Simple Solution Preparation of Cs₂SnI₆ Films and Their Applications in Solid-State DSSCs

Cs₂SnI₆ powder is, for the first time, solution-prepared via the formula CsI + SnI₂ + I₂ → Cs₂SnI₆. The product is highly pure and air/thermal stable. It is found that N,N-dimethylformamide (DMF) and methanol induce severe Cs₂SnI₆ deterioration with the appearance of a CsI phase in film preparation from Cs₂SnI₆ powder, while solvents of γ-butyrolactone (GBL) and ethylene glycol methyl ether (EGME) (Film-EGME) give better results. Then, by introducing EGME solvent, in situ preparation of Cs₂SnI₆ films (Film-1 to Film-4) is realized under solution reaction, which is found to be dominated by thermal dynamic process, i.e., highly pure/oriented Film-4 is obtained under the maximum reagent-concentration. Besides, for good reaction, the solubility of solvent should be balanced among all the reagents and products. Solid-state dye sensitized solar cells (ss-DSSCs) comprising a Cs₂SnI₆ electrolyte are investigated. The power conversion efficiencies (PCEs) of the ss-DSSCs based on solution-casted Film-EGME and the in situ-prepared Film-4 are 1.81% and 3.30%, respectively. Particularly, with the in situ prepared Cs₂SnI₆ films, it is found that the open circuit voltages of the ss-DSSCs are closely related to their gap states. When additive is added in Cs₂SnI₆ electrolyte, a PCE of 6.14% is obtained in an ss-DSSC. Our work highlights the importance of solvent in film preparation and the role of Cs₂SnI₆ gap states in device performance.