Anion Exchange Doping: Tuning Equilibrium to Increase Doping Efficiency in Semiconducting Polymers

Abstract

High electron affinity (EA) molecules p-type dope low ionization energy (IE) polymers, resulting in an equilibrium doping level based on the energetic driving force (IE-EA), reorganization energy, and dopant concentration. Anion exchange doping (AED) is a process whereby the dopant anion is exchanged with a stable ion from an electrolyte. We show that the AED level can be predicted using an isotherm equilibrium model. The exchange of the dopant anion (FeCl 3 −) for a bis(trifluoromethanesulfonamide) (TFSI −) anion in the polymers poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[3-(2,2-bithien-5yl)-2,5-bis(2-hexyldecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione-6,5-diyl] (PDPP-2T) highlights two cases in which the process is nonspontaneous and spontaneous, respectively. For P3HT, FeCl 3 provides a high doping level but an unstable counterion, so exchange results in an air stable counterion with a marginal increase in doping. For PDPP-2T, FeCl 3 is a weak dopant, but the exchange of FeCl 3 − for TFSI − is spontaneous, so the doping level increases by >10× with AED.

Impact of the anion on electrochemically doped regioregular and regiorandom poly(3‐hexylthiophene)

“…On the other hand, the fluorine functionalized tetraphenylborate derivatives are more effective in stabilizing the oxidized P3HT owing to the electron withdrawing fluorine atoms. Electrochemical doping of conjugated polymers relies in part on the ability of the electrolyte solution to slightly swell the polymer film, 27,43 which permits counterion penetration to stabilize the electrochemically doped polymer. If the anions in the electrolyte are unable to penetrate into the polymer film, there will be no balance of this positive charge and primarily surface oxidation will occur.…”

“…and electrochemical doping. 17,26,27 Chemical doping requires matching the energetics of the dopant with the OSC and it is often difficult to control the extent of doping. 28 Alternatively, electrochemical doping works by oxidizing or reducing the OSC through applying a potential to a conductive electrode in an electrochemical cell and balances the charge with the influx of anions (or cations) into the material.…”

High‐Efficiency Ion‐Exchange Doping of Conducting Polymers

“…The high carrier density and exchange efficiency achieved here derive primarily from the choice of acetonitrile (AN) as the doping solvent, as opposed to n-butyl acetate used in previous works. [24,39] These improvements stem from AN's high dielectric constant, which increases electrolyte dissociation, and a dramatic increase in the reduction potential of Fe 3+ ions in AN, which enables us to reach high carrier densities.…”

Thermoelectric Materials: Current Status and Future Challenges

“…Similarly to sequential processing techniques, anion exchange doping utilises molecular dopants in concentrated solutions of electrolytes. (Yamashita et al, 2019;Murrey et al, 2021) Deposition of the anionic exchange solution onto a polymer film allows for the reduced p-type dopant to substitute with the anion from the electrolyte, leading to higher doping levels. Yamashita et al showed that PBTTT doped using anion exchange solutions containing F4TCNQ and LiTFSI as the dopant and electrolyte, respectively, led to conductivities of 620 S cm -1 compared to 260 S cm -1 for just F4TCNQ.…”