Origin of Hole Selectivity and the Role of Defects in Low-Temperature Solution-Processed Molybdenum Oxide Interfacial Layer for Organic Solar Cells

A critical component in bulk-heterojunction (BHJ) organic photovoltaics (OPVs) is the charge-selective interfacial layer, which plays a vital role in achieving high device performance and stability. Here, we present the performance of molybdenum oxide (MoO x ) hole selective interfacial layers for BHJ OPVs based on poly(3-hexylthiophene) (P3HT) donor and [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM) acceptor, prepared by a facile, low-temperature solution process. The results showed that the MoO x films enhanced device efficiency and stability in comparison to reference devices containing the conventional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) interfacial layer. Furthermore, a high fill factor (∼69%) close to the theoretical maximum value predicted for the P3HT:PCBM BHJ system was achieved. Despite their hole selective nature, ultraviolet photoemission spectroscopy (UPS) revealed that the MoO x films were n -type. This hole selective behavior can be explained by invoking band bending at the MoO x /organic interface and electron transport through MoO x gap states. These results show that a p -type semiconductor is an essential but not necessary requirement for hole selective interfacial layers, and we believe this finding would have important implications toward the design and selection of interfacial materials for OPVs.