Effect of Molybdenum Oxide Electronic Structure on Organic Photovoltaic Device Performance: An X-ray Absorption Spectroscopy Study

While molybdenum oxide (MoO3) has been shown to be an effective hole transport layer in organic photovoltaic (OPV) devices, a complete understanding of its electronic behavior has proven elusive. In this work, thin films of substoichiometric molybdenum oxide (MoO3-x) were prepared via thermal evaporation and subjected to a variety of annealing conditions. The films were employed as the hole transport layers in organic photovoltaic devices, and the device performance was found to depend strongly on the annealing conditions: as-prepared MoO3x films produced poly(3-hexylthiophene)/[6,6]-phenyl-C61-butyric acid methyl ester devices with good performance (3.1% power conversion efficiency), while films annealed at higher temperatures or in a reducing atmosphere produced devices with very low efficiencies (=1%). Through X-ray absorption near-edge structure (XANES) measurements at the Mo L3-edge, we show that while oxygen vacancies present in the as-prepared films may play a key role in hole extraction, extensive reduction of the molybdenum ions leads to more metallic behavior that results in a pronounced drop in device efficiency. These results clearly show that careful control over the MoO3x stoichiometry is necessary in order to achieve the highest performance in OPV devices and further demonstrate the utility of XANES in correlating OPV device performance to changes in electronic structure