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Physical insights into non-fullerene organic photovoltaics

Nature Reviews Physics volume 6, pages 365–381 (2024)Cite this article

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Abstract

Boosted by the fast development of non-fullerene acceptors, organic photovoltaics (OPVs) have achieved breakthrough power conversion efficiencies — in excess of 20% and approaching those of state-of-the-art crystalline silicon photovoltaics. New physical properties, unusual phenomena and critical mechanisms have been uncovered in non-fullerene acceptors and related devices, all contributing to deliver advances in OPV technologies. In this Review, we summarize the photophysics and device physics of non-fullerene-acceptor-based OPVs, with emphasis on the comparison between fullerene and non-fullerene acceptors of the physical processes that affect device performance. We discuss the processes of exciton generation, diffusion, transport and separation and charge recombination in OPVs and present recent interpretations of the physics of non-fullerene-acceptor-based OPVs, looking at how driving energy affects exciton separation and how charge recombination affects voltage loss. Compiling these mechanisms — especially those that can overcome the intrinsic limitations imposed by the energy-gap law — we provide a strategy for minimizing voltage loss and discuss future research directions and challenges in the fundamentals and performance of OPVs, including new modes of operation for non-fullerene-acceptor-based OPVs.

Key points

  • Non-fullerene acceptor materials show strong visible and near-infrared absorption and thus can generate abundant excitons and photocurrent in organic photovoltaics.

  • Exciton diffusion coefficients for non-fullerene acceptors are much larger than those for fullerene acceptors. The spectral overlap between donor and non-fullerene acceptor enables long-range energy transfer from donor to acceptor.

  • Exciton separation in non-fullerene-acceptor-based devices mainly follows hole-transfer pathways and is typically much slower than in fullerene-based devices owing to morphological factors. Spontaneous photogeneration of charges and intra-moiety excimer states are also observed in non-fullerene acceptors.

  • Non-fullerene acceptors show lower energetic disorder, suppressed sub-bandgap states and reduced trap-assisted recombination and voltage loss, compared with fullerene acceptors.

  • When energy offsets are small, the weakly emissive charge-transfer states can hybridize with highly emissive local excited states of non-fullerene acceptors, leading to increased radiative efficiency for charge-transfer states and thus reduced non-radiative voltage loss.

  • The non-radiative voltage loss of non-fullerene-acceptor-based devices is largely governed by the radiative efficiency of local excited states, which is limited by the energy-gap law for organic molecules.

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Fig. 1: Molecular structures of donors and acceptors.
Fig. 2: Exciton generation and diffusion properties of non-fullerene acceptors.
Fig. 3: Exciton separation in non-fullerene-acceptor-based devices.
Fig. 4: Three-state vibronic model, non-radiative voltage loss and photoluminescence quantum yields of acceptors.
Fig. 5: The features of non-fullerene acceptors.

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Acknowledgements

X.Z. thanks the National Science Foundation of China (No. U21A20101). H.W. thanks the National Nature Science Foundation of China (No. 52273177).

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Authors and Affiliations

  1. Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, School of Materials Science and Engineering, Peking University, Beijing, China

    Jiayu Wang & Xiaowei Zhan

  2. Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, China

    Yuan Xie & Hongbin Wu

  3. MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, New Zealand

    Kai Chen & Justin M. Hodgkiss

  4. Robinson Research Institute, Victoria University of Wellington, Wellington, New Zealand

    Kai Chen

  5. The Dodd-Walls Centre for Photonic and Quantum Technologies, University of Otago, Dunedin, New Zealand

    Kai Chen

  6. School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand

    Justin M. Hodgkiss

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J.W., Y.X. and K.C. contributed equally to this work. All authors researched data and contributed to the writing of the article.

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Wang, J., Xie, Y., Chen, K. et al. Physical insights into non-fullerene organic photovoltaics. Nat Rev Phys 6, 365–381 (2024). https://doi.org/10.1038/s42254-024-00719-y

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