Dye-sensitized solar cells (DSC) play a leading role in the third generation photovoltaic devices due to their low cost, easy fabrication process, high conversion efficiency, and good stability. As a media of dye adsorption, electron transport, and electrolyte diffusion, the nanocrystalline semiconductor photoanode plays a key role during light-to-electricity conversion in DSC. Apparently, titania (TiO₂) has been most frequently used as a photoanode materials in DSCs. Although other semiconductors such as ZnO, SnO₂, and SrTiO₃ etc. have also been widely applied in photoanode of DSC, none of them have shown better performance than TiO₂. Up to now, a record efficiency of 12.3% for the DSC with TiO₂ photoanode has been achieved. However, further improvement of the conversion efficiency of DSC is still a major issue for its application prospects. In recent years, doping TiO₂ with metal and nonmetal elements has been considered as a promising way to tailor the electronic properties of TiO₂ photoanode in DSC and has succeeded in improving photovoltaic performance of DSC. This article presents an overview on doped nanocrystalline TiO₂, including non-metal doped TiO₂, transition metal doped TiO₂, rare earth doped TiO₂, and main-group metal doped TiO₂ employed as the photoanode for improving photovoltaic performance of DSC. The influence of foreign-elements doping on the band edge, the trap state distribution, the electron transport process, the recombination reaction, and the light harvesting of TiO₂ photoanode are discussed. Lastly, an outlook on the future challenges and prospects of doped nanocrystalline TiO₂ materials as the photoanode for DSC are also briefly brought up.

Contents
1 Introduction
2 Nonmetal-doped TiO₂ photoanode of DSC
2.1 N-doped TiO₂ photoanode of DSC
2.2 Halogen-doped TiO₂ photoanode of DSC
2.3 B-doped TiO₂ photoanode of DSC
2.4 Codoped TiO₂ photoanode of DSC
3 Transition metal doped TiO₂ photoanode of DSC
3.1 Zn-doped TiO₂ photoanode of DSC
3.2 Cr-doped TiO₂ photoanode of DSC
3.3 W-doped photoanode of DSC
3.4 Nb-doped photoanode of DSC
3.5 Ta-doped TiO₂ photoanode of DSC
4 Rare earth doped TiO₂ photoanode of DSC
5 Main-group metal doped TiO₂ photoanode of DSC
6 Conclusion and outlook