Tuning Surface Oxidation States of Nickel Oxide for Efficient Inverted Perovskite Solar Cells
Man Yang, Xueliang Zhu, Kangwei Mo, Sheng Li, Siyang Cheng, Yong Liu, Ning Yan, Zhiping Wang
Abstract
Nickel oxide (NiO x ), as an inorganic p-type semiconductor, has been widely adopted as hole-transporting layers in perovskite solar cells. Despite its superior material stability, the poor charge extraction and multi-vacancies greatly restrict the photovoltages and efficiencies. Here, we propose a facile method to tune the surface oxidation states of NiO x films by a lithium salt treatment for photovoltage enhancement. X-ray photoelectron spectroscopy measurement indicates that the lithium treatment only reduces the Ni 3+ sites at the top region rather than in the bulk of NiO x films. This graded distribution allows more efficient charge extraction at the NiO x /perovskite interface as revealed by photoluminescence studies. Through a combination of capacitance–voltage and drive-level capacitance profiling measurements, we confirm enhanced built-in potentials and decreased interface defect densities in lithium-modified devices. Further modifying the interface with a self-assembly monolayer, the energy offsets at the interface can be largely reduced. Based on these enhanced properties, the modified devices achieve a high-power conversion efficiency of 22.4% (0.07 cm 2 ) with a 120 mV enhancement in photovoltage in comparison with the untreated devices.