Inverted organic photovoltaic (OPV) cells based on poly(3-hexylthiophene (P3HT) as the electron donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the electron acceptor, were fabricated and characterized. To improve the photovoltaic performance, ZnO films were used as electron collection layers, while an under-stoichiometric molybdenum oxide MoOx was employed as the hole collection layer. Two types of ZnO layers were employed; one deposited by atomic layer deposition (ALD-ZnO) and another deposited using the sol-gel method (sg-ZnO). OPV cells with a 20 nm thick ALD-ZnO layer exhibited significant efficiency enhancement compared with those based on the sg-ZnO layer with the same thickness. The ALD-ZnO film exhibited reduced defect/trap concentration compared with the sg-ZnO counterpart, as confirmed by steady state photoluminescence spectroscopy, showing a promising interface layer for efficient organic photovoltaic devices exhibiting also improved temporal stability. By employing capacitance-voltage measurements we were able to identify a downward shift of the conduction band edge of ALD-ZnO film (or equivalently, an upward shift of the conduction band minimum of the sg-ZnO film), verified also by ultraviolet photoelectron spectroscopy measurements. This resulted in a significant decrease in the electron extraction barrier at the ALD-ZnO/organic active layer interface, as was also demonstrated by the increased current in unipolar (electron only) devices. This work highlights the importance of using the ALD method to develop conformal and defect free ZnO electron collection layers for high performance organic photovoltaics. (C) 2014 Elsevier Ltd. All rights reserved.