Dynamic Study Of Charge Carrier Mobility In A Conjugated Polymer Chain

Due to their advantages of low cost, versatility of chemical synthesis, ease of processing and considerable conducting capability, organic molecules such as conjugated polymers and oligomers have more and more potential applications in molecular electronics and performance improvements of optoelectronic devices. The organic molecules usually used as charge transport channels have attracted many attentions. However, the underlying mechanism of charge transport is not well understood partly because the charge carriers in these molecules are more complicated.We investigate elastic scattering processes of a negative polaron and a negative(or positive) bipolaron, and calculate the charge carrier mobility in a conjugated polymer chain in the presence of an external electric field. The dynamical simulations are performed based on the Su-Schrieffer-Heeger(SSH) model modified to include electron-electron interactions and a Brazovskii-Kirova symmetry-breaking term. We have found that the charge carrier transport will be greatly affected by polaron and bipolaron interaction in elastic scattering process. Basically we identify two types of polaron migration patterns in the presence of an external electric field. In the first type, the negative polaron will push the negative bipolaron and polaron transport will be impeded. For the second type, the negative polaron will be pulled to move by a field-driven positive bipolaron. We demonstrate that the charge carrier mobility in a conjugated polymer chain is determined by the charge characteristic of the polaron and the bipolaron when they concur. As the carried charge of the two species has the same sign, the carrier mobility is dominated by polaron. As the charge of the two species has distinct sign, the carrier mobility is dominated by bipoolaron.We assume that thermal disturbance introduces local fluctuations in atomic displacements in a random way. The local thermal fluctuations are found to be equivalent to a potential barrier for the polaron. The range of the thermally disturbed region in the molecule and the effective temperature difference resulted from asymmetric thermal absorptions determine the height of the barrier. It is also shown that the intra-molecule polaron mobility obeys a logarithmic law in low electric field range provided that there exist asymmetric thermal absorptions within the molecule. The results will serve to understand the intra-molecule charge transport in organic semiconductors and are in favor of the design of molecular circuits.In the first chapter, we discuss the basic structure of polymer and Peierls phase transition theory.In the second chapter, we introduce a classical model—SSH model and the adiabatic dynamic method.In the third chapter, dynamics study of thermal effects on charge transport along a conjugated polymer chain.In the fourth chapter, summaries are given.