Temperature-dependent excitonic emission characteristics of lead-free inorganic double perovskites and their third-order optical nonlinearities.

We report temperature-dependent photoluminescence (PL) in the temperature range between 77 K and 300 K, and room temperature nonlinear optical (NLO) properties of solution processed lead-free Cs 2 NaBiI 6 (CNBI) and Cs 2 KBiI 6 (CKBI) perovskite films. The de-convolution analysis of temperature-dependent PL spectra showed thermal quenching behavior of free-exciton (FX) emission, an unusual blue-shift of PL emission, and line broadening with increasing temperature as a consequence of strong exciton-phonon interaction. The nonlinear refractive index ( n 2 ) and nonlinear absorption coefficient ( β ) of both the CNBI and CKBI films are determined using a closed aperture (CA) and open aperture (OA) Z-scan technique, respectively. Both the CNBI and CKBI perovskites exhibited features of saturable absorption (SA) with β ∼ -6.23 × 10 -12 cm W -1 , and -1.14× 10 -12 cm W -1 , respectively. The CA measurements depicted a self-defocusing effect in both the samples with n 2 values ∼-1.06 × 10 -14 cm 2 W -1 and -1.337× 10 -14 cm 2 W -1 , respectively. With such emission and NLO characteristics, CNBI and CKBI perovskite films can be used for designing eco-friendly optoelectronic and NLO devices.