Decoupling the Dual Thermal Failure Pathways at the PDINN/Ag Cathode Interface in Conventional Organic Solar Cells

The conventional architecture of organic solar cells (OSCs) results in power conversion efficiency (PCE) but suffers from poor thermal stability, largely due to insufficiently understood interfacial degradation at the cathode side. Herein, we systematically investigate the thermal failure mechanisms of the widely used electron transport layer PDINN in conventional OSCs with a PM6:L8-BO/PDINN/Ag configuration. We identify a two-stage degradation pathway: In the initial heating phase, PDINN degradation induces n-type doping in the adjacent active layer near the cathode contact, reducing the short-circuit current density (JSC). Upon prolonged thermal stress, Ag- migrates through the PDINN interlayer, generating deep hole traps and shifting the cathode work function. The resulting energy level mismatch at the PM6:L8-BO/PDINN/Ag interface dramatically increases the electron extraction barrier, intensifying charge recombination and causing substantial losses in both open-circuit voltage (VOC) and fill factor (FF). Guided by this mechanistic understanding, we introduce a thermally evaporated C60/SnO2 bilayer between the active layer and PDINN as a diffusion-blocking interlayer. The modified devices retain 80% of their initial PCE (initial PCE: 16.96%) after continuous heating at 85 °C for 1000 h, in stark contrast to the rapid degradation of unmodified reference devices (T80 <50 h). This work elucidates the intrinsic thermal vulnerability of PDI-based transport layers in conventional OSCs and offers a viable interfacial engineering strategy toward thermally robust devices.