The effect of a zinc oxide (ZnO) interfacial layer on the device performance of polymer solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) was studied. When a 15 nm ZnO layer was inserted between the active layer (P3HT:PCBM) and Al electrode, the power conversion efficiency (PCE) was increased by approximately 10% as compared to that without ZnO layer. From the systematic analysis of the current–voltage and impedance characteristics of devices with and without the ZnO layer at various temperatures and light intensities, we found that the device with the ZnO layer has a lower activation energy for carrier extraction and bimolecular recombination loss compared with that without the ZnO layer. Therefore, the enhanced efficiency of the device with ZnO is mainly attributed to the enhanced extraction efficiency and thereby increased short-circuit current density.