LOW-MOLECULAR-WEIGHT AND POLYMERIC HETEROCYCLICS AS ELECTRON-TRANSPORT HOLE-BLOCKING MATERIALS IN ORGANIC LIGHT-EMITTING-DIODES

The use of low molecular weight, oligomeric and polymeric heterocyclic s as electron transport/hole-blocklayers in organic light-emitting dio des is reviewed. most widely applied materials are pi-electron deficie nt heterocyclics carrying imine nitrogen atoms in the aromatic ring, s uch as 1,3,4-oxadiazoles, 1,2,4-triazoles, 1,3,5-triazines, and 1,4-qu inoxalines. Properties such as redox potentials, ionization potential, electron affinity and charge transport mobility of the materials, if known, are taken into consideration to support the electron injection/ transport and hole-blocking effectiveness. It can be generalized that heterocyclic moieties with high reduction potential reduce the interfa ce barriers caused by the band offset between organic material and cat hode and are most suitable materials for electron injection in organic electroluminescent devices. These materials are generally characteriz ed by high ionization potential values that contribute towards the hol e-blocking property. A general comparison of devices and materials is only possible with limitations owing to the variations in device struc ture, fabrication, electrode materials, Emitter materials, etc. (C) 19 98 John Wiley & Sons, Ltd.