Side Chain Regioregularity Enables High-Performance and Sustainable Organic Electrochemical Transistors

Open AccessCCS ChemistryRESEARCH ARTICLES13 Aug 2024Side Chain Regioregularity Enables High-Performance and Sustainable Organic Electrochemical Transistors Liuyuan Lan†, Junxin Chen†, Huiqing Hou, Jiayao Duan, Yiming Wang, Yuze Lin, Iain McCulloch and Wan Yue Liuyuan Lan† State Key Laboratory of Optoelectronic Materials and Technologies, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Guangzhou Key Laboratory of Flexible Electronic Materials and Wearable Devices, School of Materials and Engineering, Sun Yat-Sen University, Guangzhou 510275 , Junxin Chen† State Key Laboratory of Optoelectronic Materials and Technologies, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Guangzhou Key Laboratory of Flexible Electronic Materials and Wearable Devices, School of Materials and Engineering, Sun Yat-Sen University, Guangzhou 510275 , Huiqing Hou Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids and Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Jiayao Duan State Key Laboratory of Optoelectronic Materials and Technologies, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Guangzhou Key Laboratory of Flexible Electronic Materials and Wearable Devices, School of Materials and Engineering, Sun Yat-Sen University, Guangzhou 510275 , Yiming Wang State Key Laboratory of Optoelectronic Materials and Technologies, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Guangzhou Key Laboratory of Flexible Electronic Materials and Wearable Devices, School of Materials and Engineering, Sun Yat-Sen University, Guangzhou 510275 , Yuze Lin Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids and Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 , Iain McCulloch Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Oxford OX1 3TA Andlinger Center for Energy and the Environment, Department of Electrical and Computer Engineering, Princeton University, Princeton, NJ 08544 and Wan Yue *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Optoelectronic Materials and Technologies, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, Guangzhou Key Laboratory of Flexible Electronic Materials and Wearable Devices, School of Materials and Engineering, Sun Yat-Sen University, Guangzhou 510275 Cite this: CCS Chemistry. 2024;0:1–14https://doi.org/10.31635/ccschem.024.202404746 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail High-performance organic electrochemical transistors (OECTs) with sustainable processes are crucial for bioelectronics and integration applications, but still face challenges in molecular design, as well as solvent-device compatibility. Herein, we introduced a unique synthetic protocol focused on regioselective chemistry for the development of a donor–acceptor polymeric mixed ionic–electronic conductor (PMIEC) with a well-defined side chain arrangement and demonstrated the superiority of side chain regioregularity in enhancing OECT performance. Furthermore, we pioneered the utilization of a green solvent, 2-methyl tetrahydrofuran (MeTHF), for depositing the active OECT channel layers. We found that the regioregular copolymer exhibited over three times higher μC* of up to 810 F cm−1 V−1 s−1 compared to its regioirregular counterpart, thanks to improved crystallinity, reduced trap density of states (tDOS), and enhanced OECT hole mobility. Notably, this was achieved without the need for additional film post-treatments or specialized polymer fractionation techniques and stood among the highest values reported to date for green-solvent-processed OECTs. Our work represents a significant advancement in sustainable OECTs and highlights the importance of precise control over side chain regioregularity in developing high-performance PMIECs. Download figure Download PowerPoint Introduction Sustainable bioelectronics not only paves the way for a more technologically advanced future but also addresses the pressing need for environmental consciousness.1–5 As we delve deeper into research in this field, it becomes increasingly crucial to integrate sustainability at every stage of development—from materials selection to processing techniques and device architectures.6–8 In this respect, the emergence of organic electrochemical transistors (OECTs) has opened up exciting opportunities by offering numerous advantages over traditional electronic components such as low operating voltage, high sensitivity, flexibility, and compatibility with aqueous media.9–12 OECTs hold tremendous promise in fields such as biosensing, medical diagnostics, artificial synapses, and neuromorphic computing.13–17 To further advance these practical applications, continuous material innovation is imperative to enhance performance and functionality while implementing sustainable manufacturing processes to minimize environmental impact. OECTs utilize organic mixed ionic–electronic conductors (OMIECs) as channel materials, enabling the penetration of ions from an electrolyte into the bulk of the channel material under a gate bias. This process, known as electrochemical doping/de-doping, plays a crucial role in altering the channel conductivity. A figure of merit commonly used for performance benchmarking is the product of charge carrier mobility and volumetric capacitance (μC*).18 From the perspective of molecular design for OMIECs, the incorporation of oligoethylene glycol (OEG) side chains in conjunction with backbone engineering offers a robust approach to enhance ionic/electronic conduction in conjugated polymers (CPs), due to its synthetic versatility and ability to fine-tune material properties by tailoring the length, branching position, and composition of hydrophilic side chains.19–21 In the past few years, driven by the development of OEG functionalized CPs, OECTs have made significant strides in terms of performance and functionality, achieving impressive μC* values exceeding 500 F cm−1 V−1 s−1.22–30 It is noteworthy that the donor–acceptor (D–A) copolymers, characterized by an alternating arrangement of D and A units along their backbones, exhibit strong interchain interactions, tunable electronic properties, and versatile design options. These attributes position them as a highly promising class of mixed conductors that have garnered significant attention in the OECT community.31–40 McCulloch's group33 reported a series of D–A copolymers based on diketopyrrolopyrrole (DPP), with the glycolated polymer p(gDPP-T2) (Figure 1a) showing improved polaron delocalization and better OECT performance (μC* = 377 F cm−1 V−1 s−1). Yue's group39 synthesized a range of D–A copolymers via post-polymerization, and the utilization of g2T2-gBT4 in OECTs yielded a μC* of 359 F cm−1 V−1 s−1. Very recently, Yoon's group40 developed DPP-based D–A copolymers through hybrid side chain engineering, resulting in a high μC* of 702 F cm−1 V−1 s−1 for PDPP-4EG-based OECTs, which was further increased to 809 F cm−1 V−1 s−1 using a unidirectional film alignment technique. Though progress in exploring D–A copolymers for OECTs, the current landscape remains limited in both quantity and diversity, with device performance lagging behind that of all-donor counterparts.30 Given their highly tunable energy levels and significant promise in mitigating noncapacitive Faradaic side reactions during OECT operation,33,41 it is worthwhile to focus more on optimizing the molecular design to fully unlock the potential of D–A copolymers for OECT applications. Figure 1 | (a) The representative D–A copolymers previously reported in the literature for OECTs. (b) The regioirregular and regioregular D–A copolymers developed in this work for OECTs. Download figure Download PowerPoint Enhancing the ionic and electrical conductivities in D–A polymeric mixed conductor necessitates comprehensive considerations involving meticulous tailoring of the donor/acceptor structure, fine-tuning of molecular packing/orientation, and precise control over intermolecular interactions. Conventional wisdom suggests that manipulating regioregularity, which refers to the orientation or orderliness of asymmetric units along the backbone vector, generally exerts a positive influence on charge transport property by enhancing intermolecular interactions and reducing energetic disorder in organic field-effect transistors (OFETs) field.42–45 Keeping in mind the fact that, in OFETs, which are solely dependent on electronic processes, the arrangement of densely packed polymer chains or crystalline domains is crucial for efficient charge carrier transport. For OECTs, however, charge conduction is primarily influenced by volumetric doping/dedoping, induced by ions injection, achieving an optimal device performance relies on maintaining a delicate balance between electronic and ionic conductivities due to their intrinsic contradictions.46–49 The fundamental difference in charge transport mechanisms between OFETs and OECTs underscores the need to further elucidate the relationship between charge/ion transport and the regioregularity effect in OMIECs, which has not yet been reported. Furthermore, it is important to note that current high-performance OECT devices rely heavily on the usage of chloroform (CHCl3), a toxic solvent for processing purposes, which poses a substantial risk to both environmental and human health. In this regard, it is becoming increasingly important to consider sustainable manufacturing processes (using environmentally friendly "green" solvents) for these devices, particularly as we delve deeper into the exploration of OECT technology for bioelectronics requiring compatibility with biological systems and adherence to stringent safety standards.1,50 Hydrophilic OEG functionalized CPs are known for their high polarity, potentially conferring solubility in polar green solvents such as acetone, 2-methyl tetrahydrofuran (MeTHF), and anisole based on the principle of "like dissolves like."51 Despite these properties, there has been very limited exploration into the utilization of these green solvents for the fabrication of OECTs,52–54 mainly due to the challenge associated with the compromise between solvent compatibility and device performance. To this end, developing OMIECs that can be processed from green solvents while achieving favorable transistor characteristics is highly desired for realizing sustainable and high-throughput transistors for real bioapplications. Herein, for the first time, we exploited the regioselective chemistry within OEG-substituted benzothiadiazole (BT) and successfully developed a novel D–A polymeric mixed conductor with side chain regioregularity (R-gBT-gCPDT, Figure 1b) for OECTs. For comparison, an irregioregular counterpart (iR-gBT-gCPDT, Figure 1b) was also synthesized, sharing the same backbone but differing in the arrangement of OEG side chains on the BT moiety. Extensive investigations were conducted to thoroughly examine the influence of side chain regioregularity on their electronic, optical, charge transport, and coupled ionic–electronic transport properties. It is noteworthy that, in line with sustainable practices, MeTHF was selected as the solvent for fabricating OECT devices due to its commercial availability and biorenewable nature derived from furfural,55 and its ability to effectively dissolve both polymers (shown in Scheme 1). We found that the regular arrangement of OEG chains on BT unit along the backbone of polymer R-gBT-gCPDT caused an edge-on packing motif, enhanced crystallinity, and reduced trap density of states (tDOS), thereby resulting in a more than 6-fold increase in hole mobility up to 5.6 cm2 V−1 s−1, as compared to the regiorandom counterpart (iR-gBT-gCPDT). Impressively, R-gBT-gCPDT achieved a maximum μC* value of 810 F cm−1 V−1 s−1 in OECTs, setting a new record for D–A copolymers and ranking highest among the reported values thus far for OECTs processed using environmentally friendly solvents. This work provides valuable insights into the development of high-performance OECTs through precise control of side chain regioregularity within CP structures while prioritizing sustainable electronics through green solvent selection. Scheme 1 | The synthetic routes for copolymers iR-gBT-gCPDT and iR-gBT-gCPDT. Next to the resultant polymer lies a photograph depicting its dissolution in MeTHF at a concentration of 5 mg/mL. Download figure Download PowerPoint Experimental Methods Materials All starting materials were purchased from commercial suppliers and used as received unless stated otherwise. The detailed synthetic procedures of CPs were described in the Supporting Information. Materials characterization 1H NMR and 13C NMR spectra were recorded in deuterated solvents on a AVANCE 400 NMR Spectrometer (Bruker, Germany). 1H NMR chemical shifts are reported in ppm downfield from tetramethylsilane (TMS) reference using the residual protonated solvent as an internal standard. The number-average molecular weight (Mn) and polydispersity index of polymers were determined by a PL-GPC 220 in hexafluoroisopropanol at 45 °C using a calibration curve with standard linear polystyrene as a reference. UV–vis-NIR absorption spectra were recorded on a Cary Win spectrophotometer (Cary 5000/6000i, Agilent Technologies). Polymer thin films were deposited onto glass substrates by spin-coating using a hexafluoroisopropanol solution (5 mg mL−1) under ambient conditions. Cyclic voltammetry (CV) measurements were performed with a CHI520E electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd., China) with a three-electrode single-compartment cell equipped with a cylindrical platinum working electrode as the working electrode, Pt wire as the counter electrode, and Ag/AgCl electrode as the reference electrode. 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) dissolved in acetonitrile and aqueous solution of 0.1 M NaCl were used as the supporting electrolytes, respectively. Spectroelectrochemistry measurements Spectroelectrochemical measurements were performed using the Cary Win spectrophotometer (Cary 5000/6000i; Agilent Technologies) coupled with an electrochemical analyzer (Shanghai Chenhua Instrument Co. LTD., CHI520E, China) in a three-electrode setup. The polymers were deposited on indium tin oxide (ITO)-coated glass slides by spin-coating a MeTHF solution (5 mg mL−1) at 1500 rpm for 30 s and 3000 rpm for 30 s, which acted as the working electrode and were placed in quartz cuvette equipped with a custom-made Teflon lid. Pt wire and Ag/AgCl electrode were used as the counter electrode and the reference electrode, respectively. The supporting electrolyte used was a 0.1 M NaCl aqueous solution. A background measurement was taken with a clean bare ITO substrate. The indicated voltages were applied for 10 s until the current stabilized prior to the spectrum measurement. Device fabrication and characterization The glass substrates were cleaned via sonication in acetone, isopropyl alcohol, and deionized water and then dried by nitrogen flow. Gold contacts were patterned and deposited onto glass slides to form source/drain electrodes by a standard photolithographic/lift-off process and thermal evaporation, respectively. The channel length (L) and width (W) were 10 and 100 μm, respectively. After cleaning the substrate using piranha solution and subsequently blow-drying with a nitrogen gun, the polymer semiconductors were deposited via spin-coating from a MeTHF solution (5 mg mL−1) at 1500 rpm for 30 s and then at 3000 rpm for another 30 s, under ambient conditions. A 0.1 M NaCl aqueous solution was used as the electrolyte, while an Ag/AgCl pellet served as the gate electrode. A small poly(dimethylsiloxane) (PDMS) well was used to confine the electrolyte. The fabricated devices were characterized in the air at room temperature using a semiconductor parameter analyzer (B1500A, Keysight Technologies, Beijing, China) and an electrical probe station. Atomic force microscopy measurements Atomic force microscopy (AFM) measurements were performed in tapping mode, using Bruker dimension icon AFM with a silicon tip (Bruker FastScan®). The polymer films were spin-coated on ITO substrates from MeTHF solution (5 mg mL−1) under ambient conditions. The doping of polymer films was conducted in 0.1 M NaCl solution with a three-electrode system using an electrochemical analyzer (CHI520E; Shanghai Chenhua Instrument Co. LTD., China), with polymer-coated ITO as working electrode, Ag/AgCl pellet as reference electrode and a Pt wire as the counter electrode and applying a voltage at 0.7 V versus Ag/AgCl for 10 mins. Then the film was washed with deionized water and dried with nitrogen flow. The cyclic dedoping-doping of polymer films was conducted by subjecting them to a square-wave voltage switching between 0 and 0.7 V versus Ag/AgCl. Electrochemical impedance spectroscopy measurements Electrochemical impedance spectroscopy (EIS) measurements were performed using an impedance analyzer (CHI520E, Shanghai Chenhua Instrument Co. Ltd., China) with a three-electrode set-up identical to that used for CV measurements. Polymer-coated gold electrode with a defined area of 0.2 cm2 served as the working electrode. A platinum wire and an Ag/AgCl were employed as the counter electrode and reference electrode, respectively. The measurements were conducted in 0.1 M NaCl at a DC offset potential of 0.8 V and a sinusoidal alternating current (AC) amplitude of 10 mV, and the frequency range was set to span from 10 kHz to 1 Hz. Capacitance was calculated using the equation below: C = 1 2 π f × | I mg ( Z ) | where C is the capacitance, f is the frequency, and |Img(Z)| is the magnitude of the imaginary part of the impedance. The capacitance values extracted at 1 Hz were divided by film volume to determine the volumetric capacitance (C*). Grazing-incidence wide-angle X-ray scattering (GIWAXS) Samples for X-ray scattering were prepared using the same procedures for the preparation of active channel layer in device fabrication except using pure silicon wafers as the substrates. An incidence angle of 0.18° and a photon energy of 8 keV were used to record the scattering patterns. The two-dimensional (2D) GIWAXS patterns were collected from films with a resistivity of 0.001–30 Ohm cm−1. Trap density of states (tDOS) tDOS were performed on Keysight 4980A precision inductance, capacitance, and resistance (LCR) meter (Keysight Technologies, Beijing, China) and analyzed by using the thermal admittance spectroscopy (TAS) method under angular frequency-dependent capacitance measurement (0.02–2000 kHz). The energetic profile of tDOS was derived according to the following equation:56 N T ( E ω ) = − V bi q W d C d ω ω k B T where kB is Boltzmann's constant, ω is the angular frequency, C is the capacitance, T is the absolute temperature, W is the depletion width, and Vbi is the built-in potential. The applied angular frequency ω is defined by the following formula:56 E ω = k B T ln ( ω 0 ω )where ω0 is the attempt-to-escape frequency. The device architecture for TAS measurement was ITO/ZnO/Polymer/MoO3/Ag. Results and Discussion The synthetic routes for regioirregular and regioregular D–A copolymers, iR-gBT-gCPDT and R-gBT-gCPDT, are illustrated in Scheme 1, and the detailed synthesis procedures can be found in the Supporting Information. The incorporation of triethylene glycol (TEG) side chains into both polymers was aimed to facilitate ions insertion/transport while taking into account their molecular packing and solution processability. For the preparation of side chain regioirregular polymer (iR-gBT-gCPDT), the asymmetric electron-deficient compound gBT was first synthesized by a nucleophilic substitution reaction of 4,7-dibromo-5-fluoro-2,1,3-benzothiadiazole (fBT) and TEG monomethyl ether using sodium hydride as a base in anhydrous tetrahydrofuran (THF), which was then subjected to a standard Stille-coupling polymerization with electron-donating monomer 4,4-di[2-[2-(2-methoxyethoxy)ethoxy]ethyl]-2,6-bis(tributylstannyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT-Sn2). To access the sidechain regioregular polymer (R-gBT-gCPDT), our focus was placed on the regiochemistry of gBT. We sought to understand and control the specific position where a one-fold cross-coupling reaction occurred between compound gBT and 4,4-di[2-[2-(2-methoxyethoxy)ethoxy]ethyl]-2-tributylstannyl-4H-cyclopenta[2,1-b:3,4-b']dithiophene (CPDT-Sn1). For our experimental attempts, we opted for Pd2(dba)3/P(o-tolyl)3 as the catalyst while employing THF as the solvent. The isolated product from this reaction was analyzed utilizing 1H–1H nuclear overhauser effect (NOE) spectroscopy. As shown in Supporting Figure it an of between the of and that a new compound with high been successfully by the on BT to TEG to the in Scheme 1). the of the on we that there be an reaction within this the electron-donating effect of the TEG made its more to This reaction be in the future design and preparation of regioregular mixed was by and then with to compound employing Pd2(dba)3/P(o-tolyl)3 as the catalyst in the of to the of polymer R-gBT-gCPDT, thereby a arrangement of the side chains on BT units along the polymer iR-gBT-gCPDT and R-gBT-gCPDT were in chloroform and can be effectively dissolved into the green solvent MeTHF enabling thin film preparation through the spin-coating The number-average molecular (Mn) and molecular of the copolymers were by the using hexafluoroisopropanol as the at 45 The values were and for iR-gBT-gCPDT and R-gBT-gCPDT, while the values were found to be for iR-gBT-gCPDT and for R-gBT-gCPDT ( Supporting and at were to the of side chain regioregularity on the and electronic properties of these To the we the OEG side chains on the and BT with and while utilizing units of the To a regiorandom for a was employed to determine the of BT side chain The of the polymers are in Figure polymers exhibited backbones, as by the angle between and BT units over The value in R-gBT-gCPDT is than that of iR-gBT-gCPDT = potentially conferring a that to the of crystalline domains and enhanced charge transport properties. We further calculated the potential of the with an density value set at as shown in Figure where of low potential are by while of high potential are in For both values were to be along the conjugated mainly the nitrogen and their In the of the and exhibited positive charge characteristics the This is particularly in the of these their ability to with in the electrolyte solution. the that R-gBT-gCPDT exhibited a of in to the value of for to facilitate intermolecular Furthermore, we the of iR-gBT-gCPDT with additional side chain and as shown in Supporting Figure The values in iR-gBT-gCPDT with and were and respectively. to iR-gBT-gCPDT with a values were along the conjugated backbone in the for both while the in their positive charge the Figure 2 | molecular based on at for (a) regioirregular iR-gBT-gCPDT and (b) regioregular Molecular of iR-gBT-gCPDT and its Molecular of R-gBT-gCPDT and its Download figure Download PowerPoint The properties of the copolymers were through UV–vis-NIR absorption measurement and the are in Supporting As in Figure both absorption of D–A polymers characterized by in solution and thin a in the of to and a from to from the charge between the and We that there was difference in the absorption between iR-gBT-gCPDT and R-gBT-gCPDT, in solution or with the in high and low energy at 400 and ( Supporting from solution to film both polymers exhibited a small in yet a into the energy in absorption that the polymer chains were more densely packed in their Furthermore, the absorption for R-gBT-gCPDT was found to be than that of iR-gBT-gCPDT an enhanced which be by the regioregular arrangement of OEG side based on these absorption the were calculated to be for iR-gBT-gCPDT and for Figure | (a) UV–vis-NIR absorption spectra of copolymers iR-gBT-gCPDT and R-gBT-gCPDT in MeTHF solution and a thin (b) Cyclic of polymer thin films in a 0.1 NaCl aqueous electrolyte. UV–vis-NIR of polymer thin films for iR-gBT-gCPDT and R-gBT-gCPDT in 0.1 M NaCl aqueous electrolyte under from to 0.8 The absolute in polaron absorption