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.