Thermionic OLEDs

The analytic theory outlined above provides valuable insight into the factors that determine the efficiency of OI.EDs. However, there is no completely analytical solution that includes diffusive transport of carriers, field-dependent mobilities, and specific injection mechanisms. Therefore, numerical simulations have been undertaken in order to provide quantitative solutions to the general case of the bipolar current problem for typical parameters of OLED materials [144—1481. Emphasis was given to the influence of charge injection and transport on OLED performance. 1. Campbell et at. [I47 found that, for Richardson-Dushman thermionic emission from a barrier height lower than 0.4 eV, the contact is able to supply... 

Figure 23 Energy level diagrams for OLED materials. Only the frontier orbitals are considered, (a) Transition from the molecular orbital energies to narrow bands in the solid state to the parallelogram image of the HOMO and LUMO energies are shown, (b) The effect of an applied bias on the energy levels (left). The red arrows illustrate carrier injection via tunneling (tu), thermionic emission (te), or midgap states (mg). The midgap states are shown in blue.

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