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 Ce4+/Ce3+ is doped in the TiO2 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 TiO2 due to the unoccupied Ce-4f trap states in the band gap, and the Ce4+ is reduced to Ce3+ 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 TiO2 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 TiO2 photoanode (7.2%). However, the Ce4+ 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 TiO2 crystalline surface, are verified and effectively passivized by TiCl4 surface treatment.