Engineered optical and electrical performance of rf-sputtered undoped nickel oxide thin films for inverted perovskite solar cells

Inverted perovskite solar cells incorporating RF sputtered NiO thin films as a hole transport layer and window layer are demonstrated. The electrical and optical properties of the NiO thin films are engineered using varied sputtering conditions. The localized states within bandgap owing to its crystal disorder and nonstoichiometric features affect the transmittance and the optical bandgap of the NiO thin films which in turn influences the J_sc of the perovskite solar cells. In addition, the electrical properties of the NiO thin films can be also varied during sputtering condition affecting the concentration of nickel vacancies and the resulting hole concentration. The conductivity largely originates from the hole concentration relating to the density of states in the NiO thin films which influence the fill factor (FF) of the solar cells. The solar cells fabricated with the NiO thin films made at 4 Pa of deposition pressure show highest performance owing to excellent transmittance and wider bandgap along with moderate conductivity. With further optimization, the perovskite solar cells exhibit ~20 mA/cm² of J_sc and a 12.4% PCE (11.3% of averaged PCE).