Hybridization of Local Exciton and Charge-Transfer States Reduces Nonradiative Voltage Losses in Organic Solar Cells
AuthorsEisner, Flurin D.
Anthopoulos, Thomas D.
Dennis, T. John S.
KAUST DepartmentKAUST Solar Center (KSC)
Material Science and Engineering Program
Physical Science and Engineering (PSE) Division
Online Publication Date2019-03-18
Print Publication Date2019-04-17
Permanent link to this recordhttp://hdl.handle.net/10754/652458
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AbstractA 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.
CitationEisner FD, Azzouzi M, Fei Z, Hou X, Anthopoulos TD, et al. (2019) Hybridization of Local Exciton and Charge-Transfer States Reduces Nonradiative Voltage Losses in Organic Solar Cells. Journal of the American Chemical Society 141: 6362–6374. Available: http://dx.doi.org/10.1021/jacs.9b01465.
SponsorsF.D.E. and M.A. thank the Engineering and Physical Sciences Research Council (EPSRC) for support via doctoral studentships. J.N. is grateful for funding from EPSRC (Grant Nos. EP/P005543/1 and EP/M025020/1) and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 742708). M.H. and Z.F. thank the British Council (337323) for support. X.H. thanks the Chinese Scholarship council for support via a PhD studentship. F.D.E., M.A., and J.N. thank Artem Bakulin, Nathaniel Gallop, Shawn Zheng, and Thomas Kirchartz for helpful discussions.
PublisherAmerican Chemical Society (ACS)