An insight into the role of oxygen vacancy in hydrogenated TiO₂ nanocrystals in the performance of dye-sensitized solar cells

Hydrogenated titanium dioxide (H-TiO₂) nanocrystals were successfully prepared via annealing TiO₂ in H₂/N₂ mixed gas flow at elevated temperatures ranging from 300 to 600 °C. Electron paramagnetic resonance (EPR) spectra were used to determine the produced oxygen vacancy in H-TiO₂. Variations in temperature were studied to investigate the concentration change of oxygen vacancy in H-TiO₂. The H-TiO₂ nanocrystals prepared at different temperatures were employed into photoanodes sensitized by N719 dye and found to have exceptional effect on the solar-to-electric energy conversion efficiency (η). Photoanodes with H-TiO₂ nanocrystals hydrogenated at 300°C show the highest short-circuit current density (J_sc) of 18.92 mA cm^-2 and photoelectrical conversion efficiency of 7.76% under standard AM 1.5 global solar irradiation, indicating a 27 and 28% enhancement in J_sc and η, respectively, in comparison to those with TiO₂. The enhancement is attributed to high donor density, narrow band gap and positive shift of flat band energy (V_fb) of H-TiO₂ that promote the driving force for electron injection. Intensity-modulated photocurrent spectroscopy (IMPS) accompanied by intensity-modulated photovoltage spectroscopy (IMVS) and other analyses were applied to shed more light on the fundamental mechanisms inside the charge transfer and transport in these systems.