The development of intrinsically stretchable electronics poses great challenges in synthesizing elastomeric conductors, semiconductors and dielectric materials. While a wide range of approaches, from special macrostructural engineering to molecular synthesis, have been employed to afford stretchable devices, this review surveys recent advancements in employing various morphological and nanostructural control methods to impart mechanical flexibility and/or to enhance electrical properties. The focus will be on (1) embedding percolation networks of one-dimensional conductive materials such as metallic nanowires and carbon nanotubes in an elastomer matrix to accommodate large external deformation without imposing a large strain along the one-dimensional materials, (2) design strategies to achieve intrinsically stretchable semiconductor materials that include direct blending of semiconductors with elastomers and synthesizing semiconductor polymers with appropriate side chains, backbones, cross-linking networks, and flexible blocks, and (3) employing interpenetrating polymer networks, bottlebrush structures and introducing inclusions in stretchable polymeric dielectric materials to improve electrical performance. Moreover, intrinsically stretchable electronic devices based on these materials, such as stretchable sensors, heaters, artificial muscles, optoelectronic devices, transistors and soft humanoid robots, will also be described. Limitations of these approaches and measures to overcome them will also be discussed.
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An Intrinsically Stretchable High‐Performance Polymer Semiconductor with Low Crystallinity
For wearable and implantable electronics applications, developing intrinsically stretchable polymer semiconductor is advantageous, especially in the manufacturing of large‐area and high‐density devices. A major challenge is to simultaneously achieve good electrical and mechanical properties for these semiconductor devices. While crystalline domains are generally needed to achieve high mobility, amorphous domains are necessary to impart stretchability. Recent progresses in the design of high‐performance donor–acceptor polymers that exhibit low degrees of energetic disorder, while having a high fraction of amorphous domains, appear promising for polymer semiconductors. Here, a low crystalline, i.e., near‐amorphous, indacenodithiophene‐
- Award ID(s):
- 1757220
- NSF-PAR ID:
- 10453412
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Functional Materials
- Volume:
- 29
- Issue:
- 46
- ISSN:
- 1616-301X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Polymer semiconductors (PSCs) are essential active materials in mechanically stretchable electronic devices. However, many exhibit low fracture strain due to their rigid chain conformation and the presence of large crystalline domains. Here, a PSC/elastomer blend, poly[((2,6‐bis(thiophen‐2‐yl)‐3,7‐bis(9‐octylnonadecyl)thieno[3,2‐b]thieno[2′,3′:4,5]thieno[2,3‐d]thiophene)‐5,5′‐diyl)(2,5‐bis(8‐octyloctadecyl)‐3,6‐di(thiophen‐2‐yl)pyrrolo[3,4‐c]pyrrole‐1,4‐dione)‐5,5′‐diyl]] (P2TDPP2TFT4) and polystyrene‐
block ‐poly(ethylene‐ran‐butylene)‐block ‐polystyrene (SEBS) are systematically investigated. Specifically, the effects of molecular weight of both SEBS and P2TDPP2TFT4 on the resulting blend morphology, mechanical, and electrical properties are explored. In addition to commonly used techniques, atomic force microscopy‐based nanomechanical images are used to provide additional insights into the blend film morphology. Opposing trends in SEBS‐induced aggregation are observed for the different P2TDPP2TFT4 molecular weights upon increasing the SEBS molecular weight from 87 to 276 kDa. Furthermore, these trends are seen in device performance trends for both molecular weights of P2TDPP2TFT4. SEBS molecular weight also has a substantial influence on the mesoscale phase separation. Strain at fracture increases dramatically upon blending, reaching a maximum value of 640% ± 20% in the blended films measured with film‐on‐water method. These results highlight the importance of molecular weight for electronic devices. In addition, this study provides valuable insights into appropriate polymer selections for stretchable semiconducting thin films that simultaneously possess excellent mechanical and electrical properties. -
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Abstract The backbone of diketopyrrolopyrrole‐thiophene‐vinylene‐thiophene‐based polymer semiconductors (PSCs) is modified with pyridine (Py) or bipyridine ligands to complex Fe(II) metal centers, allowing the metal–ligand complexes to act as mechanophores and dynamically crosslink the polymer chains. Mono‐ and bi‐dentate ligands are observed to exhibit different degrees of bond strengths, which subsequently affect the mechanical properties of these Wolf‐type‐II metallopolymers. The counter ion also plays a crucial role, as it is observed that Py‐Fe mechanophores with non‐coordinating BPh4–counter ions (Py‐FeB) exhibit better thin film ductility with lower elastic modulus, as compared to the coordinating chloro ligands (Py‐FeC). Interestingly, besides mechanical robustness, the electrical charge carrier mobility can also be enhanced concurrently when incorporating Py‐FeB mechanophores in PSCs. This is a unique observation among stretchable PSCs, especially that most reports to date describe a decreased mobility when the stretchability is improved. Next, it is determined that improvements to both mobility and stretchability are correlated to the solid‐state molecular ordering and dynamics of coordination bonds under strain, as elucidated via techniques of grazing‐incidence X‐ray diffraction and X‐ray absorption spectroscopy techniques, respectively. This study provides a viable approach to enhance both the mechanical and the electronic performance of polymer‐based soft devices.
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Abstract In order to apply polymer semiconductors to stretchable electronics, they need to be easily deformed under strain without being damaged. A small number of conjugated polymers, typically with semicrystalline packing structures, have been reported to exhibit mechanical stretchability. Herein, a method is reported to modify polymer semiconductor packing‐structure using a molecular additive, dioctyl phthalate (DOP), which is found to act as a molecular spacer, to be inserted between the amorphous chain networks and disrupt the crystalline packing. As a result, large‐crystal growth is suppressed while short‐range aggregations of conjugated polymers are promoted, which leads to an improved mechanical stretchability without affecting charge‐carrier transport. Due to the reduced conjugated polymer intermolecular interactions, strain‐induced chain alignment and crystallization are observed. By adding DOP to a well‐known conjugated polymer, poly[2,5‐bis(4‐decyltetradecyl)pyrrolo[3,4‐c]pyrrole‐1,4‐(2
H ,5H )‐dione‐(E)‐1,2‐di(2,2′‐bithiophen‐5‐yl)ethene] (DPPTVT), stretchable transistors are obtained with anisotropic charge‐carrier mobilities under strain, and stable current output under strain up to 100%. -
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Abstract Stretchable polymer semiconductors (PSCs) have seen great advancements alongside the development of soft electronics. But it remains a challenge to simultaneously achieve high charge carrier mobility and stretchability. Herein, we report the finding that stretchable PSC thin films (<100-nm-thick) with high stretchability tend to exhibit multi-modal energy dissipation mechanisms and have a large relative stretchability (
rS ) defined by the ratio of the entropic energy dissipation to the enthalpic energy dissipation under strain. They effectively recovered the original molecular ordering, as well as electrical performance, after strain was released. The highestrS value with a model polymer (P4) exhibited an average charge carrier mobility of 0.2 cm2V−1s−1under 100% biaxial strain, while PSCs with lowrS values showed irreversible morphology changes and rapid degradation of electrical performance under strain. These results suggestrS can be used as a parameter to compare the reliability and reversibility of stretchable PSC thin films.
