Strategies for stretchable polymer semiconductor layers
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on Organic electronics are advancing to devices beyond flexible electronics. This trend opens the pathway to stretchable electronics, where the electronic devices are integrated on/in stretchable substrates.1 Since the initial implementation of this technology, much attention has been given to the development of stretchable conductors,1–3 e-skin sensors,4–6 light-emitting diodes,7 energy-storage devices,8 and solar cells.9 Although stretchable field-effect transistors are the fundamental building blocks of most stretchable devices,1,10,11 they have been less studied due to a lack of stretchable semiconductors. Further exploration of organic stretchable semiconductors is necessary to bring stretchable transistors into practical use. Possible methodological approaches to fabricate stretchable semiconductors are summarized in Figure 1. The concept of imparting stretchability to organic semiconductors through geometrical engineering is shown in Figure 1a. A semiconductor in the form of a curved-line bridging two electrodes and mounted on an elastomer substrate can be stretched without mechanical failure,7 or a semiconductor film on a prestrained rubber substrate causes the structure to buckle upon releasing the stress.12 This is straightforward and reviewed in publications that address the fabrication of Si-based stretchable devices.13,14 This structural approach makes it difficult to generate uniform patterning and buckling over the entire device with miniaturization, which is the major drawback of this approach.
Film-type semiconductors are more desirable than the structural design for robust applications. Such stretchable film-type semiconductors are named “intrinsically stretchable” in this article. The intrinsically stretchable semiconductors allow the fabrication of stretchable devices in which all the device components are stretchable (Figure 1b), and the patterning can be achieved by printing or lithography. In addition, such intrinsically stretchable devices ensure high stretchability without any noticeable structural change. The development of stretchable semiconductor film is a key step toward deformable devices, but it is still in the conceptual stage. Here, we discuss technological approaches for intrinsically stretchable polymer semiconductor films. Although lowmolecular-weight organic semiconductors have the advantage of long-range order and better transport properties,15 their structures and properties are relatively sensitive to mechanical deformation. Polymer semiconductors are composed of a sequence of semiflexible chains that provide greater possibility of intrinsic stretchability.15,16 Therefore, this review focuses on polymer semiconductors.
Approaches to stretchable semiconductor layers Polythiophenes are the most studied polymer semiconductors for flexible and stretchable devices. O’Connor et al. demonstrated strain-aligned anisotropic charge transport in
Kaliannan Thiyagarajan, Department of Materials Science and Engineering, Pohang University of Science and Technology, Republic of Korea; thiyagu@poste
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