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

Cs2SnI6 powder is, for the first time, solution-preparedvia the formula CsI + SnI2 + I-2 & RARR; Cs2SnI6. The product is highly pure and air/thermalstable. It is found that N,N-dimethylformamide (DMF)and methanol induce severe Cs2SnI6 deteriorationwith the appearance of a CsI phase in film preparation from Cs2SnI6 powder, while solvents of & gamma;-butyrolactone(GBL) and ethylene glycol methyl ether (EGME) (Film-EGME) give betterresults. Then, by introducing EGME solvent, in situ preparation ofCs(2)SnI(6) films (Film-1 to Film-4) is realizedunder solution reaction, which is found to be dominated by thermaldynamic process, i.e., highly pure/oriented Film-4 is obtained underthe maximum reagent-concentration. Besides, for good reaction, thesolubility of solvent should be balanced among all the reagents andproducts. Solid-state dye sensitized solar cells (ss-DSSCs) comprisinga Cs2SnI6 electrolyte are investigated. Thepower conversion efficiencies (PCEs) of the ss-DSSCs based on solution-castedFilm-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-DSSCsare closely related to their gap states. When additive is added inCs(2)SnI(6) electrolyte, a PCE of 6.14% is obtainedin an ss-DSSC. Our work highlights the importance of solvent in filmpreparation and the role of Cs2SnI6 gap statesin device performance.