Waals forces this precludes charge transport in delocalized valence and conduction bands. Instead, charge carriers are localized at molecular entities, e.g. oligomers or segments of a conjugated polymer chain and move by incoherent hopping among adjacent sites with statistically variable energies. Quite often the site energies feature a Gaussian distribution. Also the hopping distances can vary statistically (positional disorder). A consequence of the energetic broadening of the density of states (DOS) distribution is that charge motion is both temperature and field dependent and the charge carrier mobility can be several orders of magnitude lower than in an equivalent crystalline system. This disorder effect on charge carrier motion is diminished in organic field-effect transistors because current flow is confined in a thin layer. Therefore, the tail states of the DOS distribution are already filled so that the activation energy for charge carrier hopping is diminished. For this reason the charge carrier mobility inferred from FET experiments is always higher than that determined from TOF experiments. In organic semiconductors charge carriers couple to vibrational modes and are referred to as polarons. Therefore, the activation energy for hopping motion contains an additional term due to structural site relaxation upon charging a molecular entity. It turns out, however, that usually the disorder contribution to the temperature dependence of the mobility dominates over the polaronic contribution. See also Conductive polymer Dinaphthylene dioxide Molecular electronics Organic electronics Organic field-effect transistor (OFET) Organic laser Organic light-emitting diode (OLED) Organic photonics Organic photovoltaic cell (OPVC) References Further reading Electronic Processes in Organic Semiconductors : An Introduction by Anna Köhler and Heinz Bässler, Wiley – VCH, 2015 Electronic processes in organic crystals and polymers by M. Pope and C.E.Swenberg, Oxford Science Publications, 2nd edition, 1999. Organic photoreceptors for Xerographyby P.M.Borsenberger and D.S.Weiss, Marcel Dekker, New York, 1998. External links Conductive polymers Molecular electronics Semiconductor material types