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Unification of trap-limited electron transport in semiconducting polymers

Abstract

Electron transport in semiconducting polymers is usually inferior to hole transport, which is ascribed to charge trapping on isolated defect sites situated within the energy bandgap. However, a general understanding of the origin of these omnipresent charge traps, as well as their energetic position, distribution and concentration, is lacking. Here we investigate electron transport in a wide range of semiconducting polymers by current–voltage measurements of single-carrier devices. We observe for this materials class that electron transport is limited by traps that exhibit a Gaussian energy distribution in the bandgap. Remarkably, the electron-trap distribution is identical for all polymers considered: the number of traps amounts to 3 × 1023 traps per m3 centred at an energy of ~3.6 eV below the vacuum level, with a typical distribution width of ~0.1 eV. This indicates that the electron traps have a common origin that, we suggest, is most likely related to hydrated oxygen complexes. A consequence of this finding is that the trap-limited electron current can be predicted for any polymer.

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Figure 1: Comparison of electron and hole current.
Figure 2: Electron transport in different polymers.
Figure 3: Schematic representation of the energies of the LUMO and the centre of the trap distribution.
Figure 4: The influence of water complexation on the electron affinity of PPV was studied for three oligomer conformations.
Figure 5

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Acknowledgements

The authors thank M. Lenes, Y. Zhang, M. Mandoc and M. Lu for their contributions to this work and J. Harkema for technical support. The work at the University of Groningen was supported by the European Commission under contract FP7-13708 (AEVIOM). The work at Georgia Tech was supported by the MRSEC Program of the National Science Foundation under Award Number DMR-0819885.

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P.W.M.B. and B.d.B. proposed and supervised the project. H.T.N., M.K. and G.A.H.W. carried out experiments. H.T.N. and M.K. analysed the electron transport data. J.L.B. supervised the quantum-chemical calculations. C.C. and C.R. carried out the quantum-chemical calculations and C.R. analysed the data. H.T.N., C.R. and J.L.B. wrote the manuscript.

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Correspondence to P. W. M. Blom.

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Nicolai, H., Kuik, M., Wetzelaer, G. et al. Unification of trap-limited electron transport in semiconducting polymers. Nature Mater 11, 882–887 (2012). https://doi.org/10.1038/nmat3384

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