NDI-Based Small Molecule as Promising Nonfullerene Acceptor for Solution-Processed Organic Photovoltaics

A novel naphthalene diimide (NDI)-based small molecule (BiNDI) is designed and synthesized by linking two NDI monomers via a vinyl donor moiety. The electronic structure of BiNDI is carefully investigated by ultraviolet photoelectron spectroscopy (UPS). Density functional theory (DFT) sheds further light on the molecular configuration and energy level distribution. Thin film transistors (TFT) based on BiNDI show a highest electron mobility of 0.365 cm² V^-1 s^-1 in ambient atmosphere. Organic photovoltaics (OPVs) by using BiNDI as the acceptor show a highest power conversion efficency (PCE) of 2.41%, which is the best result for NDI-based small molecular acceptors. Transmission electron microscopy (TEM), atomic force microscopy (AFM), grazing incidence wide-angle X-ray diffraction (GIXD), and X-ray photoelectron spectroscopy (XPS) characterization to understand the morphology and structure order of the bulk heterojunction film are performed. It is found that small amount of 1,8-diiodooctane (DIO) (i.e., 0.5%) in the blended film facilitates the crystallization of BiNDI into fibrillar crystals, which is beneficial for the improvement of device performance.