Organic Light-Emitting Diodes: Tailor-Made Polymers For Better Flat-Panel Displays

OLED devices consist of one or several semiconducting organic layer(s) sandwiched between two electrodes. When an electric field is applied, electrons are injected by the cathode, while at the anode they are taken from the semiconducting layer, yielding positively and negatively charged carriers, respectively. The charges migrate in opposite directions, the holes towards the cathode, the electrons towards the anode. When the two types of carriers encounter each other, the formed excitons may decay by emitting light. The emission color can be easily tuned by altering the chemical structure of the emitter.

The light exits the devices through a transparent electrode, most commonly through the anode made from indium tin oxide (ITO). Recently, "doped" (i.e., oxidized) conducting polymers (e.g. derivatives of polyaniline or polythiophene) have been used as alternative anodes for improved hole injection and extended operational life time of OLED devices.

However, the performance of OLED containing polymeric anodes varies quite strongly from device to device depending on the preparation conditions. Klaus Meerholz, the leader of the Munich team states: "We had speculated for quite some time already, that this might be a result of not properly controlling the redox state of the polymer. None of the previous studies had given or considered this aspect, though it has been long known that the doping level is correlated to the electrochemical equilibrium potential of the polymer, which in turn is often used to estimate the 'work function' of a material. Vice versa, these considerations suggested to us that it should be possible to actively influence the work function of such anodes by specifically adjusting the doping level of the polymer."

Indeed, the authors demonstrate that there is a relation between the degree of oxidation and the work function of polymeric anodes. They found, that positive carriers can be injected much more efficiently into organic semiconductors using highly doped anodes.

To take further advantage of the new principle, blue-emitting OLEDs based on a polyfluorene derivative synthesized at Max Planck Institute for Polymer Research in Mainz were investigated. By highly doping the polymeric anode not only the onset field for light emission was reduced, on top of that the efficiency of the devices at a given current density was increased. This was attributed to the more balanced number density of the two carrier types inside the luminescent layer (a ratio 1:1 would be ideal). The speed up the time consuming search for the optimum doping level of their polymer, the researchers used "combinatorial devices", a matrix of small devices which can be adjusted individually.