Charge Recombination and Band-Edge Shift in the Dye-Sensitized Mg2+-Doped TiO2 Solar Cells

The effect of Mg2+ ions substituted into the anatase lattice on the charge recombination and band-edge movement in dye-sensitized solar cells was investigated in this study. The HRTEM results indicated that Mg2+ ions incorporation into the TiO2 lattice led to the increased lattice spacing of the (101) plane of the anatase phase. Mg2+-doped TiO2 could produce a blue shift in the optical absorption edge compared with that of the untreated samples. Detailed analysis of the open-circuit photovoltage (V-oc) under different surface charge densities showed that the Mg2+-doped TiO2 samples resulted in the negative shift of the TiO2 conduction band about 70 mV in comparison with the untreated samples. From Raman spectra and light intensity-dependent variation of the short-circuit current density (J(sc)) of the solar cells, it could be concluded that the decreased efficiency of electron injection for DSCs with Mg2+-doped TiO2 was attributed to the negative shift of the band edge in the Mg2+-doped TiO2 electrode to obtain a decreased J(sc). The electron diffusion coefficient in Mg2+-doped TiO2 was found to be higher than that in TiO2 at the same photoelectron density. We present evidence that the increase of trap states in Mg2+-doped TiO2 as recombination channels to decrease the electron lifetime could compensate for the effect of band-gap widening to obtain a slightly increased V-oc., for DSCs with Mg2+-doped TiO2. It is suggested that the recombination channels should be suppressed to enhance the performance of dye-sensitized Mg2+-doped TiO2 solar cells.