Minimizing the Thickness of Ethoxylated Polyethylenimine to Produce Stable Low-Work Function Interface for Nonfullerene Organic Solar Cells

Selection of interfacial layer is critical for nonfullerene active layers to obtain high performance due to high chemical reactivity with base or amines. So far, alcohol-processed ethoxylated polyethylenimine (PEIE) interfacial layer has not been successfully used in high-performance nonfullerene solar cells. Herein, findings are reported as follows: 1) alcohol-processed PEIE as an interfacial layer can deliver high-performance (15.3%) Y6-based nonfullerene solar cells if its thickness is reduced to about 1.2nm rather than 5-10nm that is generally used in fullerene solar cells. That is comparable with the reference cells with ZnO interlayer. Slightly thicker PEIE (5nm) would significantly reduce the power conversion efficiency down to 7.5%. This is completely different from the fullerene-based solar cells. 2) The ultrathin thickness strategy is not universal for other nonfullerene acceptors. Even though the PEIE thickness is about 1.2nm, the IT-4F-based nonfullerene solar cells exhibit lower performance than reference cells with ZnO interlayer. That is because the IT-4F is more chemically reactive than Y6 with the PEIE. The observation suggests the proper selection and optimization of the interfacial layer is critical to the solar cells with different nonfullerene electron acceptors.