An interfacial layer and active layer were simultaneously fabricated through a facile one-step process using a blended solution of zinc oxide (ZnO) sol-gel as the interfacial material, and P3HT:PCBM (poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric methyl ester) as the photoactive components. The resultant devices based on the one-step process exhibited a favourable 3.56% power conversion efficiency (PCE) after a simple UV light treatment, close to the PCE of a conventionally fabricated reference device with a multilayer architecture. Using a combination of surface morphology analysis and investigation of the vertical component distribution, we confirmed that a spontaneous and significant phase separation between the ZnO and the photoactive materials occurred during the spin coating and sequential thermal annealing processes. This resulted in the automatic formation of an enriched ZnO interfacial layer at the ITO surface. The work function of the ZnO layer deposited by the one-step process on the ITO substrate was 3.76 eV, consistent with conventionally formed ZnO layers. Furthermore, the performance of the device fabricated by the one-step process demonstrated superior stability under light exposure. This successful demonstration of organic photovoltaics (OPVs) using the one-step process is an important step towards the realization of high performance, roll-to-roll compatible OPVs. (C) 2014 Elsevier B.V. All rights reserved.