Effect of Cerium Doping in the TiO2 Photoanode on the Electron Transport of Dye-Sensitized Solar Cells

Rare earth element cerium (Ce) with variable valence states Ce 4+ /Ce 3+ is doped in the TiO 2 photoanode for dye-sensitized solar cells (DSSCs). The anatase crystalline phase keeps unchanged, while the crystalline size decreases slightly after Ce doping. The Ce positively changes the conduction band minimum of TiO 2 due to the unoccupied Ce-4f trap states in the band gap, and the Ce 4+ is reduced to Ce 3+ when electrons are injected in the photoanode. The cerium-doped photoanodes with special electrochemical properties severely influence the performance of DSSCs. Due to the increased electron injection, the small Ce content (0.05% and 0.1%) doped TiO 2 photoanodes improve the performance of DSSCs which is optimized to 7.65% with Ce0.1% doping (1 sun, AM1.5) compared with the one with a pure TiO 2 photoanode (7.2%). However, the Ce 4+ states effectively trap electrons in the photoanode with further increase of Ce from 0.3 to 0.9%, which is confirmed by the charge extraction method and electrochemical impedance spectroscopy (EIS); thus, it suppresses the photocurrent and the efficiency of the DSSCs but is helpful to the fill factor improvement. The trap states, mainly resting on the TiO 2 crystalline surface, are verified and effectively passivized by TiCl 4 surface treatment.