Efficient dye-sensitized solar cells based on CNT-derived TiO2 nanotubes and Nb-doped TiO2 nanoparticles

We report a new facile strategy to enhance the efficiency of TiO2 dye-sensitized solar cells (DSSCs) using Nb-doped TiO2 photoelectrodes with carbon nanotube-derived TiO2 nanotubes (CDTs). Multi-walled carbon nanotubes (CNTs) with an average diameter of 80 nm are used as the template to produce CDTs through facile sol–gel and oxidation processes at 600 °C. Field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD) analyses reveal that the synthesized CDTs have a uniform thickness of 60–120 nm and a crystallite size of 17 nm. We prepare Nb-doped TiO2 nanoparticles with different atomic ratios of 0–4 at% Nb and test them as photo-electrodes to investigate the doping effects on electron–hole separation and electron injection. X-Ray photon spectroscopy (XPS) show that Nb5+ ions are well doped into Ti–O lattice without any secondary phases. The highest power conversion efficiency (PCE) of 7.87% is observed for 3 at% Nb-doped TiO2 nanoparticles (DN3%). Furthermore, different percentages of CNT-derived TiO2 nanotubes (0–20 wt%) are introduced into the optimal Nb-doped electrode (DN3%) to improve the electron transportation. An increase of 32% in cell efficiency is obtained for the DSSC composed of the optimal content of CDTs compared with the DSSC made of pure TiO2 nanoparticles. We obtain the highest photovoltaic performance (i.e., PCE of 9.02% and short current density of 18.83 mA cm−2) for a mixture of 3 at% Nb-doped TiO2 nanoparticles and 5 wt% CDT. Such enhancements are attributed to a higher photo-response by introducing additional energy levels within the band gap of TiO2 as well as providing a direct way for electron transportation to occur in the photoelectrode.