Polymer solar cells (PSCs) are considered promising energy power suppliers due to their light weight, printability, low-energy fabrication and roll-to-roll processability. Recently, the solution-processed NiOx nanoparticles have been a desirable interfacial material for hole transport in the PSCs, instead of organic semiconductors. However, pure NiOx films restrain the high performance of PSCs due to their poor electrical characteristics caused by the localized orbital distribution at the top of valence band. Therefore, metal ion doping has been explored as a method to endow NiOx nanoparticles with the appropriate electrical characteristics. Herein, we applied solution-processed Cu-doped NiOx (Cu:NiOx) nanoparticles as an efficient hole transport layer (HTL) in PSCs. The Cu-doped NiOx enhanced the electrical conductivity of the material and improved the interface contact with the active layer, which remarkably facilitated the hole extraction and effectively suppressed the carrier recombination at the interface. Thus, a higher power conversion efficiency of 7.05%, corresponding to an approximately 30% efficiency improvement compared with that of a pristine NiOx interlayer (5.44%) in poly[N- 9 ''-hepta-d ecanyl-2,7-carbazolealt-5,5-(4',7'-di-2-thieny1-2',1',3'-ben-zothiadiazole)]:[6,6]-phenyl-C-71-butyric acid methyl ester (PCDTBT:PC71BM)-based PSCs, was achieved by the proposed device. The developed solution-processed Cu:NiOx nanoparticles may be an excellent alternative for interfacial materials in PSCs or other optoelectronic devices requiring HTLs. (C) 2018 Elsevier Inc. All rights reserved.