The tin(ii) precursor is an active site to determine the crystal framework in CsSnI3 perovskite

Abstract

Understanding the creation of the crystal framework and finding its origin in the perovskite structure is one of the most important tasks in the perovskite research field. The fundamental observation of the formation of a perovskite crystal from a liquid phase to a solid-state promotes the realization of an ideal perovskite structure. In this work, we investigated how to determine the crystal structure of cesium tin triiodide (CsSnI₃) perovskite from the precursor solution to the crystallization film. Based on the opto-physical spectroscopy data and chemical interaction results, we found that tin(II) cations are the main active sites to determine the crystal framework of CsSnI₃ perovskite. The Sn²⁺ precursor has strong coordination bonding with the sulfinyl group of dimethyl sulfoxide (DMSO) solvent in the liquid phase, which is kept in the solid phase and determines the CsSnI₃ crystal structure. To achieve a more grain-ordered crystal structure by the coordination interaction, we introduced a new molecule, an ammonium trifluoromethanesulfonate (ATMS) anion, with a sulfonate group. Through the strong chemical interaction between the Sn²⁺ cation and ATMS anion, the Sn²⁺ precursor-based crystal phase was controlled. This facilitated unprecedented two-step synthesis of the CsSnI₃ perovskite crystal and solar cell application, which revealed better device stability than conventional methods reported to date.