Plastic electronics: Back to basics
Five years ago, MRS Bulletin published a theme issue on the then—and still—burgeoning field of stretchable and flexible electronics. Reference Wagner and Bauer1 The authors in that issue explored the mechanics of stretchable thin films, Reference Suo2 approaches to generating stretchable devices from otherwise rigid materials, Reference Sekitani and Someya3 and a range of stretchable device components, from batteries to electronic eye cameras. Reference Kim, Lu, Huang and Rogers4 The topics in that issue reflected the state of the art in the field then, which was dominated by devices based on inorganic materials (e.g., metallic films and semiconductor nanoribbons).
A complementary approach to extreme deformability is based on organic conductors and semiconductors. Reference Savagatrup, Printz, O’Connor, Zaretski and Lipomi5 While the conductivity and transport properties of inorganic materials are superior to those of organics, the advantages of these molecular materials remain—oxide-free interfaces, fabrication by printing in a roll-to-roll manner, low cost, tailorability by synthesis, and intrinsic mechanical compliance. Reference Savagatrup, Printz, O’Connor, Zaretski and Lipomi5 Deformability was one of the original goals of organic electronics, Reference Heeger6 but some of the best performing materials today remain stiff, and they crack at relatively low strains. Organic devices can be rendered stretchable in many of the same ways as can devices based on inorganic thin films (e.g., using wavy, fractal, or relief features that direct strain away from the sensitive components). Reference Fan, Yeo, Su, Hattori, Lee, Jung, Zhang, Liu, Heng, Falgout, Bajema, Coleman, Gregoire, Larsen, Huang and Rogers7 Other approaches that are applicable only to organics, however, give them a distinct advantage, such as the formation of fibers to form elastic mats and the synthesis of materials whose molecular structure or solid-state packing structure permits extreme deformation. Reference Lee, Shin, Thiyagarajan and Jeong8
A suite of tools incorporating metrology, synthesis, and fabrication has recently emerged whose goal is to achieve the original dream of organic electronics—to combine state-of-the-art electronic performance with high deformability. This research is, at present, somewhat distinct from efforts in the field to understand and to improve the electronic properties of organic semiconductors, though ultimately, the results of both spheres of inquiry must be merged. The authors of the articles in this issue of MRS Bulletin have made key contributions to developing stretchable new materials or stretchable forms of old ones, new techniques for measuring the mechanical properties of fragile thin films, and new devices that exhibit unprecedented deformability.
New materials and stretchable forms of old ones
Stretchable electronics had its beginnings in the 1990s with the work of Wagner, Suo, and others, who examined the mechanics of materials—principally metals—on flexible and stretchable substrates. Reference Suo, Ma, Gleskova and Wagner9 These materials adopted buckled, wavy, or fractured morphologies that accommodated tensile strains while retaining electronic functionality. Reference Jones, Lacour, Wagner and Suo10 In an example of applying this approach to organic electronic materials, we have made stretchable organic solar cells by buckling the devices on elastic substrates. Reference Lipomi, Tee, Vosgueritchian and Bao11 Bettinger and co-workers used a similar approach by producing the first stretchable organic transistor ( Figure 1a). Reference Wu, Kustra, Gates and Bettinger12 Kaltenbrunner, Bauer, Someya, and co-workers increased the conformability and stretchability of devices by fabricating their devices on ultrathin plastic foils (Figure 1b), noting that the bulk of a “thin-film” electronic device is embodied in its substrate. Reference Kaltenbrunner, Sekitani, Reeder, Yokota, Kuribara, Tokuhara, Drack, Schwödiauer, Graz, Bauer-Gogonea, Bauer and Someya13 This approach has been applied successfully to solar cells, Reference Kaltenbrunner, White, Głowacki, Sekitani, Someya, Sariciftci and Bauer14 light-emitting devices, Reference White, Kaltenbrunner, Głowacki, Gutnichenko, Kettlegruber, Graz, Aazou, Ulbricht, Egbe, Miron, Major, Scharber, Sekitani, Someya, Bauer and Sariciftci15 and pressure sensors. Reference Bauer, Bauer-Gogonea, Graz, Kaltenbrunner, Keplinger and Schwödiauer16 Another strategy is to deposit high-aspect-ratio organic nanostructures on (or in) an elastomeric matrix. Reference Bauer, Bauer-Gogonea, Graz, Kaltenbrunner, Keplinger and Schwödiauer16 This approach has worked for nanowires of regioregular poly(3-hexylthiophene) (P3HT) to yield all-nanofiber transistors (Figure 1c). Reference Shin, Song, Lim, Lim, Park and Jeong17 Bao et al. found the nanoconfinement effect to improve the stretchability of polymer semiconductors through lowering its modulus due to increased polymer chain dynamics. Reference Xu, Wang, Wang, Zhu, Luo, Jin, Gu, Chen, Feig, To, Rondeau-Gagné, Park, Schroeder, Lu, Oh, Wang, Kim, Yan, Sinclair, Zhou, Xue, Murmann, Linder, Cai, Tok, Chung and Bao18 Organic semiconductor films can remain functional in thin-film transistors even if they contain a large density of cracks (as long as the film retains an uninterrupted pathway for charge carriers). Reference Chortos, Lim, To, Vosgueritchian, Dusseault, Kim, Hwang and Bao19
A new strategy relies on the intrinsic stretchability of the organic material. This approach, called “intrinsically” Reference Yu, Niu, Liu and Pei20 or “molecularly” Reference Savagatrup, Printz, O’Connor, Zaretski and Lipomi5 stretchable, does not require materials to be processed into buckles or nanowires (Figure 1d). Reference Root, Savagatrup, Pais, Arya and Lipomi21 Scientifically, this approach requires circumventing the apparent incompatibility of charge transport with deformability. That is, the stiff, π-conjugated backbones and highly aggregated microstructures of organic semiconductors tend to be good for charge transport, but bad for mechanical deformability. Our groups and several of the groups represented in the articles in this issue have investigated the effects of molecular structure, molecular weight, additives, microstructural details, molecular recognition, and other phenomena, on the mechanical properties of organic semiconductors, and the ways in which intrinsic stretchability can be combined with electronic performance. Reference Wu, Benight, Chortos, Lee, Mei, To, Lu, He, Tok, Chen and Bao22,Reference Savagatrup, Printz, O’Connor, Zaretski, Rodriquez, Sawyer, Rajan, Acosta, Root and Lipomi23
Approaches to measuring the mechanical properties of organic semiconductors are subject to two constraints. First, little material is typically produced by a laboratory-scale synthesis. Second, these materials are generally processed into thin films. Traditional pull testing is difficult or impossible for materials produced by syntheses that can yield less than 100 mg of material. The stress–strain behavior can be measured directly for some materials using a highly sensitive technique developed by Kim and co-workers in which the pull test is carried out with the film supported by water ( Figure 2a). Reference Kim, Kim, Lee, Yu, Kim, Song, Shin, Oh, Jeong, Kim and Kim24 Dauskardt and co-workers have developed the four-point bend test (Figure 2b) and double-cantilever beam test for determining the cohesive and adhesive fracture energies of semiconducting polymer films. Reference Bruner and Dauskardt25
Several techniques have been developed to reconstruct the stress–strain response of thin films by assaying their behavior when strained on elastomeric substrates. Reference Printz, Zaretski, Savagatrup, Chiang and Lipomi26 The mechanical buckling metrology, as depicted in Figure 2c, Reference Stafford, Harrison, Beers, Karim, Amis, Vanlandingham, Kim, Volksen, Miller and Simonyi27 is a well-known system to derive the elastic modulus of a thin film based on its buckling pattern on an elastic substrate under small compressive strains. Basically, the modulus increases with the cube of λb/d f, where λb is the buckling wavelength and d f is the thickness of the film. Reference Stafford, Harrison, Beers, Karim, Amis, Vanlandingham, Kim, Volksen, Miller and Simonyi27 The yield point can be measured by cyclically straining a thin film by incrementally increasing displacements until buckles appear; this point corresponds to where the film has deformed plastically. Reference Printz, Zaretski, Savagatrup, Chiang and Lipomi26 The crack onset strain of a thin film on an elastic substrate is generally taken to be the strain at fracture, however, as a composite system, it is affected by the adhesion of the film to the substrate (better adhesion leads to larger crack onset strains). Figure 2d Reference Rodriquez, Savagatrup, Valle, Proctor, McDowell, Bazan, Nguyen and Lipomi28 shows the experimental configurations of measuring the crack onset strain in the high-strain regime using a linear actuator (left) and in the low-strain regime by bending with a large radius of curvature (right). Reference Sawyer, Zaretski, Printz, de los Santos, Bautista-Gutierrez and Lipomi29 The results of these film-on-elastomer techniques can be taken together to estimate the stress–strain behavior. Reference Printz, Zaretski, Savagatrup, Chiang and Lipomi26
Mechanical properties of organic semiconductors
The mechanical properties of the organics are critical to the stability of the devices in use. For conjugated polymers, increasing molecular weight tends to increase the cohesive energy, toughness, and crack-onset strain. Reference Bruner and Dauskardt25 Side chains in conjugated polymers decrease the modulus and brittleness by reducing the glass-transition temperature. Reference Savagatrup, Printz, Wu, Rajan, Sawyer, Zaretski, Bettinger and Lipomi30 Branched side chains have a similar effect, but side chains that are far enough apart to interdigitate produce stiff crystallites. Reference Roth, Savagatrup, de los Santos, Hagemann, Carle, Helgesen, Livi, Bundgaard, Sondergaard, Krebs and Lipomi31 Fused rings (e.g., thienothiophene) tend to produce stiffer thin films than isolated rings (e.g., bithiophene). Reference Roth, Savagatrup, de los Santos, Hagemann, Carle, Helgesen, Livi, Bundgaard, Sondergaard, Krebs and Lipomi31
The solid-state packing structure, however, is often a stronger predictor of mechanical properties than is the flexibility of an isolated molecule. Reference Savagatrup, Zhao, Chan, Mei and Lipomi32 Small-molecule or polymeric additives (i.e., to increase the efficiency of solar cells) tend to behave as plasticizers, which is an attractive route to achieving high performance and good mechanical compliance. Reference Savagatrup, Makaram, Burke and Lipomi33 Small-molecule semiconductors have reduced toughness compared to semiconducting polymers, but some specimens may be extended—without cracking—over a large enough range to enable applications that are truly stretchable. Reference Rodriquez, Savagatrup, Valle, Proctor, McDowell, Bazan, Nguyen and Lipomi28 Prediction of the mechanical properties of conjugated polymers has recently become the target of computational scientists. Reference Tummala, Bruner, Risko, Bredas and Dauskardt34 Atomistic molecular dynamics simulations of polymer:fullerene bulk heterojunction solar cells have been used to predict the cohesive fracture energy. Reference Savagatrup, Makaram, Burke and Lipomi33 In a recent contribution by Root et al., coarse-grained molecular dynamics simulations correctly predicted many key properties of P3HT, including the tensile modulus, strain alignment, Poisson’s ratio, and glass-transition temperature. Reference Root, Savagatrup, Pais, Arya and Lipomi21
Applications of stretchable and ultraflexible devices
The Lipomi and Kim groups Reference Kim, Kim, Kang, Lee, Kang, Shin, Wang, Ma, Jeong, Kim and Kim35 have reported several intrinsically stretchable solar cells, including wearable Reference O’Connor, Zaretski, Savagatrup, Printz, Wilkes, Diaz and Lipomi36 and biaxially stretchable ones. Reference O’Connor, Zaretski, Shiravi, Savagatrup, Printz, Diaz and Lipomi37 O’Connor et al. combined thin substrates with the selection of the most deformable active materials to produce wearable solar cells that could undergo repetitive bending strains with radii of curvature ≥100 µm ( Figure 3a). Reference O’Connor, Zaretski, Savagatrup, Printz, Wilkes, Diaz and Lipomi36 Stretchable and ultraflexible thin-film transistors have been made by a variety of techniques. For example, Wu et al. have used the buckling technique to generate microscale wrinkles in the active materials that can accommodate strain. Reference Wu, Kustra, Gates and Bettinger12 Kaltenbrunner et al. have fabricated “imperceptible” organic electronic devices by fabricating them on ultrathin plastic foils. Reference Kaltenbrunner, Sekitani, Reeder, Yokota, Kuribara, Tokuhara, Drack, Schwödiauer, Graz, Bauer-Gogonea, Bauer and Someya13 Chortos et al. have used intentionally cracked (but electrically contiguous) semiconductor films Reference Chortos, Lim, To, Vosgueritchian, Dusseault, Kim, Hwang and Bao19 (Figure 3b), and Lee et al. used another approach in which the semiconducting channel comprises a network of nanofibers in a stretchable matrix. Reference Lee, Shin, Thiyagarajan and Jeong8
We note the importance of encapsulation for improving the stability of flexible and stretchable devices. In the case of flexible applications, use of encapsulation allows the researcher to put the most sensitive components in the mechanically neutral plane; in the case of stretchable applications, encapsulation redistributes strain evenly across the active materials such that strain is not concentrated at thin areas and defects, which are ordinarily the sites where cracks initiate (Figure 3c–d). Reference Sawyer, Zaretski, Printz, de los Santos, Bautista-Gutierrez and Lipomi29
In this Issue
The work described in this theme issue is a comprehensive examination of the state of the art in highly deformable organic electronic materials and devices. The articles cover all important areas, including the design of materials, microstructural characterization, mechanical properties, and applications in devices. Thiyagarajan et al. describe several strategies for achieving stretchable active layers in polymeric semiconductors. In their article, Qi et al. cover an oft-overlooked aspect of stretchable devices—the substrate. O’Connor et al. describe the microstructural parameters that influence the stiffness and brittleness of materials in work related to the mechanical properties of materials. Kim et al. cover the mechanical behavior as it relates to reliability. Someya et al. describe the use of ultrathin substrates to achieve unprecedented levels of flexibility and “mechanical imperceptibility” on the device side. Liang et al. describe specific applications of stretchable organic semiconductors in thin-film transistors. Li et al. describe work on stretchable sensors and actuators for soft robotics.
The future of stretchable and ultraflexible organic electronic devices is promising. The field has benefitted from a highly collaborative approach, in which synthetic chemists have interacted synergistically with materials scientists and chemical, mechanical, electrical, and biomedical engineers. New materials and metrologies are being developed to overcome the challenges of minute quantities of materials measured in thin-film form. Given the interdisciplinarity of the field, it can benefit from open sourcing, of the type recently described by Bauer in a complete description of an apparatus for biaxial stretching. Reference Moser, Kettlgruber, Siket, Drack, Graz, Cakmak, Major, Kaltenbrunner and Bauer38 In this spirit of collaboration, the field is overcoming the apparent incompatibilities between charge transport and deformability, and applications in energy Reference Printz and Lipomi39 and the health sciences are at hand. Reference Leleux, Rivnay, Lonjaret, Badier, Benar, Herve, Chauvel and Malliaras40
D.J.L. acknowledges support from the Air Force Office of Scientific Research (AFOSR), Grant No. FA9550-16-1-0220. Z.B. acknowledges support from AFOSR (FA9550-15-1-0106) on work in related areas.
Darren J. Lipomi is an associate professor in the Department of NanoEngineering at the University of California, San Diego. He earned his PhD degree from Harvard University in 2010, and was a postdoctoral fellow at Stanford University from 2010 to 2012. He is the recipient of the Air Force Office of Scientific Research Young Investigator Award and the NIH Director’s New Innovator Award. His current research focuses on organic semiconductors for stretchable devices. Lipomi can be reached by phone at 858-246-1227 or by email at email@example.com.
Zhenan Bao is a professor of chemical engineering at Stanford University. She received her PhD degree in chemistry from The University of Chicago in 1995. She then joined Bell Labs, and later joined the faculty of chemical engineering at Stanford in 2004. She is a member of the National Academy of Engineering. Her research involves design and synthesis of new organic materials and processing methods for flexible and stretchable electronics. Bao can be reached by phone at 650-723-2419 or by email at firstname.lastname@example.org.