Abstract
In this work, the effect of graphene and nitrogen doping on the performance of dye-sensitized solar cells of pure TiO2 was studied. Pure and N-doped TiO2 nanoparticles were synthesized using a hydrothermal method, while graphene was prepared through the reduction of graphene oxide. The materials were characterized using x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR), Raman, Brunauer- Emmett-Teller surface area analysis (BET) and ultraviolet-visible (UV-Vis) diffusion reflectance spectroscopy. Nitrogen dopant concentration varied from 0 at.% to 1.57 at.%. The results confirmed that all N-doped samples exhibited pure anatase phase with an average diameter in the range of 7-12 nm. The pore volume and BET surface area increased with the amount of nitrogen in TiO2. XPS investigation displayed an N1s peak around 397 eV, which suggested N-Ti-O structure in the TiO2 matrix. Moreover, optical measurements showed that the optical absorption edge of N-doped TiO2 exhibited a significant shift from ultraviolet to visible light region in comparison with pure TiO2. Dye-sensitized solar cells (DSSCs) were fabricated using N719 dye and various TiO2 based photoanodes. The photoanode of N-doped TiO2 modified with graphene showed the highest energy-conversion efficiency of 6.3%, while the efficiencies of pure and N-doped TiO2 cells are 0.41% and 1.21%, respectively. The improvement in conversion efficiency of graphene-based DSSC was attributed to the formed electron bridges between TiO2 and fluorine-doped tin oxide (FTO), which led to a reduction in the recombination rate of electron-hole pairs and an increase in the rate of electron transport.