Abstract
Heterojunctions between organic semiconductors are central to the operation of light-emitting and photovoltaic diodes, providing respectively for electron–hole capture and separation. However, relatively little is known about the character of electronic excitations stable at the heterojunction. We have developed molecular models to study such interfacial excited electronic excitations that form at the heterojunction between model polymer donor and polymer acceptor systems: poly(9,9-dioctylfluorene-co-bis-N,N-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine) (PFB) with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), and poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) with F8BT. We find that for stable ground-state geometries the excited state has a strong charge-transfer character. Furthermore, when partly covalent, modelled radiative lifetimes (∼10−7 s) and off-chain axis polarization (30∘) match observed ‘exciplex’ emission. Additionally for the PFB:F8BT blend, geometries with fully ionic character are also found, thus accounting for the low electroluminescence efficiency of this system.
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Acknowledgements
We thank J. S. Kim and W. J. D. Beenken for discussions. This work was supported by the Engineering and Physical Science Research Council, and by the EU Integrated Project NAIMO (No NMP4-CT-2004-500355). The work in Mons was partly supported by the Belgian Federal Government ‘Interuniversity Attraction Pole in Supramolecular Chemistry and Catalysis, PAI 5/3’, the Belgian National Fund for Scientific Research (FNRS/FRFC) and the European STREP project MODECOM (NMP-CT-2006-016434). D.B. is a research director of FNRS.
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Y.-S.H., I.A., P.S. and C.D. carried out the modelling. S.W. and J.M.H. measured the photoluminescence anisotropy. Y.-S.H., S.W., R.H.F. and D.B. analysed data, interpreted results and wrote the paper. All authors provided comments on the manuscript. R.H.F. contributed to the interpretation of the combined modelling and experimental work. D.B. and R.H.F. directed the research.
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Huang, Ys., Westenhoff, S., Avilov, I. et al. Electronic structures of interfacial states formed at polymeric semiconductor heterojunctions. Nature Mater 7, 483–489 (2008). https://doi.org/10.1038/nmat2182
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DOI: https://doi.org/10.1038/nmat2182
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