In bulk hetero-junction organic solar cells (BHJOSCs), light absorption can be improved by employing techniques such as plasmonic effects which can lead to improved device efficiencies. In this manuscript, we demonstrate highly efficient devices obtained upon incorporation of gold nanoparticles (Au NPs) of size 10-20 nm at various concentrations into solution processed ZnO (zinc oxide) electron transport layers (ETLs). The inverted OSC devices of configuration ITO/ZnO:Au NPs/PTB7-Th:PC71BM/MoO3/Ag show the highest power conversion efficiency (PCE) for an optimum concentration of Au NPs (0.05 wt%) in ZnO ETL layer driven by plasmonic effects leading to improved light absorption and improved interface between the active layer and the ETL leading to improved carrier collection. The effects together cause enhancement in the short circuit current density (J(sc)) and fill factor (FF) leading to a maximum PCE of 11.8% with an open circuit voltage (V-oc), short circuit current density (J(sc)), and fill factor (FF) of 0.80 V, 21.75 mAcm(-2), and similar to 68%, respectively. This PCE is 24% higher than the control device (ZnO ETL without Au NPs) and is among the best observed values for widely explored bulk hetero-junction (BHJ) solar cells based on PTB7-Th:PC71BM.