Stable P3HT-PC61BM inverted organic solar cells based on cerium oxide as an electron transport layer

Abstract

The potential of cerium oxide (CeO_x) as an electron transport layer (ETL) was explored in inverted bulk heterojunction organic solar cells (BHJ OSCs), using a mixture of P3HT and PC₆₁BM as the photoactive layer, and compared with the benchmark zinc oxide (ZnO). CeO_x 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. CeO_x 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 CeO_x, with a value of x reaching 1.70. These optimized thin layers of CeO_x were successfully used as an 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 CeO_x 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 CeO_x layer, as shown by AFM, likely producing short circuits. ZnO thin films doped with 3 mol% of CeO_x were tested leading to a slight decrease of the photovoltaic efficiency. Importantly, the efficiency of the CeO_x-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 CeO_x retained 85% of their initial PCE after five months of storage. These results open up interesting perspectives for the development of stable electronic devices.