This paper describes inverted bulk heterojunction organic solar cells featuring solution-processed zinc oxide nanoparticles (ZnO NPs) as an electron extraction layer, prepared at relatively low annealing temperatures (<= 150 degrees C). A solution of ZnO NPs (average size: 25 nm) was prepared using a wet grinding method. When the ZnO interlayer was present in the solar cell, the vertical phase separation of the active layers prepared with and without solvent annealing exhibited similar gradient concentrations and, therefore, similar photocurrent generation, both of which were superior to those of conventional devices incorporating a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) hole extraction layer. We attribute this vertical phase separation to the similar surface energies of the fullerene derivative and the ZnO interlayer. Under simulated air mass (AM) 1.5 G illumination at 100 mW cm(-2), the power conversion efficiency of the optimized device was approximately 4%. (C) 2012 Elsevier B.V. All rights reserved.