Fermi OLEDs

Figure 13-4. Encigy level diagnim of a single-layer OLED, where the organic malerial is depicted as a fully depleted semiconductor. The valence band Ey corresponds to the HOMO and the conduction band Ec corresponds to the LUMO. Tile Fermi levels of the two metal electrodes are marked as Et-. Upon contact a built-in potential is established and needs to be compensated for, before the device will begin to operating.

Although the low electric field percolation model is useful in organic field effect transistors, it is not suitable for OLEDs or OPVs due to the much larger electric fields in these devices. Under an applied electric field, charge carriers hop from sites deep in the DOS to higher energy levels where the DOS is broader (see Fig. 1). The effect of the electric field is fo create a population of hot carriers above the Fermi level [18]. The aim of the effective temperature concept is to model the electric field by a nonequilibrium disfribution of elecfrons characterized by an effective temperature Teff. 

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