Effect of Lewis acid-base additive on lead-free Cs₂SnI₆ thin film prepared by direct solution coating process

Inorganic Cs₂SnI₆ 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 Cs₂SnI₆ 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 〈111〉 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 Cs₂SnI₆ from 1.52 eV to 1.07 eV. This approach provides a novel method for utilizing Cs₂SnI₆ films in high-performance optoelectronic devices.