Controlling the Band Gap to Improve Open-Circuit Voltage in Metal Chalcogenide based Perovskite Solar Cells

Both recombination and band-edge shift are important factors for the open-circuit voltage (Voc) improvement of metal chalcogenide hole-transport material (HTM) based perovskite solar cells, but it is still not clear that which aspect plays the dominant role in such devices. Herein, we addressed this aspect through employing the band-tunable metal chalcogenide Cu2ZnSnS4 (CZTS) QDs as HTM into perovskite solar cells. By replacing sulfur with selenium atom, the band gap of HTM was tuned from 1.64 eV to 1.14 eV and their influences on cell performances were further discussed. Though the Cu2ZnSnSe4 (CZTSe) device with higher hole transport ability could improve the fill factor (FF), its Voc was still remarkably lower than that of the CZTS device. Electrochemical impedance spectroscopy (EIS) measurements indicated that the Voc loss (45 mV) induced by recombination here was far less than the Voc differences between the two devices (140 mV). After analyzing the band level alignment at TiO2/CH3NH3PbI3/HTM heterojunction, we proposed that the Voc enhancement of CZTS device was mainly ascribed to the more downward valence band-edge shift of HTM. This further approves that developing a wide band gap material without hindering charge injection is more pressing rather than depressing recombination process for future metal chalcogenide HTM researches.

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