Microscopic charge transport

The electrical conductivity of a material is a macroscopic solid-state property since even in high molecular-weight polymers there is not just one conjugated chain which spans the distance between two electrodes. Then it is not valid to describe the conductivity by the electronic structure of a single chain only, because intra- and interchain charge transport are important. As with crystalline materials, some basic features of the microscopic charge-transport mechanism can be inferred from conductivity measurements [83]. The specific conductivity a can be measured as the resistance R of a piece of material with length d and cross section F within a closed electrical circuit,... 

Flowever, the charge carrier motions in many organic semiconductors are between these two limits. It is thus expected that more sophisticated microscopic charge transport theories need to be developed to unify the concepts of the band-like and hopping transport. Indeed, many work along this line have been performed." It is known, however, that most of those rigorous quantum approaehes are limited to tens of sites because of the numerical convergence problem and computer memory limitations. 

The discrepancies in the reported conductance data of Au-alkanedithiol-Au junctions attracted our attention, and we decided to carry out an in-depth experimental study of the charge transport properties of Au-a,oo-alkanedithiol-Au molecule junctions in a non-conducting solvent. The combination with quantum chemistry ab initio simulations yielded a detailed view of this archetype of molecular junctions, and helped to resolve the puzzle on the role of microscopic geometries at the contacts and in the molecular conformation. 

Figure 7.8 Centre Current density-voltage characteristics of hybrid poly-3-hexylthiophene ZnO devices with different morphology. The device based on a layer of vertically oriented ZnO nanorods outperforms the device based on ZnO nanoparticles of similar diameter, while both nanostructured films outperform the bilayer. Left Scanning electron microscope image of ZnO nanoparticle film. Right SEM image (side view) of ZnO nanorod film. The superior performance of the ZnO nanorod-based film is attributed to the paths for charge transport, which are directed towards the electrodes (Ravirajan et al, 2006).

M. Surin, P. Leclere, R. Lazzaroni, J.D. Yuen, G. Wang, D. Moses, AJ. Heeger, S. Cho, and K. Lee, Relationship between the microscopic morphology and the charge transport properties in poly(3-hexylthiophene) field-effect transistors. J. Appl. Phys., 100, 033712 (2006). 

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