DEVELOPMENT OF HIGH-PERFORMANCE THERMOELECTRIC POLYMERS VIA DOPING OR DEDOPING ENGINEERING.

It is of great significance to develop high-performance thermoelectric (TE) materials, because they can be used to harvest waste heat into electricity and there is abundant waste heat on earth. The conventional TE materials are inorganic semimetals or semiconductors like Bi2Te3 and its derivatives. However, they have problems of high cost, scare/toxic elements, high thermal conductivity, and poor mechanical flexibility. Organic TE materials emerged as the next-generation TE materials because of their merits including solution processability, low cost, abundant element, low intrinsic thermal conductivity, and high mechanical flexibility. Organic TE materials are mainly conducting polymers because of their high conductivity. Both the conductivity and Seebeck coefficient depend on the doping level, and they are interdependent. Hence, the TE properties of polymers can be improved through doping/dedoping engineering. There are three types of doping forms, oxidative (or reductive) doping, protonic acid doping, and charge transfer doping. Accordingly, they can be dedoped by different approaches. In this article, we review the methods to dope and dedope p-type and n-type TE polymers and the combination of doping and dedoping to optimize their TE properties. Secondary doping is also covered, since it can significantly enhance the conductivity of some TE polymers.

Adhesive, multifunctional, and wearable electronics based on MXene-coated textile for personal heating systems, electromagnetic interference shielding, and pressure sensing.

Adhesion between flexible devices and skin surface facilitates portability of devices and reliable signal acquisition from human body, which is essential for medical therapy devices or monitoring systems. Here, we utilize a simple, cost-effective, and scalable layer-by-layer dip-coating method to fabricate a skin-adhesive multifunctional textile-based device, consisting of three parts: low-cost and easily available airlaid paper (AP) substrate, conductive MXene sensitive layer, and adhesive polydimethylsiloxane (PDMS). The adhesive layer of lightly cross-linked PDMS enables the device to form conformal contact with skin even during human joint bending. The smart textile device exhibits excellent electro-thermal and photo-thermal conversion performance with good cycling stability and tunability. Furthermore, the textile electronics show good electromagnetic interference (EMI) shielding properties due to the good electrical conductivity, as well as sensitive and stable pressure sensing properties for human motion detection. Consequently, this efficient strategy provides a possible way to design multifunctional and wearable electronic textiles for medical applications.

Salt-induced ductilization and strain-insensitive resistance of an intrinsically conducting polymer.

High mechanical ductility and high mechanical strength are important for materials including polymers. Current methods to increase the ductility of polymers such as plasticization always cause a remarkable drop in the ultimate tensile strength. There is no report on the ductilization of polymers that can notably increase the elongation at break while not lowering the ultimate tensile strength. Here, we report the salt-induced ductilization of an intrinsically conducting polymer, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS). Treating highly conductive PEDOT:PSS with a salt such as sodium perchlorate can enhance its elongation at break from 8.5 to 53.2%, whereas it hardly affects the tensile strength. Moreover, the resistance of the ductilized PEDOT:PSS films is insensitive to the tensile strain before fracture and slightly increases by only ~6% during the cyclic tensile testing with the strain up to 30%. These effects are ascribed to the decrease in the Coulomb attraction between PEDOT+ and PSS- by the salt ions.

Soret Effect of Ionic Liquid Gels for Thermoelectric Conversion.

Cations and anions can accumulate at the two ends of an ionic conductor under temperature gradient, which is the so-called Soret effect. This can generate a voltage between the two electrodes, and the thermopower can be higher than that of the electronic conductors because of the Seebeck effect by 1-2 orders in magnitude. The thermoelectric properties of ionic conductors depend on the ionic thermopower, ionic conductivity, and thermal conductivity. Compared with other ionic conductors, like liquid electrolytes and hydrogels, ionogels made of an ionic liquid and a gelator can have the advantages of high thermopower and high stability. Great progress was recently made to improve the ionic conductivity and/or ionic thermopower of ionogels. They can be used in ionic thermoelectric capacitors (ITECs) to harvest heat. In addition, they can be integrated with electronic thermoelectric materials to harvest heat from both temperature gradient and temperature fluctuation, which can be caused by waste heat.

Stretchable and Self-Adhesive PEDOT:PSS Blend with High Sweat Tolerance as Conformal Biopotential Dry Electrodes.

Dry epidermal electrodes that can always form conformal contact with skin can be used for continuous long-term biopotential monitoring, which can provide vital information for disease diagnosis and rehabilitation. But, this application has been limited by the poor contact of dry electrodes on wet skin. Herein, we report a biocompatible fully organic dry electrode that can form conformal contact with both dry and wet skin even during physical movement. The dry electrodes are prepared by drop casting an aqueous solution consisting of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), poly(vinyl alcohol) (PVA), tannic acid (TA), and ethylene glycol (EG). The electrodes can exhibit a conductivity of 122 S cm-1 and a mechanical stretchability of 54%. Moreover, they are self-adhesive to not only dry skin but also wet skin. As a result, they can exhibit a lower contact impedance to skin than commercial Ag/AgCl gel electrodes on both dry and sweat skins. They can be used as dry epidermal electrodes to accurately detect biopotential signals including electrocardiogram (ECG) and electromyogram (EMG) on both dry and wet skins for the users at rest or during physical movement. This is the first time to demonstrate dry epidermal electrodes self-adhesive to wet skin for accurate biopotential detection.

Role of Ions in Hydrogels with an Ionic Seebeck Coefficient of 52.9 mV K-1.

Ionic thermoelectric (i-TE) material with mobile ions as charge carriers has the potential to generate large thermal voltages at low operating temperatures. This study highlights the role of ions in i-TE hydrogels employing a poly(vinyl alcohol) (PVA) polymer matrix and a number of ion providers, e.g., KOH, KNO3, KCl, KBr, NaI, KI, and CsI. The relationship between the intrinsic physical parameters of the ion and the thermoelectric performance is established, indicating the ability to influence the hydrogen bond by the ion is a crucial factor. Among these i-TE hydrogels, the PVA/CsI hydrogel exhibits the largest ionic Seebeck coefficient, reaching 52.9 mV K-1, which is the largest of all i-TE materials reported to date. In addition, our work demonstrates the influence of ions on polymer configuration and provides an avenue for ion selection in the Soret effect in ionic thermoelectrics.

Recent Progress of Electrode Materials for Flexible Perovskite Solar Cells.

Flexible perovskite solar cells (FPSCs) have attracted enormous interest in wearable and portable electronics due to their high power-per-weight and low cost. Flexible and efficient perovskite solar cells require the development of flexible electrodes compatible with the optoelectronic properties of perovskite. In this review, the recent progress of flexible electrodes used in FPSCs is comprehensively reviewed. The major features of flexible transparent electrodes, including transparent conductive oxides, conductive polymer, carbon nanomaterials and nanostructured metallic materials are systematically compared. And the corresponding modification strategies and device performance are summarized. Moreover, flexible opaque electrodes including metal films, opaque carbon materials and metal foils are critically assessed. Finally, the development directions and difficulties of flexible electrodes are given.

Tunable Soft Lens of Large Focal Length Change.

Tunable lens technology inspired by the human eye has opened a new paradigm of smart optical devices for a variety of applications due to unique characteristics such as lightweight, low cost, and facile fabrication over conventional lens assemblies. The fast-growing demands for tunable optical lenses in consumer electronics, medical diagnostics, and optical communications require the lens to have a large focal length modulation range and high compactness. Herein, for the first time, an all-solid tunable soft lens driven by highly transparent dielectric elastomer actuators (DEAs) based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and waterborne polyurethane (PEDOT:PSS/WPU) transparent electrodes is developed. The deformation of the tunable soft lens is achieved by the actuation of DEAs, mimicking the change of the surface profile of the human eye to achieve remarkable focal length variations. Upon electrical activation, this tunable soft lens can vary its original focal length by 209%, which is one of the highest among current tunable soft lenses and far beyond that of the human eye. This study demonstrates that transparent DEAs are capable of achieving focus-variation functions, and potentially useful in artificial robotic vision, visual prostheses, and adjustable glasses, which will induce significant effects on the future development of tunable optics.

Wirelessly operated bioelectronic sutures for the monitoring of deep surgical wounds.

Monitoring surgical wounds post-operatively is necessary to prevent infection, dehiscence and other complications. However, the monitoring of deep surgical sites is typically limited to indirect observations or to costly radiological investigations that often fail to detect complications before they become severe. Bioelectronic sensors could provide accurate and continuous monitoring from within the body, but the form factors of existing devices are not amenable to integration with sensitive wound tissues and to wireless data transmission. Here we show that multifilament surgical sutures functionalized with a conductive polymer and incorporating pledgets with capacitive sensors operated via radiofrequency identification can be used to monitor physicochemical states of deep surgical sites. We show in live pigs that the sutures can monitor wound integrity, gastric leakage and tissue micromotions, and in rodents that the healing outcomes are equivalent to those of medical-grade sutures. Battery-free wirelessly operated bioelectronic sutures may facilitate post-surgical monitoring in a wide range of interventions.

Stretchable and Sensitive Silver Nanowire-Hydrogel Strain Sensors for Proprioceptive Actuation.

Safer human-robot interactions mandate the adoption of proprioceptive actuation. Strain sensors can detect the deformation of tools and devices in unstructured and capricious environments. However, such sensor integration in surgical/clinical settings is challenging due to confined spaces, structural complexity, and performance losses of tools and devices. Herein, we report a highly stretchable skin-like strain sensor based on a silver nanowire (AgNW) layer and hydrogel substrate. Our facile fabrication method utilizes thermal annealing to modulate the gauge factor (GF) by forming multidimensional wrinkles and a layered conductive network. The developed AgNW-hydrogel (AGel) sensors sustain and exhibit a strain-sensitive profile (max. GF = ∼70) with high stretchability (200%). Due to its conformability, the sensor demonstrates efficacy in integration and motion monitoring with minimal mechanical constraints. We provide contextual cognizance of tooltip during a transoral procedure by incorporating AGel sensors and showing the fabrication methodology's versatility by developing a hybrid self-sensing actuator with real-time performance feedback.

Self-Adhesive, Stretchable, Biocompatible, and Conductive Nonvolatile Eutectogels as Wearable Conformal Strain and Pressure Sensors and Biopotential Electrodes for Precise Health Monitoring.

Conductive stretchable hydrogels and ionogels consisting of ionic liquids can have interesting application as wearable strain and pressure sensors and bioelectrodes due to their soft nature and high conductivity. However, hydrogels have a severe stability problem because of water evaporation, whereas ionogels are not biocompatible or even toxic. Here, we demonstrate self-adhesive, stretchable, nonvolatile, and biocompatible eutectogels that can always form conformal contact to skin even during body movement along with their application as wearable strain and pressure sensors and biopotential electrodes for precise health monitoring. The eutectogels consist of a deep eutectic solvent that has high conductivity, waterborne polyurethane that is an elastomer, and tannic acid that is an adhesive. They can have an elongation at a break of 178%, ionic conductivity of 0.22 mS/cm, and adhesion force of 12.5 N/m to skin. They can be used as conformal strain sensors to accurately monitor joint movement and breath. They can be even used as pressure sensors with a piezoresistive sensitivity of 284.4 kPa-1 to precisely detect subtle physical movements like arterial pulses, which can provide vital cardiovascular information. Moreover, the eutectogels can be used as nonvolatile conformal electrodes to monitor epidermal physiological signals, such as electrocardiogram (ECG) and electromyogram (EMG).

Fully organic compliant dry electrodes self-adhesive to skin for long-term motion-robust epidermal biopotential monitoring.

Wearable dry electrodes are needed for long-term biopotential recordings but are limited by their imperfect compliance with the skin, especially during body movements and sweat secretions, resulting in high interfacial impedance and motion artifacts. Herein, we report an intrinsically conductive polymer dry electrode with excellent self-adhesiveness, stretchability, and conductivity. It shows much lower skin-contact impedance and noise in static and dynamic measurement than the current dry electrodes and standard gel electrodes, enabling to acquire high-quality electrocardiogram (ECG), electromyogram (EMG) and electroencephalogram (EEG) signals in various conditions such as dry and wet skin and during body movement. Hence, this dry electrode can be used for long-term healthcare monitoring in complex daily conditions. We further investigated the capabilities of this electrode in a clinical setting and realized its ability to detect the arrhythmia features of atrial fibrillation accurately, and quantify muscle activity during deep tendon reflex testing and contraction against resistance.

Significant Enhancement in the Seebeck Coefficient and Power Factor of p-Type Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) through the Incorporation of n-Type MXene.

Thermoelectric (TE) materials are important for sustainable development because they can directly convert heat into electricity. Compared with inorganic TE materials, conductive polymers have demonstrated unique benefits and their irreplaceability. But their TE properties, particularly the Seebeck coefficient, must be greatly enhanced for practical application. In this work, MXene (Ti3C2Tx), an n-type two-dimensional material, is blended into p-type poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The Seebeck coefficient of the composites increases with the increasing MXene loading at the MXene loading below 33 wt % and then decreases with further increasing of the MXene loading. MXene can enhance the Seebeck coefficient from 23 up to 57.3 µV K-1 and the power factor from 44.1 up to 155 µW m-1 K-2. For the first time, enhancement in the Seebeck coefficient of a p-type TE polymer by an n-type filler has been achieved. Enhancement in the Seebeck coefficient is ascribed to energy filtering of charge carriers by the internal electric field arising from the electron transfer from MXene to PEDOT:PSS. The internal electric field can filter the charge carriers with low energy and thus enhance the Seebeck coefficient.

Biocompatible Conductive Polymers with High Conductivity and High Stretchability.

Stretchable electronic materials have drawn strong interest due to their important applications in areas such as bioelectronics, wearable devices, and soft robotics. The stretchable electrode is an integral unit of stretchable systems. Intrinsically conductive polymers such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) can have high mechanical flexibility and good biocompatibility. However, their electrical conductivity and mechanical stretchability should be greatly improved for its applications as the stretchable electrode. Here, we report highly conductive and highly stretchable PEDOT:PSS by incorporating biocompatible d-sorbitol. d-Sorbitol can serve as both the secondary dopant and plasticizer for PEDOT:PSS. It can not only significantly improve the conductivity but also the stretchability. d-Sorbitol-PEDOT:PSS (s-PEDOT:PSS) can have a conductivity of >1000 S/cm, and the conductivity could be maintained at a strain up to 60%. The resistance of s-PEDOT:PSS remains almost constant during repeated stretching-releasing cycles. The mechanism for the stretchability improvement by d-sorbitol is ascribed to the softening of PSSH chains. d-Sorbitol can position among the PSSH chains and thus destructs the hydrogen bonds among the PSSH chains. This makes the conformational change of the PSSH chains under stress become easy and thus increases the mechanical flexibility of PEDOT:PSS. This conductivity is the highest for biocompatible intrinsically conductive polymers with high stretchability.

Enhancement in the photovoltaic performance of planar perovskite solar cells by perovskite cluster engineering using an interfacial energy modifier.

The grain size and quality of hybrid organic-inorganic perovskite (HOIP) films greatly affect the performance of perovskite solar cells (PSCs). However, dripping an anti-solvent during the spin coating process induces rapid nucleation and reduces the grain size. Here, a facile method is developed to engineer clusters in precursor solution and obtain high-quality perovskite films with an enlarged grain size. A cluster interfacial modifier, chlorobenzene (CB), is added to precursor solution. The modifier increases the interfacial energy between the precursor cluster and the solvent. The increased interfacial energy suppresses the nucleation and gives rise to HOIP films with large grains and high crystallinity. The efficiency of PSCs based on this method is greatly improved from 17.55% to 19.5%.

Sequential Solution Polymerization of Poly(3,4-ethylenedioxythiophene) Using V2O5 as Oxidant for Flexible Touch Sensors.

Various in situ synthesis methods have been developed for the polymerization of 3,4-ethylenedioxythiophene monomers, such as electropolymerization, oxidative chemical vapor deposition, and vapor phase polymerization. Meeting industrial requirements through these techniques has, however, proven challenging. Here, we introduce an alternative method to fabricate highly conductive poly(3,4-ethylenedioxythiophene) (PEDOT) films in situ by solution means. The process involves sequential deposition of oxidants (V2O5 in this case) and monomers. Excess reactants and by-products can be completely removed from the PEDOT film by MeOH rinsing. The obtained PEDOT films possess good crystallinity and high doping level, with carrier concentration three orders of magnitude higher than that of the commercial product (PH1000, Heraeus GmbH). The electrical conductivity of the as-cast PEDOT film reaches up to 1,420 S/cm. In addition, this method is fully compatible with large-scale printing techniques. These PEDOT conducting films enable the realization of flexible touch sensors, which demonstrate superior flexibility and sensitivity.

Soft Electronically Functional Polymeric Composite Materials for a Flexible and Stretchable Digital Future.

Flexible/stretchable electronic devices and systems are attracting great attention because they can have important applications in many areas, such as artificial intelligent (AI) robotics, brain-machine interfaces, medical devices, structural and environmental monitoring, and healthcare. In addition to the electronic performance, the electronic devices and systems should be mechanically flexible or even stretchable. Traditional electronic materials including metals and semiconductors usually have poor mechanical flexibility and very limited elasticity. Three main strategies are adopted for the development of flexible/stretchable electronic materials. One is to use organic or polymeric materials. These materials are flexible, and their elasticity can be improved through chemical modification or composition formation with plasticizers or elastomers. Another strategy is to exploit nanometer-scale materials. Many inorganic materials in nanometer sizes can have high flexibility. They can be stretchable through the composition formation with elastomers. Ionogels can be considered as the third type of materials because they can be stretchable and ionically conductive. This article provides the recent progress of soft functional materials development including intrinsically conductive polymers for flexible/stretchable electrodes, and thermoelectric conversion and polymer composites for large area, flexible stretchable electrodes, and tactile sensors.

Recent Development of Thermoelectric Polymers and Composites.

Thermoelectric materials can be used as the active materials in thermoelectric generators and as Peltier coolers for direct energy conversion between heat and electricity. Apart from inorganic thermoelectric materials, thermoelectric polymers have been receiving great attention due to their unique advantages including low cost, high mechanical flexibility, light weight, low or no toxicity, and intrinsically low thermal conductivity. The power factor of thermoelectric polymers has been continuously rising, and the highest ZT value is more than 0.25 at room temperature. The power factor can be further improved by forming composites with nanomaterials. This article provides a review of recent developments on thermoelectric polymers and polymer composites. It focuses on the relationship between thermoelectric properties and the materials structure, including chemical structure, microstructure, dopants, and doping levels. Their thermoelectric properties can be further improved to be comparable to inorganic counterparts in the near future.

Investigation on the structural, morphological, electronic and photovoltaic properties of a perovskite thin film by introducing lithium halide.

The performance of perovskite solar cells (PSCs) including device efficiency and stability is mainly dependent on the perovskite film properties which are critically related to the organic cations used. Herein, we studied the role that the inorganic lithium (Li) cation played in perovskite thin films and its influence on crystal growth, film properties, and device performance. We found that within the threshold limit of a 1.0% molar ratio, the Li dopant had a positive effect on the film formation and properties. However, after replacing more MA+ with Li+, the device performance was degraded significantly with reduced short-circuit current density (J sc) and fill factor (FF) values. With a doping ratio of 10 mol%, the film morphology, crystallinity, photophysical, and electronic properties totally changed due to the unstable nature of the Li doped, distorted 3-D perovskite structure. The Li doping mechanism was discussed, and it was thought to contain two different doping mechanisms. One is interstitial doping at the much lower doping ratio, and the other is substitutional doping for the MA cation at the higher doping ratio.

The Effect of Methylammonium Iodide on the Supersaturation and Interfacial Energy of the Crystallization of Methylammonium Lead Triiodide Single Crystals.

It is very important to study the crystallization of hybrid organic-inorganic perovskites because their thin films are usually prepared from solution. The investigation on the growth of perovskite films is however limited by their polycrystallinity. In this work, methylammonium lead triiodide single crystals grown from solutions with different methylammonium iodide (MAI):lead iodide (PbI2 ) ratios were investigated. We observed a V-shaped dependence of the crystallization onset temperature on the MAI:PbI2 ratio. This is attributed to the MAI effects on the supersaturation of precursors and the interfacial energy of the crystal growth. At low MAI:PbI2 ratio (<1.7), more MAI leads to the supersaturation of the precursors at lower temperature. At high MAI:PbI2 ratio, the crystal growing plans change from (100)-plane dominated to (001)-plane dominated. The latter have higher interfacial energy than the former, leading to a higher crystallization onset temperature.