Improved characteristics of organic light emitting diodes with coevaporated Al-alkaline metal cathode

Abstract

The cathodes using co-evaporated Al-CsF Al-YF and Al-NaF composites are adopted to enhance the electrical and the optical properties of organic light emitting diodes (OLEDs). The hole transport layer (HTL), 50 nm thick N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4, 4'-diamine (TPD) and the electron transport layer (ETL), 50 nm thick tris(8-hydroxy-quinoline) aluminum (Alq(3)) were deposited under the base pressure of 1.6x10(-6) Torr. In depositing Al-CsF Al-KF and Al-NaF composite contacts, the mass ratio of alkaline metals is varied 1similar to10wt%. OLEDs with alkaline metal-doped cathode have luminance of about 35,000 cd/m(2) and external quantum efficiency of about 1.35%. Alkaline metal tends to diffuse into organic layer and then form AlF3 with the alkaline metal-doped surface region.