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Metal halide perovskites are an emerging class of materials that are promising for low-cost and high-quality next-generation optoelectronic devices. Despite this potential, perovskites suffer from poor thermomechanical and chemical stability that must be overcome before the technology is commercially viable. Key sources of the instabilities in perovskites are ion migration and defects that can be tied to high residual stresses accumulated in the perovskite thin films during processing. This Mini-Review serves as a general overview of residual thin-film stresses, specifically in perovskite solar cells. A brief introduction to the origin of residual stresses in thin films is followed by the effects of these stresses in perovskite films specifically. Mitigation strategies for these stresses are then highlighted, followed by potential avenues of further exploration of residual stresses in perovskite films.

Metal halide perovskites are emerging semiconducting materials with a wide range of applications, including photovoltaics, thin-film transistors, and light-emitting diodes. A key advantage of perovskites over more established semiconductor technologies is solution processability, which reduces processing costs and enables rapid production by roll-to-roll printing. However, metal halide perovskites suffer from poor thermomechanical and chemical stability that must be overcome for successful commercialization of perovskite-based technologies. These instabilities have been shown to stem from defect states present at perovskite interfaces, and particularly grain boundaries, that result in destructive trap states. This review focuses on one successful strategy—organic additive engineering—that has been demonstrated to inhibit the formation of defect sites in perovskite films. This review begins by discussing defects present at grain boundaries—e.g., undercoordinated bonds, vacancies, and antisites—that develop during crystallization, and how these defects lead to reduced device performance via non-radiative recombination and hysteresis, as well as poor chemical and thermal stability. The bulk of this review is dedicated to additive engineering strategies to enhance performance and stability in 3D, quasi-2D, and 2D perovskites. Lewis acid–base interactions between the additives and perovskite constituents are highlighted, with additives ranging from small molecules and solvents to polymers. Special attention is paid to Lewis base additives containing amino and/or carbonyl functional groups, and a short discussion on semiconducting additives for improved charge transport is also included. This review concludes by identifying several areas of further discovery in organic additives for metal halide perovskites.

Photochemical and mechanical stability are critical in the production and application of organic solar cells. While these factors can individually be improved using different additives, there is no example of studies on the combined effects of such additive-assisted stabilization. In this study, the properties of PTB7:[70]PCBM organic solar cells are studied upon implementation of two additives: the carotenoid astaxanthin (AX) for photochemical stability and the silicone polydimethylsiloxane (PDMS) for improved mechanical properties. A newly designed additive, AXcPDMS, based on astaxanthin covalently bonded to PDMS was also examined. Lifetime tests, produced in ISOS-L-2 conditions, reveal an improvement in the accumulated power generation (APG) of 10% with pure AX, of 90% when AX is paired with PDMS, and of 140% when AXcPDMS is added in the active layer blend, as compared to the control devices. Singlet oxygen phosphorescence measurements are utilized to study the ability of AX and AXcPDMS to quench singlet oxygen and its precursors in the films. The data are consistent with the strong stabilization effect of the carotenoids. While AX and AXcPDMS are both efficient photochemical stabilizers, the improvement in device stability observed in the presence of AXcPDMS is likely due to a more favorable localization of the stabilizer within the blend. The mechanical properties of the active layers were investigated by tensile testing and cohesive fracture measurements, showing a joint improvement of the photooxidative stability and the mechanical properties, thus yielding organic solar cell devices that are promising for flexible photovoltaic applications.

We report on the fabrication of transparent, conductive, mechanically robust electrode composites of poly(triarylamine) (PTAA) doped with different concentrations of carbon nanotubes (CNTs) or graphene nanoplatelets (GNPs). Additionally, the effects of nanofiller surface modification with amines were characterized by comparing the transparency, conductivity, and mechanical properties to composites with unmodified nanofillers. The optimization of the concentrations and fabrication parameters resulted in films with high transparency, improved electrical conductivity, and superior mechanical properties. Amine-functionalized nanofillers more readily dispersed into the matrix, and the addition of 1 wt% amine-CNTs resulted in a composite film with a conductivity of 1.1 S cm− 1 and a transparency of 90–95% in the visible spectrum at a sub-100 nm thickness. At low doping concentrations, composites with amine-functionalized nanofillers exhibited a 100% increase in fracture energy compared to composites with unmodified nanofillers, an effect attributed to increased plasticity of the doped polymer. These findings have applications in many electronic devices that utilize organic semiconducting layers, such as organic light emitting diodes and perovskite photovoltaics.

Organic solar cells subjected to environmental stressors such as heat, moisture, and UV radiation can undergo significant mechanical degradation, leading to delamination of layers and device failure. This paper reports the effect these stressors have on the mechanical integrity of active layers and interfaces as measured by subcritical debonding tests, and the in situ evolution of defects and fracture processes is characterized. At elevated temperatures below 50 °C in inert conditions, significant device weakening was observed, an effect we attributed to a temperature-induced P3HT:PCBM delamination mechanism from the underlying ZnO. At 50 °C in ambient conditions with UV exposure—selected to better simulate real-world environments—devices were more resistant to fracture because of an interfacial strengthening effect from increased hydrogen bonding where UV-induced Zn(OH) 2 formation reinforced the interface with P3HT:PCBM. This photoinduced hydroxylation mechanism was determined from a decrease in the Zn/O ratio with increased UVA or UVB exposure, and hydroxylation was shown to directly correlate with the resistance to fracture in devices.

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 glass transition temperature (Tg) 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 Tg 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 Tg 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 Tg, 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 spec...

The relative insensitivity of the optoelectronic properties of organometal trihalide perovskites to crystallographic defects and impurities has enabled fabrication of highly-efficient perovskite solar cells by scalable solution-state deposition techniques well suited to low-cost manufacturing. Fracture analyses of state-of-the-art devices, however, have revealed that both the perovskite active layer and adjacent carrier selective contacts are mechanically fragile—a major obstacle to technological maturity that stands to significantly compromise their thermomechanical reliability and operational lifetimes. We report a new concept in solar cell design, the compound solar cell (CSC), which addresses the intrinsic fragility of these materials with mechanically reinforcing internal scaffolds. The internal scaffold effectively partitions a conventional monolithic planar solar cell into an array of dimensionally scalable and mechanically shielded individual perovskite cells that are laterally encapsulated by the surrounding scaffold and connected in parallel via the front and back electrodes. The CSCs exhibited a significantly increased fracture energy of ∼13 J m−2—a 30-fold increase over previously reported planar perovskite (∼0.4 J m−2)—while maintaining efficiencies comparable to planar devices. Notably, the efficiency of the microcells formed within the scaffold is comparable to planar devices on an area-adjusted basis. This development is a significant step in demonstrating robust perovskite solar cells to achieve increased reliability and service lifetimes comparable to c-Si, CIGS, and CdTe solar cells.

We report on submicron organosilicate barrier films produced rapidly in air by a scalable spray plasma process that improves both the stability and efficiency of perovskite solar cells. The plasma is at sufficiently low temperature to prevent damage to the underlying layers. Oxidizing species and heat from the plasma improve device performance by enhancing both interfacial contact and the conductivity of the hole transporting layer. The thickness of the barrier films is tunable and transparent over the entire visible spectrum. The morphology and density of the barrier are shown to improve with the addition of a fluorine-based precursor. Devices with submicron coatings exhibited significant improvements in stability, maintaining 92% of their initial power conversion efficiencies after more than 3000 h in dry heat (85 °C, 25% RH) while also being resistant to degradation under simulated operational conditions of continuous exposure to light, heat, and moisture. X-ray diffraction measurements performed while heating showed the barrier film dramatically slows the formation of PbI2. The barrier films also are compatible with flexible devices, exhibiting no signs of cracking or delamination after 10 000 bending cycles on a 127 μm substrate with a bending radius of 1 cm.

The ways in which organic solar cells (OSCs) are measured and characterized are inefficient: many substrates must be coated with expensive or otherwise precious materials to test the effects of a single variable in processing. This serial, sample-by-sample approach also takes significant amounts of time on the part of the researcher. Combinatorial approaches to research OSCs generally do not permit microstructural characterization on the actual films from which photovoltaic measurements were made, or they require specialized equipment that is not widely available. This paper describes the formation of one- and two-dimensional gradients in morphology and thickness. Gradients in morphology are formed using gradient annealing, and gradients in thickness are formed using asymmetric spin coating. Use of a liquid metal top electrode, eutectic gallium–indium (EGaIn), allows reversible contact with the organic semiconductor film. Reversibility of contact permits subsequent characterization of the specific areas o...

This paper describes a large increase in the stretchability–i.e., resistance to cracking under tensile deformation–of organic semiconductor films produced by encapsulation. Specifically, encapsulation is shown to greatly suppress crack formation and growth in films of materials relevant to organic solar cells. Encapsulated films of the organic bulk heterojunction blend of poly(3-heptylthiophene) and phenyl-C61-butyric acid methyl ester (P3HpT:PCBM) exhibit greater crack-onset strain, lower crack density, and lower average crack length than unencapsulated films. Films of P3HpT:PCBM on polyurethane (PU) showed cracks at 6.6 ± 0.5% without encapsulation and 40 ± 4% with encapsulation. Films of the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) also demonstrate suppressed cracking when encapsulated, as well as reduced dependence of resistance on strain after the crack-onset strain (which indicates a greater usable range of strain for encapsulated vs. unencapsulated films). A finite element model is used to explain the mechanism suppressing crack growth, which involves delocalization of strain around minor defects in the films by the encapsulating substrate. In addition, encapsulation is used to enable the first encapsulated solar cell in which every component is intrinsically stretchable. These cells are stretchable to 9–10% strain, with some cells performing well even after their crack-onset strain of 8–9%, whereas unencapsulated cells fail at 4% strain. This work highlights the necessity to consider encapsulation–already important for protecting the electronically active components of a device from abrasion, weathering, or chemical damage–as an important factor in the mechanical robustness of stretchable devices.

Organic electronic materials have many characteristics that make them attractive for applications in which mechanical deformability—i.e., flexibility and stretchability—are required. While deformation often degrades the performance of these devices, very little is known about the effects of cyclic loading—i.e., mechanical fatigue—on the microstructure and mechanical properties of the active materials. This paper examines the evolution of microstructure, stiffness, and ductility of thin films of poly(3-heptylthiophene) (P3HpT) as the film undergoes cyclic straining using ultraviolet-visible (UV–vis) spectroscopy and film-on-elastomer techniques. Thin films of P3HpT are cyclically stretched by 5, 10, or 25 percent (i.e., below, at, and above the yield point—the point at which the polymer plastically deforms with strain) up to 10000 cycles. UV–vis absorption spectroscopy is taken in intervals and the weakly interacting H-aggregate model is used to determine the aggregate quantity (from the vibronic progression) and quality (from the exciton bandwidth) in the films. Films cyclically strained at 5 and 10 percent (below and at the yield point) do not undergo significant reduction in the aggregated fraction of polymer chains, while films strained to 25% (above the yield point) undergo a reduction in aggregated fraction of over 10% by the 2000th cycle. At 25% strain, a significant reduction in the buckling wavelength from 3.4 ± 0.4 μm to 2.4 ± 0.3 μm is observed within the first 100 strain cycles suggesting a significant reduction in the stiffness and resilience of the films. A significant decrease in ductility is observed in films cyclically strained, and the effect is found to increase with increasing levels of strain. These results suggest that materials cyclically strained below their yield point will retain a microstructure that is their most electronically favorable, and that the mechanical properties of materials strained above their yield point will evolve significantly under repeated deformation. This information can be used to inform design where accommodation of repetitive strain is required, such as outdoor, portable, and wearable devices.

Unlike existing silicon, CIGS, and multi-junction cells that exhibit remarkable mechanical durability, perovskite solar cells have been shown to delaminate when subjected to the mechanical loads that occur during processing and field exposures, limiting their potential as a reliable solar technology. Mechanical reinforcement is thus essential for durable and reliable perovskite technologies. A recent strategy to overcome the thermomechanical fragility of perovskite solar cells is to extrinsically shield them by introducing reinforcing scaffolds which partition the cell into many distinct microcells, but improvements in mechanical stability coincide with a reduced device efficiency due to parasitic absorption by the scaffold. We address this reduced efficiency by integrating concentrating immersion-lens arrays (CILAs) into scaffold-reinforced solar cells. These scaffolds are lithographically patterned by a maskless process, in which ultraviolet light is used to pattern the scaffolds through the CILAs, ensuring self-alignment and optical contact with the microcells. This process creates mm-scale scaffold patterns with a line edge roughness of 17.5 ± 4.3 µm and line width roughness of 26.4 ± 8.5 µm. Perovskite devices are deposited into the microcells, and during operation, light is concentrated into the microcells and away from the insulating scaffolds, resulting in mechanically resilient solar cells with efficiencies comparable to planar devices. The devices with the strongest lenses—i.e., lower radius of curvature—recover up to 89 ± 15% of photocurrent and 91 ± 29% power conversion efficiency that would have been otherwise lost to parasitic absorption. Ray-tracing simulations of the CILAs also demonstrate passive tracking of incident light which is critical for optimal power output as the sun moves across the sky during the day. In these simulations, devices with CILAs showed improved passive tracking compared to devices without CILAs out to 60° off-normal. The UV stability of CILAs are experimentally verified through aging tests which show no change in their mechanical integrity after 1000 h of UV exposure. The simplicity of the fabrication process and the efficiency of the resulting scaffolded devices show that a lens-integrated scaffold-reinforced structure is a potential pathway to high-performance, robust, commercially viable perovskite solar cells.

Author(s): Root, Samuel Evan | Advisor(s): Lipomi, Darren J. | Abstract: Mechanical softness and deformability underpin most of the advantages offered by semiconducting polymers. A detailed understanding of the mechanical properties of these materials is crucial for the design and manufacturing of robust, thin-film devices such as solar cells, displays, and sensors. The mechanical behavior of polymers is a complex function of many interrelated factors that span multiple scales, ranging from molecular structure, to microstructural morphology, and device geometry. This thesis builds a comprehensive understanding of the thermomechanical properties of polymeric semiconductors through the development and experimental-validation of computational methods for mechanical simulation. A predictive computational methodology is designed and encapsulated into open-sourced software for automating molecular dynamics simulations on modern supercomputing hardware. These simulations are used to explore the role of molecular structure/weight and processing conditions on solid-state morphology and thermomechanical behavior. Experimental characterization is employed to test these predictions—including the development of simple, new techniques for rigorously characterizing thermal transitions and fracture mechanics of thin films.

We report on the inclusion of 5-aminovaleric acid (5-AVA)—a bulky, long-chained organic molecule—as an additive to enhance the mechanical integrity of hybrid perovskite films. We studied a range of organic cation additives and found that 5-AVA was the most effective at improving cohesion energy, a key metric of mechanical reliability. MAPbI3 films reinforced with 5-AVA added in 5% concentration increased cohesion energy 12-fold from 0.53 J/m2 to 6.04 J/m2, an effect which is attributed to increased plasticity and crack deflection around grain boundaries of the additive-containing perovskite. The addition of 5-AVA also improves Voc in perovskite solar cells and carrier lifetimes with a minimal decrease in PCE, attributed to passivation reducing defect and trap densities. A Tauc plot analysis of the bandgap shows that 5-AVA increases the band gap of the perovskite, correlating with reduced film stress compared to MAPbI3. As a result, the usage of 5-AVA improves the intrinsic thermomechanical reliability, but this improvement comes at the penalty of slightly reduced device efficiency due to reduced charge extraction from the presence of the bulky, insulating organic additive.

We report on recent studies characterizing the intrinsic mechanical integrity of perovskite compositions and fully explore the role of various cation combinations, additives, and microstructure on perovskite cohesion. Adding cations to the perovskite decreased mechanical integrity, largely due to smaller grain sizes and increased concentration of PbI 2 . Microindentation hardness testing was performed to estimate the fracture toughness of single-crystal perovskite, and the results indicated perovskites are inherently fragile, even in the absence of grain boundaries and defects. Introducing plastically deformable cations led to a modest improvement in cohesion, and the most robust architecture was observed by infusing perovskite into a porous TiO 2 /ZrO 2 /C layer that provided extrinsic reinforcement to mechanical and environmental stressors.

Despite the intrinsic flexibility of organic electronic materials due to their thinness, a deliberate selection of materials on the basis of their mechanical—not just charge-transport—properties is required for applications with mechanically demanding form factors, such as those that exist in the field of wearable electronics. This paper describes a skin-wearable solar cell enabled by the deliberate selection or intentional plasticization of the components to enable an extreme level of stability under cyclic bending deformations. In particular, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films plasticized with 10% fluorosurfactant are used for both the anode and cathode, with the cathode layer further modified with polyethyleneimine (PEI) to lower the work function. Use of poly(3-heptylthiophene) (P3HpT) instead of the far more common poly(3-hexylthiophene) (P3HT) as the electron donor permitted extreme deformation because of its increased mechanical compliance (owing to its low glass transition temperature). Cells fabricated on 13-µm polyimide tape and adhered to human skin show stable performance when compressively strained by approximately 75%. These compressive strains produce convex and concave buckles with minimum radii of curvature of ca. 100 µm. Finite-element modeling predicts that the films require the ability to withstand a range of strains of 10%, when both convex and concave bends are considered. These devices, enabled by the stretchable semiconductor P3HpT, withstand up to one thousand cycles of compression with less than 20% degradation in power conversion efficiency, whereas devices based on P3HT show greater degradation after only five cycles, and fail catastrophically by fifty cycles. The usefulness of the wearable solar cells is demonstrated by their abilities to power an LED and a digital watch.

Colloidal particles with asymmetric surface chemistry (Janus particles) have unique bifunctional properties. The size of these particles is an important determinant for their applications in diverse fields from drug delivery to chemical catalysis. The size of Janus particles, with a core surface coated with carboxylate and a partially encapsulating silica shell, depends upon several factors, including the core size and the concentration of carboxylate coating. The role of the carboxylate coating on the Janus particle size is well-understood; however, the role of the core size is not well defined. The role of the carboxylated polystyrene (cPS) core size on the cPS-silica Janus particle morphology (its size and shape) was examined by testing two different silica sizes and five different cPS core sizes. Results from electron microscopy (EM) and dynamic light scattering (DLS) analysis indicate that the composite cPS-silica particle acquires two distinct shapes: (i) when the size of the cPS core is much smaller than the non-cPS silica (b-SiO2) sphere, partially encapsulated Janus particles are formed, and (ii) when the cPS core is larger than or equal to the b-SiO2 sphere, a raspberry-like structure rather than a Janus particle is formed. The cPS-silica Janus particles of ∼100-500 nm size were obtained when the size of the cPS core was much smaller than the non-cPS silica (b-SiO2) sphere. These scalable nanoscale Janus particles will have wide application in a multifunctional delivery platform and catalysis.

An organic solar cell based on a bulk heterojunction (BHJ) of a polymer and a methanofullerene ([60]PCBM or [70]PCBM) exhibits a complex morphology that controls both its photovoltaic and mechanical compliance (robustness, flexibility, and stretchability). Methanofullerenes are excellent electron acceptors; however, they have relatively high cost and production energy (in the purest samples) compared to other small-molecule semiconductors. Moreover, [60]PCBM and [70]PCBM-typical of van der Waals solids-can be stiff and brittle. Stiffness and brittleness may lower the yield of working modules in roll-to-roll manufacturing, shorten the lifetime against mechanical failure in outdoor conditions, and jeopardize wearable and portable applications that demand stretchability or extreme flexibility. This paper tests the hypothesis that technical grade PCBM (incompletely separated but otherwise pure blends containing >= 90% [60]PCBM or [70]PCBM) could lower the cost of manufacturing organic solar cells while simultaneously increasing their mechanical stability. Measurements of the tensile modulus of five methanofullerene samples, technical grades and 99% grades of both [60]PCBM and [70]PCBM, and a 1:1 mixture [60]PCBM and [70]PCBM, along with their blends with regioregular poly(3-hexylthiophene) (P3HT), lead to two important conclusions: (1) films of pure [70]PCBM are approximately five times more compliant than films of pure [60]PCBM; BHJ films with [70]PCBM are also more compliant than those with [60]PCBM. (2) BHJ films comprising technical grades of [60]PCBM and [70]PCBM are approximately two to four times more compliant than are films made using 99% grades. Tensile modulus is found to be an excellent predictor of brittleness: BHJs produced with technical grade methanofullerene accommodate strains 1.4-2.2 times greater than those produced with 99% grades. The smallest range of stretchability was found for BHJs with 99% [60]PCBM (fracture at 3.5% strain), while the greatest was found for technical grade [70]PCBM (11.5% strain). Mechanical properties are correlated to the microstructures of the blended films informed from analyses of UV-vis spectra using the weakly interacting H-aggregate model. Photovoltaic measurements show that solar cells made with technical grade [70]PCBM have similar efficiencies to those made with higher-grade material but with decreased cost and increased mechanical robustness.

Mechanical compliance is a critical attribute for organic semiconductors in flexible, stretchable, mechanically robust, and biologically integrated electronics. This paper substantially develops the observation that a small change in the length of the alkyl side chain of regioregular poly(3-alkylthiophene)s has a dramatic effect on the interplay between their mechanical and charge-transport properties. Specifically, the thermal, mechanical, and charge-transport properties of poly(3-heptylthiophene) (P3HpT, n = 7), which we found to be an unusual example of a stretchable semiconducting thermoplastic, are described in comparison to those of poly(3-hexylthiophene) (P3HT, n = 6) and poly(3-octylthiophene) (P3OT, n = 8). Neat P3HpT was found to have mechanical properties similar to that of P3OT, and when mixed in 1:1 blends with the fullerene [6,6]-phenyl C 61 butyric acid methyl ester (PCBM), exhibited electronic properties comparable to P3HT. However, the charge-carrier mobility of neat P3HpT is substantially inferior to that of P3HT; the good performance of P3HpT-based solar cells is the result of improved mobility in P3HpT:PCBM blends compared to the neat material. While P3HpT may be a favorable alternative to P3HT in ultra-flexible, stretchable, and mechanically robust organic solar cells, P3HpT would only make a good field-effect transistor in situations in which mechanical compliance was more important than high mobility.

Bulk heterojunction films, which typically comprise a polymer donor and fullerene acceptor, are considerably stiffer than films of the neat polymer. The increase in stiffness upon blending is dependent on the miscibility of the polymer and the fullerene, and potentially on the details of molecular mixing, in particular, intercalation of the fullerene molecules between the polymer side chains. This paper describes the effects of molecular mixing on the tensile modulus of polythiophenes in 1:1 blends with [6,6]-phenyl C61 butyric acid methyl ester (PC61BM). A series of four polymers and their blends with PC61BM are tested using mechanical, spectroscopic, and photovoltaic device-based measurements to determine if it is possible to predict trends in the tensile modulus based on the extent of molecular mixing. The four polymers are poly-2,2′:5′,2″-(3,3″-dihexyl-terthiophene) (PT2T), which forms an amorphous, molecularly mixed composite, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), which forms a well ordered blend with bimolecular crystallization, and regioregular poly(3-hexylthiophene) (P3HT) and poly(3-heptylthiophene) (P3HpT), which form a ternary blend with an amorphous mixed phase. The tensile moduli are measured by the buckling technique and correlations are found between the modulus of the neat polymer and the blends. Although spectroscopic and photovoltaic device-based measurements of P3HT:PC61BM and PT2T:PC61BM, along with literature precedent, suggest completely different extents of molecular mixing, they were found to have similar moduli (2.75±0.59 GPa and 2.61±0.39 GPa, after annealing). A strong correlation between the moduli of the blended films and the moduli of the neat polymers suggest that the stiffness of the blend is determined to a large extent by that of the polymer, and is unexpectedly insensitive to the details of molecular mixing, at least for the materials investigated.

The carbon framework common to both organic semiconductors and biological structures suggests that these two classes of materials should be easily integrated. Substantial work, however, will be required to endow synthetic electroactive materials with properties resembling those of biological tissue, which exhibits extreme elasticity, biodegradability, and the capacity for self-repair. This Highlight reviews successful integration of organic semiconductor devices with biological systems, for example, in wearable and implantable health monitors and prosthetic devices. It then points to recent work in the areas of molecularly stretchable electronics, whole devices that can degrade under physiological conditions, and conjugated polymers capable of self-healing, which together suggest the possibility of a future in which organic electronics and biological tissue can interact seamlessly.

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.

The need for increased focus on patient-centered care delivery approaches has been highlighted by numerous national organizations and health policy established over the past few years. Shared decision-making (SDM) is an approach to engaging patients in decision-making that involves the exchange of information between two or more parties, one of which must be the patient, and results in reaching a final decision by consensus. Limited studies have shown feasibility and efficacy of implementing SDM strategies within the emergency department to facilitate translation of scientific evidence to practice while improving patient satisfaction and reducing resource utilization without increasing harm. The assessment and treatment of patients with potential acute cardiovascular conditions in the emergency setting offers a number of opportunities for further testing and implementation of SDM in the emergency care setting. Future establishment of guidelines and quality of care measures focused on assessing adequacy of patient engagement may help drive this agenda.

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.

Hollow/porous nanoparticles, including nanocarriers, nanoshells, and mesoporous materials have applications in catalysis, photonics, biosensing, and delivery of theranostic agents. Using a hierarchical template synthesis scheme, we have synthesized a nanocarrier mimicking a golf ball, consisting of (i) solid silica core with a pitted gold surface and (ii) a hollow/porous gold shell without silica. The template consisted of 100 nm polystyrene beads attached to a larger silica core. Selective gold plating of the core followed by removal of the polystyrene beads produced a golf ball-like nanostructure with 100 nm pits. Dissolution of the silica core produced a hollow/porous golf ball-like nanostructure.

This Perspective describes electronic materials whose molecular structure permits extreme deformation without the loss of electronic function. This approach—“molecularly stretchable” electronics—is complementary to the highly successful approaches enabled by stretchable composite materials. We begin by identifying three general types of stretchable electronic materials: (1) random composites of rigid structures sitting atop or dispersed in an elastic matrix, (2) deterministic composites of patterned serpentine, wavy, or fractal structures on stretchable substrates, and (3) molecular materials—noncomposite conductors and semiconductors—that accommodate strain intrinsically by the rational design of their chemical structures. We then identify a short-term and a long-term goal of intrinsically stretchable organic electronics: the short-term goal is improving the mechanical stability of devices for which commercialization seems inevitable; the long-term goal is enabling of electronic devices in which every co...

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) (P...

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.

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.

Photoactive perovskite semiconductors are highly tunable, with numerous inorganic and organic cations readily incorporated to modify optoelectronic properties. However, despite the importance of device reliability and long service lifetimes, the effects of various cations on the mechanical properties of perovskites are largely overlooked. In this study, the cohesion energy of perovskites containing various cation combinations of methylammonium, formamidinium, cesium, butylammonium, and 5-aminovaleric acid is reported. A trade-off is observed between the mechanical integrity and the efficiency of perovskite devices. High efficiency devices exhibit decreased cohesion, which is attributed to reduced grain sizes with the inclusion of additional cations and PbI2 additives. Microindentation hardness testing is performed to estimate the fracture toughness of single-crystal perovskite, and the results indicated perovskites are inherently fragile, even in the absence of grain boundaries and defects. The devices found to have the highest fracture energies are perovskites infiltrated into a porous TiO2/ZrO2/C triple layer, which provide extrinsic reinforcement and shielding for enhanced mechanical and chemical stability.

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.

Mechanical buckling of thin films on elastomeric substrates is often used to determine the mechanical properties of polymers whose scarcity precludes obtaining a stress-strain curve. Although the modulus and crack-onset strain can readily be obtained by such film-on-elastomer systems, information critical to the development of flexible, stretchable, and mechanically robust electronics (i.e., the range of strains over which the material exhibits elastic behavior) cannot be measured easily. This paper describes a new technique called laser determination of yield point (LADYP), in which a polymer film on an elastic substrate is subjected to cycles of tensile strain that incrementally increase in steps of 1% (i.e., 0% → 1% → 0% → 2% → 0% → 3% → 0%, etc.). The formation of buckles manifests as a diffraction pattern obtained using a laser, and represents the onset of plastic deformation, or the yield point of the polymer. In the series of conjugated polymers poly(3-alkylthiophene), where the alkyl chain is pentyl, hexyl, heptyl, octyl, and dodecyl, the yield point is found to increase with increasing length of the side chain (from approximately 5% to 15% over this range when holding the thickness between ∼200 and 300 nm). A skin-depth effect is observed in which films of150 nm thickness exhibit substantially greater yield points, up to 40% for poly(3-dodecylthiophene). Along with the tensile modulus obtained by the conventional analysis of the buckling instability, knowledge of the yield point allows one to calculate the modulus of resilience. Combined with knowledge of the crack-onset strain, one can estimate the total energy absorbed by the film (i.e., the modulus of toughness).

This paper describes several approaches to understanding and improving the response of π-conjugated (semiconducting) polymers to tensile strain. Our principal goal was to establish the design criteria for introducing elasticity and ductility in conjugated (semiconducting) polymers through a rigorous analysis of the structural determinants of the mechanical properties of this type of material. We elucidated the details of the effect of the alkyl side chain length on the mechanical properties of regioregular polythiophene and used this analysis to select materials for stretching and transfer printing of organic solar cells to hemispherical substrates. This demonstration represents the first time that a conjugated polymer device has ever been stretched and conformally bonded to a complex 3D surface (i.e., other than a cone or cylinder, for which flexibility—as opposed to stretchability—is sufficient). We then further explored the details of the dependence of the mechanical properties on the side chain of a semiconducting polymer by synthesizing a series of hybrid materials (block and random copolymers) containing both short and long side chains. This analysis revealed the unusual semiconducting polymer, poly(3-heptylthiophene), as having an excellent combination of mechanical and electronic properties. In parallel, we explored a new method of producing “blocky” copolymers using a new procedure based on random segmentation of conjugated monomers. We found that introduction of structural randomness increased the elasticity without having detrimental effects on the photovoltaic performance. We also describe methods of synthesizing large volumes of conjugated polymers in environmentally benign ways that were amenable to manufacturing.

The primary goal of the field concerned with organic semiconductors is to produce devices with performance approaching that of silicon electronics, but with the deformability—flexibility and stretchability—of conventional plastics. However, an inherent competition between deformability and charge transport has long been observed in these materials, and achieving the extreme (or even moderate) deformability implied by the word “plastic” concurrently with high charge transport may be elusive. This competition arises because the properties needed for high carrier mobilities—e.g., rigid chains in π-conjugated polymers and high degrees of crystallinity in the solid state—are antithetical to deformability. On the device scale, this competition can lead to low-performance yet mechanically robust devices, or high-performance devices that fail catastrophically (e.g., cracking, cohesive failure, and delamination) under strain. There are, however, some observations that contradict the notion of the mutual exclusivit...

A current hypothesis holds that chromatin fragmentation into oligonucleosomal patterns is an early event during apoptosis. In contrast, induction of apoptosis in cultured hepatocytes by TGF-beta 1 was not associated with DNA fragmentation into oligonucleosomes in hepatocyte monolayers and apoptotic fragments. For a more rigorous test of the hypothesis we performed a number of experiments. We compared nuclear changes resulting from TGF-beta 1 with those induced by Ca2+, a known activator of endonuclease. The morphology of apoptotic and Ca(2+)-treated nuclei was different as judged by DNA staining with Hoechst 33258. Likewise, electron microscopy of apoptotic nuclei showed characteristic condensation of the chromatin as well as dissolution of the nucleolar structure and nuclear fragmentation, changes not seen after Ca2+ treatment, after three hours of incubation. Analysis of DNA fluorescence of nuclei by FACS revealed that treatment with Ca2+ reduced the signal by 20%. In contrast, nuclei from TGF-beta 1-treated hepatocytes did not exhibit a reduced signal and after sorting by FACS, apoptotic nuclei remained in the 2N and 4N fractions. The absence of detectable DNA fragmentation in apoptotic nuclei was further verified by in situ nick translation, not only in hepatocytes but also in a mouse lymphoma cell line. From these findings we conclude that activation of an endonuclease is not an early event on the pathway to morphologically recognizable apoptosis.

Graphene is expected to play a significant role in future technologies that span a range from consumer electronics, to devices for the conversion and storage of energy, to conformable biomedical devices for healthcare. To realize these applications, however, a low-cost method of synthesizing large areas of high-quality graphene is required. Currently, the only method to generate large-area single-layer graphene that is compatible with roll-to-roll manufacturing destroys approximately 300 kg of copper foil (thickness = 25 μm) for every 1 g of graphene produced. This paper describes a new environmentally benign and scalable process of transferring graphene to flexible substrates. The process is based on the preferential adhesion of certain thin metallic films to graphene; separation of the graphene from the catalytic copper foil is followed by lamination to a flexible target substrate in a process that is compatible with roll-to-roll manufacturing. The copper substrate is indefinitely reusable and the method is substantially greener than the current process that uses relatively large amounts of corrosive etchants to remove the copper. The sheet resistance of the graphene produced by this new process is unoptimized but should be comparable in principle to that produced by the standard method, given the defects observable by Raman spectroscopy and the presence of process-induced cracks. With further improvements, this green, inexpensive synthesis of single-layer graphene could enable applications in flexible, stretchable, and disposable electronics, low-profile and lightweight barrier materials, and in large-area displays and photovoltaic modules.

There is increasing demand for mononuclear cell (MNC) harvests not only for PBPC but also for immune therapies using dendritic cells and donor lymphocytes. We determined the collection efficiencies (CE) of various MNC subpopulations during CD34+ cell harvests using a Fenwal CS 3000 Plus Omnix system in small children and adolescents. The cell content of 140 leukapheresis products (LP) was prospectively evaluated in 45 pretreated patients with solid tumors and hematological malignancies. The median age was 12 years (range 0.8-22), and the median body weight (BW) 43 kg (range 9-92). Depending upon the BW of the patients, the media used for priming were saline (SP) in 86, human albumin (HA, HAP) in 10, and packed red blood cells (BP) in 44 apheresis procedures. The major nucleated cell (NC) fractions collected were monocytes (52% of NC) and CD3+ T cells (26%). The median cell yield for monocytes was 174 * 10(6)/kg (range 24-613) representing a CE of 55%. The median number of CD3(+) T cells was 84 * 10(6)/kg (range 5.6-380; CE = 74%). CD34+ cells represented a very small cell fraction of the LP (1.3% of NC), with a median yield of 4.2 * 10(6)/kg (range 0.2-87) and a CE of 63%. The cell yield of various MNCs was significantly correlated with the cell count in the peripheral blood (PB) and with the blood volume processed (ANOVA, P0.0001). No influence on the CE was observed for the priming procedure, the patients' age or sex, or the other adaptations used in the harvesting protocol. In conclusion, the Fenwal CS 3000 Plus OMNIX system with the CD34+ cell program and the described adaptations, is also predictably useful for harvest of monocytes or lymphocytes in pediatric patients. We present regression equations that predict the cell yield of various MNC subpopulations in apheresis products.

The monitoring of chimerism by PCR has become a routine diagnostic approach in patients after allogeneic bone marrow or peripheral blood stem cell transplantation. Nevertheless, a temporal correlation between molecular and hematologic assessment of engraftment has not been clearly established. To address this issue, and to determine the potential clinical implications of early kinetics of mixed chimerism, we have investigated 66 allogeneic stem cell transplantations (SCTs) in 58 pediatric patients suffering from different types of leukemia (n = 44) or non-malignant hematologic disorders (n = 14) by close molecular monitoring during the first days and weeks after transplantation. Patient- and donor-derived hematopoiesis were assessed at 1- to 3-day intervals in peripheral blood samples by PCR analysis of highly polymorphic microsatellite loci (STR-PCR). Detection of an increasing, and ultimately dominant donor-specific allelic pattern, which we defined as molecular engraftment, preceded hematologic engraftment by a median of 7 days (range 1-17 days) in all patients investigated. PCR analyses during the first days after transplantation facilitated detection of molecular engraftment according to the above definition by day +14 (range day +2 to day +14), thus permitting prediction of successful engraftment (upper limit of the two-sided confidence interval po = 6%) while the peripheral leukocyte counts were mostly below 200/microl. In three cases, however, the criteria for molecular engraftment were not fulfilled by day +14. These patients also failed to show hematologic engraftment, and required a second transplantation. Close monitoring by STR-PCR showed that graft rejection and autologous recovery can occur early and with very rapid dynamics. Molecular analysis of specific leukocyte subsets isolated by flow-sorting enabled sensitive assessment of changes in the pattern of chimerism which had escaped detection in assays using whole white blood cell (WBC) samples. This approach facilitated the identification of expanding or decreasing recipient cells, and permitted early detection of impending rejection or relapse. Moreover, monitoring of the dynamics of chimerism allowed rapid assessment of the response to therapy. Our observations provide support for the concept of initiating genotype analyses early after SCT and monitoring at rather short intervals to permit timely evaluation of clinically relevant processes, and to provide a basis for early implementation of treatment.

We have recently shown that CD99 (MIC2) is differentially expressed during normal early B-cell development in the bone marrow (BM). Since immature B-cell precursors (BCP) are assumed to correspond to some extent to acute lymphoblastic leukaemia (ALL) and non-Hodgkin's lymphoma (NHL) cells with respect to patterns of phenotypic differentiation, we wondered whether the particular maturation-associated expression patterns of CD99 in the normal BCP stages were conserved also in malignant cells. Therefore we compared malignant and physiological B cells from paediatric ALL/NHL and normal BM samples with respect to CD99 expression using selective gating and semi-quantitative flow cytometry. Common-ALLs (n = 45) were similar to their corresponding, very immature BCPs (stage 1) in expressing very high levels of CD99. Most pre-B ALLs (n = 16) were also CD99hi and thus differed from the patterns found in normal cytoplasmic mu-chain+ (cmu+) pre-B cells (stage 2, CD99lo). In particular, we found that those pre-B-ALL cases which were CD34+ also showed higher CD99 expression than the CD34- cases. This prompted us to investigate the levels of CD99 in those rare normal BCPs which also coexpress CD34 and cmu; these cells, which are transitory from stage 1 to stage 2, were found also CD99hi, thus precisely reflecting the patterns of CD34+ pre-B ALLs. The blasts of Burkitt-type B-cell ALL/NHL samples (n = 13) expressed considerably less CD99, similarly to the more differentiated BCP stages 2 (cmu+) and 3 (surface mu-chain+). In summary, we found that paediatric B-lineage malignancies display remarkable synchrony regarding the levels of CD99 expression compared to their putative normal counterparts.

We recently investigated samples of pediatric B-cell precursor acute lymphoblastic leukemia (BCP-ALL) and normal bone marrow (BM). We found that leukemic blasts, compared to their physiologic counterpart cells, frequently display aberrant phenotypes with respect to levels of expression of certain antigens. Using multiparameter flow cytometry, these differences enabled us to trace leukemic cells admixed to normal BM, which suggested that this approach might be a useful strategy for minimal residual disease detection. In the present study, we used the same multiparameter approach ("comparative phenotype mapping") to prove that such quantitative phenotypic differences really exist between malignant and normal BCP when simultaneously present in the BM. We demonstrate this by five exemplary follow-up BM samples from patients with BCP-ALL, all of which showed phenotypically aberrant cells according to levels of expression of CD10, CD11a, CD19, CD34, CD44, or CD45RA, as well as according to altered orthogonal light scattering properties. We confirmed the leukemic nature of these cells by polymerase chain reaction-based detection of bcr1/abl transcripts, and of leukemia clone-specific immunoglobulin heavy chain rearrangements in only the suspicious cells when sorted by flow cytometry, but not in normal BCP or non-B cells. Comparative phenotype mapping thus allows one to distinguish between normal and leukemic cells, and we show that it may enable rapid, specific, and quantitative detection of residual/resurgent leukemia in BCP-ALL.