A Nonconjugated Zwitterionic Polymer: Cathode Interfacial Layer Comparable with PFN for Narrow‐Bandgap Polymer Solar Cells

A nonconjugated, alcohol‐soluble zwitterionic polymer, poly(sulfobetaine methacrylate) (denoted by PSBMA), is employed as cathode interfacial layer (CIL) in polymer solar cells (PSCs) based on PTB7‐Th:PC 71 BM. Compared with the control device without CIL, PSCs with PSBMA CILs show significant enhancement on the resulting performance, and the highest power conversion efficiency (PCE) of 8.27% is achieved. Under parallel conditions, PSCs with PSBMA as CIL show comparable performance than those with widely used poly[(9,9‐bis(30‐( N , N ‐dimethylamino)propyl)‐2,7‐fluorene)‐ alt ‐2,7‐(9,9‐ioctylfluorene)] as CIL. The polar groups of PSBMA not only provide a solvent orthogonal solubility in the process of preparation of the devices but also lead to interfacial dipole to the electrode, which promises a better energy level alignment. In addition, PSBMA‐based devices show better abilities of hole blocking. These results indicate that the zwitterionic polymer PSBMA should be a promising CIL in PSC‐based narrow‐bandgap polymers.