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

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(Al_(q_3))(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 Li F(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/m^2 and 1 729 cd/m^2, 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.