Evaluation of surface energy state distribution and bulk defect concentration in DSSC photoanodes based on Sn, Fe, and Cu doped TiO2

Electron transfer dynamics in the oxide layers of the working electrodes in both dye-sensitized solar cells and photocatalysts greatly influences their performance. A proper understanding of the distribution of surface and bulk energy states on/in these oxide layers can provide insights into the associated electron transfer processes. Metal ions like Iron (Fe), Copper (Cu) and Tin (Sn) doped onto TiO2 have shown enhanced photoactivity in these processes. In this work, the structural, optical and transient properties of Fe, Cu and Sn doped TiO2 nanocrystalline powders have been investigated and compared using EDX, Raman spectroscopy, X-ray Photoelectron spectroscopy (XPS), and Transient Absorption spectroscopy (TAS). Surface free energy states distributions were probed using Electrochemical Impedance spectroscopy (EIS) on Dye Sensitized Solar Cells (DSSC) based on the doped TiO2 photoanodes. Raman and XPS Ti2p3/2 peak shifts and broadening showed that the concentration of defects were in the order: Cu doped TiO2 > Fe doped TiO2 > Sn doped TiO2 > pure TiO2. Nanosecond laser flash photolysis of Fe and Cu doped TiO2 indicated slower transient decay kinetics than that of Sn doped TiO2 or pure TiO2. A broad absorption peak and fast transient decay at 430 nm for Fe doped TiO2 was ascribed to an increase in surface hole concentration resulting in poor current density in the Fe doped TiO2 photoanodes relative to pure TiO2, Sn or Cu doped anodes. The charge transfer capacitance and the calculated electron lifetimes correlated well with the trend in current density of the photoanodes (Sn > Cu > pure TiO2). The poor performance of Fe doped cells is due to faster recombination of injected electrons with surface holes while those of Sn and Cu were more influenced by the concentration of their bulk defects. These results demonstrate that the choice of selected metal ions doping onto TiO2 for a desired application should take into consideration the influence of bulk defect concentrations, the energy state distribution and the electron transfer properties in/on the oxide photoanodes.