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

Effects of oxygen plasma (O-2 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 O-2 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 O-2 plasma treatment of bare ITO film was found to incorporate the polar surface species such as (O-2)(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 x 10(-2) to 4.08 x 10(2) S/cm, depending on O-2 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 (O-2)(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. (C) 2013 Elsevier B.V. All rights reserved.