Precise size control of NiO_x nanoparticles to improve the photovoltaic performance of the inverted perovskite solar cells

Abstract

As a low-cost, high stable hole transport material, nickel oxide has been widely used in inverted structure perovskite solar cells in recent years. By far, the most common method for the preparation of nickel oxide hole transport layers is spin-coating pre-prepared nickel oxide nanoparticles (NiO_x NPs), which puts forward high appeal on the particle sizes and solution processing capabilities of NiO_x NPs. In this work, the particle sizes of NiO_x NPs were precisely controlled by adjusting the pH of the system during the synthesis process, and high-quality nickel oxide hole transport layers were then prepared. The experimental results exhibit that NiO_x NPs with average sizes of 5-10 nm were obtained at a pH of 9.5-9.8. More interestingly, the obtained NiO_x NPs have good dispersion stability and can achieve long-term dispersion in aqueous solutions. Furthermore, the structural composition analysis of NiO_x NPs showed that the pH of the synthesis system did not have a significant effect on the material structure and composition of the NiO_x NPs. Surface morphological analysis shows that the NiO_x film prepared by the pH-controlled NiO_x NPs is rather dense and particularly flat with small surface roughness. It is also found that the film exhibits good hole extraction capability. We also fabricated an inverted perovskite solar cell based on the NiO_x film. The device structure is ITO/NiO_x/CH₃NH₃PbI₃/PC61BM /Bphen/Ag. It yields a good photovoltaic conversion efficiency of 17.39 %. In addition, the device was almost hysteresis-free. Our experimental results exhibited that the performance of perovskite solar cells can be effectively improved by precisely controlling the sizes of NiO_x NPs through pH values. Our work may facilitate the development of NiO_x-based perovskite solar cells.