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.