Effect of different concentration Li-doping on the morphology, defect and photovoltaic performance of Li-ZnO nanofibers in the dye-sensitized solar cells

Low concentrations of Li in Li-doped ZnO nanofibers prepared using hydrothermal method at low temperature can introduce oxygen vacancies and intrinsic Zn ions into the structure. Photo-luminance (PL) was used to investigate oxygen vacancies in the structure of ZnO nanofibers prepared by lower annealing temperature, and the XPS technique was also employed to satisfy the PL analysis results. PL analysis showed that oxygen vacancies increase in conjunction with Li concentrations. A shift in the lower angle of XRD patterns also demonstrates the defect in ZnO structure related to Li doping. Higher-efficiency DSSCs were obtained from the lower Li concentration of 0.01 M in ZnO nanofibers. Higher concentrations of Li tended to produce large amounts of cross-like nanofibers, which increase the open circuit voltage of the DSSCs. The highest open circuit voltage (Voc) obtained was 750 mV, which was higher than the best reported ZnO nanofibers-based DSSCs. Intensity modulation photocurrent spectroscopy (IMPS) and intensity modulation photo-voltage spectroscopy (IMVS) analysis showed that low amounts of Li-doping improved the electron injection efficiency of ZnO nanofibers in DSSCs. Lower recombination rates with higher electron transfer efficiency for 0.01 M Li-doped DSSCs exhibited higher efficiency of 0.59% than non-doped ZnO nanofibers DSSCs.