Splitting of the recombination zone in organic light emitting diodes by dye doping

In organic light emitting devices, doping of the electroactive organic layer with highly luminescent molecules has been shown to considerably increase device performance and lifetime. In most cases, the doping molecule does not act as a charge donor or an acceptor as in classical semiconductors, but is used to enhance light emission and to tune the emission color. By using the laser dye derivative 4-(dicyanomethylene)-2-methyl-6-{2-[(4-diphenylamino)phenyl]ethyl}-4H-pyran as dopant in a standard organic light emitting device, we have achieved highly efficient red to yellow emission depending on doping concentration. Furthermore the emission color changes with increasing current density. Using device model simulations, we have found that this color change is caused by the splitting of the recombination zone into two zones due to a decrease of the electron mobility inside the doped area.