Relation between structure and electronic properties of amorphous MEH-PPV polymers.

Classical molecular dynamics simulations were used to build several large models of the amorphous polymeric semiconductor MEH-PPV. A balanced set of approximations was determined to evaluate the electronic structure of these large systems, providing quantitative information for the understanding of the charge transport properties. We have verified that the electronic structure is largely determined by the conformational disorder of the individual chains, with little effect of electrostatic disorder and interchain coupling. The disorder is essentially static, although thermal motions cause an evolution of the single chain orbital energies. The localization length of the orbitals relevant for transport is energy-dependent, unlike what is normally assumed in variable range hopping methods. Although we have found evidence of correlation between planarity of the polymer chain and localization of the highest valence band orbitals, the correlation is moderate and exceptions are frequent. All observations are discussed in terms of the desirable characteristics that should be included in a model of transport for amorphous polymeric semiconductors.