A Dual‐Functional Ultraviolet‐Converting Anode Interfacial Modifier Unlocks Highly Efficient and Stable Organic Solar Cells

The power conversion efficiency (PCE) of organic solar cells (OSCs) has surpassed 21%; however, their long‐term stability remains constrained by ultraviolet (UV)‐induced degradation of the active layer, while the underutilization of UV photons further restricts the efficiency potential. To address these issues, a dual‐functional UV‐converting interfacial modifier, 2‐(2H‐benzotriazol‐2‐yl)‐6‐(dodecyl)‐4‐methylphenol (UV571), was utilized to integrate photon down‐conversion with active interface optimization. The benzotriazole core of UV571 absorbs UV photons through photoinduced tautomerism and transfers the energy to the donor polymer through Förster resonance energy transfer, efficiently converting UV light into visible light. Concurrently, the incorporation of a long alkyl chain serves to adjust the surface energy and enhance its miscibility with the active layer. This dual‐functional strategy has been shown to optimize hole transport by suppressing pinhole defects and enhancing charge‐transport kinetics. It also markedly mitigates UV‐triggered active layer degradation. Consequently, binary OSCs based on the D18:BTP‐eC9 system deliver an excellent PCE of 20.36%, along with significantly improved photostability, while the ternary OSCs (D18:BTP‐eC9:L8‐BO) achieve a higher PCE of 20.61%. This work successfully transforms an environmental stressor into a performance enhancer, emphasizing the crucial role of interfacial engineering in enhancing stability and providing a pragmatic approach to high‐performance OSCs.