The potential of cerium oxide (CeOx) as electron transport layer (ETL) was explored in inverted bulk heterojunction organic solar cells (BHJ OSCs), using a mixture of P3HT and PC61BM as photoactive layer, and compared with the benchmark zinc oxide (ZnO). CeOx layers on indium tin oxide or glass were obtained by spin-coating a solution prepared from Ce(OAc)3.yH2O followed by thermal annealing which was optimized. CeOx layers were characterized by different analytical techniques such as X-ray photoelectron spectroscopy showing that thermal annealing at 200°C for 15 minutes led to the formation of CeOx, with a value of x reaching 1.70. These optimized thin layers of CeOx were successfully used as ETL in P3HT-PC61BM inverted OSCs exhibiting power conversion efficiencies (PCEs) reaching 1.7% whereas ZnO-based devices showed a maximum PCE of 3.2%. Although annealing at 450°C led to CeOx layers with a higher x value of 1.79, the PCEs of the devices became significantly lower due to the appearance of large crystalline clusters in the CeOx layer, as shown by AFM, likely producing short circuits. ZnO thin films doped with 3 mol % of CeOx were tested leading to a slight decrease of the photovoltaic efficiency. Importantly, the efficiency of the CeOx-based OSCs increased up to 2.1% after two weeks and remained higher than the initial value after one month, while OSCs based on ZnO doped with CeOx retained 85% of its initial PCE after five month’s storage. These results open up interesting perspectives for the development of stable electronic devices.