Slow charge transfer from pentacene triplet states at the Marcus optimum

Abstract

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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Fig. 1: Illustrations of both the spatial and energetic dimensions of these experiments, which feature electrochemically tuned electron-acceptor molecules dispersed in a host matrix capable of efficient SF.
Fig. 2: Trends in charge yield as a function of driving force.
Fig. 3: Structures and relevant electrochemical potentials of the donor molecule (pentacene) and acceptor molecules (tetracenes, rhodanines (FBR and FTR) and PDIs) used in this study.
Fig. 4: TA spectra and kinetics for neat pentacene and pentacene/0.5 mol kg–1 PDI(F7Bu)-(CF3)3.
Fig. 5: The fluence dependence of charge carrier yield from TRMC and the excitation wavelength dependence of the TA spectra provide two independent means of testing whether charge transfer is rate-constant or diffusion limited.

Data availability

The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.

Code availability

The code generated during the current study is available from the corresponding author on reasonable request.

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Acknowledgements

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.

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N.A.P. fabricated samples; performed steady-state absorption, X-ray diffraction, TA and TRMC measurements; synthesized PDI(dodecyl)-(CN)3 and PDI(dodecyl)-(CN)4 samples; and contributed to experimental design. N.V.K. synthesized PDI(dodecyl)-(CN)3 and PDI(dodecyl)-(CN)4 samples. T.T.C., S.H.S. and O.V.B. synthesized PDI(Bu)-(CF3)2, PDI(Bu)-(CF3)3, PDI(F7Bu)-(CF3)3 and PDI(F7Bu)-(CF3)4 samples. S.H. and I.M. synthesized FTR, FBR and CN-FBR samples. D.B.G. and J.E.A. synthesized TIPS Tc and TIPS Tc COOH samples. G.M.C. performed cyclic voltammetry measurements. S.U.N. performed atomic force microscopy measurements. J.C.J., G.R. and O.G.R. contributed to overall experimental design and supervised the project.

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Correspondence to Justin C. Johnson or Garry Rumbles or Obadiah G. Reid.

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Supplementary information

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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