Resonance Raman and DFT Reveal Sulfur Oxidation and Backbone Twisting as Key Degradation Pathways in Crosslinked PM6-Based Organic Solar Cells

Elkhansa Elbashier^a^,^b

^aThe Dodd‐Walls Centre for Photonic and Quantum Technologies, Department of Chemistry, University of Otago, P.O. Box 56, Dunedin 9016, New Zealand

^bMacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, New Zealand

^cSchool of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

^dDepartment of Chemistry, Korea University, Seoul 136-713, Republic of Korea

^eDepartment of Physics and Energy Harvest Storage Research Center, University of Ulsan, Ulsan 44610, Republic of Korea

^fGraduate School of Carbon Neutrality, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

Proceedings of MATSUS Spring 2026 Conference (MATSUSSpring26)

G2 Monitoring the degradation mechanisms of photovoltaic devices by optoelectronic characterization

Barcelona, Spain, 2026 March 23rd - 27th

Organizers: Enrique H. Balaguera and Emilio J. Juarez-Perez

Poster, Elkhansa Elbashier, 974
Publication date: 15th December 2025

Organic solar cells (OSCs) have achieved high efficiencies, but the long-term photostability of donor polymers remains a key challenge. In PM6-based systems, backbone twisting and photo-oxidation, especially of sulfur, cause backbone distortion, shifts in energy levels, trap formation, and device failure. This study explores degradation mechanisms in PM6 and its crosslinkable derivative (PM6-Br) using resonance Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and time-dependent density functional theory (TD-DFT). Resonance Raman analysis shows that photo-aging causes notable decreases in backbone vibrational modes (P1–P4), suggesting loss of conjugation and increased molecular twisting.

XPS data confirm extensive S=O formation in PM6, whereas crosslinked PM6-Br exhibits significantly reduced sulfur oxidation. Simulated Raman spectra of oxidized and twisted species match the observed spectral shifts, connecting vibrational alterations to HOMO localization and electronic disorder. Quantitatively, after 100 h of irradiation, the P1 peak intensity decreases by 18% in PM6 but only 7% in crosslinked PM6-Br, indicating that crosslinking suppresses structural degradation.

Device-level analysis shows that trap density increases by 36.9% in PM6-based OSCs, compared with only 12.8% in crosslinked devices, correlating spectroscopic signatures with trap-assisted recombination and Voc loss. Under continuous 1-sun irradiation, crosslinked devices retain 81.1% of the initial PCE after 1000 h (T80 = 1590 h), significantly outperforming PM6 devices (T80 = 245 h). These findings establish resonance Raman spectroscopy as a powerful tool for diagnosing donor degradation pathways and demonstrate that suppressing backbone twisting and sulfur oxidation is essential to improving OSC operational stability [1].

References:

[1] Cho, H. W., Jee, M. H., Elbashier, E., Kim, J., Roe, J., Son, J., Lee, Y., Lee, D. C., Cho, S., Gordon, K. C., Woo, H. Y., & Kim, J. Y. (2026). Mitigating Photo-Aging of Donor Polymer via Crosslinking for Long-Term Organic Solar Cell Performance. Advanced Functional Materials, 36(10), e18984

Acknowledgements:

Elkhansa Elbashier 1,2*, Hye Won Cho3, Minhun Jee4, Jae Hyeong Kim3, Jin A Roe3, Jeong Min Son3, Yeon Jeong Lee3, Dong Chan Lee5, Shinuk Cho5, Keith C. Gordon1,2*, Han Young Woo3, Jin Young Kim3,6

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