Coordinated Interfacial Engineering and Time-Induced Bulk Trap Passivation Governing Performance Evolution in D18:Y6-Based Inverted Organic Solar Cells

Interface and bulk traps critically limit the efficiency and operational stability of organic solar cells (OSCs) by inducing energetic disorder and trap-assisted recombination. Here, we elucidate the distinct and synergistic roles of interfacial engineering and time-induced bulk trap passivation in inverted D18:Y6- based OSCs. An ultrathin ethanol-processed 2-(9H-carbazol-9-yl)ethyl phosphonic acid ( 2PACz ) interfacial layer is introduced at the anode interface (ITO/ZnO/ D18:Y6/2PACz /MoO 3 /Ag) to passivate interfacial traps and improve energetic alignment. Atomic force microscopy (AFM) and high-resolution–transmission electron microscopy (HR-TEM) reveal enhanced interfacial uniformity without perturbing the bulk morphology, while optical spectroscopy confirms unchanged light-harvesting. Photoluminescence quenching and space-charge-limited current analysis demonstrate improved interfacial charge transfer, yielding a ∼43% enhancement in hole mobility and a ∼19% reduction in hole trap density. Consequently, power conversion efficiency (PCE) increases from 14.53% for reference devices to 15.43% with interfacial engineering, accompanied by an increase in open-circuit voltage ( V OC ) from 0.81 to 0.84 V. Remarkably, upon storage, intrinsic bulk trap passivation further reduces trap density and enhances carrier mobility, leading to a PCE of 16.05% after 15 days. The combined effect of interfacial engineering and intrinsic bulk trap passivation results in ∼10.4% relative improvement in device efficiency compared to the initial reference device. These findings establish coordinated interfacial and bulk trap management as a powerful strategy for achieving high-performance organic solar cells.