Organic solar cells have attracted much attention in the recent years due to their many intrinsic advantages, such as, light weight, flexibility, low-cost, solution processing, and facile device fabrication. In this study, effective interface engineering was employed to improve all the photovoltaic parameters of an organic solar cell simultaneously by incorporating ZnO nanoparticle (ZnO-NP) interlayer in between the indium tin oxide cathode and sol-gel processed ZnO electron transport layer. The significance of incorporating a ZnO-NP/ZnO bilayer as the electron transport layer in the bulk heterojunction organic solar cells was demonstrated via systematic study, employing a high efficiency photoactive layer system (PBDB-T:ITIC). The bilayer electron transport layer demonstrated reduced work function compared to the sol-gel ZnO, which enabled effective electron transfer from the active layer to the electron transport layer. In addition, improved bilayer surface morphology, via reduction of ZnO-NP layer roughness, and better crystallinity compared to sol-gel ZnO facilitated charge separation and transmission between electron transport layer and active layer. Consequently, the devices with bilayer interlayer exhibited an enhancement of > 13% in power conversion efficiency compared to the control devices with sol-gel only ZnO as electron transport layer. The mechanisms behind the improvement in device performance were analysed using the ultraviolet and X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. The champion bilayer device exhibited 12.24% efficiency which is much higher than the efficiency of 10.69% for the control device.