Perylene diimide derivatives with diverse ionic functionality as cathode interlayer for ZnO-free inverted non-fullerene organic solar cells

Perylene diimide (PDIN) derivatives with diverse ionic functionality are prepared and applied to inverted-type non-fullerene organic solar cells (NFOSCs) as the cathode interlayer (CIL). Among PDIN derivatives, PDIN-O is generally applied as the CIL in conventional NFOSCs. As for conventional-type NFOSCs, heat treatment after the deposition of CIL is not necessary because its thermal stability is not considerable. However, the application of PDIN-O as the CIL in inverted-type devices becomes challenging due to the intrinsic property of PDIN-O being unstable under heat treatment and tending to degrade. In this study, we prepare quaternary ammonium salts (PDIN-I and PDIN-Br) and zwitterion (PDIN-BS) as the CIL to enhance thermal stability. We discover PDIN derivatives with diverse ionic functionality can substitute the ZnO layer with the comparable power conversion efficiency. In addition, these CIL retained their original power conversion efficiency after 30 hours of thermal and photo-aging, proving their effectiveness as an oxygen-blocking layer at the device interface. By introducing a diverse ionic functionality to adjust the energy level alignment at the cathode interface, the side group enhances the electron extraction in the devices. This study demonstrates the capacity to regulate CIL and thus provides insight into the fabrication of low-temperature processible ZnO-free NFOSCs.