Critical charge transport networks in doped organic semiconductors

Abstract

Intrinsic organic small molecule and polymer materials are insulators. The discovery that polymers can be made highly conductive by doping has therefore sparked strong interest in this novel class of conductors. More recently, efficient doping of small molecule materials has also been achieved and is now a key technology in the multi-billion dollar organic light emitting diode industry. Nevertheless, a comprehensive description of charge transport in the presence of doping is still missing for organic semiconductors with localized electronic states. Here, we present a theoretical and computational approach based on percolation theory and quantitatively predict experimental results from the literature for the archetype small molecule materials ZnPc, F8ZnPc and C60. We show that transport in the complex potential landscape that emerges from the presence of localized charges can be aptly analyzed by focusing on the network properties of transport paths instead of just the critical resistance. Specifically, we compute the activation energy of conductivity and the Seebeck energy and yield excellent agreement with experimental data. The previously unexplained increase of the activation energy at high doping concentrations can be clarified by our approach.