Chemical aspects of polymeric electroluminescent devices

Both empirical development and scientific understanding of chemical processes in operating organic EL devices have progressed dramatically during the last 6 years. For hermetically sealed devices, state of the art lifetimes (defined as the time to 50% of initial output, at constant current) today are well over 10 000 h at the prototypical luminance of a computer display (100 cd/m2). For certain types of PPV devices that pass primarily hole current, the lifetime appears indefinite (i.e., no degradation has occurred after more than 10,000 h). This, combined with the fact that photostability in the absence of oxygen is extremely good (quantum yield around 10-9 or less), strongly suggests that at least for one important class of materials, reactions involving the negative charge carriers are the critical problem. On the other hand, specific reactions at the cathode interface appear essential to obtain good electron injection. More extensive studies with TOF-SIMS, especially using oligomers with well-defined molar masses, could go a long way toward more fully elucidating the reactions that accompany degradation of light output, and provide a basis for molecular design principles that would reduce it. With some further progress in device lifetime, increased efficiency obtained by better light extraction techniques, and the remarkable manufacturing cost reductions that are expected from the development of roll to roll processing, organic EL may achieve commercial success in the entire range of applications where moderate average luminances are required. This development will be complementary to the revolution in high-intensity solid- state lighting based on inorganic LEDs.