Molecular Design of Stretchable Polymer Semiconductors: Current Progress and Future Directions

Abstract

Stretchable polymer semiconductors have advanced rapidly in the past decade as materials required to realize conformable and soft skin-like electronics become available. Through rational molecular-level design, stretchable polymer semiconductor films are now able to retain their electrical functionalities even when subjected to repeated mechanical deformations. Furthermore, their charge-carrier mobilities are on par with the best flexible polymer semiconductors, with some even exceeding that of amorphous silicon. The key advancements are molecular-design concepts that allow multiple strain energy-dissipation mechanisms, while maintaining efficient charge-transport pathways over multiple length scales. In this perspective article, we review recent approaches to confer stretchability to polymer semiconductors while maintaining high charge carrier mobilities, with emphasis on the control of both polymer-chain dynamics and thin-film morphology. Additionally, we present molecular design considerations toward intrinsically elastic semiconductors that are needed for reliable device operation under reversible and repeated deformation. A general approach involving inducing polymer semiconductor nanoconfinement allows for incorporation of several other desired functionalities, such as biodegradability, self-healing, and photopatternability, while enhancing the charge transport. Lastly, we point out future directions, including advancing the fundamental understanding of morphology evolution and its correlation with the change of charge transport under strain, and needs for strain-resilient polymer semiconductors with high mobility retention.

Polyurethane-Based Stretchable Semiconductor Nanofilms with High Intrinsic Recovery Similar to Conventional Elastomers

“…Stretchable polymer semiconductors are a key component of highly deformable electronics, such as stretchable organic thin film transistors (OTFTs), , for applications in wearable and implantable devices. − Common polymer semiconductors, that is, conjugated polymers (CPs), tend to exhibit brittle fracture behavior with low elongation expressed by small crack onset strain (COS), due to their rigid backbone and semicrystalline microstructure as necessities for efficient charge transport. , Strategies such as reducing backbone regioregularity, , inserting nonconjugation breaker into the backbone, − introducing flexible coblocks or side chains, , and dispersing conjugated polymers in elastomer binder − have been employed to improve the deformability of CPs to COS over 100%. , For a wider range of application scenes, a large deformability of semiconductor nanofilms with high COS far beyond 100% is actually preferable. , Besides COS, elastic recovery (ER) is another important parameter to evaluate mechanical durability of the semiconductors after multiple strain cycles, as buckles are easily formed to result in device deterioration after strain removal in case of irreversible plastic deformation . As semiconductors are supposed to apply in stretchable electronics as thin films, it is important to understand not only the apparent behaviors supported by stretchable substrates but also the intrinsic mechnical properties in thin film free from substrates. , However, in previous reports, the ER of stretchable semiconductors was measured either in the bulk state or by loading a thin film on relatively thick supportive layer, both unable to mirror the intrinsic ER of thin film satisfactorily.…”

“…7,8 Strategies such as reducing backbone regioregularity, 9,10 inserting nonconjugation breaker into the backbone, 11−13 introducing flexible coblocks 14 or side chains, 15,16 and dispersing conjugated polymers in elastomer binder 17−21 have been employed to improve the deformability of CPs to COS over 100%. 22,23 For a wider range of application scenes, a large deformability of semiconductor nanofilms with high COS far beyond 100% is actually preferable. 24,25 Besides COS, elastic recovery (ER) is another important parameter to evaluate mechanical durability of the semiconductors after multiple strain cycles, as buckles are easily formed to result in device deterioration after strain removal in case of irreversible plastic deformation.…”

Stretchable π-conjugated polymer electrets for mechanoelectric generators

“…However, π-electrets exhibit relatively high electrical conductivity, resulting in a short lifetime for trapped charges due to self-discharge, and thus are considered unsuitable for MEGs [21]. This lifetime can be improved by insulation or isolation of the π-conjugated unit at the molecular level by alkyl side chains or polymeric matrices, which is also advantageous for application to flexible devices [22,23]. Recently, we proposed stretchable MEGs using porphyrins as π-electrets, where the porphyrins are liquefied at room temperature by peripherally attached bulky yet flexible alkyl chains [24].…”

From stretchable and healable to self-healing semiconducting polymers: design and their TFT devices

“…34 Considering that the elastic properties of non-conjugated materials are influenced by the chemical nature of the monomer, branching, conformation, packing structure, extent of crosslinking, and chain length of macromolecules, exploitation of similar attributes in electrically active polymers bears great potential to achieve similar or even better mechanical properties. 35 The success of this approach can tremendously promote the potential of flexible devices, including the possibility of using low-cost fabrication methods over large-area substrates. In particular, Prof. Bao at Stanford University contributed greatly to the innovative development of this emerging field by utilizing the semiconducting polymer poly(3-hexylthiophene (P3HT)) to yield high-performance transistors.…”