Cobalt-doped nickel oxide nanoparticles as efficient hole transport materials for low-temperature processed perovskite solar cells

[Display omitted] •NiOx nanoparticles with different concertation of cobalt ions are synthesized.•Co-NiOx layers are fabricated at low-temperature process as a hole transport layer.•The band alignment is optimized by decreased work function of Co-NiOx layer.•The conductivity of Co-NiOx films is improved to 6.20×10-6Scm-1.•Conversion efficiency is improved to 14.5% using 1 mol% Co-NiOx. We have synthesized cobalt-doped NiOx nanoparticles containing 0.5, 1, 2, 5 and 10 mol% cobalt ions and have investigated their electronic properties, which can be processed into smooth and pinhole-free layers at low temperature (