Effects of dynamic disorder on thermoelectric transport in soft materials

Abstract

A model is developed that accounts for the effects of thermal disorder (both static and dynamic) in predicting the thermoelectric (TE) performance of weakly bonded semiconductors. With dynamic disorder included, the model is found to fit well with experimental results found in the literature for the density-of-states and the energy-dependent carrier mobility, which are key for assessing TE properties. The model is then used to analyze the concentration-dependent TE properties of the prototypical small molecular semiconductor rubrene. At low (e.g., intrinsic) carrier concentrations, where Fermi level pinning occurs, dynamic disorder is found to reduce electrical conductivity (σ), Seebeck coefficient (S), and thermoelectric power factor (PF) to values that are much lower than those traditionally predicted by static disorder models. As carrier concentration (p) increases, S exhibits nonlinear behavior, increasing well above the conventional S vs log(p) relationship before reaching a peak value (Speak∼1550μV/K). A critical carrier concentration (pcrit.≈4.299×10−4 molar ratio) is observed near Speak at which thermoelectric transport transitions from trap-limited behavior at low concentrations to conventional band behavior at high concentrations. Above this value, σ and PF are reduced compared to the perfect crystal and static-only conditions, causing a drop in the maximum PF by factors of 3 and 2.3, respectively. This PF reduction, while not as large as the PF reduction that occurs for low carrier concentration, is found to occur in a high concentration regime (p>pcrit.) that contains the PF maximum and has remained inaccessible to experimentalists due to dopant limitations that are worsened in the presence of dynamic disorder.