Dye-Sensitized W-Doped TiO2 Solar Cells with a Tunable Conduction Band and Suppressed Charge Recombination

W(VI) is doped into TiO2 via a sol–gel method, and the crystalline anatase structure is well preserved with the W content ranging from 0.1% to 5%. The conduction band (CB) of TiO2 moves downward (i.e., positive shift) gradually with increasing the W content from 0.1% to 2% and then stays almost unchanged with increasing the W content from 2% to 5%. Dye-sensitized solar cells (DSSCs) based on the W-doped anatase TiO2 show an advantage in repressing the charge recombination. The electron lifetime in the DSSC is significantly improved by the W-doping. As a result of the positive shift of the CB and repressed charge recombination, the short-circuit photocurrent (Jsc) of the DSSC is improved remarkably. The collective effect of the positive shift of the CB and the enhanced electron lifetime caused by the W-doping makes the open-circuit photovoltage (Voc) remain almost unchanged below 0.5% W-doping and decrease above 0.5% W-doping. The highest power conversion efficiency (η = 9.1%) is obtained at 0.2% W-doping, which shows increases by 17% in Jsc and by 20% in η, as compared with the undoped DSSC.