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

Abstract

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 (n₂) 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 n₂ values ∼−1.06 × 10⁻¹⁴ cm² W⁻¹ and −1.337× 10⁻¹⁴ cm² 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.