In this study, novel nanocomposites consisting of zinc oxide (ZnO) and a polymer polyethylenimine ethoxylated (PEIE) are fabricated by blending sol-gel ZnO dispersions with different concentrations of PEIE in solution. The photoelectric properties of ZnO:PEIE composites are characterized by a series of testing methods. The results show that PEIE as a doping agent can effectively modify and passivate the surface of ZnO nanoparticles, and ZnO:PEIE composite at 0.1 wt% PEIE possesses the highest conductivity and the lowest work function. Moreover, the ZnO and ZnO:PEIE composites with different concentrations of PEIE are used as electron transport layers (ETLs) in polymer solar cells (PSCs). Compared to pure ZnO as ETL, the power conversion efficiency (PCE) of the device based on P3HT:PCBM with ZnO:PEIE (0.1 wt% PEIE) as ETL is effectively improved from 3.01% to 3.52%. For the device based on PTB7:PCBM with ZnO:PEIE (0.1 wt% PEIE) as ETL, the PCE is improved from 6.92% to 8.03%. It is found that PEIE can not only reduce the surface defects and traps of ZnO nanoparticles, but also prevent the aggregation of ZnO precursor. Moreover, the addition of PEIE does not change the shape of synthetic nanometer ZnO materials, and the Fermi level of ZnO thin films is also gradually decreased with the addition of PEIE, which helps to reduce the electronic transmission barrier. Accordingly, ZnO:PEIE composites can be effective ETLs in PSCs.