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Charge separation in localized and delocalized electronic states in polymeric semiconductors

Nature volume 392, pages 903–906 (1998)Cite this article

Abstract

Conjugated polymers such as poly(p-phenylene vinylene)s (PPVs) allow low-cost fabrication of thin semiconducting films by solution processing onto substrates. Several polymeric optoelectronic devices have been developed in recent years, including field-effect transistors1, light-emitting diodes2, photocells3,4 and lasers5. It is still not clear, however, whether the description of electronic excitations in these materials is most appropriately formulated within a molecular or semiconductor (band-theory) picture. In the former case, excited states are localized and are described as excitons; in the latter they are delocalized and described as free electron–hole pairs. Here we report studies of the electronic states associated with optical excitations in the visible and ultraviolet range for the conjugated polymer poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV), by means of photocurrent measurements and quantum-chemical calculations. We find several photocurrent spectral features between 3 and 5 eV which are coupled with bands in the absorption spectrum. On modelling the excited states in this energy range, we have discovered an important feature that is likely to be general for materials composed of coupled molecular units: that mixing of delocalized conduction- and valence-band states with states localized on the molecular units produces a sequence of excited states in which positive and negative charges can be separated further at higher energies. In other words, these excited states facilitate charge separation, and provide a conceptual bridge between the molecular (localized) and semiconductor (delocalized) pictures.

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Figure 1
Figure 2: Short-circuit photocurrent spectra (solid line) and absorption spectra (dotted line) of different films.
Figure 3: INDO/SCI calculations for the 11-ring PPV model oligomer; the site labelling is shown at the top.
Figure 4: Scheme of molecular orbitals in poly(p-phenylene vinylene) with nodes at para-position and with nodes orthogona.

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Acknowledgements

We thank M. Harrison, S. Graham, P. Hamer and S. C. Moratti for discussions, and J.J. M. Halls for assistance with the UV photovoltaics equipment and for discussions. One of us (A.K.) thanks Wolfson College and Peterhouse in Cambridge for financial support. The Cambridge–Mons collaboration is supported by the European Commission (TMR Network ‘SELOA’ and ESPRIT project ‘LEDFOS’). Work in Mons is partly supported by the Belgian Prime Minister Services for Scientific, Technical, and Cultural Affairs (IAP in Supramolecular Chemistry and Catalysis), FNRS-FRFC, and an IBM Academic Joint Study. D.B. is Chargé de Recherches FNRS.

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Authors and Affiliations

  1. Cavendish Laboratory, University of Cambridge, Madingley Road, CB3 0HE, Cambridge, UK

    A. Köhler & R. H. Friend

  2. Service de Chimie des Matériaux Nouveaux, Centre de Recherche en Electronique et Photonique Moléculaires, Université de Mons-Hainaut, Place du Parc, 20, B-7000, Mons, Belgium

    D. A. dos Santos, D. Beljonne, Z. Shuai & J.-L. Brédas

  3. University Chemical Laboratory, Lensfield Road, CB2 1EW, Cambridge, UK

    A. B. Holmes

  4. Max-Planck-Institut für Polymerforschung, Ackermannweg 10, 55128, Mainz, Germany

    A. Kraus & K. Müllen

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  1. A. Köhler
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Correspondence to A. Kraus.

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Köhler, A., dos Santos, D., Beljonne, D. et al. Charge separation in localized and delocalized electronic states in polymeric semiconductors. Nature 392, 903–906 (1998). https://doi.org/10.1038/31901

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