Fully metal oxide charge selective layers for n-i-p perovskite solar cells employing nickel oxide nanoparticles

An alcohol based nickel oxide nanoparticle dispersion was prepared and used to deposit hole conductor medium for a polymeric substance free perovskite solar cell in mesoscopic n-i-p configuration. In contrast to conventional p-i-n configuration or inverted type perovskite solar cells, nickel oxide layer was spin coated directly on perovskite layer and the perovskite layer was sandwiched between two metal oxides, TiO2 and NiO, resulting in n-i-p configuration. High surface area nickel oxide nanoparticles were synthesized by precipitation and successfully dispersed in isopropanol with the aid of ball milling, which was confirmed to breakdown the aggregates and stabilize the dispersion without the assistance of a stabilizing agent. This strategy leads to deposition of nickel oxide nanoparticles on perovskite layer without damaging the underneath perovskite layer and inhibiting the charge transfer between individual nanoparticles, confirmed by scanning electron microscopy, photoluminescence quenching and J-V measurements. Ultraviolet photoelectron spectroscopy analysis showed excellent matching of the band alignment of nickel oxide layers with that of perovskite. An efficiency of 10.89% was achieved after optimizing the nickel oxide layer thickness and comparing with a hole conductor free device using J-V measurements and electrochemical impedance spectroscopy revealed that nickel oxide layer possesses excellent electron blocking ability and reduces the recombination rate, which also in turn stabilizes the power output and hysteresis of the cells. This strategy is believed to be applicable for other metal oxides employed in charge selective layers of perovskite and organic solar cells.