The commercially available polyelectrolyte complex poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is ubiquitous in organic and hybrid electronics. As such, it has often been used as a benchmark material for fundamental studies and the development of new electronic devices. Yet, most studies on PEDOT:PSS have focused on its electronic conductivity in dry environments, with less consideration given to its ion transport, coupled ionic-electronic transport, and charge storage properties in aqueous environments. These properties are essential for applications in bioelectronics (sensors, actuators), charge storage devices, and electrochromic displays. Importantly, past studies on mixed ionic-electronic transport in PEDOT:PSS neglected to consider how the molecular structure of PSS affects mixed ionic-electronic transport. Herein, we therefore investigated the effect of the molecular weight and size distribution of PSS on the electronic properties and morphology of PEDOT:PSS both in dry and aqueous environments, and overall performance in organic electrochemical transistors (OECTs). Using reversible addition–fragmentation chain transfer (RAFT) polymerization with two different chain transfer agents, six PSS samples with monomodal, narrow ( Đ = 1.1) and broad ( Đ = 1.7) size distributions and varying molecular weights were synthesized and used as matrices for PEDOT. We found that using higher molecular weight of PSS ( M n = 145 kg mol −1 ) and broad dispersity led to OECTs with the highest transconductance (up to 16 mS) and [ μC *] values (∼140 F cm −1 V −1 s −1 ) in PEDOT:PSS, despite having a lower volumetric capacitance ( C * = 35 ± 4 F cm −3 ). The differences were best explained by studying the microstructure of the films by atomic force microscopy (AFM). We found that heterogeneities in the PEDOT:PSS films (interconnected and large PEDOT- and PSS-rich domains) obtained from high molecular weight and high dispersity PSS led to higher charge mobility ( μ OECT ∼ 4 cm 2 V −1 s −1 ) and hence transconductance. These studies highlight the importance of considering molecular weight and size distribution in organic mixed ionic-electronic conductor, and could pave the way to designing high performance organic electronics for biological interfaces.
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The effect of side chain engineering on conjugated polymers in organic electrochemical transistors for bioelectronic applications
Bioelectronics focuses on the establishment of the connection between the ion-driven biosystems and readable electronic signals. Organic electrochemical transistors (OECTs) offer a viable solution for this task. Organic mixed ionic/electronic conductors (OMIECs) rest at the heart of OECTs. The balance between the ionic and electronic conductivities of OMIECs is closely connected to the OECT device performance. While modification of the OMIECs’ electronic properties is largely related to the development of conjugated scaffolds, properties such as ion permeability, solubility, flexibility, morphology, and sensitivity can be altered by side chain moieties. In this review, we uncover the influence of side chain molecular design on the properties and performance of OECTs. We summarise current understanding of OECT performance and focus specifically on the knowledge of ionic–electronic coupling, shedding light on the significance of side chain development of OMIECs. We show how the versatile synthetic toolbox of side chains can be successfully employed to tune OECT parameters via controlling the material properties. As the field continues to mature, more detailed investigations into the crucial role side chain engineering plays on the resultant OMIEC properties will allow for side chain alternatives to be developed and will ultimately lead to further enhancements within the field of OECT channel materials.
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- Award ID(s):
- 2104234
- NSF-PAR ID:
- 10322852
- Date Published:
- Journal Name:
- Journal of Materials Chemistry C
- Volume:
- 10
- Issue:
- 7
- ISSN:
- 2050-7526
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Four glycolated polythiophene‐based organic mixed ionic‐electronic conductors (OMIECs), PE2gTT, PE2gT, PT2gTT, and PT2gT are prepared by atom‐efficient direct arylation polymerization, avoiding the need for toxic organometallic precursors. PE2gT, PT2gTT, and PT2gT are operable in p‐type accumulation mode organic electrochemical transistors (OECTs), with PT2gT displaying the best device performance with a µ
C* product figure‐of‐merit of 290 F cm−1 V−1 s−1. A record volumetric capacitance among p‐type glycolated polythiophene OMIECs of 313 F cm−3is observed forPE2gT , ascribed to the high proportionality of polar components in its materials design. The good OECT performance ofPE2gT with µC* = 84.2 F cm−1 V−1 s−1, comparable with state‐of‐the‐art poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) devices, coupled with its synthetic accessibility and favorable accumulation mode operation makesPE2gT an ideal glycolated alternative to PEDOT:PSS in bioelectronics.PE2gT with the least negative threshold voltage also displays the best OECT operational cycling stability, linked to better resistance of its oxidized state against parasitic redox side reactions . Shelf life stability of OECTs stored (without bias) is observed to be better for materials with a more negative threshold voltage and higher average molecular weight (PT2gT ), that are less susceptible to ambient auto‐oxidation and film delamination. -
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Abstract Organic mixed ionic–electronic conductors (OMIECs) have varied performance requirements across a diverse application space. Chemically doping the OMIEC can be a simple, low‐cost approach for adapting performance metrics. However, complex challenges, such as identifying new dopant materials and elucidating design rules, inhibit its realization. Here, these challenges are approached by introducing a new n‐dopant, tetrabutylammonium hydroxide (TBA‐OH), and identifying a new design consideration underpinning its success. TBA‐OH behaves as both a chemical n‐dopant and morphology additive in donor acceptor co‐polymer naphthodithiophene diimide‐based polymer, which serves as an electron transporting material in organic electrochemical transistors (OECTs). The combined effects enhance OECT transconductance, charge carrier mobility, and volumetric capacitance, representative of the key metrics underpinning all OMIEC applications. Additionally, when the TBA+counterion adopts an “edge‐on” location relative to the polymer backbone, Coulombic interaction between the counterion and polaron is reduced, and polaron delocalization increases. This is the first time such mechanisms are identified in doped‐OECTs and doped‐OMIECs. The work herein therefore takes the first steps toward developing the design guidelines needed to realize chemical doping as a generic strategy for tailoring performance metrics in OECTs and OMIECs.
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Abstract Organic electrochemical transistors (OECTs) have exhibited promising performance as transducers and amplifiers of low potentials due to their exceptional transconductance, enabled by the volumetric charging of organic mixed ionic/electronic conductors (OMIECs) employed as the channel material. OECT performance in aqueous electrolytes as well as the OMIECs’ redox activity has spurred a myriad of studies employing OECTs as chemical transducers. However, the OECT's large (potentiometrically derived) transconductance is not fully leveraged in common approaches that directly conduct chemical reactions amperometrically within the OECT electrolyte with direct charge transfer between the analyte and the OMIEC, which results in sub‐unity transduction of gate to drain current. Hence, amperometric OECTs do not truly display current gains in the traditional sense, falling short of the expected transistor performance. This study demonstrates an alternative device architecture that separates chemical transduction and amplification processes on two different electrochemical cells. This approach fully utilizes the OECT's large transconductance to achieve current gains of 103and current modulations of four orders of magnitude. This transduction mechanism represents a general approach enabling high‐gain chemical OECT transducers.
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null (Ed.)The ability to control the charge density of organic mixed ionic electronic conductors (OMIECs) via reactions with redox-active analytes has enabled applications as electrochemical redox sensors. Their charge density-dependent conductivity can additionally be tuned via charge injection from electrodes, for instance in organic electrochemical transistors (OECTs), where volumetric charging of the OMIEC channel enables excellent transconductance and amplification of low potentials. Recent efforts have combined the chemical detection with the transistor function of OECTs to achieve compact electrochemical sensors. However, these sensors often fall short of the expected amplification performance of OECTs. Here, we investigate the operation mechanism of various OECT architectures to deduce the design principles required to achieve reliable chemical detection and signal amplification. By utilizing a non-polarizable gate electrode and conducting the chemical reaction in a compartment separate from the OECT, the recently developed Reaction Cell OECT achieves reliable modulation of the OECT channel's charge density. This work demonstrates that systematic and rational design of OECT chemical sensors requires understanding the electrochemical processes that result in changes in the potential (charge density) of the channel, the underlying phenomenon behind amplification.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1TC05229B
