Revealing\nthe Origin of Fast Electron Transfer in\nTiO₂‑Based Dye-Sensitized Solar Cells

Abstract

In dye-sensitized solar cells (DSCs),\nthe electron transfer from\nphotoexcited dye molecules to semiconductor substrates remains a major\nbottleneck. Replacing TiO₂ with ZnO is expected to enhance\nthe efficiency of DSCs, owing to the latter possesses a much larger\nelectron mobility, but similar bandgap and band positions as TiO₂ remain. However, the record efficiency of ZnO-based DSCs\nis only 7% compared with 13% of TiO₂-based DSCs due to\nthe even slower electron-transfer rate in ZnO-based DSCs, which becomes\na long-standing puzzle. Here, we computationally investigate the electron\ntransfer from the dye molecule into ZnO and TiO₂, respectively,\nby performing the first-principles calculations within the frame of\nthe Marcus theory. The predicted electron-transfer rate in the TiO₂-based DSC is about 1.15 × 10⁹ s^–1, a factor of 15 faster than that of the ZnO-based DSC, which is\nin good agreement with experimental data. We find that the much larger\ndensity of states of the TiO₂ compared with ZnO near the\nconduction band edge is the dominant factor, which is responsible\nfor the faster electron-transfer rate in TiO₂-based DSCs.\nThese denser states provide additional efficient channels for the\nelectron transfer. We also provide design principles to boost the\nefficiency of DSCs through surface engineering of high mobility photoanode\nsemiconductors.