Organic electroluminescent devices (OLEDs) have been intensively investigated since the first efficient device was demonstrated.Although there are some studies about the dependence of the performance of OELDs on the morphologies and structures of organic thin films

similar studies on Alq

3

have not been reported.In this paper

organic electroluminescent devices with a structure of ITO/Poly (9-vinylcarbazole)/tris(8-hydroxyquinoline)aluminum(Alq

3

)/Mg:Ag are fabricated at different substrate temperature(77K

298K

and 438K)during Alq

3

deposition.It is found that the surface morphologies of Alq

3

thin films greatly affect the

I-V

characteristics of the devices by the contact area between metal cathode and light-emitting layer.The more uniform Alq

3

film is the larger contact area is

and the greater current is obtained.There is a decrease in the uniformity and smoothness of the Alq

3

thin films on PVK films in the order of S

298

>

S

438

>

S

77

.Accordingly

at the same bias voltage

current through the device increases in the order of I

77

<

I

438

<

I

298

.There is a increase in the quantum efficiencies of the devices in the order of η

77

<

η

438

<

η

298

.We attribute this to different structures of Alq

3

thin films.It is confirmed by transmission electron microscopy that the film deposited at 77K consists of polycrystalline grains

and the ones deposited at 298K and 438K are both amorphous.Crystallization can induces lower quantum efficiency than the glassy state does

which answers for the lowest quantum efficiency obtained at 77K. Despite the same glassy state

the quantum efficiency of the film deposited at 438K is higher than that of the film deposited at 298K

which results from the reduction of impurities and defects in the film deposited at 438K.Additionally

photoluminescence measurements testify that there is a increase in photoluminescence efficiencies of Alq

3

films on quartz substrates in the order of E

77

<

E

298

<

E

438

which is consistent with the above conclusions.In summary

by controlling the morphology and structure of Alq

3

film

we can not only improve the quantum efficiency of device

but also reduce the current of device(equal to reduce Joule heat).Hence

it is a new method to improve the performance of device:controlling the condensation state of organic film by substrate temperature.