Charge transport processes in organic light-emitting devices

The luminous efficiency of organic light-emitting diodes depends on the recombination probability of electrons injected at the cathode and holes at the anode. We have developed a numerical model to calculate the recombination profile in single- and multilayer structures, taking into account the built-in electric field, the charge injection process at each electrode, hopping transport with field-dependent mobilities, charge diffusion, trapping and Langevin recombination. By comparison of the simulation results, as well as approximate analytic solutions, with experimental data on MEH-PPV-based devices, we find that injection is thermionic with Schottky barriers for some electrode metals that are low enough to be considered Ohmic. Except at voltages close to threshold, diffusion and trapping effects are negligible. Both electrons and holes are mobile, with a field dependence that is independently confirmed both by single-carrier space-charge-limited current measurements and transient time-of-flight techniques.