The potential benefits of polyethylenimine as an electron extraction layer for facilitating the manufacturing of inverted organic solar cells

The solvent-robustness and temporal stability of polyethylenimine (PEI) as an electron extraction layer (EEL) in inverted organic solar cells (OSCs) were studied. For that purpose, a PEI EEL is utilized in inverted OSCs with the archetypal Poly (3-hexylthiophene) (P3HT): [6,6]-Phenyl C61 butyric acid methyl ester (PC60BM) donor:acceptor system. Results show that soaking the PEI film in solvents (1-propanol and/or toluene) does not significantly impact OSC performance or photostability. As verified by X-ray photoelectron spectroscopy (XPS) measurements, the N atoms in PEI interact with indium-tin-oxide (ITO), causing PEI to strongly adhere to the surface of ITO so that potential processing solvents do not dissolve it. Shifts in N bands in the case of PEI on ITO compared to the PEI on glass confirm the presence of a strong physical interaction. In addition, comparing OSCs with fresh PEI and N2-stored PEI demonstrates that the PEI film is very stable over time, and a time gap between PEI deposition and subsequent fabrication processes does not affect OSC performance and photostability. We highlight that the utilization of PEI as a stable and robust EEL facilitates bridging between laboratory discoveries of OSCs with their practical demonstration and gives us considerable latitude in tackling the stringent requirements of OSC manufacturing.