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

We have synthesized cobalt-doped NiO_x 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 (<150 °C) as hole transport layers (HTLs) for perovskite solar cells (PSCs). We have revealed that the relationship between conductivity of HTLs and hole extraction properties to improve the photovoltaic performance. The thin film of cobalt-doped NiO_x nanoparticles showed higher conductivities compared to pristine NiO_x nanoparticles. Consequently, PSCs with power conversion efficiencies over 14.5%, an improvement from 11.5% for those with the pristine NiO_x-based HTL, have been obtained. This work will contribute to the development of doped metal oxide HTLs, which are processable at low temperature.