Enhanced Electron Injection Efficiency and Electroluminescence in Organic Light-Emitting Diodes by Using an Sn/Al Cathode

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Abstract
A series of organic light-emitting diodes (OLEDs) have been fabricated with different thicknesses of the tin (Sn) layer. The structure of the devices is indium-tin oxide (ITO)/copper phthalocyanine (CuPc) (12 nm)/N, N'-diphenyl-N, N'-bis(1-naphthyl)-(1,1'-biphenyl)-4, 4'-diamine (NPB) (60 nm)/tris-(8-hydroxyquinoline)aluminum (Alq3) (60 nm)/Sn/aluminium (Al) (120 nm). It is found that compared to OLEDs with only an Al cathode, both the electron injection efficiency and electroluminescence are improved when the thickness of the Sn layer is properly chosen. The maximum efficiency and brightness of the devices with Sn (2.1 nm)/Al and Al cathode are 0.54 lm/W and 9800 cd/m2, and 0.26 lm/W and 3000 cd/m2, respectively. One possible explanation to this phenomenon is that the enhancement of the electron density of the Sn layer due to the electron transfer from the Al to the Sn layer leads to the improvement of the electron injection efficiency and electroluminescence.