Enhancing the Built‐In Electric Field of Thickness‐Insensitive Small Molecule Cathode Interlayers for High‐Efficiency and Stable Organic Solar Cells

Abstract The built‐in electric field (BEF) is proposed as a critical design parameter for optimizing small‐molecule cathode interlayer materials (SM‐CIMs) in organic solar cells (OSCs). By strategically transforming imidazole‐functionalized triads from a donor‐acceptor‐donor (D‐A‐D) to an A‐D‐A configuration and replacing the A unit with a more electron‐deficient moiety, we developed three triads: (TBT) 2 NDI, (NDI) 2 TBT, and (PDI) 2 TBT, each exhibiting progressively enhanced BEF, along with improved conductivity, work function (WF) adjustability, energy level alignment, and crystallinity. Additionally, the A‐D‐A triads facilitate superior electronic communication with both non‐fullerene acceptors (NFAs) and polymer donors, enhancing photoexcitation utilization and reducing triplet state formation. Consequently, transitioning from (TBT) 2 NDI to (NDI) 2 TBT and then to (PDI) 2 TBT significantly boosts OSC efficiency and operational stability. Notably, devices with (PDI) 2 TBT and (NDI) 2 TBT retain 85.0% and 82.3% of their peak efficiencies, respectively, far exceeding the (TBT) 2 NDI‐based device (65.9%) at an interlayer thickness of approximately 105 nm. Furthermore, (PDI) 2 TBT exhibits excellent compatibility with various active layers, and an outstanding performance of 20.10% is recorded in the PM6:L8‐BO:BTP‐eC9 system. This comprehensive study, encompassing molecular design, theoretical simulation, device fabrication, and fundamental device physics, highlights the importance of strategic donor–acceptor (D‐A) electronic framework modifications to enhance BEF, thereby advancing the development of sophisticated SM‐CIMs for OSCs.