Exploration and Optimization of the Polymer-Modified NiO_x Hole Transport Layer for Fabricating Inverted Perovskite Solar Cells

The recombination of charge carriers at the interface between carrier transport layers such as nickel oxide (NiO_x) and the perovskite absorber has long been a challenge in perovskite solar cells (PSCs). To address this issue, we introduced a polymer additive poly(vinyl butyral) into NiO_x and subjected it to high-temperature annealing to form a void-containing structure. The formation of voids is confirmed to increase light transmittance and surface area of NiO_x, which is beneficial for light absorption and carrier separation within PSCs. Experimental results demonstrate that the incorporation of the polymer additive helped to enhance the hole conductivity and carrier extraction of NiO_x with a higher Ni³⁺/Ni²⁺ ratio. This also optimized the energy levels of NiO_x to match with the perovskite to raise the open-circuit voltage to 1.01 V. By incorporating an additional NiO_x layer beneath the polymer-modified NiO_x, the device efficiency was further increased as verified from the dark current measurement of devices.