In an organic photovoltaic (OPV) cell, the buffer layers connecting the active photovoltaic material to the electrodes strongly influence the overall energy conversion efficiency. In this work, we demonstrate that a thin layer (< 4 nm) of MoO3 formed by atomic layer deposition (ALD) of Mo, followed by oxidation in ozone, performs well as an anode buffer layer in organic photovoltaic cells based on a blend of poly(3-hexylthiophene) and phenyl-C-61-butyric acid methyl ester. Compared to the commonly used buffer layer poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), the MoO3 layer causes virtually no degradation in the transmission spectrum. The improvement in optical transmission leads to higher short-circuit current density, and practically the same power conversion efficiency as OPV cells employing PEDOT:PSS. Results from a substantial number of devices indicate that the improvement is statistically significant, demonstrating the reproducibility and reliability of the layer. Moreover, because of the conformal nature of ALD, this approach can be extended to nanostructured systems. (C) 2011 Elsevier B.V. All rights reserved.