Singlet fission promises to surpass the Shockley–Queisser limit for single-junction solar cell efficiency through the production of two electron–hole pairs per incident photon. However, this promise has not been fulfilled because singlet fission produces two low-energy triplet excitons that have been unexpectedly difficult to dissociate into free charges. To understand this phenomenon, we study charge separation from triplet excitons in polycrystalline pentacene using an electrochemical series of 12 different guest electron-acceptor molecules with varied reduction potentials. We observe separate optima in the charge yield as a function of driving force for singlet and triplet excitons, including inverted regimes for the dissociation of both states. Molecular acceptors can thus provide a strategic advantage to singlet fission solar cells by suppressing singlet dissociation at optimal driving forces for triplet dissociation. However, even at the optimal driving force, the rate constant for charge transfer from the triplet state is surprisingly small, ~107 s−1, presenting a previously unidentified obstacle to the design of efficient singlet fission solar cells.
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The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.
The code generated during the current study is available from the corresponding author on reasonable request.
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This work was produced in part by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, for the US Department of Energy under Contract No. DE-AC36–08GO28308. Funding provided by the Solar Photochemistry Program of the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences. The views expressed in the article do not necessarily represent the views of the US Department of Energy or the US Government. T.T.C., S.H.S. and O.V.B. thank the National Science Foundation (grants CHE-1012468 and CHE-1362302) for support of the synthesis of PDI acceptors. S.H. and I.M. thank BASF, EPSRC (EP/G037515/1), EPSRC (EP/L016702/1), EC FP7 Project X10D (287818) and Nanomatcell (308997) for support of the synthesis of rhodanine acceptors. D.B.G. and J.E.A. thank the National Science Foundation (DMREF-1627428) for support of the synthesis of tetracene acceptors. We thank P. Parilla for advice regarding X-ray diffraction crystallite size measurements. We thank E. Pace for creating the table of contents graphic.
The authors declare no competing interests.
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Supplementary data, including further details and analysis of the kinetic Monte Carlo model, TRMC, TA, X-ray diffraction, steady-state absorption and electrochemical measurements; synthetic methods; Figs. 1–23 and Tables 1–3.
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Pace, N.A., Korovina, N.V., Clikeman, T.T. et al. Slow charge transfer from pentacene triplet states at the Marcus optimum. Nat. Chem. 12, 63–70 (2020). https://doi.org/10.1038/s41557-019-0367-x
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