Doping is a widely-implemented strategy for enhancing the inherent electronic properties of charge transport layers in photovoltaic devices. A facile solution-processed zinc oxide (ZnO) and various cation and anion-doped ZnO layers were synthesized via the sol–gel method and employed as electron transport layers (ETLs) for inverted polymer solar cells (PSCs). The results indicated that all PSCs with doped ZnO ETLs exhibited better photovoltaic performance compared with the PSCs with a pristine ZnO ETL. By exploring the role of various anion and cation dopants (three compounds with the same Al³⁺ cation: Al(acac)₃, Al(NO₃)₃, AlCl₃ and three compounds with the same Cl⁻ anion: NH₄Cl, MgCl₂, AlCl₃), we found that the work function changed to favor electronic extraction only when the Cl anion was involved. In addition, the conductivity of ZnO was enhanced more with the Al³⁺ cation. Therefore, in inverted solar cells, doping with Al³⁺ and Cl⁻ delivered the best power conversion efficiency (PCE). The maximum PCE of 10.38% was achieved from the device with ZnO doped with Al⁺ and Cl⁻.