Valence Band Edge Shifts and Charge-transfer Dynamics in Li-Doped NiO Based p-type DSSCs

• Li-doped NiO photoelectrodes show down-shift of valance band and narrowing trap energy distribution.
• Pure and Li doped NiO photoelectrodes exhibit different order in the hole lifetimes under dark and illumination.
• Li doping exhibits markedly inhibit recombination with reduced state of dye and increase hole lifetimes under illumination.

ABSTRACTThe objective of this work is to gain deeper insight into behavior of p-type DSSCs, paying special attention to photoelectric properties of Li-doped NiO photoelectrode, and studying their effect on interfacial photoelectrochemical processes under illumination and device performance. Here we find that Li-doped NiO photoelectrodes show higher charge carrier densities, down-shift of valence band and narrowing trap energy distribution. Furthermore, the performance enhancement of p-type DSSCs is observed for the Li-doped NiO photoelectrodes with respect to the unmodified ones. These improvement is found to arise from more rapid hole transport combined with reduced recombination, contributing to improved hole collection. With regard to interfacial carriers transfer process, electrochemical impedance spectroscopy (EIS), intensity-modulated photovoltage spectroscopy (IMPS) and ultra-violet photoemission spectra (UPS) provide evidence that the decreased recombination of Li-doped NiO DSSCs is attributed to the downward shift of valence band, which increases energy barrier of interface recombination. Another reason results in narrowing trap energy distribution of Li-doped NiO photoelectrode, which can weaken energy overlap with the LUMO level of dye, and subsequently the recombination path of hole back transfer to dye is hindered. These findings suggest that it will be important to carefully optimize band edge alignment and trap states distribution of p-type oxide semiconductors film to further improve the efficiency of p-DSSCs.

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