Hybridization of local exciton and charge-transfer states reduces non-radiative voltage losses in organic solar cells

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Title: Hybridization of local exciton and charge-transfer states reduces non-radiative voltage losses in organic solar cells
Authors: Eisner, F
Azzouzi, M
Fei, Z
Hou, X
Anthopoulos, T
Dennis, TJ
Heeney, M
Nelson, J
Item Type: Journal Article
Abstract: A number of recent studies have shown that the nonradiative voltage losses in organic solar cells can be suppressed in systems with low energetic offsets between donor and acceptor molecular states, but the physical reasons underpinning this remain unclear. Here, we present a systematic study of 18 different donor/acceptor blends to determine the effect that energetic offset has on both radiative and nonradiative recombination of the charge-transfer (CT) state. We find that, for certain blends, low offsets result in hybridization between charge-transfer and lowest donor or acceptor exciton states, which leads to a strong suppression in the nonradiative voltage loss to values as low as 0.23 V associated with an increase in the luminescence of the CT state. Further, we extend a two-state CT-state recombination model to include the interaction between CT and first excited states, which allows us to explain the low nonradiative voltage losses as an increase in the effective CT to ground state oscillator strength due to the intensity borrowing mechanism. We show that low nonradiative voltage losses can be achieved in material combinations with a strong electronic coupling between CT and first excited states and where the lower band gap material has a high oscillator strength for transitions from the excited state to the ground state. Finally, from our model we propose that achieving very low nonradiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems.
Issue Date: 17-Apr-2019
Date of Acceptance: 18-Mar-2019
ISSN: 1520-5126
Publisher: American Chemical Society
Start Page: 6362
End Page: 6374
Journal / Book Title: Journal of the American Chemical Society
Volume: 141
Issue: 15
Copyright Statement: © 2019 American Chemical Society. This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of the American Chemical Society, after peer review and technical editing by the publisher. To access the final edited and published work see
Sponsor/Funder: Engineering and Physical Sciences Research Council
Keywords: 03 Chemical Sciences
General Chemistry
Publication Status: Published
Online Publication Date: 2019-03-18
Appears in Collections:Physics
Experimental Solid State
Faculty of Natural Sciences

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