Quantitative relations between interaction parameter, miscibility and function in organic solar cells

Abstract

Although it is known that molecular interactions govern morphology formation and purity of mixed domains of conjugated polymer donors and small-molecule acceptors, and thus largely control the achievable performance of organic solar cells, quantifying interaction–function relations has remained elusive. Here, we first determine the temperature-dependent effective amorphous–amorphous interaction parameter, χaa(T), by mapping out the phase diagram of a model amorphous polymer:fullerene material system. We then establish a quantitative ‘constant-kink-saturation’ relation between χaa and the fill factor in organic solar cells that is verified in detail in a model system and delineated across numerous high- and low-performing materials systems, including fullerene and non-fullerene acceptors. Our experimental and computational data reveal that a high fill factor is obtained only when χaa is large enough to lead to strong phase separation. Our work outlines a basis for using various miscibility tests and future simulation methods that will significantly reduce or eliminate trial-and-error approaches to material synthesis and device fabrication of functional semiconducting blends and organic blends in general.

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Fig. 1: Schematic morphologies and phase diagram of an amorphous model system.
Fig. 2: Quantitative relation of χaa(T) to device performance and morphology in a model system.
Fig. 3: MD simulations of PCDTBT:PCBM.
Fig. 4: Relation of χaa(T) to morphology and device FF in many systems.

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Acknowledgements

Work by NCSU was initiated with support from the US Department of Energy, Office of Science, Basic Energy Science, Division of Materials Science and Engineering under contract DE-FG02-98ER45737, and completed with support from ONR grants N00141512322 and N000141712204. X-ray data were acquired at beamlines 11.0.1.2, 7.3.3 and 5.3.2.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract DE-AC02-05CH11231. The DSC instrument, and DSC and SIMS analysis by NCSU, were supported by a UNC General Administration Research Opportunity Initiative grant. SIMS was performed at the Analytical Instrumentation Facility (AIF) at NCSU, which is partially supported by the State of North Carolina and the National Science Foundation (award number ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI). The work was partially supported by the National Basic Research Program of China (973 Program; 2013CB834705), HK JEBN Limited (Hong Kong), the Hong Kong Research Grants Council (T23-407/13-N, N_HKUST623/13 and 606012), HKUST President’s Office through the SSTSP scheme (project reference number: EP201) and the National Natural Science Foundation of China (NSFC, 21374090, 21504066, 21534003 and 51320105014). The work at KAUST was supported by generous KAUST internal funding. The work at Georgia Tech was funded by ONR grant N00014-17-1-2208. A.L.D. Kilcoyne, E. Schaible, C. Zhu, A. Hexemer, C. Wang and A. Young of the ALS (DOE) assisted with the measurements and provided instrument maintenance. S. Mukherjee and O. Awartani are acknowledged for assisting with part of the X-ray data acquisition and normalization, and C. McNeill is acknowledged for initial work on the STXM miscibility measurement of PCDTBT. The KAUST IT Research Computing Team and the KAUST Supercomputing Laboratory are acknowledged for providing computational resources. The authors acknowledge and appreciate the fruitful discussions with N. Stingelin, J. Michels, E. Gomez, and M. Balik, and thank A. Dinku for maintaining the shared device fabrication facilities at NCSU, and E. Gomez for sharing his FH code.

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H.A. conceived the scientific framework and designed the experiments with the help of L.Y., M.G., H.H., H.Y. and J.-H.K. H.Y. initiated the study on various semi-crystalline polymer:SMA systems. H.H. made those polymer materials that were not purchased and carried out initial DSC measurements. M.G. performed DSC measurement of the PCDTBT:PCBM model system, additional DSC measurements of some semi-crystalline systems, and the SIMS measurements. L.Y. proposed the model PCDTBT system and carried out χaa–ISI–FF modelling, some DSC tests, device and R-SoXS experiments on the model system, and most of the final R-SoXS analysis. L.Y. and M.G. carried out the χaa(T) analysis. K.J. made and measured the polymer:SMA devices. J.C., J.-H.K. and T.M. acquired and analysed the initial R-SoXS data and measured materials density. B.A.C. and L.Y. acquired and analysed the STXM miscibility data. J.Y.L.L., J.Z., T.M. and Z.L. synthesized the small-molecule acceptors or building blocks. J.-L.B., T.W., X.C. and H.L. performed the MD simulations. L.Y. and H.A. wrote the manuscript. All authors contributed to editing the manuscript. H.A., J.-L.B. and H.Y. supervised the projects.

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Correspondence to He Yan or Harald Ade.

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Ye, L., Hu, H., Ghasemi, M. et al. Quantitative relations between interaction parameter, miscibility and function in organic solar cells. Nature Mater 17, 253–260 (2018). https://doi.org/10.1038/s41563-017-0005-1

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