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Wearable devices benefit from the use of stretchable conjugated polymers (CPs). Traditionally, the design of stretchable CPs is based on the assumption that a low elastic modulus (E) is crucial for achieving high stretchability. However, this research, which analyzes the mechanical properties of 65 CP thin films, challenges this notion. It is discovered that softness alone does not determine stretchability; rather, it is the degree of entanglement that is critical. This means that rigid CPs can also exhibit high stretchability, contradicting conventional wisdom. To inverstigate further, the mechanical behavior, electrical properties, and deformation mechanism of two model CPs: a glassy poly(3‐butylthiophene‐2,5‐diyl) (P3BT) with anEof 2.2 GPa and a viscoelastic poly(3‐octylthiophene‐2,5‐diyl) (P3OT) with anEof 86 MPa, are studied. Ex situ transmission X‐ray scattering and polarized UV–vis spectroscopy revealed that only the initial strain (i.e., <20%) exhibits different chain alignment mechanisms between two polymers, while both rigid and soft P3ATs showed similarly behavior at larger strains. By challenging the conventional design metric of lowEfor high stretchability and highlighting the importance of entanglement, it is hoped to broaden the range of CPs available for use in wearable devices.

Conjugated polymers consist of complex backbone structures and side‐chain moieties to meet various optoelectronic and processing requirements. Recent work on conjugated polymers has been devoted to studying the mechanical properties and developing new conjugated polymers with low modulus and high‐crack onset strain, while the thin film mechanical stability under long‐term external tensile strain is less investigated. Here we performed direct mechanical stress relaxation tests for both free‐standing and thin film floated on water surface on both high‐T_gand low‐T_gconjugated polymers, as well as a reference nonconjugated sample, polystyrene. We measured thin films with a range of film thickness from 38 to 179 nm to study the temperature and thickness effect on thin film relaxation, where an apparent enthalpy–entropy compensation effect for glassy polymer PS and PM6 thin films was observed. We also compared relaxation times across three different conjugated polymers and showed that both crystalline morphology and higher modulus reduce the relaxation rate besides higher glass transition temperature. Our work provides insights into the mechanical creep behavior of conjugated polymers, which will have an impact on the future design of stable functional organic electronics.

Semiconductors with both high stretchability and self‐healing capability are highly desirable for various wearable devices. Much progress has been achieved in designing highly stretchable semiconductive polymers or composites. The demonstration of self‐healable semiconductive composite is still rare. Here, an extremely soft, highly stretchable, and self‐healable hydrogen bonding cross‐linked elastomer, amide functionalized‐polyisobutylene (PIB‐amide) is developed, to enable a self‐healable semiconductive composite through compounding with a high‐performance conjugated diketopyrrolopyrrole (DPP‐T) polymer. The composite, consisting of 20% DPP‐T and 80% PIB‐amide, shows record high crack‐onset strain (COS ≈1500%), extremely low elastic modulus (E≈1.6 MPa), and unique ability to spontaneously self‐heal atroom temperature within 5 min. Unlike previous works, these unique composite materials also show strain‐independent charge mobility. An in‐depth morphological study based on multi‐model techniques indicate that all composites show blending ratio‐ and stretching‐independent fibril‐like aggregation due to the strong hydrogen bond in elastomer to enable the unique stable charge mobility. This study provides a new direction to develop highly healable and electronically stable semiconductive composite and will enable new applications of stretchable electronics.

A backbone engineering strategy is developed to tune the mechanical and electrical properties of conjugated polymer semiconductors. Four Donor–Acceptor (D–A) polymers, named PTDPPSe, PTDPPTT, PTDPPBT, and PTDPPTVT, are synthesized using selenophene (Se), thienothiophene (TT), bithiophene (BT), and thienylenevinylenethiophene (TVT) as the donors and siloxane side chain modified diketopyrrolopyrrole (DPP) as acceptor. The influences of the donor structure on the polymer energy level, film morphology, molecular stacking, carrier transport properties, and tensile properties are all examined. The films of PTDPPSe show the best stretchability with crack‐onset‐strain greater than 100%, but the worst electrical properties with a mobility of only 0.54 cm² V⁻¹ s⁻¹. The replacement of the Se donor with larger conjugated donors, that is, TT, BT, and TVT, significantly improves the mobility of conjugated polymers but also leads to reduced stretchability. Remarkably, PTDPPBT exhibits moderate stretchability with crack‐onset‐strain ≈50% and excellent electrical properties. At 50% strain, it has a mobility of 2.37 cm²V⁻¹ s⁻¹parallel to the stretched direction, which is higher than the mobility of most stretchable conjugated polymers in this stretching state.

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.

Thin films with a nanometer‐scale thickness are of great interest to both scientific and industrial communities due to their numerous applications and unique behaviors different from the bulk. However, the understanding of thin‐film mechanics is still greatly hampered due to their intrinsic fragility and the lack of commercially available experimental instruments. In this review, we first discuss the progression of thin‐film mechanical testing methods based on the supporting substrate: film‐on‐solid substrate method, film‐on‐water tensile tests, and water‐assisted free‐standing tensile tests. By comparing past studies on a model polymer, polystyrene, the effect of different substrates and confinement effect on the thin‐film mechanics is evaluated. These techniques have generated fruitful scientific knowledge in the field of organic semiconductors for the understanding of structure–mechanical property relationships. We end this review by providing our perspective for their bright prospects in much broader applications and materials of interest.

Extensive efforts have been made to develop flexible electronics with conjugated polymers that are intrinsically stretchable and soft. We recently systematically investigated the influence of conjugation break spacers (CBS) on the thermomechanical properties of a series n‐type naphthalene diimide‐based conjugated polymer and found that CBS can significantly reduce chain rigidity, melting point, as well as glass transition temperature. In the current work, we further examined the influence of CBS on the crystallization behaviors of PNDI‐C3 to C6, including isothermal crystallization kinetics, crystal polymorphism and subsequently time‐dependent modulus, in a holistic approach using differential scanning calorimetry, X‐ray scattering, polarized optical microscopy, atomic force microscopy, and pseudo‐free‐standing tensile test. Results demonstrate that increasing the length of CBS increases the crystallization half‐time by 1 order of magnitude from PNDI‐C3 to PNDI‐C6 from approximately 10³to 10⁴ s. The crystallization rate shows a bimodal dependence on the temperature due to the presence of different polymorphs. In addition, crystallization significantly affects the mechanical response, a stiffening in the modulus of nearly three times is observed for PNDI‐C5 when annealed at room temperature for 12 h. Crystallization kinetic is also influenced by molecular weight (MW). Higher MW PNDI‐C3 crystallizes slower. In addition, an odd–even effect was observed below 50°C, odd‐number PNDI‐Cxs (C3 and C5) crystallize slower than the adjacent even‐numbered PNDI‐Cxs (C4 and C6). Our work provides an insight to design flexible electronics by systematically tuning the mechanical properties through control of polymer crystallization by tuning backbone rigidity.

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‐(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%.

Donor–acceptor (D–A) type semiconducting polymers have shown great potential for the application of deformable and stretchable electronics in recent decades. However, due to their heterogeneous structure with rigid backbones and long solubilizing side chains, the fundamental understanding of their molecular picture upon mechanical deformation still lacks investigation. Here, the molecular orientation of diketopyrrolopyrrole (DPP)‐based D–A polymer thin films is probed under tensile deformation via both experimental measurements and molecular modeling. The detailed morphological analysis demonstrates highly aligned polymer crystallites upon deformation, while the degree of backbone alignment is limited within the crystalline domain. Besides, the aromatic ring on polymer backbones rotates parallel to the strain direction despite the relatively low overall chain anisotropy. The effect of side‐chain length on the DPP chain alignment is observed to be less noticeable. These observations are distinct from traditional linear‐chain semicrystalline polymers like polyethylene due to distinct characteristics of backbone/side‐chain combination and the crystallographic characteristics in DPP polymers. Furthermore, a stable and isotropic charge carrier mobility is obtained from fabricated organic field‐effect transistors. This study deconvolutes the alignment of different components within the thin‐film microstructure and highlights that crystallite rotation and chain slippage are the primary deformation mechanisms for semiconducting polymers.