We examined the charge transfer mechanism of Cs2SnI6 and clarified the function of its surface state in photovoltaic devices. From a cyclic voltammetry study, we found that the faradaic reactions of the iodine species derived from Cs2SnI6 induce charge transfer through a surface state of Cs2SnI6, mainly at +0.9 V vs. the normal hydrogen electrode. This potential is located in the mid-gap state of Cs2SnI6 and its surface state charging was confirmed by Mott-Schottky measurements. This mid-gap charge transfer was further proved in photovoltaic devices. More specifically, we developed dye-sensitized solar cells with quasi-solid Cs2SnI6-based regenerator, or conventional liquid electrolyte. The performances of the Cs2SnI6-based regenerator were strongly dependent on the highest occupied molecular orbital (HOMO) of the organic dyes. In particular, BT-HT featuring the lowest HOMO (−5.65 eV) provided a 79% enhancement in the photocurrent density (14.1 mA cm−2) in the Cs2SnI6-based regenerator compared to that of a conventional liquid electrolyte (7.9 mA cm−2). This unprecedented finding can be explained as follows: i) fast charge transfer through Cs2SnI6; ii) efficient charge regeneration owing to lower HOMO of BH-HT than mid-gap state (−5.43 eV); and iii) lesser early recombination in the Cs2SnI6-based regenerator. To further confirm this mid-gap charge transfer, we demonstrated a correlation between performance of Cs2SnI6 as a light absorber and the TiO2 conduction band by Sn doping in TiO2. Our findings confirm the importance of surface state engineering in future designs of Cs2SnI6 lead-free perovskite devices.