关键词: Cs$_{2}$SnI$_{6}$, crystalline orientation, Lewis acid-case, additive engineering, bandgap engineering

Abstract: Inorganic Cs$_{2}$SnI$_{6}$ perovskite has exhibited substantial potential for light harvesting due to its exceptional optoelectronic properties and remarkable stability in ambient conditions. The charge transport characteristics within perovskite films are subject to modulation by various factors, including crystalline orientation, morphology, and crystalline quality. Achieving preferred crystalline orientation and film morphology via a solution-based process is challenging for Cs$_{2}$SnI$_{6}$ films. In this work, we employed thiourea as an additive to optimize crystal orientation, enhance film morphology, promote crystallization, and achieve phase purity. Thiourea lowers the surface energy of the (222) plane along the $\langle 111\rangle$ direction, confirmed by x-ray diffraction, x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy studies, and density functional theory calculations. Varying thiourea concentration enables a bandgap tuning of Cs$_{2}$SnI$_{6}$ from 1.52 eV to 1.07 eV. This approach provides a novel method for utilizing Cs$_{2}$SnI$_{6}$ films in high-performance optoelectronic devices.

Key words: Cs$_{2}$SnI$_{6}$, crystalline orientation, Lewis acid-case, additive engineering, bandgap engineering