Chemistry Reference
In-Depth Information
Figure 4.33. (a) Sketch of the basic construction of an OLED. (b) Schematic dia-
gram showing the energy-level alignment for the OLED ITO/CuPc/NPB/Alq
3
/Mg.
Adapted from Lee
et al.
, 1999.
(Lee
et al.
, 2004). We shall see an example in Section 6.3, involving single crystals,
where the highest reported
ยต
h
values have been achieved thanks to the improved
quality of the organic/metal and organic/organic interfaces.
The most extensively used devices based on organic/organic, as well as or-
ganic/inorganic, heterostructures are OLEDs. OLEDs are electroluminescent de-
vices constructed using organic materials as the emitting elements (Tang &
VanSlyke, 1987). A typical OLED consists of an ITO covered glass substrate coated
with a hole transporting layer (HTL) like NPB or TPD and an electron transporting
layer (ETL) such as Alq
3
(Fig. 4.33(a)). ITO, a highly doped wide band gap ma-
terial (
E
opt
3.5-4.0 eV), with a high work function, forms the transparent HTL
anode, and a material with low work function, such as aluminium or magnesium,
is used as a cathode to inject the electrons. ITO thin films in general exhibit high
electrical conductivity, high optical transparency and smooth surface morpholo-
gies. Holes injected from the high work function anode into the HTL and electrons
