Organic light-emitting diodes(OLED) have been investigated for over 10 years and have been in a stage ready for commercialized applications.However

the stability or the lifetime of OLED is still a major issue to be further investigated.As is known

the stability of OLED is closely related to processing techniques

and the quality of each processing steps could be evaluated by some test methods.Here

we proposed a simple method for the fabrication test of OLED—"Observation of covered and heated films"

which is completely compatible with the technique of OLED fabrication. In this method

a hole transport layer (such as N

N'-diphenyl-N

N'-bis-(3-methylphenyl)-(1

1'-biphenyl)-4'4-diamine(TPD)) is firstly deposited on a half of indium tin oxide(ITO) surface

and then the mask used for patterning was turned into 90° to let the following deposited electron transport layer (such as tris (8-hydroxyquinoline) aluminum(Alq)) be perpendicular to TPD layer.After the deposition of organic films

a layer of low air permeable film(such as Al film) was deposited on the substrate as a covering film.Therefore

four different areas were formed on the same ITO substrate with the layer structurs of Al/Alq/TPD/ITO

Al/TPD/ITO

Al/Alq/ITO

and Al/ITO

respectively.The as-prepared sample was then treated by rapid heating.Lastly

the thermal treated sample was used for microscopic observation. By using this method

gas bubble formation and organic crystallization on the sample surface were observed.The effect of annealing temperature

ITO surface treatment

and purification of organic sources on the bubble formation were also observed.As increasing the annealing temperature

the induced bubbles from organic layers were getting bigger in size(larger than 50μm) and more in numbers

while the bubbles induced from ITO surface were limited in a size less than about 10μm.No bubbles can be observed if ITO surface was thermally treated before any deposition of organic films.And bubbles can be reduced if organic sources were purified by sublimation before use.It is indicated by the experiment that the method of "observation of covered and heated films" is a useful method to evaluate the quality of fabrication

and therefore to improve the performance of OLED.