Zlatan Aksamija1
University of Utah1
Organic electronic materials, specifically conjugated polymers, are a cost-effective and environmentally friendly alternative to inorganics. However, they do not possess intrinsic free carriers, which means they must be doped to increase the number of free carriers and boost conductivity. Simultaneously, their low dielectric constant means electrostatic Coulomb interactions between free carriers and dopant counterions are poorly screened. These dopant-carrier interactions dramatically alter the energetics of the states available for transport as well as the dynamics of free carriers. The importance of Coulomb interactions and their impact on the density of states (DOS) via dopant-induced disorder is now well established. In this invited talk, I will discuss our results arising from coupling a model for modification of the electronic density of states (DOS) in the presence of dopants with a phonon-assisted carrier hopping simulation through those states. We expanded the model initially proposed by Arkhipov et al. to include the finite dopant size and minimum dopant-carrier separation, which limit the deepest traps in the tail of the DOS. We use the DOS model as input to the hopping transport simulation to analyze the transport properties of doped conjugated polymers and extract both conductivity and Seebeck coefficient at each doping concentration. We also vary the parameters of the dopants, including their size, energy level, and charge distribution on the dopant counterion to quantify their impact on conductivity.<br/><br/>However, previous studies of hopping transport within the Gaussian Disorder Model did not consider the role of screening the dielectric interactions by the carriers. In our work, we implement screening in the Debye-Hückel formalism and calculate dopant-induced disorder with the resulting electrostatic potential. For a point dopant, this gives the well-known Yukawa potential, which we generalize for dopant charge distributions other than the point charge. Then we solve Pauli Master Equation with Miller Abrahams hopping rates with states from the resulting screened DOS. Our results show that screening has a significant impact on the Seebeck coefficient and shape of the DOS. The thermoelectric power factor increases almost by a factor of 2 at higher doping. We also observe that the log slope of the Seebeck coefficient plotted against the electrical conductivity for different doping concentrations, which was previously thought to have a universal value (-1/4), increases for a more energetically disordered system. By including screening, we were able to reproduce and explain these curves obtained in measurements and connect the change in slope with the change in structure of the host polymer. We conclude that carrier screening of dopant Coulomb interactions plays an important role in transport and thermoelectric properties of polymers, especially at high doping concentrations. Our work refines the understanding of fundamental processes in doped polymers and enables their easier engineering for thermoelectric, photovoltaic and other applications.