Improved Electron Transfer between TiO2 and FTO Interface by N-Doped Anatase TiO2 Nanowires and Its Applications in Quantum Dot-Sensitized Solar Cells

The growth of anatase TiO2 nanowires (NWs) on fluorine doped tin oxide (FTO) substrates through hydrothermal reaction has attracted wide attention and research, especially in the case of the solar cells. Actually, the built-in electric field at the anatase TiO2 NWs/FTO interface leads to the photoexcited holes transfer to FTO conductive substrates because the Fermi energy of anatase TiO2 NWs film is higher than that of FTO substrates. Yet efficient transport of photoexcited electron to the FTO conductive substrates is desirable. Hence, the built-in electric field at the pure TiO2 NWs/FTO interface has prevented anatase TiO2 NWs-based solar cells from achieving a higher photoelectric performance. In this work, we elaborately design and construct the N-doped anatase TiO2 NWs/FTO interface with the desirable orientations from FTO toward N-doped anatase TiO2 NWs, which favors the photoexcited electron transfer to the FTO conductive substrates. The surface photovoltage (SPV) and Kelvin probe measurements demostrate that the N-doped anatase TiO2 NWs/FTO interface favors the photoexcited electron transfer to the FTO conductive substrates due to the fact that the orientation of the built-in electric field at the N-doped TiO2 NWs/FTO interface is from FTO toward TiO2. The photoexcited charge transfer dynamics of CdS QD-sensitized TiO2 NWs and N-doped TiO2 NWs electrodes was investigated using the transient photovoltage (TPV) and transient photocurrent (TPC) technique. Benefiting from the desirable interface electric field, CdS-based quantum dot-sensitized solar cells (QDSCs) with the optimal N doping amount exhibit a remarkable solar energy conversion efficiency of 2.75% under 1 sun illumination, which is 1.46 times enhancement as compared to the undoped reference solar cells. The results reveal that the N-doped anatase TiO2 NWs electrodes have promising applications in solar cells.