Hydrogen and nitrogen codoping of anatase TiO₂ for efficiency enhancement in organic solar cells

TiO₂ has high chemical stability, strong catalytic activity and is an electron transport material in organic solar cells. However, the presence of trap states near the band edges of TiO₂ arising from defects at grain boundaries significantly affects the efficiency of organic solar cells. To become an efficient electron transport material for organic photovoltaics and related devices, such as perovskite solar cells and photocatalytic devices, it is important to tailor its band edges via doping. Nitrogen p-type doping has attracted considerable attention in enhancing the photocatalytic efficiency of TiO₂ under visible light irradiation while hydrogen n-type doping increases its electron conductivity. DFT calculations in TiO₂ provide evidence that nitrogen and hydrogen can be incorporated in interstitial sites and possibly form N_iH_i, N_iH_O and N_TiH_i defects. The experimental results indicate that N_iH_i defects are most likely formed and these defects do not introduce deep level states. Furthermore, we show that the efficiency of P3HT:IC₆₀BA-based organic photovoltaic devices is enhanced when using hydrogen-doping and nitrogen/hydrogen codoping of TiO₂, both boosting the material n-type conductivity, with maximum power conversion efficiency reaching values of 6.51% and 6.58%, respectively, which are much higher than those of the cells with the as-deposited (4.87%) and nitrogen-doped TiO₂ (4.46%).