Recently, active research is being conducted on polymer solar cells to improve power conversion efficiency by developing new device structures, synthesizing polymer materials, and applying them to charge extraction layers. In this study, we analyzed the influence of ZnO on the performance of solar cells by using various ZnO thin films (using the high and low temperature sol-gel processes and the nanoparticle process) as an electron extraction layer in inverted polymer solar cells based on PBDB-T:ITIC. The ZnO thin film was formed by the high-temperature (450°C) sol-gel process, which resulted in an increase in the sheet resistance of the ITO electrode by five times. As a result, the power conversion efficiency of the polymersolar cells was low (4.12%). Conversely, in the nanoparticle process, butanol-based ZnO exhibited better dispersion and surface properties than the IPA-based ZnO, resulting in improved performance of the polymer solar cell (PCE of 6.35% and 4.58% with butanol and IPA-based ZnO, respectively). Additionally, a ZnO precursor solution was developed using the low-temperature (150°C) sol-gel process, which was applied as an electron extraction layer in an inverted polymer solar cell. Consequently, the device performance significantly improved(PCE of 8.89%)due to the formation of a ripple-shaped surface, which facilitated the extraction of electrons due to the excellent surface roughness.