Decoupling intrinsic and extrinsic photodegradation pathways in organic solar cells

Abstract

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.