Change of the Emission Spectra in Organic Light-Emitting Diodes by Layer Thickness Modification

Electroluminescence and photoluminescence of organic light-emitting diodes consisting of an indium tin oxide anode, N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine as a hole transport layer, tris (8-hydroxyquinoline) aluminum as emitting layer, and an Ag cathode were measured for different layer thickness values. It was found that, for a certain range of thickness values, multiple peak emission can be achieved. In addition, the emission spectra were dependent on the viewing angle. For the optimized thickness values, normal incidence chromaticity coordinates achieved were 0.32 and 0.43. Possible explanations for observed unexpected behavior are discussed.