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.

Carbon nanotubes (CNTs) promise to advance a number of real-world technologies. Of these applications, they are particularly attractive for uses in chemical sensors for environmental and health monitoring. However, chemical sensors based on CNTs are often lacking in selectivity, and the elucidation of their sensing mechanisms remains challenging. This review is a comprehensive description of the parameters that give rise to the sensing capabilities of CNT-based sensors and the application of CNT-based devices in chemical sensing. This review begins with the discussion of the sensing mechanisms in CNT-based devices, the chemical methods of CNT functionalization, architectures of sensors, performance parameters, and theoretical models used to describe CNT sensors. It then discusses the expansive applications of CNT-based sensors to multiple areas including environmental monitoring, food and agriculture applications, biological sensors, and national security. The discussion of each analyte focuses on the strategies used to impart selectivity and the molecular interactions between the selector and the analyte. Finally, the review concludes with a brief outlook over future developments in the field of chemical sensors and their prospects for commercialization.

Despite the importance of mechanical compliance in most applications of semiconducting polymers, the effects of structural parameters of these materials on their mechanical properties are typically not emphasized. This paper examines the effect of length of the pendant group on the tensile modulus and brittleness for a series of regioregular poly(3‐alkylthiophenes) (P3ATs) and their blends with a soluble fullerene derivative, PCBM. The tensile modulus decreases with increasing length of the alkyl side‐chain, from 1.87 GPa for butyl side chains to 0.16 GPa for dodecyl chains. The moduli of P3AT:PCBM blends films are greater than those of the pure polymers by factors of 2–4. A theoretical model produces a trend in the effect of alkyl side chain on tensile modulus that follows closely to the experimental measurements. Tensile modulus correlates with brittleness, as the strain at which cracks appear is 6% for P3BT and >60% for P3OT. Adhesion of the P3AT film to a polydimethylsiloxane (PDMS) substrate is believed to play a role in an apparent increase in brittleness from P3OT to P3DDT. The additive 1,8‐Diiodooctane (DIO) reduces the modulus of P3HT:PCBM blend by a factor of 3. These results could enable mechanically robust, flexible, and stretchable electronics.

Despite the ubiquity of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in applications demanding mechanical flexibility, the effect on the mechanical properties of common additives—i.e., dimethylsulfoxide (DMSO), Zonyl fluorosurfactant (Zonyl), and poly(ethyleneimine) (PEI)—has not been reported. This paper describes these effects and uses plasticized films in solar cells and mechanical sensors for the detection of human motion. The tensile moduli of films spin‐coated from solutions containing 0%, 5%, and 10% DMSO and 0.1%, 1%, and 10% Zonyl (nine samples total) are measured using the buckling technique, and the ductility is inferred from measurements of the strain at which cracks form on elastic substrates. Elasticity and ductility are maximized in films deposited from solutions containing 5% DMSO and 10% Zonyl, but the conductivity is greatest for samples containing 0.1% Zonyl. These experiments reveal enlargement of presumably PEDOT‐rich grains, visible by atomic force microscopy, when the amount of DMSO is increased from 0% to 5%. PEI—which is used to lower the work function of PEDOT:PSS—has a detrimental effect on the mechanical properties of the PEDOT:PSS/PEI bilayer films. Wearable electronic sensors employing PEDOT:PSS films containing 5% DMSO and 10% Zonyl are ­fabricated, which exhibit detectable responses at 20% strain and high mechanical robustness through elastic deformation.

Ion-conducting polymers are ideal solid electrolytes for most energy storage and conversion devices where ion transport is a critical functionality. The system performance and stability are related to the transport and mechanical properties of the ionomers, which are correlated through physiochemical interactions and morphology. Thus, there exists a balance between the chemical and mechanical energies which controls the structure−function relationship of the ionomer. In this paper, it is reported how and why thermal treatments result in different water uptakes and nanostructures for a perfluorinated sulfonic acid (PFSA) membrane. The nanostructure of the PFSA membrane is characterized using small-and wide-angle Xray scattering experiments. These changes are correlated with water content and mechanical properties and result in fundamental relationships to characterize the membrane with different thermal histories. Moreover, quasi-equilibrium water uptake and domain spacing both decrease with predrying or preconstraining the membrane, thereby suggesting that similar mechanical energies govern the structural changes via internal and external constraints, respectively. The findings suggest that heat treatments alter the balance between the chemical−mechanical energies where the interplay of the morphology and mechanical properties controls the structure−function relationship of the membrane. Finally, a model is developed using an energy-balance approach with inputs of the mechanical and structural properties; the dependence of water uptake on pretreatment is successfully predicted.

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 examines a series of poly(3-alkylthiophene)s (P3ATs), a class of materials for which mechanical compliance and electronic performance have been observed to be in competition. P3ATs with longer alkyl side chains (n ≥ 8) have high elasticity and ductility, but poor electronic performance (as manifested in photovoltaic efficiency in blends with fullerenes); P3ATs with shorter chains (n ≤ 6) exhibit the opposite characteristics. A series of four polymer films in which the average length of the side chain is n = 7 is tested using mechanical, spectroscopic, microscopic, and photovoltaic device-based measurements to determine whether or not it is possible, in principle, to maximize both mechanical and electronic performance in a single organic semiconductor (the “best of both worlds”). The four polymer samples are (1) a physical blend of equal parts P3HT and P3OT (P3HT:P3OT, n = 6 and n = 8), (2) a block copolymer (P3HT-b-P3OT), (3) a random copolymer (P3HT-co-P3OT), and (4) poly(3-heptylthiophene) (P3HpT, n = 7). The tensile moduli obtained by mechanical buckling correlate well with spectroscopic evidence (using the weakly interacting H aggregate model) of a well-ordered microstructure of the polymers. The block copolymer was the stiffest of the hybrid samples (680 ± 180 MPa), while P3HpT exhibited maximum compliance (70 ± 10 MPa) and power conversion efficiency in a 1:1 blend with the fullerene PC61BM using stretchable electrodes (PCE = 2.16 ± 0.17%) that was similar to that of P3HT:PC61BM. These analyses may permit the design of organic semiconductors with improved mechanical and electronic properties for mechanically robust and stretchable applications.

The mechanical properties of low-band-gap polymers are important for the long-term survivability of rollto-roll processed organic electronic devices. Such devices, e.g., solar cells, displays, and thin-film transistors, must survive the rigors of roll-to-roll coating and also thermal and mechanical forces in the outdoor environment and in stretchable and ultraflexible form factors. This paper measures the stiffness (tensile modulus), ductility (crack-onset strain), or both of a combinatorial library of 51 low-band-gap polymers. The purpose of this study is to systematically screen a library of low-band-gap polymers to better understand the connection between molecular structures and mechanical properties in order to design conjugated polymers that permit mechanical robustness and even extreme deformability. While one of the principal conclusions of these experiments is that the structure of an isolated molecule only partially determines the mechanical propertiesanother important codeterminant is the packing structuresome general trends can be identified. (1) Fused rings tend to increase the modulus and decrease the ductility. (2) Branched side chains have the opposite effect. Despite the rigidity of the molecular structure, the most deformable films can be surprisingly compliant (modulus ≥ 150 MPa) and ductile (crack-onset strain ≤ 68%). This paper concludes by proposing a new composite merit factor that combines the power conversion efficiency in a fully solution processed device obtained via roll and roll-to-roll coating and printing (as measured in an earlier paper) and the mechanical deformability toward the goal of producing modules that are both efficient and mechanically stable.