Introducing non-covalent crosslinking moieties to polymer semiconductors produces a stretchable and healable material suitable for wearable electronics. There is great interest and potential in the development of skin-inspired flexible and wearable electronic devices. Such devices require materials that twist, fold and bend with no loss in electronic—or material—properties. Zhenan Bao and colleagues report a conjugated polymer that also incorporates non-covalent interactions between adjacent chains, enabling the material to accommodate up to 100% strain whilst maintaining high charge-carrier mobility. In this proof-of-principle study the authors use the polymers to fabricate flexible and stretchable organic transistors that combine robustness with good electronic properties. Thin-film field-effect transistors are essential elements of stretchable electronic devices for wearable electronics1,2. All of the materials and components of such transistors need to be stretchable and mechanically robust3,4. Although there has been recent progress towards stretchable conductors5,6,7,8, the realization of stretchable semiconductors has focused mainly on strain-accommodating engineering of materials, or blending of nanofibres or nanowires into elastomers9,10,11. An alternative approach relies on using semiconductors that are intrinsically stretchable, so that they can be fabricated using standard processing methods12. Molecular stretchability can be enhanced when conjugated polymers, containing modified side-chains and segmented backbones, are infused with more flexible molecular building blocks13,14. Here we present a design concept for stretchable semiconducting polymers, which involves introducing chemical moieties to promote dynamic non-covalent crosslinking of the conjugated polymers. These non-covalent crosslinking moieties are able to undergo an energy dissipation mechanism through breakage of bonds when strain is applied, while retaining high charge transport abilities. As a result, our polymer is able to recover its high field-effect mobility performance (more than 1 square centimetre per volt per second) even after a hundred cycles at 100 per cent applied strain. Organic thin-film field-effect transistors fabricated from these materials exhibited mobility as high as 1.3 square centimetres per volt per second and a high on/off current ratio exceeding a million. The field-effect mobility remained as high as 1.12 square centimetres per volt per second at 100 per cent strain along the direction perpendicular to the strain. The field-effect mobility of damaged devices can be almost fully recovered after a solvent and thermal healing treatment. Finally, we successfully fabricated a skin-inspired stretchable organic transistor operating under deformations that might be expected in a wearable device.

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Intrinsically stretchable and healable semiconducting polymer for organic transistors

Recent progress in electronic skin or e-skin research is broadly reviewed, focusing on technologies needed in three main applications: skin-attachable electronics, robotics, and prosthetics. First, since e-skin will be exposed to prolonged stresses of various kinds and needs to be conformally adhered to irregularly shaped surfaces, materials with intrinsic stretchability and self-healing properties are of great importance. Second, tactile sensing capability such as the detection of pressure, strain, slip, force vector, and temperature are important for health monitoring in skin attachable devices, and to enable object manipulation and detection of surrounding environment for robotics and prosthetics. For skin attachable devices, chemical and electrophysiological sensing and wireless signal communication are of high significance to fully gauge the state of health of users and to ensure user comfort. For robotics and prosthetics, large-area integration on 3D surfaces in a facile and scalable manner is critical. Furthermore, new signal processing strategies using neuromorphic devices are needed to efficiently process tactile information in a parallel and low power manner. For prosthetics, neural interfacing electrodes are of high importance. These topics are discussed, focusing on progress, current challenges, and future prospects.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201904765

Electronic Skin: Recent Progress and Future Prospects for Skin‐Attachable Devices for Health Monitoring, Robotics, and Prosthetics

Xugang Guo,*,† Antonio Facchetti,*,‡,§ and Tobin J. Marks*,‡ †Department of Materials Science and Engineering, South University of Science and Technology of China, No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China ‡Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois 60077, United States

Imide- and Amide-Functionalized Polymer Semiconductors

Soft and conformable wearable electronics require stretchable semiconductors, but existing ones typically sacrifice charge transport mobility to achieve stretchability. We explore a concept based on the nanoconfinement of polymers to substantially improve the stretchability of polymer semiconductors, without affecting charge transport mobility. The increased polymer chain dynamics under nanoconfinement significantly reduces the modulus of the conjugated polymer and largely delays the onset of crack formation under strain. As a result, our fabricated semiconducting film can be stretched up to 100% strain without affecting mobility, retaining values comparable to that of amorphous silicon. The fully stretchable transistors exhibit high biaxial stretchability with minimal change in on current even when poked with a sharp object. We demonstrate a skinlike finger-wearable driver for a light-emitting diode.

/pdf/highly-stretchable-polymer-semiconductor-films-through-the-2tm94kx0ke.pdf

Highly stretchable polymer semiconductor films through the nanoconfinement effect

Organic semiconductors have attracted a lot of attention since the discovery of highly doped conductive polymers, due to the potential application in field-effect transistors (OFETs), light-emitting diodes (OLEDs) and photovoltaic cells (OPVs). Single crystals of organic semiconductors are particularly intriguing because they are free of grain boundaries and have long-range periodic order as well as minimal traps and defects. Hence, organic semiconductor crystals provide a powerful tool for revealing the intrinsic properties, examining the structure–property relationships, demonstrating the important factors for high performance devices and uncovering fundamental physics in organic semiconductors. This review provides a comprehensive overview of the molecular packing, morphology and charge transport features of organic semiconductor crystals, the control of crystallization for achieving high quality crystals and the device physics in the three main applications. We hope that this comprehensive summary can give a clear picture of the state-of-art status and guide future work in this area.

Organic semiconductor crystals

Mobility is an important charge-transport parameter in organic, inorganic and hybrid semiconductors. We outline some of the common pitfalls of mobility extraction from field-effect transistor (FET) measurements and propose practical recommendations to avoid reporting erroneous mobilities in publications.

Critical assessment of charge mobility extraction in FETs

Recent studies of the bias-stress-driven electrical instability of organic field-effect transistors (OFETs) are reviewed. OFETs are operated under continuous gate and source/drain biases and these bias stresses degrade device performance. The principles underlying this bias instability are discussed, particularly the mechanisms of charge trapping. There are three main charge-trapping sites: the semiconductor, the dielectric, and the semiconductor-dielectric interface. The charge-trapping phenomena in these three regions are analyzed with special attention to the microstructural dependence of bias instability. Finally, possibilities for future research in this field are presented. This critical review aims to enhance our insight into bias-stress-induced charge trapping in OFETs with the aim of minimizing operational instability.

25th anniversary article: microstructure dependent bias stability of organic transistors.

Despite the considerable efforts applied toward developing stretchable electronics, few intrinsically stretchable semiconductors have been reported that retain the original electrical characteristics under stretching. This study introduces an intrinsically stretchable and transparent organic semiconducting layer by blending self-assembled nanowires (NWs) of an organic semiconductor with an elastomeric and transparent polymer. Blends of poly(3-hexylthiophene) (P3HT) NWs and poly(dimethylsiloxane) (PDMS) yield P3HT NW networks embedded in the PDMS matrix. Interestingly, it is found that the vertical distribution of P3HT NWs in the blend films is sensitive to the surface characteristics of the underlying substrate. Compared to the P3HT NWs distributed on a Si substrate with vertical gradation, the P3HT NWs are evenly distributed throughout the PDMS matrix on a PDMS substrate. Organic transistors prepared with the blend active layers with various P3HT ratios exhibit device performances comparable to those of a device prepared with homo-P3HT NWs, even at 1 wt% P3HT, due to the formation of percolated networks of the P3HT NWs with a high crystallinity and a large aspect ratio. This blend active layer shows the superior electrical and mechanical properties during stretching at high strains unlike the homo-P3HT NW system.

Stretchable and Transparent Organic Semiconducting Thin Film with Conjugated Polymer Nanowires Embedded in an Elastomeric Matrix

Semiconductor-dielectric blends: A facile all solution route to flexible all-organic transistors

The phase-separation characteristics of spin-cast difluorinated-triethylsilylethynyl anthradithiophene (F-TESADT)/poly(methyl methacrylate) (PMMA) blends are investigated with the aim of fabricating transistors with a high field-effect mobility and stability. It is found that the presence of PMMA in the F-TESADT/PMMA blends prevents dewetting of F-TESADT from the substrate and provides a platform for F-TESADT molecules to segregate and crystallize at the air–film interface. By controlling the solvent evaporation rate of the spin-cast blend solution, it is possible to regulate the phase separation of the two components, which in turn determines the structural development of the F-TESADT crystals on PMMA. At a low solvent evaporation rate, a bilayer structure consisting of highly ordered F-TESAT crystals on the top and low-trap PMMA dielectric on the bottom can be fabricated by a one-step spin-casting process. The use of F-TESADT/PMMA blend films in bottom gate transistors produces much higher field-effect mobilities and greater stability than homo F-TESADT films because the phase-separated interface provides an efficient pathway for charge transport.

Hyun Ho Choi

Papers

Critical assessment of charge mobility extraction in FETs

25th anniversary article: microstructure dependent bias stability of organic transistors.

Stretchable and Transparent Organic Semiconducting Thin Film with Conjugated Polymer Nanowires Embedded in an Elastomeric Matrix

Semiconductor-dielectric blends: A facile all solution route to flexible all-organic transistors

The Influence of the Solvent Evaporation Rate on the Phase Separation and Electrical Performances of Soluble Acene‐Polymer Blend Semiconductors