Park, Soohyung Jeong, Junkyeong Yoo, Jisu Kang, Donghee Kim, Kitae Lim, Keun Yong Hwang, Do Kyung Lee, Hyunbok Yi, Yeonjin 2026-02-04T06:30:15Z 2026-02-04T06:30:15Z 2026-02-02 2026-03 0141-3910 https://pubs.kist.re.kr/handle/201004/154201 The “burn-in” degradation of organic solar cells (OSCs) is a critical obstacle to their commercialization. Although environmental factors (e.g., O2) are known contributors, the interplay between these extrinsic stressors and the inherent susceptibility of active materials to photodegradation remains poorly understood. We systematically decouple these effects in a model polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) blend system by correlating the device performance with direct electronic structure measurements via ultraviolet photoelectron spectroscopy. We identify two distinct and competing degradation pathways dependent on the fabrication environment. Under inert conditions, we uncover a fundamental intrinsic degradation pathway in which visible light alone triggers the degradation of PTB7 within the blend, which is characterized by the formation of performance-limiting midgap states. In the presence of ambient air, a far more aggressive extrinsic photo-oxidation pathway dominates, causing catastrophic and indiscriminate degradation. This dual-pathway model provides a robust explanation for a key observation: midgap states, which are signatures of the intrinsic pathway, are not observed under ambient conditions because extrinsic photo-oxidation is so rapid and destructive that it bypasses this intermediate degradation stage. Our findings underscore that the future design of long-lasting OSCs should focus on the development of materials with enhanced inherent resistance to photodegradation. English Elsevier BV Decoupling intrinsic and extrinsic photodegradation pathways in organic solar cells Article 10.1016/j.polymdegradstab.2025.111903 1 Polymer Degradation and Stability, v.245 Polymer Degradation and Stability 245 N scie scopus 001659874400001 2-s2.0-105027200908 Polymer Science Polymer Science Article CONJUGATED POLYMERS LIGHT DEGRADATION MECHANISM EFFICIENCY ENERGY STABILITY DESIGN PTB7 Ultraviolet photoelectron spectroscopy PTB7 PC 71 BM Organic solar cells Photo-oxidation