Al/LiF cathode/organic is known to form an excellent interface for electron injection into the organic active layer, resulting in excellent performing organic light-emitting diodes (OLEDs). Here, the chemical structure of the interface between the Al/LiF bilayer cathode and tris (8-hydroxyquinoline) aluminum (Alq) of working OLED devices was investigated by using x-ray photoelectron spectroscopy (XPS). Using a in situ peel-off technique, we are able to characterize the buried interface structure without disturbing the chemical states of each element probed. The data show that there are two types of F at the interface; one is attributed to LiF and the other to F attached to the Alq. This F-doped Alq layer could induce a downshift in molecular orbital levels and thus leads to a reduced electron injection barrier. XPS depth profile results show significant O diffusion through Al layer to the interface, and the diffusion of O ends abruptly at the Al/LiF interface.

A transparent-charge-injection layer consisting of LiF/Al/Al-doped SiO has been developed as (i) a cathode for top-emitting organic light-emitting diodes and as (ii) a buffer layer against damages induced by energetic ions generated during deposition of other functional thin films by sputtering, or plasma-enhanced chemical vapor deposition. A luminance of 1900 cd/m2 and a current efficiency of 4 cd/A have been achieved in a simple testing device structure of indium tin oxide (ITO) N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (60 nm)/Alq3 (40 nm)/LiF (0.5 nm)/Al (3 nm)/Al-doped SiO (30 nm). A minimum thickness of 30 nm of Al-doped SiO is also found to protect organic layers from ITO sputtering damage.

Al:SiO cermet thin films were synthesized by thermal coevaporation. Physical properties of these thin films were characterized by scanning electron microscope, transmission electron microscope, x-ray photoelectron spectroscopy, current–voltage, and optical absorption measurement. The data show that the Al:SiO films consist of crystalline Al islands embedded in an amorphous network of mixed Si, SiO2, and Al2O3 as Al weight percentage exceeds ∼50%. It is found that the size of Al islands increases with increasing Al concentration, which leads to a dramatic reduction in resisitivity and optical transmittance. A multilayered SiO:Al/Al/LiF structure has been utilized as a cathode for top-emission organic light-emitting diode (TOLED). A model based on Fabry–Pérot cavity has been used to simulate the TOLED light emission spectra. The results indicate that Al:SiO films can also be used as a semitransparent mirror for a half-wavelength planar microcavity light-emitting diode.

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