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HyeonOh Shin1 Byung-Man Kim1 Deok-Ho Roh1 Tae-hyuk Kwon1

1, Ulsan National Institute of Science and Technology, Ulsan, SE, Korea (the Republic of)

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.

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