Charge carrier transport in organic semiconductors

To understand charge carrier transport in organic semiconductors the magnitude and anisotropy, as well as the temperature and eventual electric field dependence of the electron and of the hole mobility are fundamental parameters. A number of technical applications require high mobilities. A brief review is given on different experimental methods that can either directly measure charge carrier mobilities, or at least lead to an estimate. For high purity single crystals, a steep increase of mobilities towards low temperature with the consequence of nonlinear transport and final velocity saturation at elevated electric fields has been found and traced back to temperature-dependent electron and hole masses approaching the free electron mass at low temperature. This, and additional recent reports in literature on ultrahigh mobilities—with a number of exciting consequences, such as integer and fractional quantum Hall effect and even superconductivity in such materials as anthracene, tetracene, pentacene, and C60—are clear indications of band transport. With rising temperature electron–phonon coupling, and therefore the effective masses, increase and coherent band transport is gradually destroyed; polaron-hopping transport evolves as a parallel channel and dominates at sufficiently high temperature. For crystals with orientational disorder of the molecules band transport is precluded.