The Interplay Between Dopants and Charge Carriers in Organic Semiconductors

In order to make rational improvements of the charge transport properties of doped organic semiconductors, a thorough understanding of this rather complex process is needed. The efficacy of doping is governed by a multitude of steps, including the interactions between (ionized) dopants and charge carriers. In this contribution, we explore the effects of carrier-dopant and carrier-carrier interactions and strategies to mitigate them.<br/><br/>We use kinetic Monte-Carlo simulations to better understand the effects of carrier-dopant and carrier-carrier interactions. Such simulations predict that a lager host-dopant distance can result in a higher Seebeck coefficient for a given electrical conductivity. In order to realize such a system, we use amphipathic side chains in an n-type donor-acceptor copolymer. The amphipathic side chain contains an alkyl chain segment that acts as a spacer between the polymer backbone and an ethylene glycol type chain segment. The use of this alkyl spacer can not only reduce the energetic disorder in the conjugated polymer film but can also properly control the dopant sites away from the backbone, which minimizes the adverse influence of counterions. We find that this selectively increases the Seebeck coefficient and the power factor by a factor of ~ 5.<br/><br/>While it has been recognised that Coulomb interactions between dopants and charge carriers are important, carrier-carrier interactions—interactions among charge carriers—in doped organics have received less attention. In a doped organic semiconductor, however, the number of (free and bound) charge carriers equals the number of reacted dopants. As a result, both types of interactions are of importance for a proper description of the transport properties of doped organic semiconductors.<br/><br/>To study the transport properties in the presence of dopants and including both carrier-carrier and carrier-dopant interactions we once again use kinetic Monte-Carlo simulations. While the conductivity is indeed affected by carrier-dopant interactions, the effect of carrier-carrier interactions appears to be even stronger, especially so at high doping concentrations. Experimentally, we observe that the electrical conductivity of a large number of organic semiconductors shows a maximum: Upon increased doping levels the conductivity decreases. This type of behaviour is commonly attributed to changes in the microstructure as a consequence of doping. However, we find that even when using vapour doping, where there are no observable changes in the microstructure, this behaviour persists. The experimental findings match the Monte-Carlo data very closely, which suggests that the Coulomb interactions among charge carriers is at the root of the limited conductivity at high doping densities.