In principle, organic light-emitting diodes (OLEDs) just need one layer of appropriate material sandwiched between two metal electrodes in order to create light when the device is polarized. Real life is more complicated: in order to create a long-lived (> 10,000 hr) and efficient device (tens of lm/W), it is necessary to use more complex structures involving additional layers. Just as with more conventional inorganic devices, it is important to monitor the interfaces in order to identify possible reactions between the materials involved, to check for interdiffusion, and to determine the alignment of electronic levels.
Photoelectron spectroscopy is a well-suited technique for addressing these questions. Because the interfaces encountered in such devices often involve a metal on one side and an oligomer or a polymer on the other side, it is in many cases convenient to use a tuneable light source in order to balance properly the photoemission cross sections of the different levels involved in the valence band.
The use of the technique will be illustrated by considering the case of pentacene and low-work-function metals (samarium and alkali metals), in the cases of the growth of a metal film on pentacene as well as the deposition of pentacene on metal. This will be put in parallel with a study of interfaces with high-work-function materials, such as gold or poly(3,4-ethylenedioxythiophene/polystyrenosulfonate, and three different conjugated organic materials: N,N'-bis-(1-naphthyl)-N,N'-diphenyl1-1,1biphenyl1-4,4'-diamine, para-sexiphenyl, and pentacene.