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 (>140 S cm⁻¹) with superior stretchability (>600%), elasticity, and low Young’s modulus (<7 MPa). 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.

The advent of the Internet of Things and the growing interest in continuous monitoring by wearables have created a need for conformable and stretchable displays. Electrochromic displays (ECDs) are receiving attention as a cost-effective solution for many simple applications. However, stretchable ECDs have yet to be produced in a robust, large scale and cost-efficient manner. Here we develop a process for making fully screen printed stretchable ECDs. By evaluating commercially available inks with respect to electromechanical properties, including electrochromic PEDOT:PSS inks, our process can be directly applied in the manufacturing of stretchable organic electronic devices. The manufactured ECDs retained colour contrast with useful switching times at static strains up to 50% and strain cycling up to 30% strain. To further demonstrate the applicability of the technology, double-digit 7-segment ECDs were produced, which could conform to curved surfaces and be mounted onto stretchable fabrics while remaining fully functional. Based on their simplicity, robustness and processability, we believe that low cost printed stretchable ECDs can be easily scaled up and will find many applications within the rapidly growing markets of wearable electronics and the Internet of Things.

Stretchable organic electrochemical transistors (OECTs) are promising for wearable applications within biosensing, bio‐signal recording, and addressing circuitry. Efficient large‐scale fabrication of OECTs can be performed with printing methods but to date there are no reports on high‐performance fully printed stretchable OECTs. Herein, this challenge is addressed by developing fully screen‐printed stretchable OECTs based on an architecture that minimizes electrochemical side reactions and improves long‐term stability. Fabrication of the OECTs is enabled by in‐house development of three stretchable functional screen‐printing inks and related printing processes. The stretchable OECTs show good characteristics in terms of transfer curves, output characteristics, and transient response up to 100% static strain and 500 strain cycles at 25% and 50% strain. The strain insensitivity of the OECTs can be further improved by strain conditioning, resulting in stable performance up to 50% strain. Finally, an electrochromic smart pixel is demonstrated by connecting a stretchable OECT to a stretchable electrochromic display. It is believed that the development of screen‐printed stretchable electrochemical devices, and OECTs in particular, will pave the way for their use in wearable applications and commercial products.

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 µW cm⁻². The effective Seebeck coefficient (S_eff) of a metal/polymer/metal single leg OTEG includes an interfacial contribution (V_inter/ΔT) in addition to the intrinsic bulk Seebeck coefficient of the polythiophenes, such that S_eff = S + V_inter/ΔT varies from 22.7 µV K⁻¹ [9.4 µV K⁻¹] with Al to 50.5 µV K⁻¹ [26.3 µV 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.

Poly(3,4-ethylenedioxythiophene) (PEDOT) is a bicyclic derivative of polythiophene that emerged on the scientific scene at the end of the 1980s. ¹ There are more than 10000 published manuscript reports on the synthesis, fundamentals, and applications of PEDOT, thus making this material the most ­studied and explored conducting polymer of all kinds today. When charge-compensated with molecular or polymeric anions, it can form a highly conducting solid (i.e. the highest value of 8800 S/cm up to date) ² that exhibits great stability in its positively charged (p-doped) state. Several synthesis protocols have been developed for the material to enable the formation of thin and thick films on a vast array of substrates and carriers, such as on large area flexible foils and as conformal claddings on fibers. Intrinsically, PEDOT is typically insoluble; however, when dispersion-polymerized with for instance poly(styrenesulfonate) (PSS) a processable emulsion is achieved. ³ PEDOT:PSS has been further developed into various ink and coating formulations making the production of conducting patterns and areal electrodes possible using standard printing and coating techniques. Many of these techniques, successfully utilized today for PEDOT:PSS, were developed for the graphic art and industrial printing industry during the past centuries. ⁴

The conducting polymer poly(3,4-ethylenedioxythiophene) (known as PEDOT) is routinely fabricated into doped thin films for investigation of its inherent properties as well as for a range of applications. Fabrication of PEDOT is often achieved via oxidative polymerisation, where the conducting polymer is polymerised and doped (oxidised) to yield a conductive polymer thin film. The oxidiser and the polymerisation temperature are two parameters that may influence the properties and performance of the resultant PEDOT thin film. In this study, the role of temperature for the chemical polymerisation of PEDOT using the oxidiser iron tosylate is investigated from a computational and experimental viewpoint. While computations of the doping energetics suggest increasing doping with increasing temperature, x-ray photoelectron spectroscopy of fabricated PEDOT thin films indicate doping is much more complicated. With the aid of computations of the spatial distribution functions for tosylate in PEDOT, experiments indicate that two different populations of tosylate anions exist in the PEDOT matrix. Their relative populations change as a function of the polymerisation temperature. Therefore, polymerisation temperature plays a critical role in tailoring the properties of PEDOT in pursuit of being fit-for-purpose for the desired application.