Low-Temperature, Solution-Processed Molybdenum Oxide Hole-Collection Layer for Organic Photovoltaics

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 N ₂ 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-C ₆₁ 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 Mo ⁵⁺ gap states in the MoO _x HCL, and demonstrate enhanced fill factors due to reduced series resistance in the MoO _x HCL.