Research | Dept. Blom
The fact that conjugated polymers can be processed from solution raised an enormous research interest in both academia and industry. Application of a fast roll-to-roll newspaper-printing-style process was expected to lead to a paradigm shift in the production of displays and large-area devices as lighting tiles and solar cells. However, a fundamental disadvantage of semiconducting polymers is that their charge transport is unbalanced because electron transport is hindered by traps. We found that this electron trapping seems to be universal in organic semiconductors and is dominated by a trap located at ~3.6 eV below vacuum . This universal defect also quenches the excitons in conjugated polymers. As a result, for organic light-emitting diodes (OLEDs) the efficiency criteria for commercialization have only been met by the application of complex multi-layer device architectures based on small molecules, obtained by vacuum deposition. In such a multilayer structure processes as charge injection, charge transport and emission are individually optimized in at least five different organic layers, each with their own functionality. Simultaneous optimization of all these processes in just one single solution-processed layer has been deemed impossible and further industrial efforts in this direction were abandoned after 2005.
With regard to functional polymers a unique property is their ability to form controllable structures when blended with other polymers. By doing so, their functionality can be altered or even new properties can be created, depending on what extend the polymers will phase separate. An example of such new functionality has been demonstrated in non-volatile bistable ferroelectric diodes, based on an interpenetrating network of a polymeric ferroelectric and a polymeric semiconductor . Ferroelectricity and electrical conductivity are mutually exclusive in a single inorganic compound. However, in such a blend diode poling of the ferroelectric phase modifies the charge injection of the electrode into the semiconducting phase. Furthermore, the detrimental effects of trapping and non-radiative trap-assisted recombination in OLEDs can be eliminated by blending an organic semiconductor in an insulating host matrix, so-called trap dilution .