Abstract
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.