The influence of exciton behavior on luminescent characteristics of organic light-emitting diodes

We used N,N0-bis-(1-naphthyl)-N,N0-1-diphenyl-1,10-biphenyl-4,40-diamine (NPB), 4,40-N,N0-dicarbazole-biphenyl (CBP) and tris(8-hydroxyquinoline) aluminum (Alq3) to fabricate tri-layer electroluminescent (EL) device (device structure: ITO/NPB/CBP/Alq3/Al). In photoluminescence (PL) spectra of this device, the emission from NPB shifted to shorter wavelength accompanying with the decrease of its emission intensity and moreover the emission intensity of Alq3 increased relatively with the increase of reverse bias voltage. The blue-shifted emission and the decrease in emission intensity of NPB were attributed to the polarization and dissociation of NPB excitons under reverse bias voltage. The increase of emission intensity of Alq3 benefited from the recombination of electrons (produced by the dissociation of NPB exciton) and holes (injected from the Al cathode).