Organic semiconductor Fermi level

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.

The electronic properties of organic conductors are discussed by physicists in terms of band structure and Fermi surface. The shape of the band structure is defined by the dispersion energy and characterizes the electronic properties of the material (semiconductor, semimetals, metals, etc.) the Fermi surface is the limit between empty and occupied electronic states, and its shape (open, closed, nested, etc.) characterizes the dimensionality of the electron gas. From band dispersion and filling one can easily deduce whether the studied material is a metal, a semiconductor, or an insulator (occurrence of a gap at the Fermi energy). The intra- and interchain band-widths can be estimated, for example, from normal-incidence polarized reflectance, and the densities of state at the Fermi level can be used in the modeling of physical observations. The Fermi surface topology is of importance to predict or explain the existence of instabilities of the electronic gas (nesting vector concept see Chapter 2 of this book). Fermi surfaces calculated from structural data can be compared to those observed by means of the Shubnikov-de Hass method in the case of two- or three-dimensional metals [152]. 

Finally, interface states, that is, electronic states localized at the interface, are always present in the case of semiconductors and may be very important in fixing the Fermi level at the interface (Fermi level pinning) and modify the barrier height. In the case of organic solids in general, and of CPs, the existence of a surface does not imply that of dangling (i.e.,... 

Sm is the interface parameter representing the extent of Fermi level stabilization for a given organic semiconductor (t)o is a material property... 

Here we propose a mechanism to describe the formation of 0/0 heterojunctions. The position of the Fermi level in organic semiconductors (e.g., Alqg and TPD) is observed to depend strongly on metal substrates, rather than on the thickness of the organic films [55]. Besides an abrupt VL shift at the interface with a thickness lower than 10 A, the Fermi level position... 

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