TiO2 is a wide band gap semiconductor which used as anode( working electrode ) in DSSC and it provide high surface area with mesoporous structure for dye adsorption which means more available molecules for charge generation . Due to the generation of voltage mainly depends upon the band edges,TiO2 has suitable adjust band gap edge levels to the dye and electrolyte. Also,TiO2 possess high electronic mobility for electrons that photogenerated from excited dye molecules which injected into TiO2 conduction band . TiO2 serve as a medium of electron transfer to the conductive glass substrate (FTO,ITO).TiO2 is the best one in comparison with other metal oxides semiconductors such as ZnO, SnO2. ZnO and SnO2 have low efficiency compared to TiO2 because they have less mesoporous structure .

You can also consider Al2O3 mesoporous structure as well.

Good luck.

first of all understand the mechanism behind DSSC. TiO2 will only serve as the conducting medium of electrons and it doesnt produce any free or conducting electrons. only the light sensitive dye molecules produce the excitons and inject into the CB of TiO2. the build in potential between the CB of TiO2 and electrolyte will drive the electrons to the FTO.

Thank you for your help in explaining this to me. But I believe the answer is a little bit more complicated than indicated by some of you. The fundamental difference between an insulator and a semiconductor is found in the transport properties of the conduction band. For insulators, this band is narrow and the electron mobility is low. For semiconductors, the band is wide and the electron mobility is high. This is the reason that any electrons in the conduction band of semiconductors will conduct, while for insulators this still yield high resistances. I found that TiO2 with an excess of Ti (or less O) transforms the material from an insulator to a semiconductor.