We investigated the influence of nickel oxide (NiO) nanoparticles that are incorporated into the hole-collecting buffer layer [poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)] on the performance of polymer:fullerene solar cells. To understand the optimum composition of NiO nanoparticles, the composition of NiO nanoparticles was varied from 0 wt% to 23 wt%. Results showed that the optical transmittance was gradually decreased as the NiO content increased. However, the device performance (short circuit current density, fill factor, series resistance, power conversion efficiency) exhibited a two stage trend in a boundary of approximately 9 wt% NiO content. This trend was in good agreement with the trend of sheet resistance in the presence of slight discrepancy owing to the different charge transport geometry.