We report that thermal annealing of P3HT:PCBM bulk heterojunctons (BHJs) on top of a commonly used sol-gel ZnO electron transport layer can lead to the formation of PCBM clusters, which significantly decreased short circuit current density (up to 45%), open circuit voltage (up to 35%), and fill factor (up to 15%), To understand the cause of this phenomenon, we deposited ZnO films using different sol-gel recipes and systematically studied the correlation between sol-gel ZnO processing and the morphology of the P3HT:PCBM BHJ. We showed that higher amounts of monoethanolamine (MEA), a stabilizer in the ZnO sol-gel precursor, lead to higher densities of PCBM clusters in the BHJ. In addition, we correlated the cluster area fraction with bimolecular recombination at 100 mW/cm(2), as quantified by white-light biased external quantum efficiency measurements. Finally, we show that rinsing off residual organics from the ZnO surface after pyrolysis eliminated PCBM cluster formation, avoiding poor organic photovoltaic device performance. (C) 2014 Elsevier B.V. All rights reserved.