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

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)₂NDI, (NDI)₂TBT, and (PDI)₂TBT, 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)₂NDI to (NDI)₂TBT and then to (PDI)₂TBT significantly boosts OSC efficiency and operational stability. Notably, devices with (PDI)₂TBT and (NDI)₂TBT retain 85.0% and 82.3% of their peak efficiencies, respectively, far exceeding the (TBT)₂NDI-based device (65.9%) at an interlayer thickness of approximately 105 nm. Furthermore, (PDI)₂TBT 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.