Significant Enhancement of the Electrical Conductivity of Conjugated Polymers by Post-Processing Side Chain Removal

Controlling the Conductivity of Cyclo[8]pyrrole Polyiodide Cocrystals via Minor Substituent Group Changes

Substituent group modifications can influence the conductivity of organic materials. However, achieving several orders of magnitude increases in conductivity through minor substituent changes remains a challenge. Here, we report the observation of such large changes in cocrystals of cyclo[8]pyrroles and polyiodides. Two cocrystals were prepared, one from all-ethyl-substituted cyclo[8]pyrrole (1•+), which forms a 2D stacked structure [(1•+)2⊃(I7)-•(I24)-], and the other from a methyl-ethyl-substituted analogue (2•+), which yields a 3D layered structure [2•+•(I16)-]. The methyl-ethyl-substituted cocrystal exhibited an approximately 1000-fold higher conductivity (6.1 × 10-1 S/cm) than its all-ethyl counterpart (4.2 × 10-4 S/cm). Cocrystal [2•+•(I16)-] demonstrated good stability, retaining the bulk of its conductivity after being exposed to air for four months or upon heating to 100 °C. The present findings highlight how substituent effects, a molecular feature readily amenable to modification, can have a profound effect on cocrystal conductivity. This work thus sets the stage for further optimization of high-performance organic conductors.

Side Chain Engineering toward Chemical Doping of Conjugated Polymers

ConspectusSolution-processable conjugated polymers are typically composed of two distinct structural components: rigid conjugated backbones and flexible side chains, each with unique roles and properties. The conjugated backbone forms the core framework of the polymer and is directly responsible for its optoelectronic properties, such as light absorption, emission, and charge transport. Meanwhile, the conjugated backbone can undergo chemical doping, where molecular dopants introduce charge carriers to modulate the carrier density and electrical conductivity. Therefore, the conjugated backbone is the critical determinant of the resulting optoelectronic performance. However, on the other hand, the flexible side chains, originally introduced to improve solution processability, were long considered chemically inert to the doping reaction. Recent advances have shown that the role of side chains is more than just improving solubility, demonstrating the significant impact of side chains on the packing of the conjugated backbone, film morphology, and electronic properties of conjugated polymers. Side chain engineering has become an essential design strategy for creating high-performance conjugated polymers in various applications.In this Account, we aim to emphasize the importance of side chain engineering toward controllable chemical doping of conjugated polymers, where side chain engineering allows us to tune the molecular packing, doping efficiency, and film morphology, thereby enhancing charge transport and optoelectronic performance. Specifically, the length, branching structures, and functional groups of the side chains can be systematically varied to control the solubility, miscibility, and interactions of conjugated polymers with dopants. For example, longer or branched side chains can improve solubility but may disrupt the π-π stacking between the conjugated backbones, thereby reducing the charge transport efficiency of the polymer. Shorter or linear side chains may enhance backbone packing and electronic coupling, though at the expense of reduced solubility. The impact of side chains on the doping process is particularly noteworthy. Although side chains are chemically inert to doping reactions, their design influences all three critical steps of the doping process: mixing, ionization, and carrierization. Side chains affect the spatial distribution of dopants during mixing, modulate the local environment to facilitate charge transfer during ionization, and influence the dissociation of ion pairs into free charge carriers during carrier generation. Functional side chains with polar groups, for example, can enhance dopant-polymer compatibility, while those with functional groups can modulate the dielectric environment to weaken ion pairing and promote free carrier generation. The interplay between side chains and the conjugated backbone is critical to achieving optimal optoelectronic performance in applications such as organic photovoltaics, field-effect transistors, and thermoelectrics. Rational side chain engineering provides a powerful tool to address these challenges in doping, morphology control, and charge transport, bringing more opportunities to design advanced conjugated polymers and chemical dopants tailored to specific applications.

Green Electrode Processing Enabled by Fluoro-Free Multifunctional Binders for Lithium-Ion Batteries

The eco-friendly processing of conjugated polymer binder for lithium-ion batteries demands improved polymer solubility by introducing functional moieties, while this strategy will concurrently sacrifice polymer conductivity. Employing the polyfluorene-based binder poly(2,7-9,9 (di(oxy-2,5,8-trioxadecane))fluorene) (PFO), soluble in water-ethanol mixtures, a novel approach is presented to solve this trade-off, which features integration of aqueous solution processing with subsequent controlled thermal-induced cleavage of solubilizing side chains, to produce hierarchically ordered structures (HOS). The thermal processing can enhance the intermolecular π-π stacking of polyfluorene backbone for better electrochemical performance. Notably, HOS-PFO demonstrated a substantial 6-7 orders of magnitude enhancement in electronic conductivity, showcasing its potential as a functional binder for lithium-ion batteries. As an illustration, HOS-PFO protected SiOx anodes, utilizing in situ side chain decomposition of PFO surrounding SiOx particles after aqueous processing are fabricated. HOS-PFO contributed to the stable cycling and high-capacity retention of practical SiOx anodes (3.0 mAh cm-2), without the use of any conducting carbon additives or fluorinated electrolyte additives. It is proposed that this technique represents a universal approach for fabricating electrodes with conjugated polymer binders from aqueous solutions without compromising conductivity.

A Multipotent Precursor Approach for the Preparation of High-Molecular Weight Conjugated Polymers with Redox Active Units

Conjugated polymers have long been recognized as key materials in organic electronics, yet, in many instances, their processability remains challenging due to their inherent poor solubility and limited polymerization degrees, which limit the scope of several materials in device fabrication. In this study, a multipotent precursor strategy is introduced that enables the synthesis of high-molecular-weight conjugated materials incorporating either anthracene or anthraquinone units from a single precursor. These latter, based on 9,10-dihydroanthracene units, can be polymerized to high polymerization degrees and possess high solubility and processability, thanks to the flexibility of the main chain and the presence of sacrificial side chains. Different post-polymerization transformations allow the selective generation of conjugated polymers, preserving the polymerization degree and generating, from an identical precursor, different conjugated polymers characterized by a different chemical nature and different electronic characteristics. Remarkably, these transformations can also be performed on the precursors in the solid state without affecting drastically their morphology. Finally, the potential of this methodology is demonstrated in the fabrication of organic field-effect transistors and organic cathodes for potassium-ion batteries.

A Carboxylate-based Hydrophilic Organic Photovoltaic Catalyst with a Large Molecular Dipole Moment for High-Performance Photocatalytic Hydrogen Evolution

Achieving ultrafast dissociation of photogenerated excitons and efficient charge transport within the photocatalyst is a fundamental issue. Additionally, enhancing the interaction between semiconductors and water is crucial for efficient photocatalytic water splitting. Herein, we synthesized a carboxylate-based hydrophilic polymer, hPTB7-Th. Exposed carboxylates enhance semiconductor-water interfacial compatibility, reducing contact resistance and accelerating charge transfer kinetics. Furthermore, the carboxylate substitution shifts polarity centers, amplifying the molecular dipole moment by 10-fold. This induces a giant built-in electric field, enabling ultrafast electron-transfer process (ca. 0.31 ps) in the hPTB7-Th:PCBM bulk heterojunction. Consequently, the hPTB7-Th:PCBM-based bulk heterojunction nanoparticles exhibit excellent photocatalytic activity, achieving an optimal hydrogen evolution rate of 111.5 mmol g-1 h-1, four times over the ester-based counterpart (PTB7-Th:PCBM). Moreover, the electrostatic stability imparted by the carboxylates endows hPTB7-Th:PCBM with outstanding operational stability, maintaining 81% of its initial hydrogen evolution rate after 100 h operation. This result places it among the state-of-the-art organic photovoltaic bulk heterojunction photocatalysts in terms of stability. This work establishes a molecular engineering strategy for high-performance bulk heterojunction photocatalysts, emphasizing synergistic optimization of hydrophilicity, dipole engineering, and interfacial dynamics.

Pyridylboronic Acid- and Poly(ethylene glycol)-Functionalized PEDOT Copolymers for Electrochemical Detection of Sialic Acid-Rich Cancer Biomarkers in Serum

The detection of carbohydrate antigen 19-9 (CA199), a critical biomarker for pancreatic, colorectal, and gastric cancers, is essential for early diagnosis and disease monitoring. Traditional antibody-based assays for CA199 detection are limited by high costs, time-consuming procedures, and susceptibility to nonspecific interference. This study presents an electrochemical biosensor based on a multifunctional poly(EDOT-PyBA-co-EDOT-PEG) copolymer designed to overcome these limitations. The biosensor integrates pyridylboronic acid (PyBA) moieties for specific recognition of sialic acid residues and poly(ethylene glycol) (PEG) chains for antifouling properties within a conductive PEDOT framework. By eliminating antibody dependency, the biosensor reduces costs, enhances stability, and simplifies the diagnostic process. Surface characterization confirmed the successful incorporation of PyBA and PEG units, while electrochemical impedance spectroscopy enabled sensitive detection of CA199 with a limit of detection of 0.05 U·mL-1 and a linear response range from 0.05 to 40 U·mL-1. Recovery tests in spiked human serum samples demonstrated excellent accuracy (97-109% recovery), validating the biosensor's reliability in complex biological matrices. The rationally designed copolymer provides both high sensitivity and specificity while maintaining resistance to biofouling in serum samples. This work establishes a scalable, cost-effective platform for glycoprotein biomarker detection, advancing the field of clinical diagnostics and cancer monitoring.

Controlling Ion Uptake in Carboxylated Mixed Conductors

Organic mixed ionic-electronic conductors (OMIECs) have garnered significant attention due to their capacity to transport both ions and electrons, making them ideal for applications in energy storage, neuromorphics, and bioelectronics. However, charge compensation mechanisms during the polymer redox process remain poorly understood, and are often oversimplified as single-ion injection with little attention to counterion effects. To advance understanding and design strategies toward next-generation OMIEC systems, a series of p-channel carboxylated mixed conductors is investigated. Varying side-chain functionality, distinctive swelling character is uncovered during electrochemical doping/dedoping with model chao-/kosmotropic electrolytes. Carboxylic acid functionalized polymers demonstrate strong deswelling and mass reduction during doping, indicating cation expulsion, while ethoxycarbonyl counterparts exhibit prominent mass increase, pointing to an anion-driven doping mechanism. By employing operando grazing incidence X-ray fluorescence (GIXRF), it is revealed that the carboxyl functionalized polymer engages in robust cation interaction, whereas ester functionalization shifts the mechanism towards no cation involvement. It is demonstrated that cations are pivotal in mitigating swelling by counterbalancing anions, enabling efficient anion uptake without compromising performance. These findings underscore the transformative influence of functionality-driven factors and side-chain chemistry in governing ion dynamics and conduction, providing new frameworks for designing OMIECs with enhanced performance and reduced swelling.

Acid-Triggered Side Chain Cleavage Leads to Doped Conjugated Polymers of High Conductivity

Cleavable side chain based conjugated polymers (CSCPs) represent a unique approach to offering solution processability with added benefits via the elimination of insulating side chains. This work highlights an optimally designed polythiophene-carboxylic acid based CSCP, POET-T2-COOH, which achieves a conductivity exceeding 350 S/cm in molecularly doped and side chain cleaved films, 100-100,000 times higher than three other structurally isomeric CSCPs. The high conductivity of POET-T2-COOH is accomplished via a new "cleavage with doping" methodology, synergistically combining a strong acid and a primary dopant. This hybrid method achieves the greatest conductivity in all isomeric CSCPs over conventional doping or cleavage techniques. The doped and side chain cleaved POET-T2-COOH displays a stable conductivity in inert atmospheres and a high work function of 5.3 eV, opening up new applications.

Unlocking Spatial Surface Energy in Porous Skeletons: a Pathway to Bridging Electronic Circuits from 2D to 3D Architectures

Conventional approaches to creating high-resolution electric circuits face challenges such as the requirement for skilled personnel and expensive equipment. In response, we propose an innovative strategy that leverages a photochemically modified porous polymer skeleton for in-situ circuit fabrication. By developing maskless surface energy manipulation that guides PEDOT:PSS-based conductive ink deposition, electric circuits with high precision, density, stability and adaptability are effortlessly engineered within or atop the porous skeleton, enabling transitions between 2D and 3D circuit configurations. This process simplifies prototyping while significantly reducing costs and maintaining efficiency, promising advancements across various technological sectors.

A Supramolecular Approach to Enhance the Optoelectronic Properties of P3HT-b-PEG Block Copolymer for Organic Field-Effect Transistors

This study investigates a supramolecular approach to elucidate the interaction between an organic semiconducting molecule, specifically butyric acid-functionalized perylene diimide, and a block copolymer comprising poly-3-hexyl thiophene-b-polyethylene glycol. This interaction results in the formation of a precisely structured nanoarchitecture within the supramolecular block copolymer, driven by the ionic interplay between the block copolymer and small organic molecules. The optical properties of the synthesized supramolecular block copolymer were characterized by using ellipsometry. Additionally, further characterization employing atomic force microscopy, differential scanning calorimetry, and X-ray diffraction provided detailed insights into the crystallinity and morphology of the nanostructure. The characterization data showed that this approach significantly influenced the tuning of morphology, crystallinity, and optical and electronic properties of the resulting nanostructure. The demonstrated methodology holds considerable promise as a strategic tool for broadening the spectrum of attainable nanomorphologies in semiconducting polymers, particularly for applications in electronics or photovoltaics.

Highly Transmissive, Processable, Highly Conducting and Stable Polythiophene Derivatives via Direct (Hetero)arylation Polymerization

The development of modern optoelectronic devices increases the need for lightweight and flexible transparent conductors. It is thus essential to develop new eco-friendly materials that can be easily processed for the fabrication of such devices. For this purpose, the synthesis of self-doped, highly conducting, transmissive, and water-processable polythiophene derivatives was performed via the direct heteroarylation polymerization method and a protection/deprotection strategy. Stable conductivities up to 1000 S cm-1 have been obtained. Champion materials (P1 and P7) were scaled and processed via the roll-to-roll compatible slot-die coating method to demonstrate their large area applicability. The best coated films exhibited optical transmittance greater than 79% at 550 nm with sheet resistances of 116 Ω □-1. These values are comparable to indium-tin oxide on plastic and thus present a viable alternative to metal oxide-based electrodes.

Self-Templating Copolymerization to Produce Robust Conductive Nanocoatings Based on Conjugated Polymer Brushes with Implementable Memristive Characteristics

An effective synthesis of conductive polymer brushes, i.e., self-templating surface-initiated copolymerization (ST-SICP), is developed. It proceeds through copolymerization of pendant thiophene groups in the precursor multimonomer poly(3-methylthienyl methacrylate) (PMTM) brushes with free 3-methylthiophene (3MT) monomers leading to PMTM-co-P3MT brushes. This approach leads to improved conformational freedom of generated conjugated poly(thiophene)-based chains and their higher share in the brushes with respect to conjugation of pendant thiophene groups only. As a result, best performing conjugated PMTM-co-P3MT brushes demonstrate high ohmic conductivity in both out-of-plane and in-plane direction. Furthermore, thanks to the covalent anchoring as well as intra- and intermolecular connections, highly stable and mechanically robust nanocoatings are produced which can survive mechanical cleaning and long-term storage under ambient conditions. Grafting of ionic poly(sodium 4-styrenesulfonate) (PSSNa) in between PMTM-co-P3MT chains brings new properties to such binary mixed brushes that can operate as thin-film memristive coating with switchable conductance. It is worth mentioning that the crucial synthetic steps, i.e., grafting of precursor PMTM brushes by surface-initiated organocatalyzed atom transfer radical polymerization (SI-O-ATRP) and PSSNa chains by surface-initiated photoiniferter-mediated polymerization (SI-PIMP) are conducted under ambient conditions using only microliter volumes of reagents providing methodology that can be considered for use beyond the laboratory scale.

Enhancing the Thermoelectric Properties of Conjugated Polymers by Suppressing Dopant-Induced Disorder

Doping is a crucial strategy to enhance the performance of various organic electronic devices. However, in many cases, the random distribution of dopants in conjugated polymers leads to the disruption of the polymer microstructure, severely constraining the achievable performance of electronic devices. Here, it is shown that by ion-exchange doping polythiophene-based P[(3HT)1-x-stat-(T)x] (x = 0 (P1), 0.12 (P2), 0.24 (P3), and 0.36 (P4)), remarkably high electrical conductivity of >400 S cm-1 and power factor of >16 µW m-1 K-2 are achieved for the random copolymer P3, ranking it among highest ever reported for unaligned P3HT-based films, significantly higher than that of P1 (<40 S cm-1, <4 µW m-1 K-2). Although both polymers exhibit comparable field-effect transistor hole mobilities of ≈0.1 cm2 V-1 s-1 in the pristine state, after doping, Hall effect measurements indicate that P3 exhibits a large Hall mobility up to 1.2 cm2 V-1 s-1, significantly outperforming that of P1 (0.06 cm2 V-1 s-1). GIWAXS measurement determines that the in-plane π-π stacking distance of doped P3 is 3.44 Å, distinctly shorter than that of doped P1 (3.68 Å). These findings contribute to resolving the long-standing dopant-induced-disorder issues in P3HT and serve as an example for achieving fast charge transport in highly doped polymers for efficient electronics.

Role of Side-Chain Free Volume on the Electrochemical Behavior of Poly(propylenedioxythiophenes)

Mixed ionic/electronic conducting polymers are versatile systems for, e.g., energy storage, heat management (exploiting electrochromism), and biosensing, all of which require electrochemical doping, i.e., the electrochemical oxidation or reduction of their macromolecular backbones. Electrochemical doping is achieved via electro-injection of charges (i.e., electronic carriers), stabilized via migration of counterions from a supporting electrolyte. Since the choice of the polymer side-chain functionalization influences electrolyte and/or ion sorption and desorption, it in turn affects redox properties, and, thus, electrochemically induced mixed conduction. However, our understanding of how side-chain versus backbone design can increase ion flow while retaining high electronic transport remains limited. Hence, heuristic design approaches have typically been followed. Herein, we consider the redox and swelling behavior of three poly(propylenedioxythiophene) derivatives, P(ProDOT)s, substituted with different side-chain motifs, and demonstrate that passive swelling is controlled by the surface polarity of P(ProDOT) films. In contrast, active swelling under operando conditions (i.e., under an applied bias) is dictated by the local side-chain free volume on the length scale of a monomer unit. Such insights deliver important design criteria toward durable soft electrochemical systems for diverse energy and biosensing platforms and new understanding into electrochemical conditioning ("break-in") in many conducting polymers.

Main-Chain Cationic Polyelectrolytes: Design, Synthesis, and Applications

Polyelectrolytes have attracted a lot of attention spanning across disciplines, including polymer chemistry, materials chemistry, chemical biology, chemical engineering, as well as device physics, as a result of their widespread applications in sensing, biomedicine, food industry, wastewater treatment, optoelectronic devices, and renewable energy. In this review, we focus on the crucial synthetic strategies of structurally different classes of main-chain cationic polyelectrolytes. As a result of the presence of charged moieties in the main polymeric backbone, their solubility and photophysical properties can be easily tuned. Main-chain cationic polyelectrolytes provide various unique characteristics, including solubility in aqueous and organic solvents, easy processability, ease of film formation, ionic interaction, main-chain-directed charge transport, high conductivity, and aggregation. These properties make the main-chain polyelectrolyte a potential candidate for numerous applications ranging from chemo- and biosensing, antibacterial activity, optoelectronics, electrocatalysis, water splitting, ion conduction, to dye-sensitized solar cells.

Control of Properties through Hydrogen Bonding Interactions in Conjugated Polymers

Molecular design is crucial for endowing conjugated polymers (CPs) with unique properties and enhanced electronic performance. Introducing Hydrogen-bonding (H-bonding) into CPs has been a broadly exploited, yet still emerging strategy capable of tuning a range of properties encompassing solubility, crystallinity, electronic properties, solid-state morphology, and stability, as well as mechanical properties and self-healing properties. Different H-bonding groups can be utilized to tailor CPs properties based on the applications of interest. This review provides an overview of classes of H-bonding CPs (assorted by the different H-bond functional groups), the synthetic methods to introduce the corresponding H-bond functional groups and the impact of H-bonding in CPs on corresponding electronic and materials properties. Recent advances in addressing the trade-off between electronic performance and mechanical durability are also highlighted. Furthermore, insights into future directions and prospects for H-bonded CPs are discussed.

Solution-Processed Semiconductor Materials as Cathode Interlayers for Organic Solar Cells

Cathode interlayers (CILs) play a crucial role in improving the photovoltaic efficiency and stability of OSCs. CILs generally consists of two kinds of materials, interfacial dipole-based CILs and SPS-based CILs. With good charge transporting ability, excellent compatibility with large-area processing methods, and highly tunable optoelectronic properties, the SPS-based CILs exhibit remarkable superiorities to their interfacial dipole-based counterparts in practical use, making them promising candidate in developing efficient CILs for OSCs. This mini-review highlights the great potential of SPS-based CILs in OSC applications and elucidates the working mechanism and material design strategy of SPS materials. Afterward, the SPS-based CIL materials are summarized and discussed in four sections, including organic small molecules, conjugated polymers, nonconjugated polymers, and TMOs. The structure-property-performance relationship of SPS-based CIL materials is revealed, which may provide readers new insight into the molecular design of SPS-based CILs. The mechanisms to endow SPS-based CILs with thickness insensitivity, resistance to environmental erosion, and photo-electric conversion ability are also elucidated. Finally, after a brief summary, the remaining issues and the prospects of SPS-based CILs are suggested.

Improving the Conductivity of Amide-Based Small Molecules through Enhanced Molecular Packing and Their Application as Hole Transport Mediators in Perovskite Solar Cells

Organic-inorganic hybrid halide perovskite solar cells (PSCs) have attracted substantial attention from the photovoltaic research community, with the power conversion efficiency (PCE) already exceeding 26%. Current state-of-the-art devices rely on Spiro-OMeTAD as the hole-transporting material (HTM); however, Spiro-OMeTAD is costly due to its complicated synthesis and expensive product purification, while its low conductivity ultimately limits the achievable device efficiency. In this work, we build upon our recently introduced family of low-cost amide-based small molecules and introduce a molecule (termed TPABT) that results in high conductivity values (∼10-5 S cm-1 upon addition of standard ionic additives), outperforming our previous amide-based material (EDOT-Amide-TPA, ∼10-6 S cm-1) while only costing an estimated $5/g. We ascribe the increased optoelectronic properties to favorable molecular packing, as shown by single-crystal X-ray diffraction, which results in close spacing between the triphenylamine blocks. This, in turn, results in a short hole-hopping distance between molecules and therefore good mobility and conductivity. In addition, TPABT exhibits a higher bandgap and is as a result more transparent in the visible range of the solar spectrum, leading to lower parasitic absorption losses than Spiro-OMeTAD, and has increased moisture stability. We applied the molecule in perovskite solar cells and obtained good efficiency values in the ∼15% range. Our approach shows that engineering better molecular packing may be the key to developing high-efficiency, low-cost HTMs for perovskite solar cells.

Evaluation on the application of conjugate materials in the sound effect and stage effect of modern dance

The emergence and application of conjugate materials provide a broader space for the performance of sound and presentation effects on the modern music stage. This article compared and analyzed the application of conjugated materials and traditional methods in modern dance sound effects and stage presentation effects through experiments, found that the application of conjugated materials on modern stages had the effect of enhancing visual effects. Its overall reflectivity, color saturation, brightness, transparency, etc. remain in the range of 78%-97%, which is better than traditional methods. In addition, the use of conjugated materials can also improve auditory performance, have greater penetration and durability, and reduce the impact of external noise; in terms of audience experience and dancer experience, the average proportions also reached 87.8893% and 89.3867% respectively. In addition, it also has high temperature resistance and antibacterial effects, with a maximum temperature resistance value of 314.28°C and an antibacterial effect of 95.86%, indicating that it can still maintain stability under high temperature conditions and has a good inhibitory effect on the proliferation of bacteria and viruses. These findings will lay the foundation for further expanding the application of conjugated materials on the modern dance stage.

Elucidating Design Rules toward Enhanced Solid-State Charge Transport in Oligoether-Functionalized Dioxythiophene-Based Alternating Copolymers

This study investigates the solid-state charge transport properties of the oxidized forms of dioxythiophene-based alternating copolymers consisting of an oligoether-functionalized 3,4-propylenedioxythiophene (ProDOT) copolymerized with different aryl groups, dimethyl ProDOT (DMP), 3,4-ethylenedioxythiophene (EDOT), and 3,4-phenylenedioxythiophene (PheDOT), respectively, to yield copolymers P(OE3)-D, P(OE3)-E, and P(OE3)-Ph. At a dopant concentration of 5 mM FeTos₃, the electrical conductivities of these copolymers vary significantly (ranging between 9 and 195 S cm^-1) with the EDOT copolymer, P(OE3)-E, achieving the highest electrical conductivity. UV-vis-NIR and X-ray spectroscopies show differences in both susceptibility to oxidative doping and extent of oxidation for the P(OE3) series, with P(OE3)-E being the most doped. Wide-angle X-ray scattering measurements indicate that P(OE3)-E generally demonstrates the lowest paracrystallinity values in the series, as well as relatively small π-π stacking distances. The significant (i.e., order of magnitude) increase in electrical conductivity of doped P(OE3)-E films versus doped P(OE3)-D or P(OE3)-Ph films can therefore be attributed to P(OE3)-E exhibiting both the highest carrier ratios in the P(OE3) series, along with good π-π overlap and local ordering (low paracrystallinity values). Furthermore, these trends in the extent of doping and paracrystallinity are consistent with the reduced Fermi energy level and transport function prefactor parameters calculated using the semilocalized transport (SLoT) model. Observed differences in carrier ratios at the transport edge (c_t) and reduced Fermi energies [η(c)] suggest a broader electronic band (better overlap and more delocalization) for the EDOT-incorporating P(OE3)-E polymer relative to P(OE3)-D and P(OE3)-Ph. Ultimately, we rationalize improvements in electrical conductivity due to microstructural and doping enhancements caused by EDOT incorporation, a structure-property relationship worth considering in the future design of highly electrically conductive systems.

Quantifying Charge Carrier Localization in PBTTT Using Thermoelectric and Spectroscopic Techniques

Chemically doped poly[2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) shows promise for many organic electronic applications, but rationalizing its charge transport properties is challenging because conjugated polymers are inhomogeneous, with convoluted optical and solid-state transport properties. Herein, we use the semilocalized transport (SLoT) model to quantify how the charge transport properties of PBTTT change as a function of iron(III) chloride (FeCl₃) doping level. We use the SLoT model to calculate fundamental transport parameters, including the carrier density needed for metal-like electrical conductivities and the position of the Fermi energy level with respect to the transport edge. We then contextualize these parameters with other polymer-dopant systems and previous PBTTT reports. Additionally, we use grazing incidence wide-angle X-ray scattering and spectroscopic ellipsometry techniques to better characterize inhomogeneity in PBTTT. Our analyses indicate that PBTTT obtains high electrical conductivities due to its quickly rising reduced Fermi energy level, and this rise is afforded by its locally high carrier densities in highly ordered microdomains. Ultimately, this report sets a benchmark for comparing transport properties across polymer-dopant-processing systems.

Highly Conductive and Solution-Processable n-Doped Transparent Organic Conductor

Transparent conductors (TCs) play a vital role in displays, solar cells, and emerging printed electronics. Here, we report a solution-processable n-doped organic conductor from copper-catalyzed cascade reactions in the air, which involves oxidative polymerization and reductive doping in one pot. The formed polymer ink is shelf-stable over 20 days and can endure storage temperatures from -20 to 65 °C. The optimized n-doped thin-film TC exhibits a low sheet resistance of 45 Ω/sq and a high transmittance (T₅₅₀ > 80%), which can rival indium tin oxide. The transparent organic conductor exhibits excellent durability under accelerated weathering tests (85 °C/85% RH). Furthermore, the n-doped polymer film can also function as an electrode material with a high volumetric capacity. When it is paired with p-doped PEDOT:PSS, a record-high coloration efficiency is obtained in a dual-polymer electrochromic device.

A guide for the characterization of organic electrochemical transistors and channel materials

The organic electrochemical transistor (OECT) is one of the most versatile devices within the bioelectronics toolbox, with its compatibility with aqueous media and the ability to transduce and amplify ionic and biological signals into an electronic output. The OECT operation relies on the mixed (ionic and electronic charge) conduction properties of the material in its channel. With the increased popularity of OECTs in bioelectronics applications and to benchmark mixed conduction properties of channel materials, the characterization methods have broadened somewhat heterogeneously. We intend this review to be a guide for the characterization methods of the OECT and the channel materials used. Our review is composed of two main sections. First, we review techniques to fabricate the OECT, introduce different form factors and configurations, and describe the device operation principle. We then discuss the OECT performance figures of merit and detail the experimental procedures to obtain these characteristics. In the second section, we shed light on the characterization of mixed transport properties of channel materials and describe how to assess films' interactions with aqueous electrolytes. In particular, we introduce experimental methods to monitor ion motion and diffusion, charge carrier mobility, and water uptake in the films. We also discuss a few theoretical models describing ion-polymer interactions. We hope that the guidelines we bring together in this review will help researchers perform a more comprehensive and consistent comparison of new materials and device designs, and they will be used to identify advances and opportunities to improve the device performance, progressing the field of organic bioelectronics.

Effects of Side-Chain Length and Functionality on Polar Poly(dioxythiophene)s for Saline-Based Organic Electrochemical Transistors

Understanding the impact of side chains on the aqueous redox properties of conjugated polymers is crucial to unlocking their potential in bioelectrochemical devices, such as organic electrochemical transistors (OECTs). Here, we report a series of polar propylenedioxythiophene-based copolymers functionalized with glyme side chains of varying lengths as well as an analogue with short hydroxyl side chains. We show that long polar side chains are not required for achieving high volumetric capacitance (C*), as short hydroxy substituents can afford facile doping and high C* in saline-based electrolytes. Furthermore, we demonstrate that varying the length of the polar glyme chains leads to subtle changes in material properties. Increasing the length of glyme side chain is generally associated with an enhancement in OECT performance, doping kinetics, and stability, with the polymer bearing the longest side chains exhibiting the highest performance ([μC*]_OECT = 200 ± 8 F cm^-1 V^-1 s^-1). The origin of this performance enhancement is investigated in different device configurations using in situ techniques (e.g., time-resolved spectroelectrochemistry and chronoamperometry). These studies suggest that the performance improvement is not due to significant changes in C* but rather due to variations in the inferred mobility. Through a thorough comparison of two different architectures, we demonstrate that device geometry can obfuscate the benchmarking of OECT active channel materials, likely due to contact resistance effects. By complementing all electrochemical and spectroscopic experiments with in situ measurements performed within a planar OECT device configuration, this work seeks to unambiguously assign material design principles to fine-tune the properties of poly(dioxythiophene)s relevant for application in OECTs.

Metal-like Charge Transport in PEDOT(OH) Films by Post-processing Side Chain Removal from a Soluble Precursor Polymer

Herein, a route to produce highly electrically conductive doped hydroxymethyl functionalized poly(3,4-ethylenedioxythiophene) (PEDOT) films, termed PEDOT(OH) with metal-like charge transport properties using a fully solution processable precursor polymer is reported. This is achieved via an ester-functionalized PEDOT derivative [PEDOT(EHE)] that is soluble in a range of solvents with excellent film-forming ability. PEDOT(EHE) demonstrates moderate electrical conductivities of 20-60 S cm^-1 and hopping-like (i.e., thermally activated) transport when doped with ferric tosylate (FeTos₃ ). Upon basic hydrolysis of PEDOT(EHE) films, the electrically insulative side chains are cleaved and washed from the polymer film, leaving a densified film of PEDOT(OH). These films, when optimally doped, reach electrical conductivities of ≈1200 S cm^-1 and demonstrate metal-like (i.e., thermally deactivated and band-like) transport properties and high stability at comparable doping levels.

Head-to-Tail and Head-to-Head Molecular Chains of Poly(p-Anisidine): Combined Experimental and Theoretical Evaluation

Poly(p-anisidine) (PPA) is a polyaniline derivative presenting a methoxy (-OCH3) group at the para position of the phenyl ring. Considering the important role of conjugated polymers in novel technological applications, a systematic, combined experimental and theoretical investigation was performed to obtain more insight into the crystallization process of PPA. Conventional oxidative polymerization of p-anisidine monomer was based on a central composite rotational design (CCRD). The effects of the concentration of the monomer, ammonium persulfate (APS), and HCl on the percentage of crystallinity were considered. Several experimental techniques such as X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), multifractal analysis, Nuclear Magnetic Resonance (13C NMR), Fourier-transform Infrared spectroscopy (FTIR), and complex impedance spectroscopy analysis, in addition to Density Functional Theory (DFT), were employed to perform a systematic investigation of PPA. The experimental treatments resulted in different crystal structures with a percentage of crystallinity ranging from (29.2 ± 0.6)% (PPA1HT) to (55.1 ± 0.2)% (PPA16HT-HH). A broad halo in the PPA16HT-HH pattern from 2θ = 10.0-30.0° suggested a reduced crystallinity. Needle and globular-particle morphologies were observed in both samples; the needle morphology might have been related to the crystalline contribution. A multifractal analysis showed that the PPA surface became more complex when the crystallinity was reduced. The proposed molecular structures of PPA were supported by the high-resolution 13C NMR results, allowing us to access the percentage of head-to-tail (HT) and head-to-head (HH) molecular structures. When comparing the calculated and experimental FTIR spectra, the most pronounced changes were observed in ν(C-H), ν(N-H), ν(C-O), and ν(C-N-C) due to the influence of counterions on the polymer backbone as well as the different mechanisms of polymerization. Finally, a significant difference in the electrical conductivity was observed in the range of 1.00 × 10-9 S.cm-1 and 3.90 × 10-14 S.cm-1, respectively, for PPA1HT and PPA16HT-HH.

Mapping Out the Nonconjugated Organic Radical Conductors via Chemical or Physical Pathways

Stable, nitroxide-based organic radicals have gained tremendous attention in a wide range of research fields, ranging from solid-state electronics to energy storage devices. While the success of these organics has been their designer flexibility and the processability that can fully potentiate the open-shell chemistry, a significant knowledge gap exists on the solid-state electronics of small-molecular radicals. Herein, we examine the structure-property relationship that governs the solid-state electronics of a model nitroxide and its derivatives by seeking the connection to their well-established, electrolyte-based chemistry. Further, we propose a general strategy of enhancing their solid-state conductivity by systematic humidity control. This study demonstrates an open-shell platform of the device operation and underlying principles thereof, which can potentially be applied in a number of future radical-based electronic devices.

Synthetic Route to Conjugated Donor–Acceptor Polymer Brushes via Alternating Copolymerization of Bifunctional Monomers

Alternating donor–acceptor conjugated polymers, widely investigated due to their applications in organic photovoltaics, are obtained mainly by cross-coupling reactions. Such a synthetic route exhibits limited efficiency and requires using, for example, toxic palladium catalysts. Furthermore, the coating process demands solubility of the macromolecules, provided by the introduction of alkyl side chains, which have an impact on the properties of the final material. Here, we present the synthetic route to ladder-like donor–acceptor polymer brushes using alternating copolymerization of modified styrene and maleic anhydride monomers, ensuring proper arrangement of the pendant donor and acceptor groups along the polymer chains grafted from a surface. As a proof of concept, macromolecules with pendant thiophene and benzothiadiazole groups were grafted by means of RAFT and metal-free ATRP polymerizations. Densely packed brushes with a thickness up to 200 nm were obtained in a single polymerization process, without the necessity of using metal-based catalysts or bulky substituents of the monomers. Oxidative polymerization using FeCl₃ was then applied to form the conjugated chains in a double-stranded (ladder-like) architecture.

Iron(III) Dopant Counterions Affect the Charge-Transport Properties of Poly(Thiophene) and Poly(Dialkoxythiophene) Derivatives

This study investigates the charge-transport properties of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(ProDOT-alt-biEDOT) (PE₂) films doped with a set of iron(III)-based dopants and as a function of dopant concentration. X-ray photoelectron spectroscopy measurements show that doping P3HT with 12 mM iron(III) solutions leads to similar extents of oxidation, independent of the dopant anion; however, the electrical conductivities and Seebeck coefficients vary significantly (5 S cm^-1 and + 82 μV K^-1 with tosylate and 56 S cm^-1 and +31 μV K^-1 with perchlorate). In contrast, PE₂ thermoelectric transport properties vary less with respect to the iron(III) anion chemistry, which is attributed to PE₂ having a lower onset of oxidation than P3HT. Consequentially, PE₂ doped with 12 mM iron(III) perchlorate obtained an electrical conductivity of 315 S cm^-1 and a Seebeck coefficient of + 7 μV K^-1. Modeling these thermoelectric properties with the semilocalized transport (SLoT) model suggests that tosylate-doped P3HT remains mostly in the localized transport regime, attributed to more disorder in the microstructure. In contrast perchlorate-doped P3HT and PE₂ films exhibited thermally deactivated electrical conductivities and metal-like transport at high doping levels over limited temperature ranges. Finally, the SLoT model suggests that PE₂ has the potential to be more electrically conductive than P3HT due to PE₂'s ability to achieve higher extents of oxidation and larger shifts in the reduced Fermi energy levels.