Charge polarity-dependent ion-insertion asymmetry during electrochemical doping of an ambipolar π-conjugated polymer

Abstract

Electrochemical doping is central to a host of important applications such as bio-sensing, neuromorphic computing and charge storage. However, the mechanisms that enable electrochemical dopability and the various parameters that control doping efficiencies are poorly understood. Here, employing complementary electrochemical and spectroelectrochemical measurements, we report a charge-polarity dependent ion insertion asymmetry in a diketopyrrolopyrrole-based ambipolar π-conjugated polymer. We argue that electrostatic interactions are insufficient to fully account for the observed charge-specific ion insertion into the polymer matrix. Using polymer side-chain dependent electrochemical doping studies, we show that electron density donating and accepting tendencies of polymer side-chains sufficiently describe the observed charge-polarity dependent electrochemical doping. Our observations are akin to the solvation of dopant ions by polymer side-chains. We propose that Gutmann donor/acceptor number framework qualifies the 'solvent-like' properties of polymer side-chains and provides a rational basis for designing π-conjugated polymers with favorable mixed ionic electronic transport properties.

Fig. 1. 2DPP-OD-TEG-based n- and p-type OECT operation.

a Chemical structure of 2DPP-OD-TEG polymer. b Measurement configuration used for OECT measurements. Typical n-type (c) and p-type (d) OECT transfer characteristics of 2DPP-OD-TEG OECTs measured in 0.1 M aqueous NaCl. Transfer characteristics were measured by scanning the gate voltage at a scan rate of 20 mV/s from 0 to 1.2 V with the drain-source bias voltage (V_DS) set to 0.4 V for n-type and from 0 to −1.2 V at V_DS = −0.4 V for p-type electrochemical doping.

Fig. 2. Anion size dependence for p-type electrochemical doping of 2DPP-OD-TEG.

a Typical p-type OECT transfer characteristics of 2DPP-OD-TEG OECTs measured in 0.1 M aqueous solutions of sodium salts with different anions as mentioned in the plot legend. b Box plot showing I_D,max measured for identical OECTs for insertion of different anions along with plot of mean I_D,max as a function of the ionic radius of the respective inserted anion. Error bars represent standard deviation. c Five cycles of CV at a scan rate of 100 mV/s in electrolytes containing each of the four anions from 0.2 V to V_CV,max corresponding to V_G,max used for OECT measurements in Fig. 2a. d Mean charge under the reductive wave of the CVs and corresponding volumetric doping density. Dashed black lines indicate the charge that would correspond to charging of typical electrostatic double layer (C_EDL = 40 μF/cm²) at respective V_CV,max. Error bars represent standard deviation. For the box plots, center line is median; box limits are 25th and 75th percentiles; whiskers are outliers within 25th and 75th percentile + 1.5x interquartile range; ‘□’ represents mean value; ‘⨯’ represents maximum and minimum values.

Fig. 3. Cation size dependence for n-type electrochemical doping of 2DPP-OD-TEG.

a Typical n-type OECT transfer characteristics of 2DPP-OD-TEG OECTs measured in 0.1 M aqueous solutions of chloride salts with different cations as mentioned in the plot legend. b Box plot showing I_D,max measured for identical OECTs for insertion of different cations along with plot of mean I_D,max as a function of the ionic radius of the respective inserted cation. Error bars represent standard deviation. c Five cycles of CV at a scan rate of 100 mV/s in electrolytes containing each of the four cations. d Volumetric capacitance, C^*_eff estimated from EIS for different cations. For the box plots, center line is median; box limits are 25th and 75th percentiles; whiskers are outliers within 25th and 75th percentile + 1.5x interquartile range; ‘□’ represents mean value; ‘⨯’ represents maximum and minimum values.

Fig. 4. Evolution of UV-vis-NIR absorption spectra of 2DPP-OD-TEG films during electrochemical doping as a function of the applied redox potential.

a UV-vis-NIR spectra collected during p-type electrochemical doping from electrolytes containing different anions at oxidative potentials ranging from 0 V to 1 V in steps of 50 mV. b UV-vis-NIR spectra during n-type electrochemical doping from electrolytes containing different cations at reductive potentials ranging from 0 V to −1.1 V in steps of 50 mV.

Fig. 5. N- and p-type OECTs using 2DPP-OD-HEX, a polymer with all alkyl side-chains.

a Chemical structure of 2DPP-OD-HEX polymer repeat unit. b Mean drain current for p-OECT and n-OECT operation as a function of ionic radii of inserted ions. Error bars represent standard deviation.

Fig. 6. DN and AN values of selected solvents sorted according to their functional groups.

Each family of molecules represents the variation of DN and AN values across different alkyl group sizes. The dotted horizontal and vertical lines represent the DN and AN values of water.