Degradation in organic light-emitting diodes

Light-emitting diodes using tris(8-hydroxyquinoline) aluminum complex (Alq 3) as an active layer and N,N′-diphenyl- N,N′-bis(3-methyl-phenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) and 2-(4-biphenyl)-5-(4- tert-butylphenyl)-1,3,4-oxadiazole (PBD) as hole and electron transport layers, respectively, have been studied by electrical and optical measurements. The degradation effects of these diodes have been examined by measuring the current density-applied field characteristics as a function of time. It has been demonstrated that the three-layer diodes have the best stability and that the degradation occurred through the formation of dark points did not modify the injection mechanism. A discussion on the degradation mechanism is given, and the results are compared to those reported in the literature.