Effect of Al:Ag alloy cathode on the performance of transparent organic light-emitting devices

Abstract

Transparent organic light-emitting devices (TOLEDs) based on a stacked alloy cathode of LiF/Al:Ag are investigated. The devices have a structure of indium-tin-oxide (ITO)/4,4′,4″-Tris[2-naphthyl(phenyl)amino]triphenylamine (2T-NATA) (25 nm)/N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-1,1’-biphenyl-4,4′-diamine (NPB) (40 nm)/tris-(8-hydroxyquinoline) aluminum (Alq₃) (50 nm)/LiF (1 nm)/Al:Ag (1:3) (x), where the thicknesses of cathode metal layers (Al:Ag) are adjusted, respectively, from 70 nm to 100 nm. In the experiment, it is found that the LiF (1 nm)/Al:Ag (1:3) (75 nm) has good electron injection efficiency. Compared with an Al-only cathode, the turn-on voltage is lowered. At the voltage of 10 V, the luminances for bottom emission from ITO anode side and top emission from metal cathode side are 2 459 cd/m² and 1 729 cd/m², respectively. Thanks to electron injection enhancement by using Al:Ag cathode, we can obtain a better energy level matching between the cathode and the organic layer, thus the devices have lower turn-on voltage and higher luminance. The total transmittance of the devices can achieve about 40% at the wavelength of 550 nm.

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