Charge transport processes in organic light-emitting devices

The luminous efficiency of organic light-emitting diodes depends on the rec ombination probability of electrons injected at the cathode and holes at th e anode. We have developed a numerical model to calculate the recombination profile in single- and multilayer structures, taking into account the buil t-in electric field, the charge injection process at each electrode, hoppin g transport with field-dependent mobilities, charge diffusion, trapping and Langevin recombination. By comparison of the simulation results, as well a s approximate analytic solutions, with experimental data on MEH-PPV-based d evices, we find that injection is thermionic with Schottky barriers for som e electrode metals that are low enough to be considered Ohmic. Except at vo ltages close to threshold, diffusion and trapping effects are negligible. B oth electrons and holes are mobile, with a field dependence that is indepen dently confirmed both by single-carrier space-charge-limited current measur ements and transient time-of-flight techniques. (C) 2000 Elsevier Science S .A. All rights reserved.