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 Cs2NaBiI6 (CNBI) and Cs2KBiI6 (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 (beta) 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 beta similar to -6.23 x 10(-12) cm W-1, and -1.14x 10(-12) cm W-1, respectively. The CA measurements depicted a self-defocusing effect in both the samples with n(2) values similar to-1.06 x 10(-14) cm(2) W-1 and -1.337x 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.