Organic light-emitting devices based on new heterocyclic compounds

One approach to increase the overall performance of organic light emitting devices is to separate the light-emitting volume from the ones which are assigned to charge injection or transport. We realized such polymer hetero-layer structures by combination of hole transporting materials like polyparaphenylenevinylene (PPV) with new electron transporting materials, i.e. heterocyclic polymers and heterocyclic low molecular compounds, especially phenyl quinoxalines (PQs). The electronic properties of these heterocyclic compounds have been investigated by various methods including ultraviolet photoelectron spectroscopy. PQs show electron affinities near 3.5 eV and ionisation potentials below 6 eV. Measurements of thermally stimulated depolarisation currents (TSDC) were carried out in order to study both dipolar relaxation and charge transport processes in single layer devices. The TSDC spectra revealed the prominence of both dipolar relaxation and of charge transport processes. The dipolar processes show activation energies between 0.4 eV and 1 eV which are typical values for small relaxing entities like polymer side groups. Current-voltage and current-luminance characteristics were used to study the prepared heterolayer devices. Double layers made of PPV and polyphenylquinoxaline (PPQ) are characterized by low onset voltages near 2 V and high luminous efficiency of more than 0.8 ed/A. The experimental findings show that PPQs are promising materials in the field of organic electroluminescence.