This paper describes an inverted bulk heterojunction organic solar cell featuring solution-processed zinc oxide nanoparticles (ZnO NPs) as the electron transport layer. ZnO NPs with an average diameter of ~5 nm are synthesized by a wet chemistry method. The ZnO NP layers are deposited from colloidal ZnO NPs dispersed in a mixed solvent having various volumetric ratios of chlorobenzene (CB) and isopropyl alcohol (IPA). After the coating step, the surface roughness of the coating films was reduced and that was related to the dispersion of the ZnO NP solution. The maximum performance of the solar cell was achieved when the ZnO NPs were suspended within a volumetric ratio (50 vol%:50 vol%) of CB and IPA, with a solar power conversion efficiency of 3.74% at AM 1.5 G, 100 mW/cm2 illumination.