In this work, efficient bulk heterojunction (BHJ) organic solar cells (OSC) in inverted configuration have been demonstrated. Power conversion efficiency (PCE) of 3.7% is reported for OSC employing silver top electrodes, molybdenum trioxide (MoO3) as the hole-transport interlayer (HTL), active layer comprising of poly-3-hexylthiophene (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as well as a nanocrystalline solution-synthesized zinc oxide (ZnO) nanoparticle (NP) film as the electron-transport layer (ETL). By using solution-processable ZnO crystalline NPs as ETL, we can eliminate the typical high temperature processing/annealing step, which is widely adopted in the conventional ZnO ETL fabrication process via the sol-gel method. Such highly crystalline ZnO NP films can enhance charge collection at the electrodes. It is also found that inverted OSCs exhibit greater air stability and lifetime performance compared to the OSC employing the normal structure.