Carrier Mobilities in Organic Semiconductors

Two effects are usually discussed with respect to the field dependence of the charge carrier mobility in organic semiconductors [1,2, 4-7]. One of them is the apparent decrease of the drift mobility with rising field at relatively high temperatures [1, 2, 4-7, 68-70]. The other is the strong increase of the drift mobility with rising field at low temperatures. 

An unconventional method of determination of charge carrier mobility in organic semiconductors is based on admittance... 

As a result of the above, and of the direct competition between molecule-substrate and intermolecular interactions, the presence of the metal or insulator can induce interface polymorphs which do not exist in the bulk. Examples for this are the specific thin film phases of pentacene on insulators [16, 74, 75] or metals (e.g., Cu(llO) [16]), the a- and 3-phases of tetraeene on Ag(l 11) [69], or the square phases of PTCDA on Ag(l 11) [30] and Au(l 11) [84]. It is evident that the charge carrier mobilities of organic semiconductors will depend on the crystal phase. 

An alternative model to account for low mobilities in organic semiconductors is the multitrapping and release model. Traps are locahzed levels as lattice defects or impurities in which charge carriers are immobilized. The ones localized near the center of the bandgap are called deep traps, while the ones close to the conduction or valence band... 

Recently, new organic semiconductors have been synthesized to improve carrier mobility, sensitivity, and stability. Some of them are listed in Fig. 30 along with their mobilities [112-116]. The carrier mobilities of organic semiconductors have reached the range of 0.1 to 3 cm /Vs, which rivals amorphous silicon (a-Si) devices. 

Klenkler RA, Xu G, Aziz H, Popovic ZD (2006) Charge-carrier mobility in an organic semiconductor thin film measured by photoinduced electroluminescence. Appl Phys Lett 88 242101... 

Arkhipov VI, Emelianova EV, Heremans P, Bassler H (2005) Analytic model of carrier mobility in doped disordered organic semiconductors. Phys Rev B 72 235202... 

Researchers have used various models to explain the charge carrier transport mechanism in organic semiconductors. Two models have been used frequently, (i) the trapping model, which assumes a certain distribution of traps in the energy space and (ii) the field dependent mobility model, which assumes an exponential dependence of mobility on square root of electric field. 

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