An ultrathin iridium layer was treated with O(2)-plasma to form an iridium oxide (IrO(x)), employed as a hole extraction layer in order to replace poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) in organic photovoltaic (OPV) cells with poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM). The IrO(x) layer affects the self-organization of the P3HT:PCBM photo-active layer due to its hydrophobic nature, inducing a well-organized intraplane structure with lamellae oriented normal to the substrate. Synchrotron radiation photoelectron spectroscopy results showed that the work function increased by 0.57 eV as the It layer on ITO changed to IrO(x) by the O(2)-plasma treatment. The OPV cell with IrO(x) (2.0 nm) exhibits increased power conversion efficiency as high as 3.5% under 100 mW cm(-2) illumination with an air mass (AM 1.5G) condition, higher than that of 3.3% with PEDOT:PSS. (C) 2010 Elsevier B.V. All rights reserved.