Organic Solar Cells Fabricated by One-Step Deposition of a Bulk Heterojunction Mixture and TiO₂/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-C₆₁-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 TiO₂ 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