Low-temperature, solution-processed molybdenum oxide hole-collection layer for organic photovoltaics

Abstract

We have utilized a commercially available metal–organic precursor to develop a new, low-temperature, solution-processed molybdenum oxide (MoO_x) hole-collection layer (HCL) for organic photovoltaic (OPV) devices that is compatible with high-throughput roll-to-roll manufacturing. Thermogravimetric analysis indicates complete decomposition of the metal–organic precursor by 115 °C in air. Acetonitrile solutions spin-cast in a N₂ atmosphere and annealed in air yield continuous thin films of MoO_x. Ultraviolet, inverse, and X-ray photoemission spectroscopies confirm the formation of MoO_x and, along with Kelvin probe measurements, provide detailed information about the energetics of the MoO_x thin films. Incorporation of these films into conventional architecture bulk heterojunction OPV devices with poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁ butyric acid methyl ester afford comparable power conversion efficiencies to those obtained with the industry-standard material for hole injection and collection: poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The MoO_x HCL devices exhibit slightly reduced open circuit voltages and short circuit current densities with respect to the PEDOT:PSS HCL devices, likely due in part to charge recombination at Mo⁵⁺ gap states in the MoO_x HCL, and demonstrate enhanced fill factors due to reduced series resistance in the MoO_x HCL.