Low Temperature Optical Properties of Novel Lead-Free Cs2NaFeCl6 Perovskite Single Crystals

Lead-free double perovskites have attracted much attention as possible alternatives to lead halide based perovskites in photovoltaic applications. However, to date only few double perovskites have been successfully employed in optoelectronic device prototypes. Therefore, the search for stable and lead-free materials is ongoing. Here, we present the successful growth of high-quality Cs2NaFeCl6 single crystals and their temperature-dependent structural and optical properties. By combining electron paramagnetic resonance (EPR), crystal structure analysis and density functional theory (DFT) we could determine a cubic crystal structure with a spin of 5/2 for this material, showing strongly spin polarized character. Furthermore, combining photoluminescence (PL) and optical absorption measurements we find a bandgap of approximately 2.1 eV at room temperature as well as the presence of excitonic states. Using Elliot's formula, we are able to extract the temperature-dependent behavior of the bandgap as well as an estimated exciton binding energy of only 20 meV at 80 K.