MoO3/Au/MoO3 structure with a protective barrier Al2O3 was developed to suppress the reactions between MoO3 and the PEDOT:PSS film in organic solar cells (OSCs). Though the maximum optical transmittance of this structure was 66% at 550 nm wavelength, the power conversion efficiency of a MoO3/Au/MoO3/Al2O3/PEDOT:PSS based OSCs was 2.77%, comparable to the 2.89% of an ITO-based OSCs. The introduction of a very thin Al2O3 layer between the MoO3 and the acidic PEDOT:PSS film effectively protected the MoO3 from the acidic and water dispersed PEDOT:PSS film, increasing the J(sc), V-oc and FF of the structure above those of the MoO3/Au/MoO3/PEDOT:PSS structure. The Al2O3 (1 nm) introduced to the MoO3/Au/MoO3 structure improved Js, because it suppressed the reactions between MoO3 and PEDOT:PSS and lowered the work function of the PEDOT:PSS film. The MoO3/Au/MoO3/Al2O3 electrode was shown to be a promising replacement of ITO for use in flexible optoelectronic devices. (C) 2014 Elsevier Ltd. All rights reserved.