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

Graphical abstract: - Highlights: • The structural, optical and optoelectronic properties of 1 mol.% Fe, Sn and Cu doped TiO₂ have been compared. • Transient lifetimes for pure TiO₂ and Sn doped TiO₂ were considerably shorter than Fe and Cu doped TiO₂. • A good correlation between the bulk defects and transient decay for the doped TiO₂ powders was observed. • Photon to current conversion efficiency of DSSC based on the metal doped TiO₂ were in order Sn-TiO₂ > Cu-TiO₂ > Pure >> Fe-TiO₂. • DSSC based on Fe doped photoanodes is limited by a high concentration of surface free holes observed at 433 nm. - Abstract: 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 Ti2p_3/2 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