Origin of bandgap bowing in Cs2Na1−x Ag x BiCl6 double perovskite solid-state alloys: a paradigm through scanning tunneling spectroscopy

Bandgap bowing has recently been emerged as an effective strategy toward band-engineering in metal halide perovskites. In this work, we report extensive studies on the bowing phenomenon in Cs2NaBiCl6double perovskite upon alloying with silver at the sodium site. Through optical spectroscopy, composition-dependent bandgap in Cs2Na1-xAgxBiCl6(0 ⩽x⩽ 1) evidenced bandgap bowing with an upward-concave nature. Further from the quadratic fit, the bowing coefficient (b= 0.74 eV) turned out to be independent of composition and is close to the theoretically predicted value. From scanning tunneling spectroscopy and associated studies on band-energies, we have observed that the bandgap-lowering originated from a significant change of the valence-band position. The behavior of band-energies has been explained through the orbital-contributions of the constituent elements responsible in forming the two bands. Formation of an internal type-I band-alignment between the two end-members, namely Cs2NaBiCl6and Cs2AgBiCl6could be visualized. Based on experimental evidences, we could infer that the bow-like evolution of bandgap in Cs2Na1-xAgxBiCl6alloys is principally dominated by structural distortions in the Cs2NaBiCl6host-lattice upon incorporation of silver.