Spectroscopic and Microscopic Analysis of Degradation Pathways in PTQ10:IDIC Solar Cells

We report a comprehensive spectroscopic, microscopic, and device-level investigation of the ambient-driven degradation of PTQ10:IDIC bulk-heterojunction organic solar cells (BHJ-OSCs), up to 500 h. The power conversion efficiency dropped from 9.51% to 7.69% (≈19% relative loss), primarily due to a decrease in short-circuit current density (J_SC 15.93 to 13.82 mA cm^-2), while the open-circuit voltage remained largely stable (0.92 to 0.90 V). Atomic force microscopy reveals surface smoothing upon ageing, with the root-mean-square roughness decreasing from 4.29 to 3.45 nm, and the UV-vis absorption spectra show negligible changes, indicating preserved bulk light-harvesting capability. In contrast, X-ray photoelectron spectroscopy indicates pronounced surface compositional evolution, with a decrease in oxygen (5.18 to 3.18%) and a substantial increase in fluorine content (3.23 to 7.23%), consistent with fluorine-rich surface segregation or reorientation. Ultraviolet photoelectron spectroscopy further reveals a 0.48 eV reduction in surface work function, indicative of surface dipole modification and near-surface electronic reorganization. Collectively, these results demonstrate that ambient ageing primarily impacts interfacial chemistry and morphology rather than bulk optoelectronic properties, highlighting interfacial engineering and encapsulation as effective strategies for improving long-term device stability.