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Observation of long-range exciton diffusion in highly ordered organic semiconductors


Excitons in polycrystalline and disordered films of organic semiconductors have been shown to diffuse over distances of 10–50 nm. Here, using polarization- and wavelength-dependent photoconductivity in the highly ordered organic semiconductor rubrene, we show that the diffusion of triplet excitons in this material occurs over macroscopic distances (2–8 μm), comparable to the light absorption length. Dissociation of these excitons at the surface of the crystal is found to be the main source of photoconductivity in rubrene. In addition, we observe strong photoluminescence quenching and a simultaneous enhancement of photoconductivity when the crystal surface is functionalized with exciton splitters. In combination with time-resolved measurements, these observations strongly suggest that long-lived triplet excitons are indeed generated in molecular crystals by fission of singlets, and these triplets provide a significant contribution to the surface photocurrent generated in organic materials. Our findings indicate that the exciton diffusion bottleneck is not an intrinsic limitation of organic semiconductors.

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Figure 1: Polarization-resolved absorption coefficient and photoconductivity measured at the (ab) facet of rubrene.
Figure 2: Effect of surface functionalization on the photoconductivity of rubrene.
Figure 3: Modelling of the exciton diffusion length based on the experimental data and equations (1) and (2).
Figure 4: Photocurrent excitation spectra of rubrene.
Figure 5: PL quenching in rubrene single crystals.


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This work has been financially supported by NSF-DMR-0843985 and the Industrial Technology Research Grant Program 09E51007d (NEDO). We are very grateful to A. Zakhidov, V. M. Agranovich and E. Garfunkel for helpful discussions, and especially grateful to I. Biaggio for fruitful discussions and for access to the time-resolved PL set-up.

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H.N. and V.P. designed and carried out experiment and performed data analysis; B.L. and Q.Z. fabricated samples; L.C.F. provided important input on data analysis and presentation, and V.P. wrote the paper.

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Correspondence to V. Podzorov.

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Najafov, H., Lee, B., Zhou, Q. et al. Observation of long-range exciton diffusion in highly ordered organic semiconductors. Nature Mater 9, 938–943 (2010).

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