Facile Approach to Preparing a Vanadium Oxide Hydrate Layer as a Hole-Transport Layer for High-Performance Polymer Solar Cells

We demonstrate a facile and green approach to preparing a vanadium oxide hydrate (VO_x·nH₂O) layer to serve as the hole-transport layer (HTL) in high-performance polymer solar cells (PSCs). The VO_x·nH₂O layer was in situ prepared by a combined H₂O₂ and ultraviolet-ozone (UVO) processing on a VO_x layer. The as-prepared VO_x·nH₂O layer featured a work function of 5.0 ± 0.1 eV, high transmittance, and better interface properties compared to those of the generally prepared VO_x (UVO or thermal annealing) layers. PSCs based on poly­[(ethylhexyl-thiophenyl)-benzodithiophene-(ethylhexyl)-thienothiophene]/[6,6]-phenyl-C₇₁-butyric acid methyl ester using the VO_x·nH₂O layer as the HTL yielded high power conversion efficiencies (PCEs) up to 8.11%, outperforming the devices with VO_x layers (PCE of 6.79% for the UVO-processed VO_x layer and 6.10% for the thermally annealed VO_x layer) and conventional polyethylenedioxythiophene–polystyrenesulfonate (PEDOT:PSS) layers (PCE of 7.67%). The improved PCE was attributed to the enhanced J_SC and/or fill factor, which mainly correlate to the improved interfacial contact between the photoactive layer and the indium tin oxide/HTL or cathode when using the VO_x·nH₂O layer as the HTL. A similar improvement in the PCE was also observed for the PSCs based on poly­(3-hexylthiophene)/[6,6]-phenyl-C₆₁-butyric acid methyl ester. In addition, PSCs with a VO_x·nH₂O layer as the HTL showed a higher stability than that of those with a PEDOT:PSS layer. Hence, it would be possible to use this simply and in situ prepared VO_x·nH₂O layer as an inexpensive HTL for high-performance PSCs.