Charge transport and recombination are important processes with regard to the understanding and optimization of electronic devices made from organic semiconductors. It has been demonstrated that the hole transport is governed by hopping between localized states, characterized by a mobility that depends on density, electric field and temperature. The transport of electrons is strongly reduced by traps that are Gaussianly distributed in energy in the band gap. Remarkably, the electron trap distribution is identical for a large number of organic semiconductors. This universal defect also quenches the excitons in conjugated polymers.
The recombination processes in polymer light-emitting diodes (PLEDs) are investigated. Photogenerated current measurements on PLED device structures reveal that next to the known bimolecular Langevin recombination also trap-assisted recombination is an important recombination channel in PLEDs. Numerical modeling of the current-voltage characteristics incorporating both Langevin and trap-assisted recombination yields a correct and consistent description of the PLED. However, electronic properties of a conjugated polymer can be changed, or even new ones created, by blending the polymer with other functional materials. We have found that by blending poly(p-phenylene vinylene) (PPV) derivatives with wide band gap polymers the electron traps are deactivated. PLEDs made from such a blend exhibit a balanced transport and enhanced efficiency due to the strong reduction of non-radiative trap-assisted recombination.