Electrochemical Doping in Ordered and Disordered Domains of Organic Mixed Ionic–Electronic Conductors

Abstract

Conjugated polymers are increasingly used as organic mixed ionic–electronic conductors in electrochemical applications for neuromorphic computing, bioelectronics, and energy harvesting. The design of efficient electrochemical devices relies on large modulations of the polymer conductivity, fast doping/dedoping kinetics, and high ionic uptake. In this work, structure–property relations are established and control of these parameters by the co‐existence of order and disorder in the phase morphology is demonstrated. Using in situ time‐resolved spectroelectrochemistry, resonant Raman, and terahertz (THz) conductivity measurements, the electrochemical doping in the different morphological domains of poly(3‐hexylthiophene) (P3HT) is investigated. The main finding is that bipolarons are found preferentially in disordered polymer regions, where they are formed faster and are thermodynamically more favored. On the other hand, polarons show a preference for ordered domains, leading to drastically different bipolaron/polaron ratios and doping/dedoping dynamics in the distinct regions. A significant enhancement of the electronic conductivity is evident when bipolarons start forming in the disordered regions, while the presence of bipolarons in the ordered regions is detrimental for transport. This study provides significant advances in the understanding of the impact of morphology on the electrochemical doping of conjugated polymers and the induced increase in conductivity.

Crystallinity Determines Ion Injection Kinetics and Local Ion Density in Organic Mixed Conductors

“…If there were no shift observed for the π–π* peak, we would hypothesize that the dopants enter the crystalline and amorphous regions simultaneously, but that is not the case with the P3HT blends. We note that our observation of a blueshift in the absorption spectrum of P3HT upon electrochemical doping is consistent with previous studies. , Figure S2 displays ex situ UV–Vis–NIR spectra of each RR/RRa P3HT blend at 0.7 V for a larger wavelength range (300–2200 nm) to show the bipolaron peak, which exhibits a redshift as more RR P3HT is added. The bipolaron peak position is similar to that of prior absorbance measurements of doped P3HT. ,, The GIWAXS diffraction patterns of the different RR/RRa P3HT blends in their undoped (0.0 V) and doped (0.7 V) states indicate ion entry into the crystalline regions by showing a shift in the 100 diffraction peak to lower q xy values that correspond to an increase in the polymer lamellar spacing (Figure B). , We chose 0.0 V as the bias for the undoped state because the CVs in Figure A show no current at 0.0 V for all ratios of RR/RRa P3HT.…”

“…Scanning probe imaging studies have also shown that ions preferentially enter amorphous regions . However, contrary to these studies, Cavassin, et al demonstrated through spectroelectrochemistry and in situ electrochemical resonant Raman spectroscopy (ERRS) that ions and polarons prefer crystalline regions of the polymer and bipolarons form in amorphous regions …”

Impact of the dopant location in the semi-crystalline structure of alternated donor–acceptor copolymers on the polarity switching p → n mechanism

“…Clearly, there is a progressive decrease of the ratio with increasing [FeCl 3 ]. Following the work of Cavassin et al and Enengl et al on the doping of P3HT, a similar decrease in Abs(P2)/Abs(P1) was attributed to the formation of bipolarons in the disordered domains of the polymer at higher doping level 34,40,41 The situation for FeCl 3 -doped PDPP2T-TVT is slightly different (Fig. 3e).…”

Interplay between Side Chain Density and Polymer Alignment: Two Competing Strategies for Enhancing the Thermoelectric Performance of P3HT Analogues

“…We studied the solid-state electrochemical properties across the polymer series using cyclic voltammetry (CV) and spectroelectrochemistry on non-aligned films (Figures , S12, and S14). P3HT and T ref thin films drop-cast onto the carbon working electrode exhibited major electrochemical oxidation waves with peak currents at 0.5 and 0.4 V vs Fc/Fc + , respectively, along with minor oxidation waves with onsets at 0.05 V for both films when measuring at a scan rate of 50 mV s –1 . Cyclic voltammograms of T19 and T24 revealed both these oxidative waves shifting to lower potentials, thereby indicating slightly smaller ionization potentials by ∼30 meV.…”

“…Much work has been carried out recently on polythiophenes, trying to understand where dopants reside in the solid state and how they may affect the electrical conductivity. − Jacobs et al recently showed that the overall crystalline structure of the film is the major driving force for high conductivity . They conducted a study on a range of semiconducting polymers, including P3HT and PBTTT, and showed that increasing the anion size when using ion exchange doping disrupted the packing to a greater extent, leading to lower conductivity.…”