The performance of organic solar cells (OSCs) based on the widely used zinc oxide (ZnO) cathode buffer layer (CBL) is severely affected by the surface defects, energy level mismatch with the common acceptor materials and its intrinsic hydrophilic nature. Developing novel materials to passivate surface defects and improve the properties of ZnO CBL is one of the prime areas of research in OSCs. In this work, hydrothermally synthesized SnS2 nanoplates of average size 26 nm were successfully used to modify the surface of ZnO CBL in PTB7:PC70BM based OSCs. The average power conversion efficiency of the OSCs increased from 6.13% to 6.84% after the surface modification. The XPS and DFT studies revealed passivation of oxygen deficiency defects and reduction in work function of CBL leading to decrease in energy barrier for flow of electrons from the active layer to cathode. Due to the efficient passivation of surface defects by the SnS2 nanoplates, the conductivity of ZnO increased from 0.92 × 10−4 S/m to 1.63 × 10−4 S/m. Moreover, the SnS2 modification significantly enhanced the hydrophobicity of ZnO, benefitting the formation of a good interfacial contact with the organic active layer. Significantly low leakage current observed in the case of OSCs based on ZnO/SnS2 composite CBL reveals substantial decrease in charge recombination as a result of defect passivation. The present study demonstrates that interface engineering using metal dichalcogenide nanostructures is a promising approach to improve the properties of ZnO CBL/active layer interface in OSCs.