Efficient molecular doping of polymeric semiconductors driven by anion exchange

The efficiency with which polymeric semiconductors can be chemically doped-and the charge carrier densities that can thereby be achieved-is determined primarily by the electrochemical redox potential between the pi-conjugated polymer and the dopant species(1,2). Thus, matching the electron affinity of one with the ionization potential of the other can allow effective doping(3,4). Here we describe a different process-which we term 'anion exchange'- that might offer improved doping levels. This process is mediated by an ionic liquid solvent and can be pictured as the effective instantaneous exchange of a conventional small p-type dopant anion with a second anion provided by an ionic liquid. The introduction of optimized ionic salt (the ionic liquid solvent) into a conventional binary donor-acceptor system can overcome the redox potential limitations described by Marcus theory(5), and allows an anion-exchange efficiency of nearly 100 per cent. As a result, doping levels of up to almost one charge per monomer unit can be achieved. This demonstration of increased doping levels, increased stability and excellent transport properties shows that anion-exchange doping, which can use an almost infinite selection of ionic salts, could be a powerful tool for the realization of advanced molecular electronics.