Modification of zinc oxide interlayers with naphthalene diimide-based polymer zwitterions for efficient organic solar cells

Zinc oxide (ZnO) has emerged as a promising electron transport layer (ETL) for optoelectronic devices due to its excellent charge carrier mobility and high optical transparency. However, the performance of ZnO-based devices is often limited by surface and bulk defects in the ZnO film that act as charge trapping sites. To address these limitations, this study designed and synthesized two novel polymer zwitterions with benzene rings (BZ) or C=C double bonds (CZ) by integrating naphthalene diimide (NDI) conjugated units and sulfobetaine (SB) zwitterion pendant groups. The zwitterionic components can effectively passivate ZnO defects, thereby enhancing film conductivity, while the NDI units have ultraviolet (UV) light absorption properties that can protect the active layers from photodegradation. Importantly, these polymer zwitterions significantly reduced the work function of the ZnO film, facilitating efficient charge extraction and transport. When implemented as ETLs in PM6:BTP-eC9-based inverted organic solar cells (OSCs), the BZ- and CZ-modified ZnO films demonstrated a remarkable power conversion efficiency (PCE) of nearly 18% as well as substantially improved device stability. This study shows that polymer zwitterion modification is an effective strategy for optimizing interfacial properties and energy-level alignment in organic electronic devices and has considerable potential for applications in next-generation wearable optoelectronics.