Voltage dependence of light-emitting zone in aluminum-hydroxyquinoline layers of organic heterojunction EL devices

Spectra of EL emission from the devices having a thick layer of aluminum-hydroxyquinoline as the light-emitting material were altered by the voltages applied to the organic EL devices. From the analysis of the spectra, it was concluded that under the normal operational conditions the emissive zone locates in the light-emitting layer in the region about 22 nm in depth from the organic/organic interface. At low applied voltages, however, the emissive zone extends into the bulk of the light-emitting layer. Such a change in the emissive zone was explained on the basis of the difference in the barrier heights for the injections of electrons and holes from electrodes and the difference in the mobilities of electrons and holes, At low voltages, injection of electrons becomes difficult owing to the large barrier height. As a result, the holes diffuse into the bulk of the light-emitting layer before they recombine with electrons, leading to the expansion of the light-emitting zone.