Fermi OFETs

An important issue for the performance of an organic electronic device like an OFET is the injection of charge carriers, electrons or holes, from the electrode into the organic material. In case of the commonly used metal electrodes an efficient electron injection is possible only if the Fermi level of the metal and the energy of the lowest unoccupied molecular orbital (LUMO) of the organic material differs by a small amount only. A similar statement applies for hole injection, in this case the position of the highest occupied molecular orbital (HOMO) has to match with the position of the Fermi level. When noble metals, in particular Au, are being used for an electrode one may naively assume... 

Furthermore, there could have been negative effeets on the OFETs by the deposition of the metal layers. In [33] a diffusion of metal atoms into the or-ganie semieonduetor or a formation of a mixing layer on the rough surface of the pentaeene film was reported. This can excite an additional shift of the Fermi level that need not shift in direction to improve the charge carrier injection. 

A density-independent ft has been observed in devices based on single crystals of rubrene [30,35,36], pentacene [33,34], tetracene [31], and TCNQ [39]. This important characteristic of single-crystal OFETs contrasts sharply with a strongly Vg-dependent mobility observed in organic TFTs [64] and a-Si H FETs [65]. In the latter case, the density of localized states within the gap is so high that the Fermi level remains in the gap even at high jl/gl values. 

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