Organic solar cells (OSCs) have achieved power conversion efficiencies exceeding 20%, yet their long-term operational stability remains a major challenge for commercial applications. This study investigates the photodegradation of state-of-the-art OSC materials and their impact on device performance using a combination of spectroscopy and microscopy techniques. UV-vis absorption spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM), synchrotron-based X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy were employed to study chemical, structural, and morphological changes in photoactive layers exposed to AM 1.5 illumination in air.

The first part of the thesis focuses on the photodegradation mechanisms of donor and acceptor materials. Thin films of PBDB-T, Y5, PF5-Y5, and PYT were exposed to AM 1.5 illumination in air. The results show that the BDT-T unit present in PBDB-T and PF5-Y5 accelerates photobleaching, while its replacement with thiophene improves the photostability of PYT. Studies of PM6:Y6 blend revealed distinct degradation pathways. By employing long-wavelength band-pass filter illumination that selectively excites the acceptor, electron-transfer-induced superoxide formation was suppressed, and the remaining degradation occurred due to singlet oxygen via energy transfer. 

The second part of the thesis addresses the influence of photodegradation on device performance. The choice of processing solvent affected the morphology and molecular orientation in PM6:Y6 blends, but devices exhibited similar degradation rates. Incorporation of PC₇₀BM into PTQ10:Y6 blends reduced the formation of carbonyl species and improved photostability. Overall, this work provides insights into molecular- and device-level degradation pathways in high-performance OSC systems and identifies structural and compositional strategies to mitigate photooxidation processes. 

Organic solar cells (OSCs) have attracted significant attention due to their lightweight, flexibility, and potential for low-cost solution-based production process. Advances in molecular design and novel charge-transport materials have pushed power conversion efficiencies towards 20%. However, long-term stability, mainly due to materials degradation, remains a challenge for the commercial production of OSCs, making it crucial to understand the photodegradation of the active layer to enhance their long-term performance.

In the first part of this thesis, we investigate how the properties of state-of-the-art active-layer materials are affected by exposure to simulated sunlight in ambient conditions. Spectroscopy and microscopy techniques were employed to unveil changes in molecular structure and composition, as well as film morphology, and identify degradation pathways. In the second part of the thesis, we explore the effect of photodegradation in ambient conditions on the electrical performance of OSCs. The choice of solvent and additives used in OSCs is a crucial factor influencing both device performance and stability. Collectively, these findings provide insights to guide the design of next-generation donor and acceptor materials with superior photostability and strategies to mitigate photodegradation.