An inverted organic solar cell, consisting of an ultrathin Ca layer for electron transport, the blend of poly(3-hexylthiophene) and 1-(3-methoxycarbony1)-propyl-1-phenyl-(6,6)C(61) for photon absorption, and a MoO(3) layer for hole transport, is optimized. The insertion of MoO(3) layer between photoactive layer and Ag anode has been demonstrated to improve both the photocurrent and the fill factor. By optical modeling and experiment, we find that the MoO(3) layer also functions as an optical spacer for thin devices. For the optimized device with an active layer thickness of 85 nm, Ca thickness of 0.5 nm, and MoO(3) layer thickness of 5 nm, a maximum power conversion efficiency of 3.86% was achieved. (C) 2010 Elsevier B.V. All rights reserved.