Since Tang and Van Slyke reported the first high efficiency organic light-emitting diodes (OLED) in

1987

indium tin oxide (ITO) has been the most investigated OLED anode material due to its high transparency

high conductivity

and

particularly

high work function. The treatment of the ITO surface usually has a strong influence on the performance of the OLED device

suggesting that the interface between the ITO and the organic layer is quite important. Furthermore

the hole mobility in the hole-transporting layer (TPD) and electron mobility in the electron transporting layer (Alq

3

) are 1×10

-3

and 5×10

-5

cm

2

·V

-1

·s

-1

respectively

resulting in an imbalance in electron and hole concentrations in the emission layer. It is very necessary to improve the balance of holes and electrons injected to the emitter layer. One efficient method to improve the performance of OLED is to insert a buffer layer at the interface between the ITO anode layer and organic layer. We report OLEDs using ZnS ultra-thin film by RF magnetron sputtering as a hole injecting buffer layer with different thickness. The device with a typical structure of ITO/ZnS/TPD/Alq

3

/Al (TPD:N

N′-diphenyl-N

N′-bis (3-methylphenyl)-1

1′-biphenyl-4

4′-diamine

Alq

3

: tris(8-quinolinolato)-aluminum) performed a good electroluminescent properties compared with that without ZnS buffer layer. The investigation on the effects of the ZnS thickness showed that the diode with 5 nm ZnS buffer layer double its current density and luminance

and the current efficiency of the devices with 10 nm ZnS is improved by about a factor of eighteen percent compared with the devices without buffer layer. ZnS can be expected as a good buffer layer material due to its wide band gap

thus blocks part of the injected holes from the ITO anode to the organic layer (TPD) and improves the balance of hole and electron in jections in the devices.