Accumulation-mode Organic Electrochemical Transistors Research Articles

Understanding asymmetric switching times in accumulation mode organic electrochemical transistors.

Understanding the factors underpinning device switching times is crucial for the implementation of organic electrochemical transistors in neuromorphic computing, bioelectronics and real-time sensing applications. Existing models of device operation cannot explain the experimental observations that turn-off times are generally much faster than turn-on times in accumulation mode organic electrochemical transistors. Here, using operando optical microscopy, we image the local doping level of the transistor channel and show that turn-on occurs in two stages-propagation of a doping front, followed by uniform doping-while turn-off occurs in one stage. We attribute the faster turn-off to a combination of engineering as well as physical and chemical factors including channel geometry, differences in doping and dedoping kinetics and the phenomena of carrier-density-dependent mobility. We show that ion transport limits the operation speed in our devices. Our study provides insights into the kinetics of organic electrochemical transistors and guidelines for engineering faster organic electrochemical transistors.

Toward Sustainability in All‐Printed Accumulation Mode Organic Electrochemical Transistors

AbstractThis study reports on the first all‐printed vertically stacked organic electrochemical transistors (OECTs) operating in accumulation mode; the devices, relying on poly([4,4′‐bis(2‐(2‐(2‐methoxyethoxy)ethoxy)ethoxy)‐2,2′‐bithiophen‐5,5′‐diyl]‐alt‐[thieno[3,2‐b]thiophene‐2,5‐diyl]) (pgBTTT) as the active channel material, are fabricated via a combination of screen and inkjet printing technologies. The resulting OECTs (W/L ≈5) demonstrate good switching performance; gm, norm ≈13 mS cm−1, µC* ≈21 F cm−1 V−1 s−1, ON–OFF ratio > 104 and good cycling stability upon continuous operation for 2 h. The inkjet printing process of pgBTTT is established by first solubilizing the polymer in dihydrolevoglucosenone (Cyrene), a non‐toxic, cellulose‐derived, and biodegradable solvent. The resulting ink formulations exhibit good jettability, thereby providing reproducible and stable p‐type accumulation mode all‐printed OECTs with high performance. Besides the environmental and safety benefits of this solvent, this study also demonstrates the assessment of how the solvent affects the performance of spin‐coated OECTs, which justifies the choice of Cyrene as an alternative to commonly used harmful solvents such as chloroform, also from a device perspective. Hence, this approach shows a new possibility of obtaining more sustainable printed electronic devices, which will eventually result in all‐printed OECT‐based logic circuits operating in complementary mode.

Development of Synthetically Accessible Glycolated Polythiophenes for High‐Performance Organic Electrochemical Transistors

AbstractFour 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 µC* 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−3 is observed for PE2gT, ascribed to the high proportionality of polar components in its materials design. The good OECT performance of PE2gT 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 makes PE2gT 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.

Naphthalene diimide-based n-type small molecule organic mixed conductors for accumulation mode organic electrochemical transistors.

Organic mixed ionic-electronic conductors (OMIECs), which transport both ionic and electronic charges, development are important for progressing bioelectronic and energy storage devices. The p-type OMIECs are extensively investigated and used in various applications, whereas the n-type ones lag far behind due to their moisture and air instability. Here, we report the synthesis of the novel n-type naphthalene diimide (NDI)-based small-molecule OMIECs for organic electrochemical transistors (OECTs). The electro-active NDI molecule with the linear ethylene glycol side chains is a promising candidate for n-type channel material to obtain accumulation mode OECTs. This NDI-based small-molecule OMIEC, gNDI-Br2, demonstrates ion permeability due to the attachment of the glycol side chains with optimized ionic-electronic conductions. OECT devices with gNDI-Br2 channel material displays excellent performance in water and ambient stability. OECTs fabricated with two different concentrations, 50 mg mL-1 and 100 mg mL-1 of gNDI-Br2 demonstrate a transconductance value of 344 ± 19.7 μS and 814 ± 124.2 μS with the mobility capacitance product (μC*) of 0.13 ± 0.03 F cm-1 V-1 s-1 and 0.23 ± 0.04 F cm-1 V-1 s-1, respectively. These results demonstrate the n-type OMIEC behaviour of the NDI-based small-molecule and its applicability as an OECT channel material.

Side-chain engineering of self-doped conjugated polyelectrolytes for organic electrochemical transistors

Varied-length conjugated polyelectrolytes (2–5 methylene units) containing CPDT-alt-BT were synthesized to study the optical, electrochemical, and morphological properties. These were used in accumulation mode organic electrochemical transistors.

Organic Electrochemical Transistors Based on the Conjugated Polyelectrolyte PCPDTBT-SO3 K (CPE-K).

PCPDTBT-SO3 K (CPE-K), a conjugated polyelectrolyte, is presented as a mixed conductor material that can be used to fabricate high transconductance accumulation mode organic electrochemical transistors (OECTs). OECTs are utilized in a wide range of applications such as analyte detection, neural interfacing, impedance sensing, and neuromorphic computing. The use of interdigitated contacts to enable high transconductance in a relatively small device area in comparison to standard contacts is demonstrated. Such characteristics are highly desired in applications such as neural-activity sensing, where the device area must be minimized to reduce invasiveness. The physical and electrical properties of CPE-K are fully characterized to allow a direct comparison to other top performing OECT materials. CPE-K demonstrates an electrical performance that is among the best reported in the literature for OECT materials. In addition, CPE-K OECTs operate in the accumulation mode, which allows for much lower energy consumption in comparison to commonly used depletion mode devices.

Semiconducting Small Molecules as Active Materials for p‐Type Accumulation Mode Organic Electrochemical Transistors

AbstractA series of semiconducting small molecules with bithiophene or bis‐3,4‐ethylenedioxythiophene cores are designed and synthesized. The molecules display stable reversible oxidation in solution and can be reversibly oxidized in the solid state with aqueous electrolyte when functionalized with polar triethylene glycol side chains. Evidence of promising ion injection properties observed with cyclic voltammetry is complemented by strong electrochromism probed by spectroelectrochemistry. Blending these molecules with high molecular weight polyethylene oxide (PEO) is found to improve both ion injection and thin film stability. The molecules and their corresponding PEO blends are investigated as active layers in organic electrochemical transistors (OECTs). For the most promising molecule:polymer blend (P4E4:PEO), p‐type accumulation mode OECTs with µA drain currents, μS peak transconductances, and a µC* figure‐of‐merit value of 0.81 F V−1 cm−1 s−1 are obtained.

Small molecule additive for low-power accumulation mode organic electrochemical transistors

A small molecule additive, dodecylbenzenesulfonate (DBSA), is added to the electrolyte in OECTs to improve the device performance.

Tuning PEDOT:PSS conductivity to obtain complementary organic electrochemical transistor

Abstract Organic electrochemical transistors (OECT) have been gaining importance due to their applications in biological and chemical sensing along with logic circuitry. Most OECT studies have utilized poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the channel material which operates in depletion mode. Here, we have used PEDOT:PSS as the channel material and have demonstrated both depletion and accumulation mode OECTs by tuning its conductivity with over-oxidation. Over-oxidized PEDOT:PSS revealed that while the PEDOT degraded PSS remained intact after oxidation leading to a reduction in the electrical conductivity leaving the ionic conductivity intact. To the best of our knowledge this is the first demonstration of the PEDOT:PSS as an accumulation OECT device. The depletion mode device exhibits a threshold voltage of −0.5 V with a transconductance of 323.6 μS as compared to the accumulation device with a threshold of 0.52 V with a transconductance of 5.19 μS. Thus, we have demonstrated the use of a single organic semiconductor for fabricating both depletion and accumulation mode OECTs. This is technologically advantageous to achieve complementary for development of sensors, logic circuits and for reduction of low power consumption of devices.

Redox‐Stability of Alkoxy‐BDT Copolymers and their Use for Organic Bioelectronic Devices

AbstractOrganic semiconductors can be employed as the active layer in accumulation mode organic electrochemical transistors (OECTs), where redox stability in aqueous electrolytes is important for long‐term recordings of biological events. It is observed that alkoxy‐benzo[1,2‐b:4,5‐b′]dithiophene (BDT) copolymers can be extremely unstable when they are oxidized in aqueous solutions. The redox stability of these copolymers can be improved by molecular design of the copolymer where it is observed that the electron rich comonomer 3,3′‐dimethoxy‐2,2′‐bithiophene (MeOT2) lowers the oxidation potential and also stabilizes positive charges through delocalization and resonance effects. For copolymers where the comonomers do not have the same ability to stabilize positive charges, irreversible redox reactions are observed with the formation of quinone structures, being detrimental to performance of the materials in OECTs. Charge distribution along the copolymer from density functional theory calculations is seen to be an important factor in the stability of the charged copolymer. As a result of the stabilizing effect of the comonomer, a highly stable OECT performance is observed with transconductances in the mS range. The analysis of the decomposition pathway also raises questions about the general stability of the alkoxy‐BDT unit, which is heavily used in donor–acceptor copolymers in the field of photovoltaics.

Conjugated Polyelectrolyte Blends for Highly Stable Accumulation-Mode Electrochemical Transistors

Counterion exchange has been introduced as a method to modify properties of anionic conjugated polyelectrolyte (CPE) blends. Blending of two self-doped CPEs having metallic and semiconducting behavior has been achieved from two different solvents, by exchanging the counterion of the metallic component. Different blending conditions lead to films exhibiting different optical properties, depending on the aggregation states of the CPEs. Conductance responses for the blends showed the opportunity to tune threshold voltage of the films both by blending and counterion exchange. Therefore, the blends have been exploited for the fabrication of accumulation mode organic electrochemical transistors. These devices exhibit short switching times and high transconductance, up to 15.3 mS, as well as high stability upon fast pulsed cycles, retaining 88% of the drain currents after 2 × 103 cycles.