NLO responses and defect-dependent NLO behavior of Cs2AgBiBr6. Specifically, a thin film of pristine Cs2AgBiBr6 exhibits strong reverse saturable absorption (RSA), while a film of Cs2AgBiBr6 with defects (denoted as Cs2AgBiBr6(D)) shows saturable absorption (SA). The nonlinear absorption coefficients are ca. 4.0 x 104 cm GW-1 (515 nm laser excitation) and 2.6 x 104 cm GW-1 (800 nm laser excitation) for Cs2AgBiBr6 and -2.0 x 104 cm GW-1 (515 nm laser excitation) and -7.1 x 103 cm GW-1 (800 nm laser excitation) for Cs2AgBiBr6(D). The optical limiting threshold of Cs2AgBiBr6 is 8.1 x 10-4 J cm-2 (515 nm laser excitation). The samples show excellent long-term performance stability in air. The RSA of pristine Cs2AgBiBr6 correlates with excited-state absorption (515 nm laser excitation) and excited-state absorption following two-photon absorption (800 nm laser excitation), while the defects in Cs2AgBiBr6(D) strengthen the ground-state depletion and Pauli blocking, resulting in SA.