Inverted bulk heterojunction organic solar cells have generated significant interest in organic photovoltaic research due to much improved life times. Efficiency maximization for a given polymer-acceptor blend requires control of thickness and morphology of various constituent layers. In this manuscript, we present our results on the thickness variation of various constituent layers, viz. ZnO (electron transport layer), blend layer of poly(3-hexyl thiophene) and [6,6]-phenyl-C61 butyric acid methyl ester (active layer) and MoO3 (hole extraction layer) vis-a-vis effect of morphology of layers and optical field distribution on the electrical performance of inverted organic solar cells. The peak performance of the devices occurs for 30 nm thick ZnO films, 100 nm thick active layer and 10 nm of MoO3 layers with an average power conversion efficiency of approximately 3.2% and a fill factor of 61%. The maximization of the device performance upon thickness variation of device constituents appears to result from the combined effects of morphology and the optical electric field distribution in the active layer. (C) 2013 Elsevier B.V. All rights reserved.