Carrier Transport In Organic Semiconductors Research Articles

Control of carrier transport in organic semiconductors by aluminum doping

Control of carrier transport in organic semiconductors by aluminum doping is realized in organic light-emitting devices (OLEDs) for which electroluminescence can sensitively reflect the status of carrier transport. It is found that an Al-doped layer with proper thickness (∼1–10nm) may block hole transport completely and enhance electron transport to some extent regardless of its location in the organic carrier transport layers. Improvement in the efficiency of OLEDs with an aluminum cathode is achieved upon the introduction of a very thin (∼3nm) Al-doped region near the light-emitting area. The current efficiency obtained with such Al-doped devices is about 30% higher than that with undoped devices.

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 C 60—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.

Charge carrier transport in diluted hopping systems

Weak-field equilibrium charge carrier transport in organic semiconductors is considered. It is shown that there is no interplay between the energy and distance in these systems as far as the jump to the nearest hopping neighbor is concerned, since the nearest neighbor is always determined by a distance rather than by difference of energies. As a result the dependences of the charge carrier mobility and diffusivity on concentration and temperature can be factorized into terms dependent on concentration and temperature only with the temperature dependence coinciding with that in systems with high concentration of hopping sites. It is also shown that the concept of the field-dependent effective temperature cannot be directly applied to the carrier hopping transport in diluted systems.