Darren J. Lipomi

Stretchable, elastic materials and devices for solar energy conversion

This Perspective reviews stretchable, elastic materials and devices of use for the conversion of solar energy. Stretchable and extremely flexible photovoltaic and photoelectrochemical cells would be useful as, for example, power sources for rugged and collapsible electronic and biomedical devices, foldable, weather-resistant solar roofs for applications in the field, and conformable solar panels for one-time bonding to the curved surfaces of buildings and vehicles. The review discusses elastic substrates, semiconductors, and electrodes. It then describes examples of whole devices comprising these elastic components. The Perspective tends to emphasize materials of interest for excitonic solar cells—e.g., organic solar cells, those based on semiconductor nanocrystals, and dye sensitized solar cells—because of their potential for stretchability and extreme flexibility, but also discusses crystalline photovoltaic devices and nanostructured photoelectrochemical cells.

Continuous wireless pressure monitoring and mapping with ultra-small passive sensors for health monitoring and critical care

Continuous monitoring of internal physiological parameters is essential for critical care patients, but currently can only be practically achieved via tethered solutions. Here we report a wireless, real-time pressure monitoring system with passive, flexible, millimetre-scale sensors, scaled down to unprecedented dimensions of 1 × 1 × 0.1 cubic millimeters. This level of dimensional scaling is enabled by novel sensor design and detection schemes, which overcome the operating frequency limits of traditional strategies and exhibit insensitivity to lossy tissue environments. We demonstrate the use of this system to capture human pulse waveforms wirelessly in real time as well as to monitor in vivo intracranial pressure continuously in proof-of-concept mice studies using sensors down to 2.5 × 2.5 × 0.1 cubic millimeters. We further introduce printable wireless sensor arrays and show their use in real-time spatial pressure mapping. Looking forward, this technology has broader applications in continuous wireless monitoring of multiple physiological parameters for biomedical research and patient care.

Green chemistry for organic solar cells

Essentially all methods of energy production—e.g., fracking, damming, drilling, nuclear fission, and excavation of rare elements for photovoltaics—are associated with some degree of environmental degradation. Organic solar cells (OSCs) are regarded as low-cost and potentially environmentally benign sources of power. π-Conjugated (semiconducting) polymers—the components of OSCs responsible for absorbing light and transporting charge—are not typically synthesized in laboratories in ways that are amenable to manufacturing with low environmental impact. This article discusses strategies for producing conjugated polymers using green chemistry. That is, reaction methodology with low energy intensity, with minimal production of toxic waste, and at low cost. This article briefly reviews the major findings in the literature on the energy intensity and carbon emissions associated with fabricating OSCs on the laboratory scale, and identifies several strategies and materials invented by the community to lower the cost and environmental impact of the components of the devices. The principles of green chemistry, applied to the synthesis of conjugated polymers, are identified as important guidelines for the multi-tonne manufacturing of these materials. A general theme in both green chemistry and process research is that low cost can be correlated to environmental benignity when the costs of disposing wastes are high. This Perspective then highlights five synthetic strategies that satisfy several of the criteria of green chemistry: (1) polymerization using metal-mediated cross-coupling reactions that reduce or eliminate stoichiometric organotin waste; (2) the use of heterogeneously catalyzed polymerizations; (3) polymerization involving activation of C–H bonds; (4) use of biofeedstock-derived starting materials; and (5) polycondensation reactions that evolve water as a byproduct.

Mechanical degradation and stability of organic solar cells: molecular and microstructural determinants

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 properties—principally 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.

Patterning the Tips of Optical Fibers with Metallic Nanostructures Using Nanoskiving

Convenient and inexpensive methods to pattern the facets of optical fibers with metallic nanostructures would enable many applications. This communication reports a method to generate and transfer arrays of metallic nanostructures to the cleaved facets of optical fibers. The process relies on nanoskiving, in which an ultramicrotome, equipped with a diamond knife, sections epoxy nanostructures coated with thin metallic films and embedded in a block of epoxy. Sectioning produces arrays of nanostructures embedded in thin epoxy slabs, which can be transferred manually to the tips of optical fibers at a rate of approximately 2 min(-1), with 88% yield. Etching the epoxy matrices leaves arrays of nanostructures supported directly by the facets of the optical fibers. Examples of structures transferred include gold crescents, rings, high-aspect-ratio concentric cylinders, and gratings of parallel nanowires.

Structural Transformation by Electrodeposition on Patterned Substrates (STEPS): A New Versatile Nanofabrication Method

Arrays of high-aspect-ratio (HAR) nano- and microstructures are of great interest for designing surfaces for applications in optics, bio-nano interfaces, microelectromechanical systems, and microfluidics, but the difficulty of systematically and conveniently varying the geometries of these structures significantly limits their design and optimization for a specific function. This paper demonstrates a low-cost, high-throughput benchtop method that enables a HAR array to be reshaped with nanoscale precision by electrodeposition of conductive polymers. The method-named STEPS (structural transformation by electrodeposition on patterned substrates)-makes it possible to create patterns with proportionally increasing size of original features, to convert isolated HAR features into a closed-cell substrate with a continuous HAR wall, and to transform a simple parent two-dimensional HAR array into new three-dimensional patterned structures with tapered, tilted, anisotropic, or overhanging geometries by controlling the deposition conditions. We demonstrate the fabrication of substrates with continuous or discrete gradients of nanostructure features, as well as libraries of various patterns, starting from a single master structure. By providing exemplary applications in plasmonics, bacterial patterning, and formation of mechanically reinforced structures, we show that STEPS enables a wide range of studies of the effect of substrate topography on surface properties leading to optimization of the structures for a specific application. This research identifies solution-based deposition of conductive polymers as a new tool in nanofabrication and allows access to 3D architectures that were previously difficult to fabricate.

Increased elasticity of a low-bandgap conjugated copolymer by random segmentation for mechanically robust solar cells

Despite the necessity of organic electronic materials to undergo large deformations in flexible, ultra-thin, and stretchable applications, many high-performance organic semiconductors are mechanically fragile. This paper describes an approach to increase the elasticity of low-bandgap conjugated polymers by statistical incorporation of unlike monomers. The material under study is PDPP2FT, an alternating copolymer. Synthesized by the Stille polymerization, it comprises an N-alkylated diketopyrrolopyrrole (DPP) unit flanked by two furan rings (2F) alternating with thiophene (T). In the modified (“segmented”) polymer, PDPP2FT-seg-2T, the DPP is exchanged for a tail-to-tail coupled unit of two 3-hexylthiophene rings (bithiophene, 2T) in an average of one of approximately five repeat units. 1H NMR spectroscopy, ultraviolet-visible spectroscopy, and gel-permeation chromatography confirm the presence and covalent incorporation of the 2T units within the conjugated backbone of the segmented polymer. The tensile modulus of the segmented polymer, 0.93 ± 0.16 GPa, is lower than that of the homopolymer, 2.17 ± 0.35 GPa. When blended with PC61BM, the segmented material produces devices with power conversion efficiencies of 2.82 ± 0.28%, which is similar to that of PDPP2FT, 2.52 ± 0.34%. These results suggest that it is possible to increase the mechanical resiliency of semiconducting polymers for solar cells without having a deleterious effect on the photovoltaic properties.

Stretching and conformal bonding of organic solar cells to hemispherical surfaces

This paper describes the stretching and conformal bonding (i.e., decal-transfer printing) of organic solar cells in both the “conventional” and “inverted” configurations to hemispherical glass surfaces with radii of 8 mm. This action produces equivalent biaxial tensile strains of 24%, which many materials used in organic electronic devices cannot accommodate without fracture. Consideration of the mechanical properties of conjugated polymers reveals a surprising effect of a single structural parameter—the length of the alkyl side chain—on the elasticity and ductility of regioregular polythiophene. This analysis enables selection of materials that can accommodate sufficient tensile strain for non-planar applications. For polymer–fullerene solar cells, devices based on the elastic and ductile poly(3-octylthiophene) (P3OT) exhibit typical photovoltaic properties when bonded to hemispherical glass substrates, while those based on the relatively brittle poly(3-hexylthiophene) (P3HT) exhibit extensive cracking, which degrades the photovoltaic effect significantly. The results suggest that mechanical properties should be taken into account when designing and selecting organic semiconductors for applications that demand significant deformation.

Use of Thin Sectioning (Nanoskiving) to Fabricate Nanostructures for Electronic and Optical Applications

This Review discusses nanoskiving--a simple and inexpensive method of nanofabrication, which minimizes requirements for access to cleanrooms and associated facilities, and which makes it possible to fabricate nanostructures from materials, and of geometries, to which more familiar methods of nanofabrication are not applicable. Nanoskiving requires three steps: 1) deposition of a metallic, semiconducting, ceramic, or polymeric thin film onto an epoxy substrate; 2) embedding this film in epoxy, to form an epoxy block, with the film as an inclusion; and 3) sectioning the epoxy block into slabs with an ultramicrotome. These slabs, which can be 30 nm-10 μm thick, contain nanostructures whose lateral dimensions are equal to the thicknesses of the embedded thin films. Electronic applications of structures produced by this method include nanoelectrodes for electrochemistry, chemoresistive nanowires, and heterostructures of organic semiconductors. Optical applications include surface plasmon resonators, plasmonic waveguides, and frequency-selective surfaces.

Metallic Nanoislands on Graphene as Highly Sensitive Transducers of Mechanical, Biological, and Optical Signals

This article describes an effect based on the wetting transparency of graphene; the morphology of a metallic film (≤20 nm) when deposited on graphene by evaporation depends strongly on the identity of the substrate supporting the graphene. This control permits the formation of a range of geometries, such as tightly packed nanospheres, nanocrystals, and island-like formations with controllable gaps down to 3 nm. These graphene-supported structures can be transferred to any surface and function as ultrasensitive mechanical signal transducers with high sensitivity and range (at least 4 orders of magnitude of strain) for applications in structural health monitoring, electronic skin, measurement of the contractions of cardiomyocytes, and substrates for surface-enhanced Raman scattering (SERS, including on the tips of optical fibers). These composite films can thus be treated as a platform technology for multimodal sensing. Moreover, they are low profile, mechanically robust, semitransparent and have the potential for reproducible manufacturing over large areas.