Unveiling Electron–Phonon Interaction That Influences the Photoluminescence Properties of Cs2Na x Ag1–x BiCl6 Mixed Halide Double Perovskites

Owing to its indirect band gap, an all-inorganic, lead-free halide double perovskite generally tends to exhibit a low photoluminescence efficiency, thus restricting its applicability as a photovoltaic material. Cs2AgBiCl6 is one of the stable halide double perovskites with an indirect band gap of 2.7 eV. Its optoelectronic properties can be enhanced by partially replacing some of the Ag ions with the isovalent Na ions, forming Cs2Na x Ag1–x BiCl6 compositions. The origin of enhanced photoluminescence in these mixed compositions is speculated to be the competitive interaction of both radiative and nonradiative transition, highly influenced by electron–phonon coupling. However, a concrete understanding of the electron–phonon coupling phenomenon and an estimate of its strength is still lacking. Our investigations reveal that the de-excitation process involves decay through trap states formed in the substituted compositions. A quantitative analysis of Raman spectra along with the composition-dependent Urbach tail states and Fano broadening reveals a better understanding of trap state stabilization via strong electron–phonon interaction.