Abstract
Electroluminescence in organic light-emitting diodes arises from a charge-transfer reaction between the injected positive and negative charges by which they combine to form singlet excitons that subsequently decay radiatively. The quantum yield of this process (the number of photons generated per electron or hole injected) is often thought1 to have a statistical upper limit of 25 per cent. This is based on the assumption that the formation cross-section of singlet excitons, σS, is approximately the same as that of any one of the three equivalent non-radiative triplet exciton states, σT; that is, σS/σT ≈ 1. However, recent experimental2 and theoretical3 work suggests that σS/σT may be greater than 1. Here we report direct measurements of σS/σT for a large number of π-conjugated polymers and oligomers. We have found that there exists a strong systematic, but not monotonic, dependence of σS/σT on the optical gap of the organic materials. We present a detailed physical picture of the charge-transfer reaction for correlated π-electrons, and quantify this process using exact valence bond calculations. The calculated σS/σT reproduces the experimentally observed trend. The calculations also show that the strong dependence of σS/σT on the optical gap is a signature of the discrete excitonic energy spectrum, in which higher energy excitonic levels participate in the charge recombination process.
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References
Friend, R. H. et al. Electroluminescence in conjugated polymers. Nature 397, 121–128 ( 1999).
Cao, Y., Parker, I. D., Yu, G., Zhang, C. & Heeger, A. J. Improved quantum efficiency for electroluminescence in semiconducting polymers. Nature 397, 414–417 (1999).
Shuai, Z., Beljonne, D., Silbey, R. J. & Bredas, J. L. Singlet and triplet exciton formation rates in conjugated polymer light-emitting diodes. Phys. Rev. Lett. 84, 131– 134 (2000).
Campbell, D. K., Bishop, A. R. & Fesser, K. Polarons in quasi-one-dimensional systems. Phys. Rev. B 26, 6862–6874 (1982).
Soos, Z. G. & Ramasesha, S. Valence bond theory of linear Hubbard and Pariser-Parr-Pople models. Phys. Rev. B 29, 5410–5422 (1984).
Lavrentiev, M. Yu., Barford, W., Martin, S. J., Daly, H. & Bursill, R. J. Theoretical investigation of the low-lying electronic structure of poly(p-phenylene vinylene). Phys. Rev. B 59, 9987–9994 (1999).
Rohlfing, M. & Louie, S. G. Optical excitations in conjugated polymers. Phys. Rev. Lett. 82, 1959– 1962 (1999).
Monkman, A. P. et al. The triplet state of the ladder-type methyl-poly(p-phenylene) as seen by pulse radiolysis-energy transfer. Chem. Phys. Lett. 327, 111–116 ( 2000).
Kim, J., Ho, P. K. H., Greenham, N. C. & Friend, R. H. Electroluminescence emission pattern of organic light-emitting diodes: Implications for device efficiency calculations. J. Appl. Phys. 88, 1073–1081 (2000).
Vardeny, Z. V. & Wei, X. in Handbook of Conducting Polymers II (eds Skotheim, T. A., Elsenbaumer, R. L. & Reynolds, J. R.) 639–666 (Marcel Dekker, New York, 1997).
Graupner, W. et al. Direct observation of ultrafast field-induced charge generation in ladder-type poly(para-phenylene). Phys. Rev. Lett. 81, 3259–3262 (1998).
Wohlgenannt, M., Graupner, W., Leising, G. & Vardeny, Z. V. Photogeneration and recombination processes of neutral and charged excitations in films of a ladder-type poly(para-phenylene). Phys. Rev. B 60, 5321–5330 (1999).
Soos, Z. G., Ramasesha, S. & Galvão, D. S. Band to correlated crossover in alternating Hubbard and Pariser-Parr-Pople chains: Nature of the lowest singlet excitation of conjugated polymers. Phys. Rev. Lett. 71, 1609–1612 (1993).
Pariser, R. & Parr, R. G. A semi-empirical theory of the electronic spectra and electronic structure of complex unsaturated molecules II. J. Chem. Phys. 21, 767–776 (1953).
Pople, J. A. Electron interaction in unsaturated hydrocarbons. Trans. Faraday Soc. 49, 1375–1385 ( 1953).
Campbell, D. K., Gammel, J. T. & Loh, E. Y. Bond-charge Coulomb repulsion in Peierls-Hubbard models. Phys. Rev. B 38, 12043– 12046 (1988).
Gallagher, F. & Mazumdar, S. Excitons and optical absorption in one-dimensional extended Hubbard models with short- and long-range interactions. Phys. Rev. B 56, 15025– 15039 (1997).
Acknowledgements
We thank E.J.W. List for supplying the mLPPP polymer, and X.M. Jiang and E.J.W. List for their support in performing the experiments. The work at the University of Utah was supported in part by the DOE and the NSF. Work in Arizona was partially supported by the ONR through the MURI centre (CAMP) at the University of Arizona. Work in Bangalor was funded by CSIR, India.
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Wohlgenannt, M., Tandon, K., Mazumdar, S. et al. Formation cross-sections of singlet and triplet excitons in π-conjugated polymers. Nature 409, 494–497 (2001). https://doi.org/10.1038/35054025
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DOI: https://doi.org/10.1038/35054025
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