Electronic and Vibrational Spectroscopy of Photodegradation Processes in High-Performance Organic Photovoltaic Materials
Authors:   , Prasad Suraj 
Published: February 2025

Organic solar cells (OSCs) have shown a record power conversion efficiency of over 19%. However, ensuring their long-term operational stability remains a significant challenge for commercial production. In the research presented in this thesis, I have employed a combination of spectroscopy and microscopy techniques, including UV-vis absorption spectroscopy, Fourier-transform infrared (FTIR), X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), and atomic force microscopy (AFM), to investigate the degradation of the photoactive layer under exposure to light and air. The research presented in this thesis explains that the photoactive layer of the OSCs is composed of solution-processed blends of electron-donor and electron-acceptor molecules.

In a first study, we compared the photodegradation of thin films of donor polymer PBDB-T, the small molecule non-fullerene acceptors (NFAs) Y5, and their copolymers PF5-Y5 and PYT. The UV-vis absorption spectra show that PBDB-T and PF5-Y5 films are more prone to photodegradation compared to Y5 and PYT after 30 hours of exposure. The FTIR spectra reveal the formation of new carbonyl peaks in PBDB-T and PF5-Y5, whereas no such new peaks were observed in Y5 or PYT. Furthermore, the C1s, O1s, and S2p XPS spectra of PBDB-T and PF5-Y5 confirm the formation of photooxidation products. In contrast, Y5 shows no significant changes in composition upon exposure. The comparison suggests that the BDT-T unit, present in PBDB-T and PF5-Y5, accelerates their photodegradation. The replacement of the BDT-T unit in PF5-Y5 with thiophene significantly enhanced the photostability of the acceptor PYT. These results demonstrate that the choice of co-mer has a substantial impact on the intrinsic photostability of Y5-based copolymers.

In a second study, we investigated the light-induced photodegradation of the donor materials PM6 and PTQ10, the small-molecule NFA Y6, and their blends, under different illumination conditions in air. UV-vis spectra reveal that the photobleaching of Y6 under unfiltered AM 1.5 light is accelerated in the blend films PTQ10:Y6 and PM6:Y6, compared to pristine Y6 films. To distinguish the different photodegradation pathways that are active when blend films are exposed to simulated sunlight, we study the blends under filtered light conditions. Using long-wavelength light that only the acceptor can absorb, suppresses the electron transfer pathway from the donor and blocks the formation of superoxide radicals, so that the remaining photodegradation under these illumination conditions can be ascribed to an energy transfer process from the photosensitizing acceptor to oxygen, feeding the singlet oxygen formation. These insights can inspire the design of new donor and acceptor materials with improved photostability by tuning the positions of their singlet and triplet states to minimize the formation of oxygen-mediated reactive species.

Covertext

Organic Solar Cells (OSCs) have gained significant attention within the scientific community due to their lightweight, transparency, flexibility, and low-cost production. With recent breakthroughs in the design of advanced materials, particularly non-fullerene acceptor (NFAs) molecules, the record power conversion efficiency of the OSCs has reached 19.2%. 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.

This thesis focuses on understanding the photodegradation mechanisms of the active layer materials used in OSCs by investigating thin films of a selection of electron donors and NFAs, intentionally exposed to simulated sunlight in air. By employing spectroscopy and microscopy techniques, this work identifies the influence of molecular structure and possible photodegradation pathways, insights that may contribute to improved lifetime of OSCs.