Charge Transport in Organic Materials With Small Energetic Disorder

We have studied the charge carrier transport in organic semiconductors with high mobility close to the value, mu similar to 1cm(2)/Vs, using the Monte Carlo simulation. These organic semiconductors exhibit poly-crystal or liquid-crystal phase with high order. Although their conduction regime is explained by Marcus or Holstein type of small-polaron hopping (i.e., Arrhenius-type of temperature dependence of the mobility), we suggest that their charge hopping occurs in the presence of energetic-state disorder. The "Gaussian disorder model" (GDM) under the supposition of Gaussian-type of density of states (DOS) is first proposed by Bassler [1] for the explanation of the charge transport in amorphous organic semiconductors. Although the energetic disorder is very small (<= 50meV) in the present organic semiconductors, our simulation shows that the small value of energetic disorder affects temperature dependence of the mobility especially in low temperature domain (<room temperature). They exhibit a steep decrease of mobility with decreasing temperature as denoted by similar to exp[(c sigma/k(B)T)(2)] and a very week temperature dependence around room temperature. Polaron-binding energy as well as energetic disorder in localized states cannot be neglected for understanding a charge transfer process in this regime. The model can explain the unique temperature dependence found in organic polycrystals and liquid crystals. Some experimental data are well fitted by our proposed formalism.