Naphthodithiophene Diimide (NDTI)‐Based Cathode Interlayer Material Enables 19% Efficiency Binary Organic Solar Cells

Abstract A fused naphthodithiophene diimide (NDTI) derivative is first used as cathode interlayer materials (CIMs) in organic solar cells, by introducing two dimethylamine‐functionalized fluorenes on both sides, namely NDTI1. Meanwhile, two non‐fused naphthalene diimide (NDI) derivatives are synthesized as the control CIMs to validate the design strategy of fused NDI. All three CIMs show high thermal stability, robust adhesion, and strong electrode modification capability. Compared with two NDI‐based materials, NDTI1 possesses excellent film‐forming capacity and strong crystallinity, simultaneously. Besides, NDTI1 presents a strong self‐doping effect and distinct intermolecular interaction with non‐fullerene acceptors. As expected, the NDTI1‐based OSCs achieve a power conversion efficiency (PCE) of 18.02% using the PM6:Y6 active layer and a champion PCE of 19.01% employing the active layer PM6:L8‐BO, which is attributed to improve charge transport and extraction, and suppressive charge recombination. More importantly, NDTI1 retains 91% of the optimal PCE when the film thickness increases from 7 to 20 nm. Furthermore, NDTI1 also exhibits satisfactory universality for different active layer materials and excellent device stability.