Exciton Delocalization Induced by Aggregation in Polymer Donor for Efficient Non-fullerene Organic Photovoltaics

Robert Westbrook^a, Kui Jiang^b^,^e^,^h, Francis Lin^b^,^e^,^h, Cheng Zhong^c, Jianxun Lu^d, Sei-Hum Jang^f, Jie Zhang^g, Yuqing Li^d, Zhanhua Wei^d, David Ginger^a, Jen Alex^a^,^b^,^e^,^f^,^h

^aDepartment of Chemistry, University of Washington, Seattle, WA, 98195-1700, USA

^bDepartment of Materials Science and Engineering, City University of Hong Kong, Kowloon 999077, Hong Kong

^cDepartment of Chemistry, Wuhan University, Wuhan, Hubei 430072, China

^dXiamen Key Laboratory of Optoelectronic Materials and Advanced Manufacturing, Institute of Luminescent Materials and Information Displays, College of Materials Science and Engineering, Huaqiao University, Xiamen, Fujian 361021, Chin

^eDepartment of Chemistry, City University of Hong Kong, Kowloon 999077, Hong Kong.

^fDepartment of Materials Science and Engineering, University of Washington, Seattle, Washington 98195-2120, United States

^gCenter for Photonics Information and Energy Materials, Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Shenzhen, Guangdong 518055, China

^hHong Kong Institute for Clean Energy (HKICE), City University of Hong Kong, Kowloon 999077, Hong Kong

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV23)

London, United Kingdom, 2023 June 12th - 14th

Organizers: Tracey Clarke, James Durrant and Trystan Watson

Oral, Robert Westbrook, presentation 122
DOI: https://doi.org/10.29363/nanoge.hopv.2023.122
Publication date: 30th March 2023

Intimate pi-stacking in organic semiconductors is known to form aggregates, which drive dissociation of photogenerated excitons through wavefunction delocalization. In particular, the onset of non-fullerene acceptors (NFAs) such as Y6 has seen a dramatic rise in discussion of exciton delocalization and its relation to (acceptor-donor) hole transfer. Here, we revisit the concept of donor exciton delocalization and show that such treatment is necessary to accurately describe the (donor-acceptor) electron transfer channel in high performance OPV blends. Specifically, we evaluate how pi-interactions in donor polymer aggregates contribute to delocalization strength, revealing that the formation of a luminescent, delocalized exciton in strongly p-interacting donor materials opens a pathway for free carrier generation. As a result, the electron transfer pathway partially bypasses the formation of performance-limiting singlet charge-transfer states during electron transfer in OPV blends. Moreover, we observe that such aggregation induced delocalization leads to a reduction of the triplet charge transfer state density. These mechanisms improve the internal quantum efficiency in OPVs to realize a power conversion efficiency of 19.2%. Ultimately, we provide insight into overcoming the fundamental limits of OPVs associated with intrinsic material properties. Designing materials with more pronounced delocalization character should maximize the exciton dissociation efficiency and minimize terminal back recombination, pushing OPVs closer to theoretical efficiency limits.

References:

[1] Rao, A. et al. The role of spin in the kinetic control of recombination in organic photovoltaics. Nature 500, 435-439 (2013)

[2] Gélinas, S. et al. Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes. Science 343, 512-516 (2014)

[3] Tamai, Y. et al. Ultrafast Long-Range Charge Separation in Nonfullerene Organic Solar Cells. ACS Nano 11, 12473-12481 (2017)

[4] Price, M. B. et al. Free charge photogeneration in a single component high photovoltaic efficiency organic semiconductor. Nat. Commun. 13, 2827 (2022)

[5] Zhang, G. et al. Delocalization of exciton and electron wavefunction in non-fullerene acceptor molecules enables efficient organic solar cells. Nat. Commun. 11, 3943 (2020)

[6] Wang, R., Zhang, C., Li, Q., Zhang, Z., Wang, X., Xiao, M. Charge Separation from an Intra-Moiety Intermediate State in the High-Performance PM6:Y6 Organic Photovoltaic Blend. J. Am. Chem. Soc. 142, 12751-12759 (2020)

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