Since the discovery of efficient organic light-emitting devices (OLEDs)

there has been considerable interest in developing OLEDs with high brightness

high efficiency

and long lifetime for display applications. This has resulted in substantial research activity to increase the carrier mobility

color gamut

and electroluminescence efficiency of organic materials

as well as to improve electron and hole injection by the contacts. The brightness and efficiency of OLEDs depend on the number density of electrons and holes in the emission layer

so that effective charge injection into the organic materials is critical for optimum device performance. For the hole injecting contact

indium tin oxide (ITO) is often used because of its transparency and high work function (5.1eV). On the other hand

an efficient electron-injecting contact is usually a low work function material such as magnesium

calcium

or lithium. These materials are typically alloyed with metal such as aluminum or silver to form contact that is both more stable and more resistant to corrosion upon exposure to air. At the same time inserting an insulating layer between cathode and organic material is effective means to improve current injection and EL. In this paper

some multilayers OLEDs were fabricated using Al

LiF/Al and Mg:Ag as cathode respectively while ITO-coated glass as anode

1

2

3

4

5-pentaphenyl-1

3-cyclopentadiene(PPCP) as emitter

N

N′-diphenyl-N

N′-bis(3-methyl-phenyl)-(1

1′-biphenyl)-4

4′-diamine(TPD) as hole-transport layer

tris(8-hydroxyquinoline)aluminum(Alq)

2-(4-biphenylyl)-5-(4-tertbutyl-phenyl)-1

3

3-oxadiazole(PBD) or 2

5-bis(5-tert-butyl-2-benzoxazolyl)thiophene(BBOT) as electron-transport layer

respectively. And effect of cathode on performance of blue OLEDs was studied. All organic layers were prepared by a high vacuum multisource type organic molecular deposition system. From result

it is found that the performance of device with LiF/Al as cathode excels that of the device with Al or Mg:Ag as cathode. Devices with LiF/Al show better

I-V

characteristics and higher EL efficiency. The presence of LiF at the metal-organic material interface cause band bending of organic material

thus lower the electron-injection barrier height.