Solution-processed nickel oxide (s-NiOx) was synthesized for use as hole-transport layers (HTLs) in the fabrication of polymer solar cell (PSC) devices. The s-NiOx thin-films were deposited using spin-coating and post-annealed at 300 degrees C, 400 degrees C, or 500 degrees C. With increased annealing temperature, the nickel acetate precursor decomposes more fully and forms s-NiOx films that show larger crystalline grain sizes with lower root mean square surface roughness. Bulk heterojunction solar cells fabricated with the new random polymer RP(BDT-PDBT) and [6,6]-phenyl-C-70-butyric acid methyl ester (PC70BM) using s-NiOx as HTLs exhibit a 4.46% enhancement in power conversion efficiency and better stability compared to conventional PSCs using poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) as HTLs. We believe that the solution-processable and highly stable s-NiOx could be a potential alternative for functional interface materials in optoelectronic devices. (C) 2017 Elsevier B.V. All rights reserved.