A Low‐Swelling Polymeric Mixed Conductor Operating in Aqueous Electrolytes

Abstract

Organic mixed conductors find use in batteries, bioelectronics technologies, neuromorphic computing, and sensing. While great progress has been achieved, polymer‐based mixed conductors frequently experience significant volumetric changes during ion uptake/rejection, i.e., during doping/de‐doping and charging/discharging. Although ion dynamics may be enhanced in expanded networks, these volumetric changes can have undesirable consequences, e.g., negatively affecting hole/electron conduction and severely shortening device lifetime. Here, the authors present a new material poly[3‐(6‐hydroxy)hexylthiophene] (P3HHT) that is able to transport ions and electrons/holes, as tested in electrochemical absorption spectroscopy and organic electrochemical transistors, and that exhibits low swelling, attributed to the hydroxylated alkyl side‐chain functionalization. P3HHT displays a thickness change upon passive swelling of only +2.5%, compared to +90% observed for the ubiquitous poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate, and +10 to +15% for polymers such as poly(2‐(3,3′‐bis(2‐(2‐(2‐methoxyethoxy)ethoxy)ethoxy)‐[2,2′‐bithiophen]‐5‐yl)thieno[3,2‐b]thiophene) (p[g2T‐TT]). Applying a bias pulse during swelling, this discrepancy becomes even more pronounced, with the thickness of P3HHT films changing by <10% while that of p(g2T‐TT) structures increases by +75 to +80%. Importantly, the initial P3HHT film thickness is essentially restored after de‐doping while p(g2T‐TT) remains substantially swollen. The authors, thus, expand the materials‐design toolbox for the creation of low‐swelling soft mixed conductors with tailored properties and applications in bioelectronics and beyond.

Molecular Design Strategies toward Improvement of Charge Injection and Ionic Conduction in Organic Mixed Ionic–Electronic Conductors for Organic Electrochemical Transistors

“…These include the hybrid side chains, discussed above, as well as sulfonated, hydroxylated, carboxylic acid, and lysine-based side chains, respectively. 179 − 182 Nevertheless, the low cost and widespread commercial availability of ethylene glycol chains, which can be readily varied in terms of both chain length and functionalities, renders glycol-based side chains as the current gold standard for OMIEC design. Indeed, the highest performing OECT materials each feature glycol side chains as the ionic conduction component.…”

From p‐ to n‐Type Mixed Conduction in Isoindigo‐Based Polymers through Molecular Design

“…[15][16][17][18][19] The design of semiconducting materials for bioelectronic applications is heavily influenced by the rich body of literature in the broader field of organic electronics, evidenced by the development of e.g. oligo-and polythiophenes, [8,11,20,21] fullerenes, [18] diketopyrrolopyrrole-, [12,[22][23][24] and naphthalene diimide-based [15,16] systems as efficient OMIECs. With that in mind, another versatile and highly efficient charge transporting moiety, isoindigo, has received very little attention for bioelectronic applications.…”

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dominated by poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and closely related derivatives as the active material, [4][5][6][7] new semiconducting materials have started to enter the arena in recent years. [8][9][10][11][12] In particular, hole-transporting (p-type) mixed conductors have emerged with properties on par or even better than benchmark PEDOT:PSS formulations, [9,13,14] whereas electron-transporting (n-type) materials are still lacking considerably in terms of mixed conduction performance. [15][16][17][18][19] The design of semiconducting materials for bioelectronic applications is heavily influenced by the rich body of literature in the broader field of organic electronics, evidenced by the development of, for example, oligo-and polythiophenes, [8,11,20,21] fullerenes, [18] diketopyrrolopyrrole-, [12,[22][23][24] and naphthalene-diimide-based [15,16] systems as efficient OMIECs.

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How side‐chain hydrophilicity modulates morphology and charge transport in mixed conducting polymers

“…The side-chain length, branching, frequency, and placement have also been shown to affect OMIEC figures of merit. These factors have been explored for specific backbone and electrolyte combinations across many recent experimental studies, 15,[20][21][22][23][24][25][26][27][28][29][30] and to a lesser extent by simulations. 31,32 Among the salient trends from these studies, are that branched side-chains exhibit higher capacitance 20 but lower conductivity, 21 intermediate length side-chains (3 glycol units) lead to the highest transconductance, 21,23 and tail-to-tail sidechain linkage shows higher transconductance than headto-head linkage.…”

“…24 An additional strategy that has recently gained traction is to include an alkyl linker between polar side-chains and the backbone to limit backbone-electrolyte interactions and improve polymer packing and conductivity. [25][26][27][28][29] However, these side-chain design rules are not universal and strongly depend on the backbone chemistry. 15,30 Since the transconductance is proportional to the product of the mobility and carrier concentration, it is expected that several of these trends are dominated by the increase in net doping that occurs when including more polar side-chains, which also compensates for the potential decreases of mobility or doping efficiency that might occur upon polymer swelling by the electrolyte.…”