Brief introduction of the results
With the development of soft electronics technology, stretchable polymer semiconductors (PSCS) have made tremendous progress. However, achieving high carrier mobility and stretchability at the same time remains a challenge.
Academician Zhenan Bao, Stanford University, Mingqian He, Corning Corporationet al. reported that stretchable PSC films with high stretchability (thickness <100 nm) tend to exhibit a multimodal energy dissipation mechanism and have a large relative stretch (RS, defined by the ratio of entropy energy dissipation to enthalpy energy dissipation under strain). This property can effectively restore the original molecular order and electrical properties after the strain disappears. Among them, the highest RS value of the model polymer (P4) was 02 cm2 V-1 S-1, while PSCs with low RS values exhibit irreversible morphological changes and rapid degradation of electrical properties under strain. These results suggest that RS can be used as a parameter to compare the reliability and reversibility of stretchable PSC films.
Related work is based on ".highly stretchable polymer semiconductor thin films with multi-modal energy dissipation and high relative stretchability" as the title in ".nature communications**.
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Achieving high film deformability without compromising electrical properties has been a long-term challenge for PSCS. Previous work has focused on the effects of uniaxial strains. A systematic correlation based on the morphological, mechanical, and electrical behavior of molecular structures needs to be developed, especially for stretchable PSCs that may withstand multidirectional deformation in applications (Figure 1A).
Figure 1Properties of molecular order-induced PSCs
Here, this paper reveals the type of PSC film morphology that gives it stable performance under biaxial strain cycling (Figure 1C). The mean molecular weights of P2TBDPP2TBFT4-based PSCs were 22 (p (p (p3) and 97 (p4) kg mol-1, respectively (Figure 1A). Above the critical molecular weight, the solution viscosity slope of these polymers increases significantly as a function of molecular weight, indicating the presence of topological entanglement, chain aggregation, or solubility changes, while the polymer film becomes more disordered.
After stretching, the mechanical energy can be dissipated by one or more of the following mechanisms: (i) elastic or plastic deformation of the amorphous domains(ii) molecular arrangement or reorientation of crystals;(iii) amorphization of crystal domains and (iv) final bond cleavage and crack formation. Having a variety of possible energy dissipation modes can potentially produce more stable electrical performance under repeated strain cycles. The changes in RToc and DR are two parameters that can be used to characterize the changes that occur in the film during strain due to the energy dissipation mechanism (I)-(III).
Specifically, the change in RDoc represents the presence or absence of amorphization in ordered domains under strain, while DR reflects the ability of polymer chains (in crystalline and amorphous domains) to deform and align in the direction of strain (Figure 1B). Interestingly, the ratio of the change in polymer chain arrangement measured with DR to the change in RDOC under strain was found to correlate well with the crack onset strain of the film and the reversibility of the mechanical and electrical properties (Figure 1D). Therefore, this ratio is defined as relative tensile (RS) to capture the contribution of entropy energy dispersion and enthalpy energy dissipation to the polymer's ability to withstand strain.
Overall, polymers with higher RS were found to tend to have better electronic stability and mechanical durability under strain, as more mechanical energy can be dissipated under strain through non-harmful processes, such as chain alignment or crystal alignment (Figure 1E). In addition, based on published data, RS values for other reported stretchable PSCs were calculated at 0Between 3 and 3. These RS values correlate well with the trends in electrical properties under strain reported in the literature (Fig. 1f, g). Therefore, RS provides a reasonable comparison of the morphological responses of stretchable PSCs based on different designs.
Figure 2StrainMolecular order of PSC films
The conformation and crystallinity of the molecule play an important role in the tensile properties of PSC. For the same polymer chemical structure, simply changing the molecular weight increases the RS value by a factor of 4. GIXD was used to characterize the evolution of the overall molecular order (including RDOC, coherence length, and packaging orientation) of PSC films under strain. The RDOC values at different strain levels are determined by integrating the intensity of the layered diffraction peaks of the two-dimensional diffraction pattern collected from the in-plane azimuth of the PSC film from 0° to 90°, as shown in Figure 2a. The effect of applied strain on the crystallinity of the film is shown in Figure 2b. The RDOC value for each polymer is normalized based on its DOC at 0% strain. Overall, the RDOC decreases with the deformation of the polymer film, indicating that the tensile strain leads to crystal amorphization.
Here, strain-induced crystallization may also have occurred in the polymer, but the overall change in the observed RDOC is a combination of strain-induced crystallization and strain-induced amorphization, with strain-induced amorphization predominant. Among these conjugated polymers, the more predominant cause of strain-induced amorphization may be attributed to the high stiffness of their structure, which may exert higher stresses on the crystalline region under strain compared to more flexible unconjugated polymers. At 50 (P1 film) and 100% (P2 film), the RDOC decreased by about 40%, respectively, and the crystallinity did not recover after release.
In addition, a decrease in the coherence length of the layer was observed in the low molecular weight polymer (i.e., P2) film under strain (Figure 2C). The reduction in coherence length is particularly pronounced in the direction perpendicular to the strain (about 20% at 100% strain), suggesting that the ordered domains may also fracture due to compression orthogonal to the tensile direction. In contrast, films based on high molecular weight polymers (P4) have almost no constant average coherence length of RToC under strain, with a much smaller decrease (about 10% at 100% strain). After removing the strain, the RDoc returned to about 95% of the initial value, indicating that the crystal domain can be almost reversibly recovered.
Figure 3Biaxial simultaneous stretching of PSC films
P4 is a promising candidate for stretchable electronics, which requires the active semiconductor layer to withstand non-uniform stresses (stretching in any combination of directions). So far, most of the reported work has only involved the study of PSC thin films under uniaxial strain.
Here, simultaneous biaxial stretching of the polymer film was studied, as it is closer to real-world conditions. Figure 3A shows the surface topography of the P2 and P4 films observed by AFM under biaxial 100% strain. It is worth noting that even at 100% strain (equivalent to 400% area expansion), the P4 film remained smooth and no cracks were observed. In contrast, the P2 film is completely destroyed by biaxially stretching, and cracks can be observed at the microscale. In high molecular weight P4 films, the order of the molecules appears to be maintained even at high strain levels.
Due to the special ability of the P4 film to resist applied strain damage during biaxial stretching, stable charge transport performance is obtained (Fig. 3B). At a biaxial strain of 0-100%, P4 maintains 015 cm2 V-1 S-1 has an almost constant field-effect mobility with negligible hysteresis.
Compared to P4 FET devices exposed to uniaxial, isotropic biaxial and anisotropic biaxial strains, the mobility is 01-0.25 cm2 V-1 S-1, which indicates that charge transport is well maintained regardless of the direction of force applied on the PSC film (Figure 3C).
Finally, a fully stretchable FET device was fabricated using P4 with high RS as the active material, and about 0Carrier mobility of 2 cm2 V-1 S-1 (Figure 3D). The operational stability of this fully stretchable device under multi-directional deformation is shown in Figure 3e. The attenuation of the source leakage current before and after stretching is negligible, indicating that the electrical properties can still be maintained under irregular strain.
Bibliographic information
highly stretchable polymer semiconductor thin films with multi-modal energy dissipation and high relative stretchability，nature communications，2023.