Influence of Backbone Curvature on the Organic Electrochemical Transistor Performance of Glycolated Donor–Acceptor Conjugated Polymers

Abstract

Two new glycolated semiconducting polymers PgBT(F)2gT and PgBT(F)2gTT of differing backbone curvatures were designed and synthesised for application as p-type accumulation mode organic electrochemical transistor (OECT) materials. Both polymers demonstrated stable and reversible oxidation, accessible within the aqueous electrochemical window, to generate polaronic charge carriers. OECTs fabricated from PgBT(F)2gT featuring a curved backbone geometry attained a higher volumetric capacitance of 170 F cm À3 . However, PgBT(F)2gTT with a linear backbone displayed overall superior OECT performance with a normalised peak transconductance of 3.00 10 4 mS cm À1 , owing to its enhanced order, expediting the charge mobility to 0.931 cm 2 V À1 s À1 . Currentdeviceresearchwithintheemergentfieldoforganicbioelectronics is centred around the organic electrochemical transistor (OECT), [1] which is recognised as a functional amplifier for biosensing [2] as well as a materials testbed device from which we can springboard to other bioelectronic functionalities. [3][4][5] Unlike organic field-effect transistors (OFETs), the modulation of charge carrying polarons/bipolarons in an OECT active material is achieved throughout the bulk of the film by gate potential induced electrochemical oxidation or reduction, giving rise to its superior volumetric capacitance. [6] The electrochemical redox switching of OECTs necessitates volumetric and stoichiometric active material counterion accessibility, raising unique challenges associated with the design of OECT active materials. [7] Earlier OECT conjugated polymers integrated ionic components either onto the sidechains (e.g. poly(6-(thiophene-3-yl)hexane-1-sulfonate); PTHS) [8] or within separate but intimately mixed domains (e.g. poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate); PEDOT:PSS), [9,10] to engender mixed ionic and electronic conductivity. Aqueous solubility was a drawback of these ionic OECT materials, requiring performance diminishing cross-linkers to be implemented into the active layer. [11,12] Thus, OECT active material designs pivoted towards the incorporation of oligomeric glycol sidechains, as these facilitate ionic diffusion without conferring aqueous solubility. [2,13,14] Several all donor thiophene-centric glycolated conjugated polymers have been reported for p-type OECT applications. [15] Recently, developments in OECT polymer designs have progressed towards donor-acceptor (D-A) conjugated backbones. [16][17][18][19] Refined energy level tuning is an important advantage of applying D-A backbones, which can be exploited to improve electrochemical/OECT stability by avoidance of undesirable redox side-reactions, [20,21] as well as to ensure OECT operation in favourable accumulation mode, where channel conductivity is negligible at resting gate potential, and grows with increased gate bias (c.f. depletion mode with vice versa operational characteristics). [22] However, these advantages have come at the cost of lower OECT transconductances than those observed for (less s...

High‐Performance n‐Type Organic Electrochemical Transistors Enabled by Aqueous Solution Processing of Amphiphilicity‐Driven Polymer Assembly

“…Overall, P(NDIMTEG‐T) has advantageous crystallinity and orientation for efficient horizontal charge transport. [ 29 ]…”

“…Overall, P(NDIMTEG-T) has advantageous crystallinity and orientation for efficient horizontal charge transport. [29] As shown in Figure 3, all polymers were dissolved in chloroform to fabricate OECTs for electrical/electrochemical characterization. The representative OECT device structure is shown in Figure 3a (see Experimental Section for details).…”

Cyano‐Functionalized n‐Type Polymer with High Electron Mobility for High‐Performance Organic Electrochemical Transistors

“…[24] Till now, tremendous progress has been made in developing high-performance p-type (hole-transporting) OECT channel material, μC* of several hundred F cm −1 V −1 s −1 is reported for several p-type polymers. [25][26][27][28][29][30][31][32][33][34][35][36][37] For example, a remarkable μC* >500 F cm −1 V −1 s −1 has been achieved in polymer pgBTTT [29] with a high hole mobility of 3.44 cm 2 V −1 s −1 and C* of 164 F cm −3 . In contrast, the progress in developing n-type (electron-transporting) OECT channel material has greatly lagged behind.…”

High‐Current‐Density Organic Electrochemical Diodes Enabled by Asymmetric Active Layer Design

“…Although the operation of the proposed electrochemical diode device can be demonstrated using various types of OMIECs (vide infra), we selected PgBT(F)2gTT (Figure 1c; see Experimental section for details) as the active layer of the representative device owing to its large volumetric capacitance (>150 F cm −3 ), high carrier mobility (>1 cm 2 V −1 s −1 ), and decent operational stability in air. [ 49 ] Since PgBT(F)2gTT is a p‐type semiconductor in an accumulation‐mode OECT (Figure S1, Supporting Information), the active layer becomes doped under negative gate voltage ( V GS , i.e., positive potential on the active layer; E ), resulting in an increase not only in carrier mobility ( µ ) but also in volumetric capacitance ( C * ), which indicates the specific capacitance per volume of the active layer, as depicted in the µ – C * plot (Figure 1d). Considering the OECT operation under negative V GS , the potential of the applied V GS is fully engaged on the active material via ionic‐conduction pathway (Figure 1e), [ 50 ] resulting in an energy level shift of the active layer toward the valence band (VB).…”

“…Lastly, the E 0 of a given active material should be carefully designed to satisfy the aforementioned requirements, and should be sufficiently stable under ambient conditions to prevent uncontrolled oxidation/reduction, thereby, minimizing potential drift during the OECD operation. [ 49,54 ]…”