Carrier Screening of Dopant Coulomb Interactions in Organic Electronics

The importance of Coulomb interactions and their impact on the density of states (DOS) via dopant-induced disorder is now well established. However, previous studies of hopping transport within the Gaussian Disorder Model did not consider the role of screening of the interactions by the carriers. Here we implement screening in the Debye-Hückel formalism and calculate dopant-induced disorder with the Yukawa potential. Then we solve the Pauli Master Equation with Miller Abrahams hopping rates with states from the resulting doped and screened DOS. Our results show that screening has a significant impact on the Seebeck coefficient and shape of the DOS. The thermoelectric (TE) 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. Including screening we were able to reproduce and explain these curves obtained in measurements and connect the change in slope with change in structure of the host polymer. We conclude that carrier screening of dopant interactions plays an important role in transport and TE properties of polymers, especially at high doping concentrations. Our study refines the understanding of fundamental processes in doped polymers and enable their easier engineering for electronic, thermoelectric, and photovoltaic applications.