Tuning of the elastic modulus of a soft polythiophene through molecular doping

Poly(3-hexylthiophene) as a versatile semiconducting polymer for cutting-edge bioelectronics

Semiconducting polymers (SPs), widely used in organic optoelectronics, are gaining interest in bioelectronics owing to their intrinsic optical properties, conductivity, biocompatibility, flexibility, and chemical tunability. Among them, poly(3-hexylthiophene) (P3HT) has attracted great attention as a versatile SP, being both optically active and conductive, for the fabrication of smart materials (e.g., films and nanoparticles), allowing the modulation of their performance and final biomedical applications. This review article provides an overview of the design of different kinds of P3HT-based materials, from chemical properties to structural engineering, to be used as key opto-electronic components in the development of opto-transducers for the modulation of cell fate, as well as biosensors such as organic electrochemical transistors (OECTs) and organic field effect transistors (OEFTs). Finally, their foremost applications in the biomedical field ranging from tissue engineering to biosensing will be discussed, including the future perspectives of P3HT derivatives towards cutting-edge applications for bioelectronics, in which optoceutics plays a key role.

Benchmarking the Elastic Modulus of Conjugated Polymers with Nanoindentation

The elastic modulus is a critical parameter for the design of conjugated polymers for wearable electronics and correlates with electrical and thermal transport. Yet, widely different values have been reported for the same material because of the influence of processing and measurement conditions, including the temperature, mode, direction, and time scale of deformation. Thus, results obtained via different methods are usually not considered to be comparable. Here, disparate techniques from nanoindentation to tensile testing of free-standing films or films on water, buckling analysis, dynamic mechanical thermal analysis, oscillatory shear rheometry, and atomic force microscopy are compared. Strikingly, elastic modulus values obtained for the same batch of regioregular poly(3-hexylthiophene) differ by a factor of less than four, which suggests that an approximate comparison is possible. Considering the small amount of material that is typically available, nanoindentation in combination with creep analysis is identified as a reliable method for probing the elastic modulus of films with widely different elastic moduli ranging from less than 0.1 GPa in the case of a polythiophene with oligoether side chains to several GPa for polymers without side chains. Since films can display anisotropic elastic modulus values, it is proposed that nanoindentation is complemented with an in-plane technique such as tensile testing to ensure a full characterization using different modes of deformation.

Impact of Sequential Chemical Doping on the Thin Film Mechanical Properties of Conjugated Polymers

Conjugated polymer (CP) films with nanometer-scale thickness exhibit unique properties distinct from their bulk counterparts, which is an important consideration for their end application as thin film devices. In the realm of organic electronic devices, enabling high electrical conductance properties of CPs often necessitates doping. However, the impact of doping on intrinsic polymer mechanical properties, such as the elastic modulus, in ultrathin films at device-relevant thicknesses is not well understood and has not been directly measured. In this study, we quantified the effect of doping on the mechanical properties of poly(3-alkylthiophenes) (P3ATs) using pseudofree-standing tensile testing. We observed modulation of the mechanical properties of ultrathin CP films through sequential doping of P3ATs thin films (60-80 nm thick) with the molecular dopant F4TCNQ. Our findings reveal that, despite the ease of doping all P3ATs with F4TCNQ, the resulting changes in mechanical properties are highly dependent on the side-chain lengths of the P3ATs. Specifically, the elastic modulus of rubbery P3ATs with side-chain lengths of six carbons or more (e.g., P3HT and P3OT) increases significantly-by one to two times-upon F4TCNQ doping, while the modulus of the glassy poly(3-butylthiophene-2,5-diyl) (P3BT) remains nearly unchanged. Such a phenomenon is linked to the changes in the glass transition temperature (T g) of the doped film, where the rise of T g results in a large change in the modulus for P3HT samples. However, the P3BT remained in a glassy state before and after doping, exhibiting a minimal change in its mechanical properties. These insights into the mechanical behavior of doped ultrathin CP films are crucial for the design and optimization of flexible electronic devices.

Design Principles for Enhancing Both Carrier Mobility and Stretchability in Polymer Semiconductors via Lewis Acid Doping

With the rise of skin-like electronics, devices are increasingly coming into close contact with the human body, creating a demand for polymer semiconductors (PSCs) that combine stretchability with reliable electrical performance. However, balancing mechanical robustness with high carrier mobility remains a challenge. To address this, tris(pentafluorophenyl)borane (BCF) for Lewis acid doping is proposed to improve charge mobility while enhancing stretchability by increasing structural disorder. Through systematic investigation, several key structural principles have been identified to maximize the effectiveness of BCF doping in stretchable PSCs. Notably, increasing the lamellar stacking distance and reducing crystallinity facilitate the incorporation of BCF into the alkyl side-chain regions, thereby enhancing both mobility and stretchability. Conversely, stronger Lewis base groups in the main chain negatively impact these improvements. These results demonstrate that with a small addition of BCF, a two-fold increase in carrier mobility is achieved while simultaneously enhancing the crack onset strain to 100%. Furthermore, doped PSCs exhibit stable mobility retention under repeated 30% strains over 1000 cycles. This method of decoupling carrier mobility from mechanical properties opens up new avenues in the search for high-mobility stretchable PSCs.

Impact of Oligoether Side-Chain Length on the Thermoelectric Properties of a Polar Polythiophene

Conjugated polymers with oligoether side chains make up a promising class of thermoelectric materials. In this work, the impact of the side-chain length on the thermoelectric and mechanical properties of polythiophenes is investigated. Polymers with tri-, tetra-, or hexaethylene glycol side chains are compared, and the shortest length is found to result in thin films with the highest degree of order upon doping with the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). As a result, a stiff material with an electrical conductivity of up to 830 ± 15 S cm-1 is obtained, resulting in a thermoelectric power factor of about 21 μW m-1 K-2 in the case of as-cast films. Aging at ambient conditions results in an initial decrease in thermoelectric properties but then yields a highly stable performance for at least 3 months, with values of about 200 S cm-1 and 5 μW m-1 K-2. Evidently, identification of the optimal side-chain length is an important criterion for the design of conjugated polymers for organic thermoelectrics.

Dopant Enhanced Conjugated Polymer Thin Film for Low-Power, Flexible and Wearable DMMP Sensor

Conjugated polymer has the potential to be applied on flexible devices as an active layer, but further investigation is still hindered by poor conductivity and mechanical stability. Here, this work demonstrates a dopant-enhanced conductive polymer thin film and its application in dimethyl methylphosphonate (DMMP) sensor. Among five comparable polymers this work employs, poly(bisdodecylthioquaterthiophene) (PQTS12) achieves the highest doping efficiency after doped by FeCl3, with the conductivity increasing by about five orders of magnitude. The changes in Young's modulus are also considered to optimize the conductivity and flexibility of this thin film, and finally the decay of conductivity is only 9.2% after 3000 times of mechanical bending. This work applies this thin film as the active layer of the DMMP gas sensor, which could be operated under 1 mV driving voltage and 28 nW power consumption, with a sustainable durability against bending and compression. In addition, this sensor is provided with alarm capability while exposed to the DMMP atmospheres at different hazard levels. This work expects that this general approach could offer solutions for the fabrication of low-power and flexible gas sensors, and provide guidance for next-generation wearable devices with broader applications.

Electrochemical modulation of mechanical properties of glycolated polythiophenes

Electrochemical doping of organic mixed ionic-electronic conductors is key for modulating their conductivity, charge storage and volume enabling high performing bioelectronic devices such as recording and stimulating electrodes, transistors-based sensors and actuators. However, electrochemical doping has not been explored to the same extent for modulating the mechanical properties of OMIECs on demand. Here, we report a qualitative and quantitative study on how the mechanical properties of a glycolated polythiophene, p(g3T2), change in situ during electrochemical doping and de-doping. The Young's modulus of p(g3T2) changes from 69 MPa in the dry state to less than 10 MPa in the hydrated state and then further decreases down to 0.4 MPa when electrochemically doped. With electrochemical doping-dedoping the Young's modulus of p(g3T2) changes by more than one order of magnitude reversibly, representing the largest modulation reported for an OMIEC. Furthermore, we show that the electrolyte concentration affects the magnitude of the change, demonstrating that in less concentrated electrolytes more water is driven into the film due to osmosis and therefore the film becomes softer. Finally, we find that the oligo ethylene glycol side chain functionality, specifically the length and asymmetry, affects the extent of modulation. Our findings show that glycolated polythiophenes are promising materials for mechanical actuators with a tunable modulus similar to the range of biological tissues, thus opening a pathway for new mechanostimulation devices.

Impact of doping on the mechanical properties of conjugated polymers

Conjugated polymers exhibit a unique portfolio of electrical and electrochemical behavior, which - paired with the mechanical properties that are typical for macromolecules - make them intriguing candidates for a wide range of application areas from wearable electronics to bioelectronics. However, the degree of oxidation or reduction of the polymer can strongly impact the mechanical response and thus must be considered when designing flexible or stretchable devices. This tutorial review first explores how the chain architecture, processing as well as the resulting nano- and microstructure impact the rheological and mechanical properties. In addition, different methods for the mechanical characterization of thin films and bulk materials such as fibers are summarized. Then, the review discusses how chemical and electrochemical doping alter the mechanical properties in terms of stiffness and ductility. Finally, the mechanical response of (doped) conjugated polymers is discussed in the context of (1) organic photovoltaics, representing thin-film devices with a relatively low charge-carrier density, (2) organic thermoelectrics, where chemical doping is used to realize thin films or bulk materials with a high doping level, and (3) organic electrochemical transistors, where electrochemical doping allows high charge-carrier densities to be reached, albeit accompanied by significant swelling. In the future, chemical and electrochemical doping may not only allow modulation and optimization of the electrical and electrochemical behavior of conjugated polymers, but also facilitate the design of materials with a tunable mechanical response.

Flexible switch matrix addressable electrode arrays with organic electrochemical transistor and pn diode technology

Due to their effective ionic-to-electronic signal conversion and mechanical flexibility, organic neural implants hold considerable promise for biocompatible neural interfaces. Current approaches are, however, primarily limited to passive electrodes due to a lack of circuit components to realize complex active circuits at the front-end. Here, we introduce a p-n organic electrochemical diode using complementary p- and n-type conducting polymer films embedded in a 15-μm -diameter vertical stack. Leveraging the efficient motion of encapsulated cations inside this polymer stack and the opposite doping mechanisms of the constituent polymers, we demonstrate high current rectification ratios ([Formula: see text]) and fast switching speeds (230 μs). We integrate p-n organic electrochemical diodes with organic electrochemical transistors in the front-end pixel of a recording array. This configuration facilitates the access of organic electrochemical transistor output currents within a large network operating in the same electrolyte, while minimizing crosstalk from neighboring elements due to minimized reverse-biased leakage. Furthermore, we use these devices to fabricate time-division-multiplexed amplifier arrays. Lastly, we show that, when fabricated in a shank format, this technology enables the multiplexing of amplified local field potentials directly in the active recording pixel (26-μm diameter) in a minimally invasive form factor with shank cross-sectional dimensions of only 50×8 [Formula: see text].

Spontaneous Modulation Doping in Semi-Crystalline Conjugated Polymers Leads to High Conductivity at Low Doping Concentration

The possibility to control the charge carrier density through doping is one of the defining properties of semiconductors. For organic semiconductors, the doping process is known to come with several problems associated with the dopant compromising the charge carrier mobility by deteriorating the host morphology and/or introducing Coulomb traps. While for inorganic semiconductors these factors can be mitigated through (top-down) modulation doping, this concept has not been employed in organics. Here, this work shows that properly chosen host/dopant combinations can give rise to spontaneous, bottom-up modulation doping, in which the dopants preferentially sit in an amorphous phase, while the actual charge transport occurs predominantly in a crystalline phase with an unaltered microstructure, spatially separating dopants and mobile charges. Combining experiments and numerical simulations, this work shows that this leads to exceptionally high conductivities at relatively low dopant concentrations.

Mechanically Adaptive Mixed Ionic-Electronic Conductors Based on a Polar Polythiophene Reinforced with Cellulose Nanofibrils

Conjugated polymers with oligoether side chains are promising mixed ionic-electronic conductors, but they tend to feature a low glass transition temperature and hence a low elastic modulus, which prevents their use if mechanical robust materials are required. Carboxymethylated cellulose nanofibrils (CNF) are found to be a suitable reinforcing agent for a soft polythiophene with tetraethylene glycol side chains. Dry nanocomposites feature a Young's modulus of more than 400 MPa, which reversibly decreases to 10 MPa or less upon passive swelling through water uptake. The presence of CNF results in a slight decrease in electronic mobility but enhances the ionic mobility and volumetric capacitance, with the latter increasing from 164 to 197 F cm^-3 upon the addition of 20 vol % CNF. Overall, organic electrochemical transistors (OECTs) feature a higher switching speed and a transconductance that is independent of the CNF content up to at least 20 vol % CNF. Hence, CNF-reinforced conjugated polymers with oligoether side chains facilitate the design of mechanically adaptive mixed ionic-electronic conductors for wearable electronics and bioelectronics.