Molecular geometry fluctuations and field-dependent mobility in conjugatedpolymers - art. no. 085202

Many conjugated polymers exhibit an electric field-dependent mobility of ap proximately the Poole-Frenkel form. We propose a model to describe transpor t in dense films of these materials in which thermal fluctuations in the mo lecular geometry modify the energy levels of localized electronic charged s tates in the material. Based on quantum chemistry calculations we argue tha t the primary restoring force for these fluctuations in molecular geometry is steric in origin, which leads to spatially correlated fluctuations in th e on-site energy of the charged electronic states. The phenylene ring torsi on, in PPV-like conjugated polymers, is an example of this kind of spatiall y correlated thermal fluctuation. Using a Master equation approach to calcu late the mobility, we show that the model can quantitatively explain the ex perimentally observed field-dependent mobility in conjugated polymers. We e xamine typical paths taken by carriers and find that at low fields, the pat hs are three-dimensional, whereas at high fields the paths become essential ly one-dimensional along the applied field. Thus, one-dimensional transport models can be valid at high fields but not at low fields. Effects of deep traps, the site energy correlation length, temperature, and asymmetric and small polaron rates are studied.