The Selective Metallization Technique shows promise for roll-to-roll in-line patterning of flexible electronics using evaporated metals, but challenges arise when applied to sputtering functional materials. This study overcomes these challenges with simultaneous sputtering of Bi-Sb-Te and evaporation of metal (Ag or Cu) for thermoelectric layers when using Selective Metallization Technique. Large-scale manufacturing is demonstrated through roll-to-roll processing of a 0.8 m wide polymer web at 25 m/min, achieving high-throughput production of functional thin-film patterns with nanometer thickness. The room-temperature-deposited material system exhibits significantly enhanced thermoelectric performance and facilitates an n-type-to-p-type transition in the Cu- or Ag-containing Bi-Sb-Te-based composite film. Here, we show that while applying Selective Metallization Technique, the evaporation of metal modifies the impact of residual oil on Bi-Sb-Te, which can be effectively removed with a few seconds of plasma exposure, and the fabricated thermoelectric devices are validated in wearable applications utilizing a coiled-up wristband design.

The High Target Utilisation Sputtering technique (HiTUS) is of interest for industrial processes, including in roll-to-roll manufacturing. This study marks the first application of HiTUS to thermoelectric materials, exemplified by bismuth telluride. The HiTUS technique separates the sputtering power into the plasma power and the target power, with additional kinetic energy in the sputtering particles from the applied electrical field, thus enabling a much wider sputter parameter space to modify the film performance. This study investigates how plasma power, target power, and substrate bias in HiTUS intricately influence crystal orientation/size, elemental composition, surface morphology, and other film properties. These factors subsequently affect carrier density/mobility, and consequently the thermoelectric performance of the bismuth telluride film. These deposited films reach a power factor of 6.5 x 10-4 W m-1 K-2 with a figure of merit approximate to 0.14 at room temperature, the highest value for room-temperature sputtered un-doped bismuth telluride. Subsequent post-deposition annealing significantly enhances the crystallinity of the film (highly polycrystalline), further improving the power factor to 23.5 x 10-4 W m-1 K-2, with a figure of merit approximate to 0.45 at room temperature. The excellent performance of the HiTUS fabricated thermoelectric film opens opportunities for the large-area manufacture of thin-film thermoelectric materials and devices. High Target Utilisation Sputtering technique is for the first time applied to sputter thermoelectric thin films, in this case bismuth telluride. By separating the sputtering parameters, it enables more optimisations on the film performance through adjustments to thermoelectric parameters resulting in the successful fabrication of excellent thermoelectric materials. image

Nanofibrous active layers offer hierarchical control over molecular structure, and the size and distribution of electron donor:acceptor domains, beyond conventional organic photovoltaic architectures. This structure is created by forming donor pathways via electrospinning nanofibers of semiconducting polymer, then infiltrating with an electron acceptor. Electrospinning induces chain and crystallite alignment, resulting in enhanced light-harvesting and charge transport. Here, the charge transport capabilities are predicted, and charge separation and dynamics are evaluated in these active layers, to assess their photovoltaic potential. Through X-ray and electron diffraction, the fiber nanostructure is elucidated, with uniaxial elongation of the electrospinning jet aligning the polymer backbones within crystallites orthogonal to the fiber axis, and amorphous chains parallel. It is revealed that this structure forms when anisotropic crystallites, pre-assembled in solution, become oriented along the fiber- a configuration with high charge transport potential. Competitive dissociation of excitons formed in the photoactive nanofibers is recorded, with 95%+ photoluminescence quenching upon electron acceptor introduction. Transient absorption studies reveal that silver nanoparticle addition to the fibers improves charge generation and/or lifetimes. 1 ns post-excitation, the plasmonic architecture contains 45% more polarons, per exciton formed, than the bulk heterojunction. Therefore, enhanced exciton populations may be successfully translated into additional charge carriers.

Polyvinyl alcohol (PVA) is considered to have great potential in medical, pharmaceutical, and packaging applications because of its outstanding biocompatibility, water solubility, low density and relatively low cost. PVA crystallinity, central to the materials properties, has been studied by X-ray diffraction, but two possible crystal structures are mooted. Electron microscopic techniques can potentially image PVA at high resolution. Still, it is challenging for conventional electron microscopies because of the relatively low crystallinity of PVA, its severe beam sensitivity, and the poor contrast of light elements. Electron ptychography makes use of a 4D STEM dataset comprising the intensity in the STEM detector plane recorded as a function of each probe position and has lower sample damage and better phase-contrast compared to traditional techniques. Here, we use electron ptychography to image PVA crystallinity. The reconstructed images, which show good agreement in the unit cell dimension with X-ray diffraction data, can show how the atoms order in the materials, however, deviations from previous models derived from X-ray diffraction are observed. To interpret the data, we propose a series of changes based on previous models to formulate a description of PVA crystal structure. Simulated results from this new model accord well with the experimental images. This study manages to image both carbon and oxygen atoms in PVA, which has not previously been achieved by any conventional method. The results are expected to bring a new and deeper understanding of PVA crystal structure, and illustrate the opportunity presented by this approach for directly imaging molecular order in polymer crystals.

Electrospun photoactive nanofibers hold significant potential for enhanced photon absorption and charge transport in organic photovoltaics. However, electrospinning conjugated polymers with fiber diameters comparable to exciton diffusion lengths for efficient dissociation, is difficult. Previously, spinning sub-100 nm poly(3-hexylthiophene) (P3HT) fibers has required the auxiliary polymer, poly(ethylene oxide) (PEO), and large antisolvent additions. Therefore, its success differs considerably across donor polymers, due to variable antisolvent addition limits before precipitation. Herein, plasmonic nanoparticle infusion into P3HT nanofibers is used to modulate viscosity and deliver a novel and unrivaled strategy to achieve reduced fiber diameters. Following PEO removal, the fibers measure 55 nm in diameter, 30% lower than any previous report – providing the shortest exciton diffusion pathways to the heterojunction upon electron acceptor infiltration. The nanoparticle-containing nanofibers present a 58% enhancement over their pristine thin-film counterparts. ~17% is ascribed to plasmonic effects, demonstrated in thin-films, and the remainder to along-fiber polymer chain alignment, introduced by electrospinning. The anisotropy of light absorbed when polarized parallel versus perpendicular to the fibers increases from 0.88 to 0.62, suggesting the diameter reduction improves the alignment, resulting in greater electrospinning-induced enhancements. Controlled by the electrospinning behavior of PEO, our platform may be adapted to contemporary donor-acceptor systems. Graphical Abstract A dramatic reduction in the diameters of electrospun photoactive nanofibers is achieved by introducing nanoparticles, offering shorter exciton pathways towards the heterojunction in nanofibrous OPVs. Thinner fiber diameters enhance the alignment of the polymer chains along the fiber, manifesting in greater photon absorption. Alongside plasmonic effects, the dual-mode enhancement within the fibers offers 58% additional light harvesting versus their thin-film counterparts.

Strain-induced failure is a major concern in wearable electronics. Herein, a -90-nm thick bismuth telluride film is fabricated on 125-mu m thick polyethylene terephthalate as the basis for a flexible thermoelectric generator. To simulate induced mechanical strains during wearable operation and roll-to-roll manufacture, both tensile and buckling strains are studied. To improve the mechanical resilience, two types of substrate-to-coating interlayers with varying thicknesses are investigated: 0.4-7.7 mu m-thick Poly-tripropyleneglycol diacrylate (acrylate) and 2.5-16.8 mu m-thick Poly-dimethyl siloxane (PDMS). The thermoelectric performance of the coating is influenced through the stress, applied on the polymer substrate, being transferred through to the coating. Samples exposed to tensile testing recover once the load is released, mitigating against change on the film morphology and thermoelectric performance, in contrast to those measured in-situ under buckling strain. Both interlayers are shown to assist in maintaining the film performance, as fewer cracks are formed in the semiconductor. A thicker interlayer is shown to better mitigate the impact of deformation of the substrate. Furthermore, in fatigue-bending tests, the film grown on a PDMS interlayer shows better cyclical fatigue performance.

Exploring processes for industrial large-scale manufacture of low-cost electronic devices is timely for the upcoming era of the Internet of Things. In this study, a roll-to-roll process using selective metallisation (a combination of flexographic printing of an oil liquid-mask and physical vapour deposition: sputtering and evaporation) is explored for the large-scale manufacture of flexible electronics, with a wearable thermoelectric generator as an exemplar device. Two post-treatments (electron beam and infrared) are trialled to remove the residual oil after selective metallisation. Both are effective for the removal of oil, however, the oil used in selective metallisation degrades the functional thermoelectric material severely, causing a dramatic increase in the internal resistance of the device. This is attributable to (1) the doping and oxidation issues in the semiconductor film; (2) the existence of oil at the interface between the two coatings. The fabricated thermoelectric generator with post-treatments shows a good voltage output comparable to a standard device fabricated using shadow mask, providing an option for portable devices requiring a voltage source. This study reports an industrially compatible roll-to-roll manufacturing process for large-scale manufacture of flexible electronics. Further studies are under way to optimise the device performance.

High-throughput roll-to-roll processes are desirable to scale up the manufacture of flexible thermoelectric generators. While vacuum deposition onto a heated dynamic substrate presents a considerable engineering challenge, viable postdeposition in-line annealing processes are considered as an alternative to improve the functional performance of as-deposited films. The effect of infrared and electron-beam irradiations of 1 μm thick bismuth telluride thin films, produced by a vacuum roll-to-roll process for use as thermoelectric materials, was examined. A static vacuum oven and pulsed high-energy electron beam were also studied as control groups. All annealing strategies increased the crystallite size and decreased the Te content. Only the static vacuum oven treatment was shown to significantly improve the film's crystallinity. After 1 h annealing, the power factor improved by 400% (from 2.8 to 14 × 10-4 W/mK2), which, to the knowledge of the authors, is the highest reported thermoelectric performance of postannealed or hot-deposited Bi-Te films. As for in-line annealing, infrared and electron-beam post treatments improved the power factor by 146% (from 2.8 to 6.9 × 10-4 W/mK2) and 64% (from 2.8 to 4.6 × 10-4 W/mK2), respectively.

In this work, we investigated the use of in-line linear electron beam irradiation (LEB) surface treatment integrated into a commercially compatible roll-to-roll (R2R) processing line, as a single fluorocarbon cleaning step, following flexography oil masking used to pattern layers for devices. Thermoelectric generators (TEGs) were selected as the flexible electronic device demonstrator; a green renewable energy harvester ideal for powering wearable technologies. BiTe/BiSbTe-based flexible TEGs (f-TEGs) were fabricated using in-line oil patterned aluminium electrodes, followed by a 600 W LEB cleaning step, in which the duration was optimised. A BiTe/BiSbTe f-TEG using an oil-patterned electrode and a 15 min LEB clean (to remove oil prior to BiTe/BiSbTe deposition) showed similar Seebeck and output power (S ~ 0.19 mV K−1 and p = 0.02 nW at ΔT = 20 K) compared to that of an oil-free reference f-TEG, demonstrating the success of using the LEB as a cleaning step to prevent any remaining oil interfering with the subsequent active material deposition. Device lifetimes were investigated, with electrode/thermoelectric interface degradation attributed to an aluminium/fluorine reaction, originating from the fluorine-rich masking oil. A BiTe/GeTe f-TEG using an oil-patterned/LEB clean, exceeded the lifetime of the comparable BiTe/BiSbTe f-TEG, highlighting the importance of deposited material reactivities with the additives from the masking oil, in this case fluorine. This work therefore demonstrates (i) full device architectures within a R2R system using vacuum flexography oil patterned electrodes; (ii) an enabling Electron beam cleansing step for removal of oil remnants; and (iii) that careful selection of masking oils is needed for the materials used when flexographic patterning during R2R.

The optimization of flexible thin‐film thermoelectric generators (TEGs) suitable for large‐area roll‐to‐roll (R2R) processing is investigated. The selection of suitable contact materials, in‐line patterning of connections, and dimension of the thermoelectric (TE) strip are studied. As a result, copper is selected for contacts because it possesses a similar performance to gold while being cheaper. Both in‐series‐ and in‐parallel‐connected devices are found to work well and provide a voltage‐dominant and current‐dominant power source, respectively. The Seebeck coefficient and internal resistance of a device are extracted from the fit line to the measured power data. The in‐parallel‐connected TEG has a much smaller internal resistance and is thus suitable for wearable/portable devices with a small load resistance. A shorter and wider TE strip generates more power. To the authors’ knowledge, this is the first study that experimentally proves a downward trend of power output with increasing strip length. In addition, an industrially feasible/continuous process is proposed for large‐scale manufacture of flexible TEGs, by R2R sputtering TE materials on polymer webs, inkjet printing contacts, and segmenting using a laser. A segmented configuration is able to relieve internal strains in the device, while subjected to bending (e.g., within a wristband) to retain performance.

A stacked thermoelectric generator on a flexible polymer sheet is investigated that can utilize a low‐cost high throughput roll‐to‐roll process, employing a metal–insulator–semiconductor structure of <100 nm thick Cu and bismuth telluride films with a ≈1 µm thick acrylate insulating coating. Thermoelectric strips can be stacked and connected in the out‐of‐plane direction, which significantly decreases the size required in the substrate plane and also gives rise to the opportunity for greatly extending power output by stacking thousands of layers. A smooth surface of stacked layers is confirmed due to the nature of the acrylate layer. Room‐temperature sputtering can produce good quality/crystalline films, indicated by X‐ray diffraction and transmission electron microscope. Both experimental and simulation results observe a small temperature gradient across the stack from the bottom heat source to the top free surface. A stacked thermoelectric generator shows comparable performance to an in‐plane device, and most notably, the stacked architecture allows a higher power output without increasing the dimension of the device in the substrate plane, while the thickness is increased within only a µm range. Cyclic buckling fatigue tests suggest that the performance of stacked functional strips can be protected under deformation within the acrylate matrix.

The crystal structure and orientation in polymers, which depends on parameters including temperature, interfacial interactions and strain, will vary locally. To resolve the detailed microstructure, high resolution imaging is required because diffraction methods only generate average structural information from the characterization region. In a study of annealed thin films of PEN [1], crystalline domains were found to vary with the depth from a surface. Here, PEN is used as a model system to demonstrate high resolution imaging of polymer crystals.

High-throughput roll-to-roll processing could be used to scale up the manufacture of flexible thermoelectric generators. Very thin thermoelectric layers can be manufactured at high throughput speed and low cost and, most importantly, are predicted to possess better thermoelectric properties than thicker layers. Here we present a study on a series of bismuth telluride films of different thickness (few nm to 370 nm), deposited on polymer substrates at room temperature using DC magnetron sputtering. Unlike previous studies of deposition of bismuth telluride films onto heated substrates, an island-growth mode, indicated by AFM, was observed for Bi-Te films grown at room temperature. A period of growth in which the layer only partially coats the substrate, with only imperfect connections between islands, was observed. In this partially coated region, the coating exhibited an extremely high Seebeck coefficient. An energy barrier mechanism, similar to the interface effect in nanomaterials, is proposed to explain this phenomenon, along with a possible quantum confinement effect. We found that a thinner Bi-Te film could generate a greater power factor because of a quasi-decoupling of Seebeck coefficient and electrical resistivity. In addition, ensuring that the sample passed directly under the sputtering target, and using a substrate smoothed with an acrylate layer were found to improve film properties, thus enhancing thermoelectric behaviour.

Excessive stress is one of the main causes of mental illness. Long-term exposure of stress could affect one's physiological wellbeing (such as hypertension) and psychological condition (such as depression). Multisensory information such as heart rate variability (HRV) and pH can provide suitable information about mental and physical stress. This paper proposes a novel approach for stress condition monitoring using disposable flexible sensors. By integrating flexible amplifiers with a commercially available flexible polyvinylidene difluoride (PVDF) mechanical deformation sensor and a pH-type chemical sensor, the proposed system can detect arterial pulses from the neck and pH levels from sweat located in the back of the body. The system uses organic thin film transistor (OTFT)-based signal amplification front-end circuits with modifications to accommodate the dynamic signal ranges obtained from the sensors. The OTFTs were manufactured on a low-cost flexible polyethylene naphthalate (PEN) substrate using a coater capable of Roll-to-Roll (R2R) deposition. The proposed system can capture physiological indicators with data interrogated by Near Field Communication (NFC). The device has been successfully tested with healthy subjects, demonstrating its feasibility for real-time stress monitoring.

HYPOTHESIS:Selective ozone treatment of Polydimethylsiloxane (PDMS) print-stamps may facilitate local de-wetting of Krytox®1506 oil; the resulting printed pattern can be used as a masking liquid during roll-to-roll vacuum-metallization, exemplified with Ag. This novel method may exploit high-throughput manufacture without chemical etchants or elevated temperatures for thin-film electronics. EXPERIMENTS:The mechanism for selective wetting arose from O3 treatment of PDMS through a shadow-mask to vary surface-energy due to formation of polar silanol (Si-OH) replacing surface methyl groups leading to contact angle reduction from 40°-9° for oil on PDMS. Oiled PDMS was (1) metalized itself and (2) used as a stamp to print onto polyethylene-terephthalate, consisting of oil pick-up/de-wetting/transfer-to-substrate/metallization. FINDINGS:Ag (520-568 nm) thick was deposited outside oiled regions, surpassing ~20 μm resolution of commercial printing. On metalized PDMS, minimum line widths were 2.6 μm (with 10 μm edge-grading from centrifugal oil spreading) or widths of 24 μm (no Ag grading) following spin-coating/roll-coating oil respectively. The progressive effect of thinning oil via five successive stamp-to-substrate impressions, produced line widths of 14 μm (with graded edge of 7.6 μm via spreading from stamp-substrate compression). Developments may reduce reliance on laser engraving/photocuring, and could enhance micro-contact printing through liquid dynamics vs. topographical relief structures.

Here we show in-line-patterning of Al (37 nm)/Cu (58 nm)/Ag (55 nm) electrodes on flexible poly(ethylene-terephthalate) at 1-25 m min(-1), using a flexography printing apparatus retrofitted to a commercial roll-to-roll vapor deposition system. A Krytox (R) 1506 lift-off mask was printed for simultaneous evaporation during metal deposition. Heat through Al thermal evaporation facilitated single-step patterning with 99.99% masking efficiency, and 5% area shrinkage compared to the 4 x 6 mm photopolymer stamp. Al with nominal line widths of 28-58 mu m (average. 47 mu m) was also demonstrated using a commercial nitrile sleeve. Sputtering was trialed using powers 1-2 kW, substrate speeds of 1-25 m min(-1), and was unable to achieve mask lift-off, leading to metallic over-coating, removed via secondary isopropanol cleaning. Dynamic deposition rates of thermal evaporation exceeded sputtering (928 vs. 775 nm.m min(-1) respectively, with ten inline/confocal sputtering sources theoretically required). Resistivities of Al/Cu/Ag were 7.2 x 10(-8)/8.2 x 10(-7)/6.8 x 10(-8) Omega.m; 2.6/48.2/4.2x greater than respective bulk values whilst work functions of electrodes varied 4.21/4.93/5.22 eV. Resistivities were critically impaired in proximity/on sputtered Cu/Ag. Electrodes were "printed" without inks/post-deposition heat-treatments, and whilst challenges remain, selective metallization is ready for high-throughput flexible electronics whilst sputtering may be used in a two-step process, requiring additional development for single-step patterning.

Bismuth telluride was deposited onto a dynamic (25 m min(-1)) polyethylene terephthalate substrate at room temperature using direct current magnetron sputtering in preparation for roll-to-roll manufacture of flexible, low dimensional thermoelectric generators. This study explored the effect of sputtering pressure ranging from 0.03 to 0.6 Pa by adjusting argon flow rate from 50 to 500 sccm. Decreasing argon pressure from 0.6 to 0.03 Pa led to a more stoichiometric target-to-substrate atomic transfer. The coatings, deposited from a Te:Bi = 1.5 atomic ratio target, varied in composition ratio from 1.9 to 3.2, attributed to an obstructive phenomenon of sputtered Bi atoms during transport through the plasma region, under a higher working pressure. In addition, films grown under a lower pressure had wider and flatter grains (the aspect ratio of island width/height decreased from 40 (+/- 1) at 50 sccm to 10 (+/- 1) at 500 sccm for a similar to 80-nm coating), as indicated by images in atomic force microscopy. Electrical resistivity increased with pressure (0.9 +/- 0.01 to 8.1 +/- 0.2 m Omega.cm in a similar to 80-nm coating) due to a stronger carrier scattering mechanism and variations in the film composition and band gap. Seebeck coefficient increased with pressure (49.7 +/- 0.9 to 84.0 +/- 0.5 mu V/K) attributable to an increased band gap and a possible energy barrier mechanism at grain boundaries leading to a carrier filtering effect. Power factor of the thermoelectric film was enhanced by decreasing pressure until the argon flow rate was below 250 sccm. The maximum power factor of the Bi-Te thin film achieved was 4.1 ( +/- 0.1) x 10(-4) W/mK(2) under 0.055 ( +/- 0.004) Pa of argon for a similar to 55 nm coating, which was achieved here by a real industrial-scale manufacturing process.

The prospect of roll-to-roll (R2R) processable Organic Thin Film Transistors (OTFTs) and circuits has attracted attention due to their mechanical flexibility and low cost of manufacture. This work will present a flexible electronics application for pH sensing with flexible and wearable signal processing circuits. A transimpedance amplifier was designed and fabricated on a polyethylene naphthalate (PEN) substrate prototype sheet that consists of 54 transistors. Different types and current ratios of current mirrors were initially created and then a suitable simple 1:3 current mirror (200nA) was selected to present the best performance of the proposed OTFT based transimpedance amplifier (TIA). Finally, this transimpedance amplifier was connected to a customized needle-based pH sensor that was induced as microfluidic collector for potential disease diagnosis and healthcare monitoring.

A series of PET (poly(ethylene terephthalate)) / PEN (poly(ethylene 2,6-naphthalate)) copolyesters were synthesized by molten transesterification, and the surface crystallization behaviour of their thin films investigated by AFM with an in-situ heating stage. Force-distance measurements detected a surface glass transition ( T gS ) of the copolymers several tens of degrees below their bulk glass transition ( T gB ) obtained by DSC. The surface crystalline morphologies as a function of annealing temperature and film thickness were summarised as surface morphology diagrams. The surface crystallization temperature ( T cS ) was found to be several degrees lower than the bulk crystallization ( T cB ), and the films thinner than ~100 nm showed significant increase in T cB . The lamellar crystalline morphology of copolymers with high randomness and short sequence length deviated from that of the homopolymers, reflecting the composition and degree of randomness. Highly random PET/PEN=75/25wt% copolymers exhibited unique lamellar curvature with arbitrary growth directions. Sharp boundaries between the crystals and amorphous suggested an absence of large amounts of rejected material at the growth front. In the case of copolymers with high randomness and short sequence length, no bulk crystallization morphology was observed even at 190ºC, with the relatively thick surface crystalline layer totally covering the emergence of any bulk crystals.