The interfacial energetics are known to play a crucial role in organic diodes, transistors, and sensors. Designing the metal-organic interface has been a tool to optimize the performance of organic (opto)electronic devices, but this is not reported for organic thermoelectrics. In this work, it is demonstrated that the electrical power of organic thermoelectric generators (OTEGs) is also strongly dependent on the metal-organic interfacial energetics. Without changing the thermoelectric figure of merit (ZT) of polythiophene-based conducting polymers, the generated power of an OTEG can vary by three orders of magnitude simply by tuning the work function of the metal contact to reach above 1000 mu W cm(-2). The effective Seebeck coefficient (S-eff) of a metal/polymer/metal single leg OTEG includes an interfacial contribution (V-inter/Delta T) in addition to the intrinsic bulk Seebeck coefficient of the polythiophenes, such that S-eff = S + V-inter/Delta T varies from 22.7 mu V K-1 [9.4 mu V K-1] with Al to 50.5 mu V K-1 [26.3 mu V K-1] with Pt for poly(3,4-ethylenedioxythiophene):p-toluenesulfonate [poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)]. Spectroscopic techniques are used to reveal a redox interfacial reaction affecting locally the doping level of the polymer at the vicinity of the metal-organic interface and conclude that the energetics at the metal-polymer interface provides a new strategy to enhance the performance of OTEGs.

The development of an ideal solution-processable transparent electrode has been a challenge in the field of all-solution-processed semitransparent organic solar cells (ST-OSCs). We present a novel poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) top electrode for all-solution-processed ST-OSCs through in situ doping of PEDOT:PSS. A strongly polarized long perfluoroalkyl (n = 8) chain-anchored sulfonic acid effectively eliminates insulating PSS and spontaneously crystallizes PEDOT at room temperature, leading to outstanding electrical properties and transparency of PEDOT top electrodes. Doped PEDOT-based ST-OSCs yield a high power conversion efficiency of 10.9% while providing an average visible transmittance of 26.0% in the visible range. Moreover, the strong infrared reflectivity of PEDOT enables ST-OSCs to reject 62.6% of the heat emitted by sunlight (76.7% from infrared radiation), outperforming the thermal insulation capability of commercial tint films. This light management approach using PEDOT enables ST-OSCs to simultaneously provide energy generation and energy savings, making it the first discovery toward sustainable energy in buildings.

Ionic thermoelectric supercapacitors (ITESCs) produce orders of magnitude higher voltages than those of con-ventional thermoelectrics (TEs) based on the thermo-diffusion of electrons/holes and are therefore attractive for converting low-grade heat into electricity. The stretchability and stability of the whole ITESC are important for wearable heat harvesting applications. Recent studies on ITESC have focused on stretchable ionic TE electrolytes with a giant Seebeck coefficient, but there are no reports of fully stretchable ITESCs for wearable heat harvesting devices due to the lack of stretchable electrodes and stretchable ionic TE electrolytes with stability. Herein, we present a fully stretchable ITESC composed of stable high-performance ionic thermoelectric elastomer (ITE) electrolyte and stretchable gold nanowire (AuNW) electrodes. The ITE shows excellent air stability (> 60 d) in comparison to hydrogel-based electrolytes that are susceptible to dehydration in ambient conditions. Further-more, the ITE exhibits an apparent thermopower up to 38.9 mV K-1 and ionic conductivity of 3.76 x 10-1 mS cm-1, which both are maintained up to a tensile strain of 250%. Finally, a fully stretchable ITESC with AuNW electrodes is developed which can harvest energy from thermal gradients during deformations.

Next-generation wearables will interface intimately with the human body either on-skin, implanted or woven into clothing. This requires electrical components that match the mechanical properties of biological tissues - stretchability (up to 60% strain) and softness (Youngs modulus of similar to 1 kPa to 1 MPa). As wearables become increasingly complex, the energy and mechanical requirements will increase, and an integrated power supply unit such as a soft and stretchable battery is needed to achieve autonomy and wireless operation. However, two key challenges remain for current stretchable battery technology: the mechanical performance (softness and stretchability) and its relation to the size and charge storage capacity (challenge I), and the sustainability and biocompatibility of the battery materials and its components (challenge II). Integrating all these factors into the battery design often leads to a trade-off between the various properties. This perspective will evaluate current strategies for achieving sustainable stretchable batteries and provide a discussion on possible avenues for future research. Stretchable battery technology still faces several challenges to progress the development of next-generation wearables. This perspective will evaluate current strategies and provide a discussion on possible avenues for future research.

Chemical and electronic sensitization in metal oxide gas sensors are severely limited by poor dimension controls of metal oxide nanostructure and their electric/electronic properties. These limitations are overcome using hydrothermal-induced heterointerface engineering approaches. This work demonstrates that forming spherical titanium dioxide nanoparticles on a substrate significantly reduce a surface energy barrier of nucleation and induces novel mesophorous hierarchical TiO2 structure during hydrothermal synthesis, consequently increasing the surface area of the structure by similar to 3 times compared to that of control. In addition, we succeeded in tailoring the energetics of hierarchical TiO2 nanosctructure by decorating with the nickel oxide and silver nanoparticles, which results in a desirable semiconductors/metal heterointerface for fast charge transfer where silver nanoparticles bridge nickel oxide and TiO2 nanostructure, and silver nanoparticles serve as preferential sites for chemisorption and migration of oxygen anions. The resulting heterostructure sensing properties such as sensitivity, limit of detection and selectivity are studied as a function of operating temperature (30-150 degrees C), relative humidity (RH) and various volatile organic analytes concentrations. The TiO2/Ag/NiO heterostructure finally exhibits a high gas response of similar to 2.1 for acetone with a limit of detection of 34 ppb at 30 degrees C (or 21 ppb at 90 degrees C), and retains an excellent selectivity of acetone even at 90 % relative humidity. It exhibited a highly stable and speedy gas response for acetone toward various gases such as formaldehyde, ethanol, hydrogen sulfide, carbon monoxide even operating at 90 degrees C. Our results suggest a potential of constructed TiO2/Ag/NiO heterostructure for superior sensing volatile organic acetone and will also stimulate research on hetero-structured gas sensors with high sensitivity and selectivity.

Internet-of-things which requires electronics, energy convertor and storage must be low-cost, recyclable and environmentally friendly. In the development of printed batteries, ideally all the components (electrode and electrolyte) must be printable to ensure low-cost manufacturing via printing technologies. Most of the printed batteries suffer with low power. One of the reasons is the poor ionic conductivity of the electrolyte due to the high viscosity needed for printing relatively thick layers. In the present work we have demonstrated a new class of electrolyte promising for printed organic batteries following the concept of water-in-polymer salt electrolytes (WIPSEs). These highly concentrated electrolytes of potassium polyacrylate are non-flammable, low cost and environmentally friendly. They possess high ionic conductivities (45-87 mS/cm) independent on the macroscopic viscosities varying from 7 to 33000 cP. The decoupling between ionic transport and macroscopic viscosity enables us to demonstrate organic batteries based on WIPSEs that can deliver a high and constant power (similar to 4.5 kW/kg; 7.1-11 mW/cm(2)) independent on the viscosity of the electrolytes. The tunability of the viscosity presents a prerequisite for printed technology manufacturing and compatibility with printed batteries.

In electrochemical energy storage devices (ESDs), organic electrolytes are typically used for wide operational potential window, yet they suffer with cost, environmental, flammability issues, and low ionic conductivity when compared with water-based electrolytes. Hence, for large-scale applications that require high power and safety, presently there is no true solution. Though water-based electrolytes have higher ionic conductivities, and are cost-effective and nonflammable, their high self-discharge rate with organic/carbon-based electrodes impedes their commercialization. It is found out that highly concentrated polymer electrolytes on the concept of "water-in-salt electrolyte" lead to extremely low leakage current within the electrochemical stability window (ESW) of water, thus solving the issue of self-discharge in organic/carbon-based ESDs. Herein, potassium polyacrylate (PAAK) is prepared as "water-in-polymer salt electrolyte" (WIPSE) and tested for one of most abundant wood-based biopolymer lignin and polyimide as positive and negative electrodes, respectively, in both half-cell and full-cell. The device shows an open-circuit voltage drops <0.45V in 100h setting a record for organic batteries using aqueous electrolyte. The high ionic conductivity (40-120mScm(-1)) nonflammability of PAAK with high ESW (3.1V) opens a new direction for truly safe, sustainable, and high power (6.8kWkg(-1)) organic ESD manufactured by printing technologies.

Implantable electronically controlled drug delivery devices can provide precision therapeutic treatments by highly spatiotemporally controlled delivery. Iontronic delivery devices rely on the movement of ions rather than liquid, and can therefore achieve electronically controlled precision delivery in a compact setting without disturbing the microenvironment within the tissue with fluid flow. For maximum precision, the delivery device needs to be closely integrated into the tissue, which is challenging due to the mechanical mismatch between the soft tissue and the harder devices. Here we address this challenge by developing a soft and stretchable iontronic delivery device. By formulating an ink based on an in-house synthesized hyperbranched polyelectrolyte, water dispersed polyurethane, and a thickening agent, a viscous ink is developed for stencil patterning of soft ion exchange membranes (IEMs). We use this ink for developing soft and stretchable delivery devices, which are characterized both in the relaxed and stretched state. We find that their functionality is preserved up to 100% strain, with small variations in resistance due to the strain. Finally, we develop a skin patch to demonstrate the outstanding conformability of the developed device. The presented technology is attractive for future soft implantable delivery devices, and the stretchable IEMs may also find applications within wearable energy devices.

Organic-based energy harvesting devices can contribute to a sustainable solution for the transition to renewable energy sources. The concept of ionic thermoelectrics (iTE) has been recently proposed and motivated by the high values of thermo-voltage in electrolytes. So far, most research has focused on developing new electrolytes with high Seebeck coefficient. Despite the major role of the electrode materials in supercapacitors and batteries, the effect of various electrodes on energy harvesting in iTE devices has not been widely studied. In this work, the conducting polymer poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is investigated as the functional electrodes in iTE supercapacitors. Through investigating the thermo-voltage of iTEs of the same electrolyte with varying composition of PEDOT electrodes, it is identified that the different PSS content greatly affects the overall thermo-induced voltage coefficient, S-eff (i.e., effective thermopower). The permselective polyanion in the electrode causes cation concentration differences at the electrode/electrolyte interface and contributes to an interfacial potential drop that is temperature dependent. As a result, the overall thermo-voltage of the device possesses both an interfacial and a bulk contribution. The findings extend the fundamental understanding of iTE effect with functional electrodes, which could lead a new direction to enhance the heat-to-electricity conversion.

The rapid growth of wearables has created a demand for lightweight, elastic and conformal energy harvesting and storage devices. The conducting polymer poly(3,4-ethylenedioxythiophene) has shown great promise for thermoelectric generators, however, the thick layers of pristine poly(3,4-ethylenedioxythiophene) required for effective energy harvesting are too hard and brittle for seamless integration into wearables. Poly(3,4-ethylenedioxythiophene)-elastomer composites have been developed to improve its mechanical properties, although so far without simultaneously achieving softness, high electrical conductivity, and stretchability. Here we report an aqueously processed poly(3,4-ethylenedioxythiophene)-polyurethane-ionic liquid composite, which combines high conductivity (>140Scm(-1)) with superior stretchability (>600%), elasticity, and low Youngs modulus (<7MPa). The outstanding performance of this organic nanocomposite is the result of favorable percolation networks on the nano- and micro-scale and the plasticizing effect of the ionic liquid. The elastic thermoelectric material is implemented in the first reported intrinsically stretchable organic thermoelectric module. Though deformable thermoelectric materials are desirable for integrating thermoelectric devices into wearable electronics, typical thermoelectric materials are too brittle for practical application. Here, the authors report a high-performance elastic composite for stretchable thermoelectric modules.

Conducting polymers (CPs) constitute a promising building block to establish next-generation stretchable electronics. However, achieving CPs with both high electrical conductivity and outstanding mechanical stretchability beyond flexibility is still a major challenge. Therefore, understanding the key factors controlling such characteristics of CPs is required. Herein, a method to simultaneously manipulate the mechanical and electrical properties of a representative CP, PEDOT:PSS, by modifying ionic liquid (IL) additives is reported. The cation/anion modification of ILs distinctly improves the electrical conductivity of PEDOT:PSS up to ≈1075 S cm−1, and the PEDOT:PSS/IL films showing higher conductivity also exhibit superior electromechanical stretchability, enabling them to maintain their initial conductivity under a tensile strain of 80%. Based on grazing incidence wide angle X-ray scattering and Fourier transform infrared spectroscopy analyses, it is found that the cation/anion-modified ILs control the crystallinity and π–π stacking density of conjugated PEDOT chains and the growth of amorphous PSS domains via IL-induced phase separation between PEDOT and PSS, which can be the origin of the significant conductivity and stretchability improvements in PEDOT:PSS/IL composites. This study provides guidance to develop highly stretchable CP-based conductors/electrodes.

The tailoring of the chromaticity of white organic light-emitting diodes (WOLEDs) has presented a significant challenge in their application in smart lighting sources to improve the quality of life and human performance. Here, a new microcavity WOLED (M-WOLED) structure to modulate the chromaticity of the emitted light is demonstrated by only adjusting the thickness of the white light-emitting layer. By introducing a polymer-metal hybrid electrode that functions both as a partially reflective mirror and a transparent electrode, a very simple microcavity architecture that does not require additional outer mirrors, such as distributed Bragg reflectors is developed. The resulting M-WOLEDs exhibit reddish-, greenish-, and bluish-white colors with different thicknesses of the single white light-emitting layer.

The gelation of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has gained popularity for its potential applications in three dimensions, while possessing tissue-like mechanical properties, high conductivity, and biocompatibility. However, the fabrication of arbitrary structures, especially via inkjet printing, is challenging because of the inherent gel formation. Here, microreactive inkjet printing (MRIJP) is utilized to pattern various 2D and 3D structures of PEDOT:PSS/IL hydrogel by in-air coalescence of PEDOT:PSS and ionic liquid (IL). By controlling the in-air position and Marangoni-driven encapsulation, single droplets of the PEDOT:PSS/IL hydrogel as small as a diameter of approximate to 260 mu m are fabricated within approximate to 600 mu s. Notably, this MRIJP-based PEDOT:PSS/IL has potential for freeform patterning while maintaining identical performance to those fabricated by the conventional spin-coating method. Through controlled deposition achieved via MRIJP, PEDOT:PSS/IL can be transformed into different 3D structures without the need for molding, potentially leading to substantial progress in next-generation bioelectronics devices.

In this chapter, the authors summarize their understanding of Poly(3,4-ethylenedioxythiophene) (PEDOT), with respect to its chemical and physical fundamentals. They focus upon the structure of several PEDOT systems, from the angstrom level and up, and the impact on both electronic and ionic transport. The authors discuss the structural properties of PEDOT:X and PEDOT:poly(styrenesulfonate) based on experimental data probed at the scale ranging from angstrom to submicrometer. The morphology of PEDOT is influenced by the nature of counter-ions, especially at high oxidation levels. The doping anions intercalate between PEDOT chains to form a “sandwich” structure to screen the positive charges in PEDOT chains. The authors provide the main transport coefficients such as electrical conductivity s, Seebeck coefficient S, and Peltier coefficient σ, starting from a general thermodynamic consideration. The optical conductivity of PEDOT has also been examined based on the effective medium approximation, which is normally used to describe microscopic permittivity properties of composites made from several different constituents.

Since their discovery in the seventies, conducting polymers have been chemically designed to acquire specific optical and electrical properties for various applications. Poly(3,4-ethylenedioxythiophene) (PEDOT) is among the most successful polymers as indicated by approximate to 12 000 articles mentioning it to date. PEDOT is found as transparent polymer electrodes in solar cells and light-emitting diodes, as printed electrodes in transistors, and as the main component of electrochromic displays, supercapacitors, and electrochemical transistors. For around seven years, PEDOT has been classified as the first thermoelectric polymer that converts heat flow into electricity. This has triggered a renewed interest in the scientific community, with about 400 publications including the keyword "PEDOT" and "thermoelectric." Among the topics covered by those scientific works are: i) the optimization of the thermoelectric properties, ii) understanding of the interplay between electrical properties and morphology, iii) the origin of the Seebeck coefficient, iv) the characterization of its thermal conductivity; and v) the design of thermoelectric devices. This work aims to be a pedagogical introduction to PEDOT but also to review the state-of-the art of its thermoelectric properties and thermoelectric devices. Hopefully, this work will inspire scientists to find chemical design rules to bring organic thermoelectrics beyond PEDOT.

The growing energy demands for wearable electronic devices has shifted the attention of scientific community towards flexible thermoelectric materials and devices; the main goal being the enhancement of the thermoelectric and conducting properties of such systems, without sacrificing their flexibility. This paper reports the enhancement of the thermoelectric properties of flexible Poly(3,4-ethylenedioxythiophene), (PEDOT), films with acid (HCl) exposure. Relative high conductive, flexible and uniform PEDOT/polyurethane(PU) and (PEDOT)/polyvinylidene fluoride (PVDF) films were prepared, separately. The films were dipped into acid solution with the exposure time of 5, 10, and 15 min. The sheet resistance (Omega/sq), electrical conductivity (sigma), Seebeck coefficient (S) and thermoelectric power factor (sigma S-2) were measured for those systems. The thermoelectric behavior of both films was optimized with different exposure times in acid solution, while the thermoelectric properties of the PEDOT/PVDF films remained unchanged with this treatment. The Seebeck coefficient and thermoelectric power of PEDOT/PU enhanced from 9.01 to 12.6 mu V/K and from 7.4x10(-2) to 12.2x10(-2) mu W/mK(2), respectively for a 10 min exposure. The origin of this enhancement was tracked down to modifications in the surface morphology of the films, identified through AFM microscopy. The presented results indicate that acid treatment is a potential and promising approach to enhance the thermoelectric properties of PEDOT/PU films for flexible, conformable and low-cost TE applications.

Due to the trade-off between their electrical/electrochemical performance and underwater stability, realizing polymer-based, high-performance direct cellular interfaces for electrical stimulation and recording has been very challenging. Herein, we developed transparent and conductive direct cellular interfaces based on a water-stable, high-performance poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) film via solvent-assisted crystallization. The crystallized PEDOT:PSS on a polyethylene terephthalate (PET) substrate exhibited excellent electrical/electrochemical/optical characteristics, long-term underwater stability without film dissolution/delamination, and good viability for primarily cultured cardiomyocytes and neurons over several weeks. Furthermore, the highly crystallized, nanofibrillar PEDOT:PSS networks enabled dramatically enlarged surface areas and electrochemical activities, which were successfully employed to modulate cardiomyocyte beating via direct electrical stimulation. Finally, the high-performance PEDOT:PSS layer was seamlessly incorporated into transparent microelectrode arrays for efficient, real-time recording of cardiomyocyte action potentials with a high signal fidelity. All these results demonstrate the strong potential of crystallized PEDOT:PSS as a crucial component for a variety of versatile bioelectronic interfaces.

Owing to the mixed electron/hole and ion transport in the aqueous environment, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)-based organic electrochemical transistor has been regarded as one of the most promising device platforms for bioelectronics. Nonetheless, there exist very few in-depth studies on how intrinsic channel material properties affect their performance and long-term stability in aqueous environments. Herein, we investigated the correlation among film microstructural crystallinity/composition, device performance, and aqueous stability in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) films. The highly organized anisotropic ordering in crystallized conducting polymer films led to remarkable device characteristics such as large transconductance (similar to 20 mS), extraordinary volumetric capacitance (113 F.cm(-3)), and unprecedentedly high [mu C*] value (similar to 490 F.cm(-1) V-1 s(-1)). Simultaneously, minimized poly(styrenesulfonate) residues in the crystallized film substantially afforded marginal film swelling and robust operational stability even after amp;gt;20-day water immersion, amp;gt;2000-time repeated on-off switching, or high-temperature/pressure sterilization. We expect that the present study will contribute to the development of long-term stable implantable bioelectronics for neural recording/stimulation.

Despite the high expectation of deformable and see-through displays for future ubiquitous society, current light-emitting diodes (LEDs) fail to meet the desired mechanical and optical properties, mainly because of the fragile transparent conducting oxides and opaque metal electrodes. Here, by introducing a highly conductive nanofibrillated conducting polymer (CP) as both deformable transparent anode and cathode, ultraflexible and see-through polymer LEDs (PLEDs) are demonstrated. The CP-based PLEDs exhibit outstanding dual-side light-outcoupling performance with a high optical transmittance of 75% at a wavelength of 550 nm and with an excellent mechanical durability of 9% bending strain. Moreover, the CP-based PLEDs fabricated on 4 µm thick plastic foils with all-solution processing have extremely deformable and foldable light-emitting functionality. This approach is expected to open a new avenue for developing wearable and attachable transparent displays.