Synthesis of Ni/NiO core-shell nanoparticles for wet-coated hole transport layer of the organic solar cell

Nickel oxide (NiO) nanoparticle is a promising material as a p-type semiconductor to replace PEDOT:PSS which is frequently used as a hole transport layer in organic solar cells. In this study, we synthesized Ni/NiO core-shell nanoparticles via direct thermolysis of Ni(acac)2 in the presence of trioctylphosphine oxide ligand and subsequent air oxidation to increase NiO shell thickness. Transmission electron microscopy analysis revealed the core-shell structure of Ni/NiO with average size of ~ 10 nm as well as an increased shell thickness by air-oxidation. The oxidized nanoparticles were well dispersed in organic solvent such as toluene, which can directly form a hole transport layer of organic solar cell by wet coating method. By spin coating various Ni/NiO nanoparticles on the ITO surface, organic solar cells with P3HT:PCBM bulk heterojunction as an active layer were fabricated. Solar cells utilizing air-oxidized Ni/NiO core-shell nanoparticle interlayer exhibited a better performance compared to those utilizing the as-synthesized Ni/NiO layer, or with no interlayer in between ITO and active layer.