(Invited) Probing Charge-Carrier Transport in Chemically Doped Polymeric Semiconductors

Abstract

With organic semiconductors demonstrating potential for use in various electronic and optoelectronic applications, some attention has turned to an understanding of charge-carrier doping and transport processes in such material systems. Here we demonstrate a novel electrical transport measurement technique, based on microwave spectroscopy, to probe redox-based chemical p-type doping in conjugated semiconducting polymers. Through a comparison of these measurements with macroscopic charge-transport probes, we illustrate that charge injection from a prototypical dopant like 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) initially results in charge-carriers that are bound to the reduced F4-TCNQ counterion, but that the coulomb attraction can be overcome by doping to high charge-carrier concentrations. In contrast, a series of novel charge-transfer dopants based on dodecaborane clusters appended with various organic moieties results in injected charge-carriers that are spatially separated from the counterion. This behavior results from the unique chemical and electronic structure of these dopants, which localizes the negative counter-charge on the center of the dodecaborane cluster, thereby reducing the coulomb attraction for all measurable charge-carrier concentrations and resulting in enhanced charge-carrier transport. These observations point to chemical control over the charge-injection and transport processes that can yield highly doped organic semiconductors with impressive electrical transport properties, in spite of the limitations that their low dielectric constants would suggest.