Simultaneously modulating the optical and electrical properties of the hole transporting layer (HTL) plays a vital role in the performance of p-i-n perovskite solar cells (PSCs). Here, a negligible hysteresis inverted planar PSC with 17.30% power conversion efficiency was successfully fabricated by employing a thin and non-stoichiometric dopant-free nickel oxide (DF-NiOx) film as the HTL. The DF-NiOx film was formed by a radio frequency magnetron sputtering method. The Ni3+ content as well as the crystallization of the DF-NiOx thin films were tuned by regulating the post-annealing treatment, leading to enhanced optical transmittance and electrical conductivity. The optimized DF-NiOx films exhibit a high transmittance of more than 90%, appropriate conductivity, uniform surface morphology, and a better energy level match with the perovskite layer. This study not only highlights the importance of optimizing the Ni3+ content and crystallization for the formation of high-performance DF-NiOx HTLs, but also provides an excellent device platform for making large area high-performance PSCs and tandem solar cells. (C) 2019 Elsevier Ltd. All rights reserved.