Endothermic Charge Separation Occurs Spontaneously in Non-Fullerene Acceptor/Polymer Bulk Heterojunction

Kushal Rijal; Neno Fuller; Fatimah Rudayni; Nan Zhang; Xiaobing Zuo; Cindy L. Berrie; Hin-Lap Yip; Wai-Lun Chan

doi:10.1002/adma.202400578

Endothermic Charge Separation Occurs Spontaneously in Non-Fullerene Acceptor/Polymer Bulk Heterojunction

Kushal Rijal^*, Neno Fuller, Fatimah Rudayni, Nan Zhang, Xiaobing Zuo, Cindy L. Berrie, Hin-Lap Yip, Wai-Lun Chan^*

^*Corresponding author for this work

Research output: Journal Publications and Reviews › RGC 21 - Publication in refereed journal › peer-review

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Abstract

Organic photovoltaics (OPVs) based on non-fullerene acceptors (NFAs) have achieved a power conversion efficiency close to 20%. These NFA OPVs can generate free carriers efficiently despite a very small energy level offset at the donor/acceptor interface. Why these NFAs can enable efficient charge separation (CS) with low energy losses remains an open question. Here, the CS process in the PM6:Y6 bulk heterojunction is probed by time-resolved two-photon photoemission spectroscopy. It is found that the CS, the conversion from bound charge transfer (CT) excitons to free carriers, is an endothermic process with an enthalpy barrier of 0.15 eV. The CS can occur spontaneously despite being an endothermic process, which implies that it is driven by entropy. It is further argued that the morphology of the PM6:Y6 film and the anisotropic electron delocalization restrict the electron and hole wavefunctions within the CT exciton such that they can primarily contact each other through point-like junctions. This configuration can maximize the entropic driving force. © 2024 Wiley-VCH GmbH.

Original language	English
Article number	2400578
Journal	Advanced Materials
Volume	36
Issue number	31
Online published	31 May 2024
DOIs	https://doi.org/10.1002/adma.202400578
Publication status	Published - 1 Aug 2024

Funding

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under award number DE-SC0024525. W.-L.C. acknowledges the support from the University of Kansas General Research Fund allocation #2151080. C.L.B. was supported by the National Science Foundation under award CHE-2108448 as well as the University of Kansas General Research Fund #2112080. F.R. acknowledges the scholarship support from Jazan University. This research used the beamline 12-ID-B of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility at Argonne National Laboratory, and is based on research supported by the U.S. DOE Office of Science-Basic Energy Sciences, under contract no. DE-AC02-06CH11357. H.-L.Y. acknowledges the support from the University Grants Committee of Hong Kong (GRF 11307323).

Research Keywords

charge separation
excitons
organic semiconductors
photovoltaics
ultrafast spectroscopy

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abstract = "Organic photovoltaics (OPVs) based on non-fullerene acceptors (NFAs) have achieved a power conversion efficiency close to 20%. These NFA OPVs can generate free carriers efficiently despite a very small energy level offset at the donor/acceptor interface. Why these NFAs can enable efficient charge separation (CS) with low energy losses remains an open question. Here, the CS process in the PM6:Y6 bulk heterojunction is probed by time-resolved two-photon photoemission spectroscopy. It is found that the CS, the conversion from bound charge transfer (CT) excitons to free carriers, is an endothermic process with an enthalpy barrier of 0.15 eV. The CS can occur spontaneously despite being an endothermic process, which implies that it is driven by entropy. It is further argued that the morphology of the PM6:Y6 film and the anisotropic electron delocalization restrict the electron and hole wavefunctions within the CT exciton such that they can primarily contact each other through point-like junctions. This configuration can maximize the entropic driving force. {\textcopyright} 2024 Wiley-VCH GmbH.",

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AU - Rijal, Kushal

AU - Fuller, Neno

AU - Rudayni, Fatimah

AU - Zhang, Nan

AU - Zuo, Xiaobing

AU - Berrie, Cindy L.

AU - Yip, Hin-Lap

AU - Chan, Wai-Lun

PY - 2024/8/1

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N2 - Organic photovoltaics (OPVs) based on non-fullerene acceptors (NFAs) have achieved a power conversion efficiency close to 20%. These NFA OPVs can generate free carriers efficiently despite a very small energy level offset at the donor/acceptor interface. Why these NFAs can enable efficient charge separation (CS) with low energy losses remains an open question. Here, the CS process in the PM6:Y6 bulk heterojunction is probed by time-resolved two-photon photoemission spectroscopy. It is found that the CS, the conversion from bound charge transfer (CT) excitons to free carriers, is an endothermic process with an enthalpy barrier of 0.15 eV. The CS can occur spontaneously despite being an endothermic process, which implies that it is driven by entropy. It is further argued that the morphology of the PM6:Y6 film and the anisotropic electron delocalization restrict the electron and hole wavefunctions within the CT exciton such that they can primarily contact each other through point-like junctions. This configuration can maximize the entropic driving force. © 2024 Wiley-VCH GmbH.

AB - Organic photovoltaics (OPVs) based on non-fullerene acceptors (NFAs) have achieved a power conversion efficiency close to 20%. These NFA OPVs can generate free carriers efficiently despite a very small energy level offset at the donor/acceptor interface. Why these NFAs can enable efficient charge separation (CS) with low energy losses remains an open question. Here, the CS process in the PM6:Y6 bulk heterojunction is probed by time-resolved two-photon photoemission spectroscopy. It is found that the CS, the conversion from bound charge transfer (CT) excitons to free carriers, is an endothermic process with an enthalpy barrier of 0.15 eV. The CS can occur spontaneously despite being an endothermic process, which implies that it is driven by entropy. It is further argued that the morphology of the PM6:Y6 film and the anisotropic electron delocalization restrict the electron and hole wavefunctions within the CT exciton such that they can primarily contact each other through point-like junctions. This configuration can maximize the entropic driving force. © 2024 Wiley-VCH GmbH.

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KW - photovoltaics

KW - ultrafast spectroscopy

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ER -

Endothermic Charge Separation Occurs Spontaneously in Non-Fullerene Acceptor/Polymer Bulk Heterojunction

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GRF: Simulation-guided Design and Fabrication of High-performance Perovskite/Organic Tandem Solar Cells

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Endothermic Charge Separation Occurs Spontaneously in Non-Fullerene Acceptor/Polymer Bulk Heterojunction

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Research Keywords

UN SDGs

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GRF: Simulation-guided Design and Fabrication of High-performance Perovskite/Organic Tandem Solar Cells

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