Device optimization of tris-aluminum (Alq3) based bilayer organic light emitting diode structures

In this work we present a detailed analysis of the emitted radiation spectrum from tris(8-hydroxyquinoline) aluminum (Alq(3)) based bilayer organic light emitting diodes (OLEDs) as a function of the choice of cathode, the thickness of the organic layers, and the position of the hole transport layer/Alq(3) interface. The calculations fully take into account dispersion in the glass substrate, the indium tin oxide anode, and in the organic layers, as well as the dispersion in the metal cathode. The influence of the incoherent transparent substrate (I mm glass substrate) is also fully accounted for. Four cathode structures have been considered: Mg/Ag, Ca/Ag, LiF/Al, and Ag. For the hole transport layer, N,N-diphenyl-N,N'-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) and N,N'-di(naphthalene-1-yl)-N,N'-diphenylbenzidine (NPB) were considered. As expected, the emitted radiation is strongly dependent on the position of the emissive layer inside the cavity and its distance from the metal cathode. Although our optical model for an OLED does not explicitly include exciton quenching in the vicinity of the metal cathode, designs placing the emissive layer near the cathode are excluded to avoid unrealistic results. Guidelines for designing devices with optimum emission efficiency are presented. Finally, several different devices were fabricated and characterized and experimental and calculated emission spectra were compared.