Solution-processed-MoO3 hole extraction layer on oxygen plasma-treated indium tin oxide in organic photovoltaics

Effects of oxygen plasma (O2 plasma) treatment of ITO on the characteristics of solution-processed molybdenum oxide (MoO3) hole extraction layer in bulk hetero-junction organic photovoltaics (OPVs) are studied. The chemical composition of O2 plasma-treated ITO was determined using monochromatic X-ray photoelectron spectroscopy (XPS). The valence band energies were investigated by ultraviolet photoemission spectroscopy (UPS) measurements. XPS and UPS measurements reveal that O2 plasma treatment of bare ITO film was found to incorporate the polar surface species such as (O2)2−, resulting in an increase of both workfunction from 4.62 to 5.05 eV and polar surface energy from 27 to 38 mN/m. The high work function results in efficient hole transport at the ITO/MoO3 interfaces. The highly polar surface is readily available for uniform coating of MoO3 on ITO. Electrical conductivity of oxidized ITO changes four orders of magnitude from 2.4×10-2 to 4.08×102 S/cm, depending on O2 plasma pressure conditions. Thus, the ITO/MoO3 interface dominates the series resistance of OPVs fabricated with poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The presence of (O2)2− states in the ITO/MoO3 interface in OPVs is suggested to play a significant role in controlling the device lifetime as well as the efficiency of OPV.