Stretchable semiconducting polymers have been developed as a key component to enable skin-like wearable electronics, but their electrical performance must be improved to enable more advanced ...functionalities. Here, we report a solution processing approach that can achieve multi-scale ordering and alignment of conjugated polymers in stretchable semiconductors to substantially improve their charge carrier mobility. Using solution shearing with a patterned microtrench coating blade, macroscale alignment of conjugated-polymer nanostructures was achieved along the charge transport direction. In conjunction, the nanoscale spatial confinement aligns chain conformation and promotes short-range π-π ordering, substantially reducing the energetic barrier for charge carrier transport. As a result, the mobilities of stretchable conjugated-polymer films have been enhanced up to threefold and maintained under a strain up to 100%. This method may also serve as the basis for large-area manufacturing of stretchable semiconducting films, as demonstrated by the roll-to-roll coating of metre-scale films.
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.
Molecular additives are often used to enhance dynamic motion of polymeric chains, which subsequently alter the functional and physical properties of polymers. However, controlling the chain dynamics ...of semiconducting polymer thin films and understanding the fundamental mechanisms of such changes is a new area of research. Here, cycloparaphenylenes (CPPs) are used as conjugated molecular additives to tune the dynamic behaviors of diketopyrrolopyrrole‐based (DPP‐based) semiconducting polymers. It is observed that the addition of CPPs results in significant improvement in the stretchability of the DPP‐based polymers without adversely affecting their mobility, which arises from the enhanced polymer dynamic motion and reduced long‐range crystalline order. The polymer films retain their fiber‐like morphology and short‐range ordered aggregates, which leads to high mobility. Fully stretchable transistors are subsequently fabricated using CPP/semiconductor composites as active layers. These composites are observed to maintain high mobilities when strained and after repeated applied strains. Interestingly, CPPs are also observed to improve the contact resistance and charge transport of the fully stretchable transistors. ln summary, these results collectively indicate that controlling the dynamic motion of polymer semiconductors is proved to be an effective way to improve their stretchability.
Conjugated carbon cyclic nanoring compounds are used as molecular additives to enhance the stretchability of semiconducting polymers without compromising mobility. The additives are shown to significantly decrease long‐range crystalline order, while short‐range ordered aggregates are well‐maintained. Fully stretchable transistors fabricated with the newly established polymer semiconductor/molecular additive blend films exhibit improved mobility retention under strain and after repeated applied strain.
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, poly2,5‐bis(4‐decyltetradecyl)pyrrolo3,4‐cpyrrole‐1,4‐(2H,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%.
Bulky additive molecules residing between amorphous chain networks can disrupt the crystalline ordering of polymer semiconductors. The long‐range crystalline domains are suppressed while short‐range aggregations of conjugated polymers are promoted, leading to an improved thin‐film stretchability without affecting charge transport under an external strain up to 100%.
The understanding of the structure‐mechanical property relationship for semiconducting polymers is essential for the application of flexible organic electronics. Herein pseudo free‐standing tensile ...testing, a technique that measures the mechanical property of thin films floating on the surface of water, is used to obtain the stress–strain behaviors of two semiconducting polymers, poly(3‐hexylthiophene) (P3HT) and poly(2,5‐bis(2‐decyltetradecyl)‐3,6‐di(thiophen‐2‐yl)diketopyrrolo3,4‐cpyrrole‐1,4‐dione‐alt‐thienovinylthiophene (DPP‐TVT) donor–acceptor (D–A) polymer. To our surprise, DPP‐TVT shows similar viscoelastic behavior to P3HT, despite DPP‐TVT possessing a larger conjugated backbone and much higher charge carrier mobility. The viscoelastic behavior of these polymers is due to sub room temperature glass transition temperatures (Tg), as shown by AC chip calorimetry. These results provide a comprehensive understanding of the viscoelastic properties of conjugated D–A polymers by thickness‐dependent, strain rate dependent, hysteresis tests, and stress‐relaxation tests, highlighting the importance of Tg for designing intrinsically stretchable conjugated polymers.
A single stretch test and cyclic hysteresis test are conducted using pseudo‐free standing tensile test and the stress‐strain curves are shown for poly(2,5‐bis(2‐decyltetradecyl)‐3,6‐di(thiophen‐2‐yl)diketopyrrolo3,4‐cpyrrole‐1,4‐dione‐alt‐thienovinylthiophene (DPP‐TVT) donor‐acceptor (D‐A) polymers.
Mechanical failure of π‐conjugated polymer thin films is unavoidable under cyclic loading conditions, due to intrinsic defects and poor resistance to crack propagation. Here, the first tear‐resistant ...and room‐temperature self‐healable semiconducting composite is presented, consisting of conjugated polymers and butyl rubber elastomers. This new composite displays both a record‐low elastic modulus (<1 MPa) and ultrahigh deformability with fracture strain above 800%. More importantly, failure behavior is not sensitive to precut notches under deformation. Autonomous self‐healing at room temperature, both mechanical and electronic, is demonstrated through the physical contact of two separate films. The composite film also shows device stability in the ambient environment over 5 months due to much‐improved barrier property to both oxygen and water. Butyl rubber is broadly applicable to various p‐type and n‐type semiconducting polymers for fabricating self‐healable electronics to provide new resilient electronics that mimic the tear resistance and healable property of human skin.
A mechanically durable and electronically stable semiconducting composite is engineered by introducing a blend of donor–acceptor polymer and butyl rubber elastomer. The composite exhibits ultralow modulus, ultrahigh deformability, tear resistance, and self‐healing performance, as well as ambient stable device stability. This method is widely applicable to different semiconducting polymers.
Diketopyrrolopyrrole (DPP)-based donor–acceptor conjugated polymers, with increasing amount of weak H-bonding units, namely 2,6-pyridinedicarboxamide (PDCA), inserted as end groups in alkyl side ...chains were prepared and investigated. In contrast to previously reported DPP polymers containing PDCA units as conjugation breakers along the polymer backbone, PDCA in the alkyl side chains readily produced almost quantitative formation of intermolecular H-bonding even at low PDCA unit content (<10 mol %) as shown by Fourier transform infrared spectroscopy (FTIR). The efficient intermolecular H-bonding was further supported by the appearance of a pronounced vibronic shoulder in the UV–vis spectra and a reduction of interlamellar spacing (from 24.02 to 22.87 Å) compared to the neat DPP polymer. Increasing mol % of PDCA units in side chains of DPP conjugated polymers also has a clear effect on the thermal and mechanical properties of the films as investigated by dynamic mechanical analysis (DMA). Polymers with a high loading of PDCA showed a linear increase in both tan delta intensity and temperature at which softening of film cross-linking occurs. In particular, at a comparable mol %, polymers with PDCA units along the conjugated backbone showed a lower transition intensity and on average a 10–20 °C higher temperature required for H-bonding breaking. FTIR coupled with crack onset measurements showed that H-bonding breaking during tensile deformation happens only at strains close to crack onset. All these observations suggest that molecular engineering of conjugated polymers bearing H-bonding units has a strong influence on microstructure, thermal and mechanical properties of solution processed films, and final energy dissipation mechanisms in stretchable electronics applications.
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.
Incorporation of metal–ligand coordination bonds into polymer semiconductors is able to simultaneously improve thin film deformability and charge transport efficiency. Such metal‐coordination bonds prove to be both dynamic and reversible under external forces, indicating that PSCs with metallated mechanophores are promising candidates for future high performance mechanically robust electronics.
Organic semiconducting donor–acceptor polymers are promising candidates for stretchable electronics owing to their mechanical compliance. However, the effect of the electron‐donating thiophene group ...on the thermomechanical properties of conjugated polymers has not been carefully studied. Here, thin‐film mechanical properties are investigated for diketopyrrolopyrrole (DPP)‐based conjugated polymers with varying numbers of isolated thiophene moieties and sizes of fused thiophene rings in the polymer backbone. Interestingly, it is found that these thiophene units act as an antiplasticizer, where more isolated thiophene rings or bigger fused rings result in an increased glass transition temperature (Tg) of the polymer backbone, and consequently elastic modulus of the respective DPP polymers. Detailed morphological studies suggests that all samples show similar semicrystalline morphology. This antiplasticization effect also exists in para‐azaquinodimethane‐based conjugated polymers, indicating that this can be a general trend for various conjugated polymer systems. Using the knowledge gained above, a new DPP‐based polymer with increased alkyl side chain density through attaching alky chains to the thiophene unit is engineered. The new DPP polymer demonstrates a record low Tg, and 50% lower elastic modulus than a reference polymer without side‐chain decorated on the thiophene unit. This work provides a general design rule for making low‐Tg conjugated polymers for stretchable electronics.
This paper investigates the effect of isolated/fused thiophene units on the thermomechanical properties of donor–acceptor conjugated polymers. In diketopyrrolopyrrole‐based polymers, it is observed that thiophene units in the main chain structure serve as the antiplasticizer, which increase the stiffness and glass transition temperature of polymer chains. This allows for the development of a much softer conjugated polymer.
Flexible alkyl side chain in conjugate polymers (CPs) improves the solubility and promotes solution processability, in addition, it affects interchain packing and charge mobilities. Despite the ...well‐known charge mobility and morphology correlation for these semi‐crystalline polymers, there is a lack of fundamental understanding of the impact of side chain on their crystallization kinetics. In the present work, isothermal crystallization of five poly(3‐alkylthiophene‐2,5‐diyl) (P3ATs) with different side‐chain structures were systematically investigated. To suppress the extremely fast crystallization and trap the sample into amorphous glass, an advanced fast scanning chip calorimetry technique, which is able to quench the sample with few to tens thousands of K/s, was applied. Results show that the crystallization of P3ATs was greatly inhibited after incorporation of branched side chains, as indicated by a dramatic up to six orders of magnitude decrease in the crystallization rate. The suppressed crystallization of P3ATs were correlated with an increased π–π stacking distance due to unfavorable side‐chain steric interaction. This work provides a pathway to use side‐chain engineering to control the crystallization behavior for CPs, thus to control device performance.
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