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A molecular interaction–diffusion framework for predicting organic solar cell stability

Abstract

Rapid increase in the power conversion efficiency of organic solar cells (OSCs) has been achieved with the development of non-fullerene small-molecule acceptors (NF-SMAs). Although the morphological stability of these NF-SMA devices critically affects their intrinsic lifetime, their fundamental intermolecular interactions and how they govern property–function relations and morphological stability of OSCs remain elusive. Here, we discover that the diffusion of an NF-SMA into the donor polymer exhibits Arrhenius behaviour and that the activation energy Ea scales linearly with the enthalpic interaction parameters χH between the polymer and the NF-SMA. Consequently, the thermodynamically most unstable, hypo-miscible systems (high χ) are the most kinetically stabilized. We relate the differences in Ea to measured and selectively simulated molecular self-interaction properties of the constituent materials and develop quantitative property–function relations that link thermal and mechanical characteristics of the NF-SMA and polymer to predict relative diffusion properties and thus morphological stability.

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Fig. 1: Phase diagram, chemical structure, schematic of the device structure and degradation of polymer:NF-SMA devices.
Fig. 2: SIMS profiles, phase diagram, diffusion properties and activation energy of polymer:NF-SMA.
Fig. 3: Thermal and mechanical properties of neat polymers and neat NF-SMAs.
Fig. 4: Interrelations of parameters, with the χ and D dependence on Tg or Tc and EF.

Data availability

The data represented in Fig. 3a,b are provided with the paper as source data. Other datasets generated and/or analysed during the current study are available from the corresponding authors upon request.

Code availability

The code used for the simulation of the Flory–Huggins phase diagram is available from the corresponding author upon request.

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Acknowledgements

Y.Q., Z.P., H.H., H.A. and initial work by M.G. was supported by Office of Naval Research (ONR) grant no. N000141712204 and KAUST’s Center Partnership Fund (no. 3321). N.B. and B.T.O. acknowledge support by a National Science Foundation (NSF) grant (no. CMMI-1554322). T.K., A.A. and recent work by M.G. was supported by NCSU start-up funds to A.A., J.R. and W.Y. acknowledge support by an NSF grant (no. CBET-1639429). C.R. and W.M. acknowledge the support of the ONR (N00014-18-1-2448) and the NSF under Cooperative Agreement no. 1849213; supercomputing resources were provided by the Department of Defense (DoD) through the DoD High-Performance Computing Modernization Program (project no. ONRDC40433481) and by the University of Kentucky Information Technology Department and Center for Computational Sciences. SIMS measurements were performed at the Analytical Instrumentation Facility at NCSU, which is partially supported by the State of North Carolina and the National Science Foundation. C. Zhou is acknowledged for providing support for SIMS measurements. The DSC instrument was purchased with UNC-GA ROI funds. C. Zhu, A. Hexemer and C. Wang of the ALS provided instrument maintenance. E. Gomez and J. Litofsky are acknowledged for providing the initial Flory–Huggins program code. L. Ye and M. Balik (NCSU) are acknowledged for fruitful discussion and input. A. Dinku is acknowledged for maintaining shared ORaCEL facilities and sharing some PBDB-T2F:Y6 stability data for reference. F. He and T. Zhao are acknowledged for help with attaining molecular weight data via high temperature gel permeation chromatography. H. Yan is acknowledged for providing ITIC-4Cl NF-SMA. I. Angunawela is acknowledged for performing complementary shelflife measurements of P3HT:EH-IDTBR devices.

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Contributions

H.A. and B.T.O. conceived the scientific framework with the help of M.G. M.G. designed experimental protocols, coordinated the experimental work, performed the SIMS, DSC measurements and analysed the SIMS and DSC data with the help of Z.P. Z.P. performed the complementary SIMS measurements of P3HT:NF-SMA. M.G. fabricated solar cell devices and performed subsequent stability tests with the help of H.H., T.K. and Y.Q., and with supervision by A.A. B.T.O. and N.B. designed the mechanical test experiments. N.B. prepared the films needed for mechanical test measurements and performed DMA and elastic modulus measurements. H.H. fabricated the complementary FTAZ:IT-M devices. J.J.R. synthesized the FTAZ polymers, supervised by W.Y. M.B. synthesized P3HT, supervised by I.M. W.M. performed molecular dynamics simulations, supervised by C.R. M.G., H.A. and B.T.O. drafted the paper. All authors contributed to the editing and interpretation.

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Correspondence to Brendan T. O’Connor or Harald Ade.

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Peer review information Nature Materials thanks Mats Andersson, Andrew T. Kleinschmidt, Darren Lipomi and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. 1–13, equations (1)–(15), Tables 1–9 and Discussion.

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Source data

Source Data Fig. 3

DSC thermograms of neat polymers and NF-SMAs and time-temperature superposition of neat polymers.

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Ghasemi, M., Balar, N., Peng, Z. et al. A molecular interaction–diffusion framework for predicting organic solar cell stability. Nat. Mater. 20, 525–532 (2021). https://doi.org/10.1038/s41563-020-00872-6

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