Surface State‐Mediated Charge Transfer of Cs 2 SnI 6 and Its Application in Dye‐Sensitized Solar Cells

A vacancy‐ordered double perovskite, Cs 2 SnI 6 , has emerged as a promising lead‐free perovskite in the optoelectronic field. However, the charge transfer kinetics mediated by its surface state remains unclear. Here, the charge transfer mechanism of Cs 2 SnI 6 is reported and the role of its surface state in the presence of a redox mediator is clarified. Specifically, charge transfer through the surface state of Cs 2 SnI 6 and its subsequent surface state charging are demonstrated by cyclic voltammetry and Mott–Schottky measurements, respectively. Because it is expected that the surface state of Cs 2 SnI 6 is capable of regenerating oxidized organic dyes, a Cs 2 SnI 6 ‐based regenerator is developed for a dye‐sensitized solar cell composed of fluorine‐doped tin oxide (FTO)/dyed mesoporous TiO 2 /regenerator/poly(3,4‐ethylenedioxythiophene)/FTO. As expected, the performance of the Cs 2 SnI 6 ‐based regenerator is strongly dependent on the highest occupied molecular orbital of the dyes. Consequently, Cs 2 SnI 6 shows efficient charge transfer with a thermodynamically favorable charge acceptor level, achieving a 79% enhancement in the photocurrent density (14.1 mA cm −2 ) compared with that of a conventional liquid electrolyte (7.9 mA cm −2 ). The results suggest that the surface state of Cs 2 SnI 6 is the main charge transfer pathway in the presence of a redox mediator and should be considered in future designs of Cs 2 SnI 6 ‐based devices.