Organic solar cells fabricated by one-step deposition of a bulk heterojunction mixture and TiO ₂/NiO hole-collecting agents

Organic solar cells (OSCs) were fabricated using a one-step deposition of a mixture of NiO nanoparticles, region-regular poly(3-hexylthiophene) (P3HT), and [6,6]-phenyl-C ₆₁-butyric methyl ester (PCBM) without poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Although the intended NiO layer was successfully formed at the interface between indium tin oxide (ITO) and the photoactive layer, only a marginal increase in the power conversion efficiency (PCE) of the OSCs (from 0.773 to 1.171%) was found by addition of NiO nanoparticles to the solution of the P3HT/PCBM mixture. Using X-ray photoelectron spectroscopy, it was evidenced that P3HT was oxidized at interfaces of P3HT and NiO, which can decrease the photovoltaic performance of an OSC. Ultrathin TiO ₂ wrapping layers (thickness ∼ 2 nm) on the surface of NiO nanoparticles prepared by atomic layer deposition quenched oxidation of P3HT resulted in a significant increase in PCE up to 2.684%. Our result shows that, in OSCs, oxidation of active polymers at oxide/polymer interfaces should be of concern, and a strategy for avoiding such degradation of polymers is required. Fabrication of various core-shell nanostructures as oxide buffers can be useful for quenching the oxidation of active polymers and increasing photovoltaic performances.