Atomic layer deposition (ALD) is an effective coating technique for angstrom- to nanometer-scale film deposition; its advantages include uniform and conformal coverage, controlled thickness, high reproducibility, and facile synthesis of various functional materials. In this study, we analyzed sol-gel-processed ZnO films coupled with interface-engineered ALD ZnS as electron buffer layers (EBLs) for inverted polymer solar cells (IPSCs). The thickness of the ZnO film was optimized to 10 nm by adjusting the solution concentration. Subsequently, we investigated the effect of the thickness of the ALD ZnS (formed on top of the ZnO film using diethyl zinc and H2S gas) on the photovoltaic properties of the IPSCs. The IPSC device fabricated with 1.8 nm-thick ALD ZnS on ZnO EBL (ZnS C) exhibited a power conversion efficiency (PCE) of 3.17%, which represents a 22% increase over that of equivalent reference cell devices containing only a pristine ZnO EBL. Characterization of the ZnO and ALD ZnS on the ZnO films revealed that the ALD ZnS films reduced the electron resistivity and surface defects of the ZnO films; this in turn reduced the interfacial carrier recombination in the IPSCs. Overall, we demonstrated that the interface engineering of ALD ZnS favorably influenced the electrical properties of the ZnO films.