Importance of Oxygen Partial Pressure in Annealing NiO Film for High Efficiency Inverted Perovskite Solar Cells

The effect of annealing atmosphere and importance of oxygen partial pressure upon annealing NiO film for achieving high efficiency inverted perovskite solar cells (PSCs) is reported. The solution-process NiO films are deposited on an FTO (fluorine-doped tin oxide) substrate and annealed at 400 °C under different atmospheres of air, O₂, N₂, and Ar. The devices using air- and O₂-annealed NiO films show better photovoltaic performance than the N₂- and Ar-annealed ones, mostly due to large difference in photocurrent density (J_sc) of ≈20 mA cm⁻² for air and O₂ vs ≈15 mA cm⁻² for N₂ and Ar. Oxygen-excess condition leads to more p-type characteristics along with better electrical and interfacial properties, leading to higher photovoltaic performance. When comparing air and O₂ condition, the air-annealed NiO film shows slightly better power conversion efficiency (PCE) (15.68% for air vs. 14.93% for O₂), being indicative of importance of oxygen partial pressure. By carefully modifying oxygen content, the best photovoltaic performance is achieved from the NiO film annealed at the O₂/(O₂+N₂) ratio of 30%, delivering a PCE of 16.32%.