Organic electroluminescent devices using fullerene C

60

as a hole-injecting buffer layer with different thickness have been prepared.The structure of the device is ITO/C

60

/TPD/Alq

3

/LiF/Al(TPD: N

N-diphenyl-N

N-bis(3-methylphenyl)-1

1- biphenyl-4

4-diamine

Alq

3

:tris(8-quinolinolato)-aluminum).The driven voltage are higher at the same current density with the increment of C

60

thickness.The driven voltages are 8.1V

8.4V

9.2V

10.3V for the devices with 0nm

12nm

16nm and 20nm thickness of C

60

at the current density of 4mA/cm

2

respectively.The effects of the thickness of C

60

on the efficiencies of the devices are investigated.The device with 12nm C

60

buffer has the highest luminance while the current efficiency is not the optimum for this device

because the current density for this device is much greater than that for the device with 16nm C

60

buffer.The optimum thickness of C

60

buffer

16nm

which produces the highest efficiency

is obtained.EL efficiency of the device

which

c

ontains 16nm C

60

buffer

has been improved by about a factor of one in comparison with the device without buffer at the current density of 100mA/cm

2

.The undoped C

60

with high resistivity and first ionized potential using as a hole injecting buffer layer blocks the hole injection at the interface of anode and hole transporting layer.As the buffer becomes thicker

the current density is suppressed by decreased tunneling of holes under opplied voltage.As a result

the current densities are reduced at the same driven voltage.In the devices

Alq

3

which is an electron transporting and emissive layer

has a much higher electron mobility than hole mobility.Accordingly

in Alq

3

electrons are majority carriers and holes are minority carriers.In order to obtain high efficiency in organic electroluminescent devices the balanced electrons and holes injected from the cathode and anode into the emissive layer is required.Electroluminescence efficiency is controlled by the hole injection from the TPD into the Alq

3

.Part of the driven voltage drops across the C

60

buffer

resulting in acceleration of injected holes and more accelerated holes injected into emissive layer.Thus

the reason for the improvement of efficiencies for the devices with suitable thickness of C

60

can be attributed to the more balanced carriers concentration in emissive layer.