(161ab) Improved Doping Efficiency for Organic Semiconductors Via Anion Exchange

Organic semiconductors offer a low input cost and energy alternative to inorganic semiconductors. One of the most important advantages of organic semiconductors is that they can be reversibly doped from solution whereas dopants cannot be removed from inorganic semiconductors. This reversibility is exploited in a new method called anion exchange doping that uses a solution method to replace a reactive dopant anion with a different anion salt that cannot be reversibly oxidized. This process depends on the exchange of a radical dopant anion with an electrolyte anion in solution to yield more stable doped polymer films and higher doping efficiencies. Here we show excitingly high doping levels within thick (~200 nm) Poly(3-hexylthiophene) (P3HT) films with ferric chloride (FeCl3) in the presence of an electrolyte, Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI). High conductivity values around 150 S/cm2 and accompanying UV-vis-NIR spectra confirm that anion exchange does increase doping efficiency in this system. In addition to experimental data, a theoretical framework for anion exchange is presented to explore the equilibrium composition of the polymer-dopant-electrolyte system. The model reports ΔG values around -4.4 kJ mol-1 and a doping efficiency around 12.5%. Both shed light on the thermodynamic driving force behind anion exchange doping.