Volume 79, Number 4, August 2007
Article Number 47011
Number of page(s) 6
Section Condensed Matter: Electronic Structure, Electrical, Magnetic and Optical Properties
Published online 25 July 2007
EPL, 79 (2007) 47011
DOI: 10.1209/0295-5075/79/47011

Evidence of fractal structure for charge transport in carbon-nanotube/conjugated-polymer composites

M. Koehler1, C. D. Canestraro1, M. C. Schnitzler2, M. M. Oliveira1, A. J. G. Zarbin2, L. S. Roman1 and M. G. E. da Luz1

1  Departamento de Física, Universidade Federal do Paraná - C.P. 19044, 81531-990, Curitiba-PR, Brazil
2  Departamento de Química, Universidade Federal do Paraná - 81531-990 Curitiba-PR, Brazil

received 23 April 2007; accepted in final form 2 July 2007; published August 2007
published online 25 July 2007

We measure the current-voltage (J-V) characteristics of organic composites as a function of carbon nanotubes concentration dispersed in a poly-3-hexilthiophene (P3HT) matrix. From a drift-diffusion-space-charge model and adapting the general effective medium and classical percolation theories, we quantify the system transport features. We find a drastic increase of the injection current due to drain channels provided by the nanotubes, probably a universal mechanism for charge injection in such type of system. We identify a percolation transition (with t = 0.3, the lowest critical exponent so far reported in the literature) and a fractal structure for transport after the percolation. This nearly 1D structure is surprising since the composites do not have any peculiar orientation along some preferential direction. Supported by transmission electron microscopy we explain the fractal behavior in terms of the morphology of the conductivity paths.

72.60.+g - Mixed conductivity and conductivity transitions.
72.80.Le - Polymers; organic compounds (including organic semiconductors).
73.61.Ph - Polymers; organic compounds.

© Europhysics Letters Association 2007