Mechanical deformability underpins many of the advantages of organic semiconductors. The mechanical properties of these materials are, however, diverse, and the molecular characteristics that permit charge transport can render the materials stiff and brittle. This review is a comprehensive description of the molecular and morphological parameters that govern the mechanical properties of organic semiconductors. Particular attention is paid to ways in which mechanical deformability and electronic performance can coexist. The review begins with a discussion of flexible and stretchable devices of all types, and in particular the unique characteristics of organic semiconductors. It then discusses the mechanical properties most relevant to deformable devices. In particular, it describes how low modulus, good adhesion, and absolute extensibility prior to fracture enable robust performance, along with mechanical "imperceptibility" if worn on the skin. A description of techniques of metrology precedes a discussion of the mechanical properties of three classes of organic semiconductors: π-conjugated polymers, small molecules, and composites. The discussion of each class of materials focuses on molecular structure and how this structure (and postdeposition processing) influences the solid-state packing structure and thus the mechanical properties. The review concludes with applications of organic semiconductor devices in which every component is intrinsically stretchable or highly flexible.

The mechanical properties of organic semiconductors and the mechanical failure mechanisms of devices play critical roles in the yield of modules in roll-to-roll manufacturing and the operational stability of organic solar cells (OSCs) in portable and outdoor applications. This paper begins by reviewing the mechanical propertiesprincipally stiffness and brittleness-of pure films of organic semiconductors. It identifies several determinants of the mechanical properties, including molecular structures, polymorphism, and microstructure and texture.Next, a discussion of the mechanical properties of polymer-fullerene bulk heterojunction blends reveals the strong influence of the size and purity of the fullerenes, the effect of processing additives as plasticizers, and the details of molecular mixing-i.e., the extent of intercalation of fullerene molecules between the side chains of the polymer. Mechanical strain in principle affects the photovoltaic output of devices in several ways, from strain-evolved changes in alignment of chains, degree of crystallinity, and orientation of texture, to debonding, cohesive failure, and cracking, which dominate changes in the high-strain regime. These conclusions highlight the importance of mechanical properties and mechanical effects on the viability of OSCs during manufacture and in operational environments. The review-whose focus is on molecular and microstructural determinants of mechanical properties-concludes by suggesting several potential routes to maximize both mechanical resilience and photovoltaic performance for improving the lifetime of devices in the near term and enabling devices that require extreme deformation (i.e., stretchability and ultra-flexibility) in the future. Broader contextOrganic solar cells (OSCs) are potentially an inexpensive source of renewable energy that can be manufactured at speeds that dwarf the rate at which wafer-based devices (i.e., silicon) can be fabricated. While low efficiencies of OSCs have historically been regarded as a major roadblock, the performance of this class of printable devices is improving rapidly, and module efficiencies of ten percent now seem possible. The susceptibility of polymer-based active layers to undergo thermally activated phase separation, photochemical damage, and other forms of degradation has motivated large and expanding literature devoted to understanding and improving the long-term stability of modules. Conspicuously absent from the literature, however, is a similar effort directed toward understanding the mechanical properties of organic semiconductors and their effects on the lifetime of devices against mechanical failure. The principal advantage of OSCs and all printed electronic devices is, nonetheless, roll-to-roll manufacturing on exible substrates. Manufacturing, installation, and use of these devices will thus require substantial mechanical resilience. Moreover, the ability to make devices on ultrathin plastic sheets-necessary to achieve low production energy for whole modules-requires that the acti...

This paper describes a comparison of two characterization techniques for determining the mechanical properties of thin-film organic semiconductors for applications in soft electronics. In the first method, the film is supported by water (film-on-water, FOW), and a stress-strain curve is obtained using a direct tensile test. In the second method, the film is supported by an elastomer (film-on-elastomer, FOE), and is subjected to three tests to reconstruct the key features of the stress-strain curve: the buckling test (tensile modulus), the onset of buckling (yield point), and the crack-onset strain (strain at fracture). The specimens used for the comparison are four poly(3-hexylthiophene) (P3HT) samples of increasing molecular weight (M = 15, 40, 63, and 80 kDa). The methods produced qualitatively similar results for mechanical properties including the tensile modulus, the yield point, and the strain at fracture. The agreement was not quantitative because of differences in mode of loading (tension vs compression), strain rate, and processing between the two methods. Experimental results are corroborated by coarse-grained molecular dynamics simulations, which lead to the conclusion that in low molecular weight samples (M = 15 kDa), fracture occurs by chain pullout. Conversely, in high molecular weight samples (M > 25 kDa), entanglements concentrate the stress to few chains; this concentration is consistent with chain scission as the dominant mode of fracture. Our results provide a basis for comparing mechanical properties that have been measured by these two techniques, and provide mechanistic insight into fracture modes in this class of materials.

The glass transition temperature (T g ) of a conjugated polymer can be used to predict its morphological stability and mechanical properties. Despite the importance of this parameter in applications from organic solar cells to wearable electronics, it is not easy to measure. The T g is often too weak to detect using conventional differential scanning calorimetry (DSC). Alternative methodse.g., variable temperature ellipsometryrequire specialized equipment. This paper describes a technique for measuring the T g of thin films of semicrystalline conjugated polymers using only a hot plate and an ultraviolet−visible (UV−vis) spectrometer. UV−vis spectroscopy is used to measure changes in the absorption spectrum due to molecular-scale rearrangement of polymers when heated past T g , corresponding to the onset of the formation of photophysical aggregates. A deviation metric, defined as the sum of the squared deviation in absorbance between as-cast and annealed films, is used to quantify shifts in the absorption spectra. The glass transition is observed as a change in slope in a plot of the deviation metric versus temperature. To demonstrate the usefulness of this technique, a variety of semiconducting polymers are tested: P3BT, PBTTT-C14, F8BT, PDTSTPD, PTB7, PCDTBT, TQ1, and MEH-PPV. These polymers represent a range of solid-state morphologies, from highly ordered to predominantly amorphous. A successful measurement of T g depends on the ability of the polymer to form photophysical aggregates. The results obtained using this method for P3BT, PBTTT-C14, F8BT, and PDTSTPD are in agreement with values of T g that have been reported in the literature. Molecular dynamics simulations are used to show how the morphology evolves upon annealing: above the T g , an initially kinetically trapped morphology undergoes structural rearrangement to assume a more thermodynamically preferred structure. The temperature at which onset of this rearrangement occurs in the simulation is concomitant with the spectroscopically determined value of T g .

The ability to predict the mechanical properties of organic semiconductors is of critical importance for roll-to-roll production and thermomechanical reliability of organic electronic devices. Here, we describe the use of coarse-grained molecular dynamics simulations to predict the density, tensile modulus, Poisson ratio, and glass transition temperature for poly­(3-hexyl­thiophene) (P3HT) and its blend with C60. In particular, we show that the resolution of the coarse-grained model has a strong effect on the predicted properties. We find that a one-site model, in which each 3-hexyl­thiophene unit is represented by one coarse-grained bead, predicts significantly inaccurate values of density and tensile modulus. In contrast, a three-site model, with one coarse-grained bead for the thiophene ring and two for the hexyl chain, predicts values that are very close to experimental measurements (density = 0.955 g cm–3, tensile modulus = 1.23 GPa, Poisson ratio = 0.35, and glass transition temperature = 290 K). The model also correctly predicts the strain-induced alignment of chains as well as the vitrification of P3HT by C60 and the corresponding increase in the tensile modulus (tensile modulus = 1.92 GPa, glass transition temperature = 310 K). We also observe a decrease in the radius of gyration and the density of entanglements of the P3HT chains with the addition C60 which may contribute to the experimentally noted brittleness of the composite material. Although extension of the model to poly­(3-alkyl­thiophenes) (P3ATs) containing side chains longer than hexyl groupsnonyl (N) and dodecyl (DD) groupscorrectly predicts the trend of decreasing modulus with increasing length of the side chain measured experimentally, obtaining absolute agreement for P3NT and P3DDT could not be accomplished by a straightforward extension of the three-site coarse-grained model, indicating limited transferability of such models. Nevertheless, the accurate values obtained for P3HT and P3HT:C60 blends suggest that coarse graining is a valuable approach for predicting the thermomechanical properties of organic semiconductors of similar or more complex architectures.