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Silver nanowires (AgNWs) are excellent candidate electrode materials in next-generation wearable devices due to their high flexibility and high conductivity. In particular, patterning techniques for AgNWs electrode manufacture are very important in the roll-to-roll printing process to achieve high throughput and special performance production. It is also essential to realize a non-contact mode patterning for devices in order to keep the pre-patterned components away from mechanical damages. Here, we report a successful non-contact patterning of AgNWs-based stretchable and transparent electrodes by laser-induced forward transfer (LIFT) technique. The technique was used to fabricate a 100% stretchable electrode with a width of 200 μm and electrical resistivity 10−4 Ωcm. Experiments conducted integrating the stretchable electrode on rubber substrate in which LED was pre-fabricated showed design flexibility resulting from non-contact printing. Further, a patterned transparent electrode showed over 80% in optical transmittance and less than 100 Ω sq−1 in sheet resistance by the optimized LIFT technique.

The reliability of a commercially available isotropic conductive adhesive (ICA) deposited via laser induced forward transfer (LIFT) printing is reported. ICAs are particularly important for surfacemount device (SMD) integration onto low-cost, large-area system-in-foil (SiF) applications such as radio frequency identification (RFID) transponder tags. For such tags, and for SiF in general, the reliability of the printed interconnects under harsh circumstances is critical. In this study, the reliability of surface mounted resistors bonded onto screen-printed conductive circuitry on polymer foil was assessed. The prepared samples were subjected to thermal shock testing (TST), accelerated humidity testing (AHT) and flexural testing, while electrical measurements were conducted at regular intervals. Die shear testingwas performed to evaluate the bond strength. The reliability characteristics of the LIFT-printed sampleswere benchmarked against current industry standard stencil printing process. Finally, the applicability of the LIFT–ICA process for practical applications is demonstrated using RFID transponder integration and testing.

Flexible semi-transparent organic photovoltaic (OPV) modules were manufactured by roll-to-roll slot–die coating of three functional layers [ZnO, photoactive layer, and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)] and either the screen printing or inkjet printing of the top electrodes. A poly(3-hexylthiophene):[6,6] phenyl C61-butyric acid methyl ester (P3HT:PCBM) layer deposited from non-chlorinated solvents was used as the absorber layer. The modules were realized by slot–die coating of the layers onto a laser-patterned polyethylene terephthalate/indium-tin oxide (PET/ITO) substrate, followed by laser structuring of all coated layers. The top electrodes were realized by high-resolution printing, which, combined with laser patterning of other layers, enables manufacturing of the modules with high geometrical fill factor (92.5%). The modules have an active area of 156 cm2, and contain 13 serially interconnected cells. Two semitransparent electrodes (ITO from the bottom and PEDOT:PSS/Ag-grid from the top side) allow the absorption of photons incident from both sides. The performance of the modules was evaluated and compared among the modules by considering the following factors: (i) roll-to-roll slot–die coated vs. spin-coated layers,(ii) inkjet-printed vs. screen-printed top electrodes, (iii) top vs. bottom illumination. The demonstrated technology is one of the proven feasible ways towards industrial manufacturing of the OPV modules.

This paper presents an alcohol vapor sensor realized using stretchable optical waveguides doped with commercially available fluorescent dyes. The fabrication technology is based on a cost-efficient replication method, employing polydimethylsiloxane materials mixed with the dye Nile red. Upon introduction of ethanol vapors, the fluorescent emission was found to have a wavelength shift of ~20 nm with a response time of ~10 s. Observing the fluorescence intensity of the shifted emission spectrum in a periodically varying environment inside a gas-sensing setup showed a respective variation with introduction of ethanol vapor. The intensity variation also showed the reversibility of the sensor. The sensing platform is found to hold much promise for further integration and multiplexing.

The localized growth of carbon nanotube structures has potential in many applications such as interconnects, field emitters and sensors. Using a laser to locally heat the substrate offers a highly versatile process compatible with a broad range of substrates and devices. However, for laser-assisted CNT growth, detailed process information, such as temperature evolution and process monitoring over time are often unavailable. Here, we report on a miniaturized laser-CVD reactor, where a precise control of the gas flow and composition on the laser-growth site is provided. The particular design of this miniaturized reactor results in a high reproducibility as well as faster growth time. A multi-parameter finite element method (FEM) model is implemented to link substrate temperature at the laser spot with emitted radiation, taking into account the gas flow, the process time and other temperature-dependent physical parameters such as forced convection, thermal- conductivity and heat capacity. The resulting growth is assessed using Scanning Electron Microscopy. Combined with results from the thermal model, process information is used to calculate activation energy for the nanotube nucleation. Finally, several demonstrations of possible applications are given, exploiting the potentials of the miniaturized reaction chamber.

Selective laser patterning of thin organic films is an important aspect in the roll-to-roll production of organic electronic devices such as organic light emitting diodes (OLEDs). An excimer laser is well suited for the patterning and structuring of polymer thin films as their UV absorption is significant. Selective removal of a transparent conducting polymer PEDOT:PSS (poly(3,4-ethylene dioxythiophene): polystyrene sulfonate) on a multilayered (inorganic-organic-inorganic) barrier and a flexible PEN (polyethylene napthalate) substrate has been studied using a KrF excimer laser. The ablation craters were characterized with electron microscopy and profilometry. For the first time, chemical surface analysis of the patterned area was performed with Time-Of-Flight Static Secondary Ion Mass Spectrometry (TOF-S-SIMS), providing a detailed insight of the surface composition after laser ablation and plasma post-treatments. (C) 2013 Elsevier B.V. All rights reserved.

We present a spectroscopy technique to measure temperature locally in a polydimethylsiloxane (PDMS) micro-optofluidic chip with integrated optical fibers and minimal optical components. The device was fabricated in one step with fiber coupler grooves followed by manual integration of the optical fibers. The experimental setup consists of a micro-optofluidic chip with a pair of optical fibers for excitation and fluorescence collection, a laser module and a spectrometer. The laser module is coupled to one of the optical fibers to guide the light into the microchannel. The fluorescence signal is collected by a second integrated optical fiber placed orthogonally. A spectroscopy technique is used to measure the local temperature in a microchannel (500 μm wide and 125 μm in height) using Rhodamine B as a temperature indicator. It is shown that for a flow rate between 200 and 400 μL/min, the local temperature can be determined., We present a spectroscopy technique to measure temperature locally in a polydimethylsiloxane micro-optofluidic chip with integrated optical fibers and minimal optical components. The device was fabricated in one step with fiber coupler grooves followed by the manual integration of the optical fibers. The experimental setup consists of a micro-optofluidic chip with a pair of optical fibers for excitation and fluorescence collection, a laser module, and a spectrometer. The laser module is coupled to one of the optical fibers to guide the light into the microchannel. The fluorescence signal is collected by a second integrated optical fiber placed orthogonally. A spectroscopy technique is used to measure the local temperature in a microchannel (500 μm wide and 125 μm in height) using Rhodamine B as a temperature indicator. It is shown that for a flow rate between 200 and 400 μL/min, the local temperature can be determined.

Thin film photovoltaics can be beneficial for specific applications like building integrated photovoltaics. To fully exploit the differentiator of form freedom, the interconnections in thin film modules can be tuned depending on the required module output. Traditionally, an alternation of coating and scribing steps is applied, determining the form from the start. Here, we present a set of techniques to define the module design from a master substrate with homogeneously coated electroactive layers. By applying subtractive and additive laser-based processes, the size and form of the module are only fixed after the manufacturing of the whole solar cell stack. By laser-induced forward transfer, an isolating dielectric material and a conductive top electrode are deposited in laser ablated scribes to enable the interconnection between two adjacent cells. After optimization of the laser settings for ablation and forward transfer, the optimal annealing time and temperature for the curing of the silver top electrode were determined. The proof of principle was demonstrated by constructing a 4-cell organic solar module of 1.0% efficiency on an area of over 3 cm2 showing the anticipated short-circuit current and open-circuit voltage.

Non-contact printing techniques are receiving increasing interest in the field of printed electronics, because they can be used to pattern various inks on arbitrary substrates without applying mechanical pressure or damaging pre-patterned components. The ink-jet process is frequently used for non-contact printing of various conductive inks. However, the ink-jet printing process is restricted by the viscosity of the ink, because the nozzle can become clogged by high-viscosity inks. Here we successfully demonstrate a non-contact printing technique for high-viscosity and high-concentration silver precursor inks using the laser-induced forward transfer (LIFT) process. The process conditions for LIFT printing, including the triazene polymer sacrificial layer thickness, the laser fluence, and the donor-acceptor distance, have been investigated in detail. LIFT printing of a hexylamine-based 70 wt% silver precursor ink with viscosity of 60 mPa s was achieved, and produced fine conductive lines with widths of 141 μm, thicknesses of 490 nm, and volume resistivity of 11.6 μΩ cm. It is envisaged that this non-contact printing method can pave the way towards non-contact and maskless printing of high viscosity inks in the manufacture of printed electronics. ©2015 The Royal Society of Chemistry.

In this paper we introduce a microfluidic device ultimately to be applied as a wearable sweat sensor. We show proof-of-principle of the microfluidic functions of the device, namely fluid collection and continuous fluid flow pumping. A filter-paper based layer, that eventually will form the interface between the device and the skin, is used to collect the fluid (e.g., sweat) and enter this into the microfluidic device. A controllable evaporation driven pump is used to drive a continuous fluid flow through a microfluidic channel and over a sensing area. The key element of the pump is a micro-porous membrane mounted at the channel outlet, such that a pore array with a regular hexagonal arrangement is realized through which the fluid evaporates, which drives the flow within the channel. The system is completely fabricated on flexible polyethylene terephthalate (PET) foils, which can be the backbone material for flexible electronics applications, such that it is compatible with volume production approaches like Roll-to-Roll technology. The evaporation rate can be controlled by varying the outlet geometry and the temperature. The generated flows are analyzed experimentally using Particle Tracking Velocimetry (PTV). Typical results show that with 1 to 61 pores (diameter = 250 μm, pitch = 500 μm) flow rates of 7.3 × 10-3 to 1.2 × 10-1 μL/min are achieved. When the surface temperature is increased by 9.4 °C, the flow rate is increased by 130 %. The results are theoretically analyzed using an evaporation model that includes an evaporation correction factor. The theoretical and experimental results are in good agreement. Electronic supplementary material The online version of this article (doi:10.1007/s10544-015-9948-7) contains supplementary material, which is available to authorized users.

In this work, we present a digital microflow meter operating in the range 30–250 nl min-1 for water. The principle is based on determining the evaporation rate of the liquid via reading the number of wetted pore array structures in a microfluidic system, through which continuous evaporation takes place. A proof-of-principle device of the digital flow meter was designed, fabricated, and tested. The device was built on foil-based technology. In the proof-of-principle experiments, good agreement was found between set flow rates and the evaporation rates estimated from reading the number of wetted pore structures. The measurement range of the digital flow meter can be tuned and extended in a straightforward manner by changing the pore structure of the device.

Laser-assisted chemical vapour deposition (CVD) growth is an attractive mask-less process for growing locally aligned carbon nanotubes (CNTs) in selected places on temperature sensitive substrates. The nature of the localized process results in fast carbon nanotube growth with high experimental throughput. Here, we report on detailed investigation of growth kinetics related to physical and chemical process characteristics. Specifically, the growth kinetics is investigated by monitoring the dynamical changes of reflected laser beam intensity during growth. Benefiting from the fast growth and high experimental throughput, we investigate a wide range of experimental conditions and propose several growth regimes. Rate-limiting steps are determined using rate equations linked to the proposed growth regimes, which are further characterized by Raman spectroscopy and Scanning Electron Microscopy (SEM), therefore directly linking growth regimes to the structural quality of the CNTs. Activation energies for the different regimes are found in the range of 0.3 - 0.8 eV., 12 pages, 17 figures

In this paper, we present a method to locally control the wettability behavior of PDMS surfaces by excimer laser ablation. In the ablation process, a micrometer scale roughness is formed in the irradiated regions while a nanometer scale roughness is formed by the redeposits surrounding the irradiated regions. The increase in surface roughness results in a change of the wettability behavior of the PDMS surface. By using a hexagonal pattern and tuning the patterning pitch, two different wetting behaviors were realized. A pitch smaller than 300 μm resulted in a superhydrophobic surface with an advancing contact angle of θadv = 165° and a receding contact angle of θrec = 160°. A pitch between 300 and 500 μm resulted in a sticky superhydrophobic surface with θadv = 120–150° and θrec = 80°. The contact angle hysteresis for the latter was larger than for untreated PDMS resulting in very sticky surfaces with high sliding angles. This gives the method great versatility since the two wetting behaviors are very different. By combining both behaviors, local surface features like pinning sites, non-wetting sites, barriers and guides can all be fabricated by a single method. As an application demonstrator of the method, we show that drops can be caught and released depending on size and tilting angle by creating slippery surfaces with sticky barriers. Additionally, the method is ideal for rapid prototyping as it consist of only a single step. It is a direct write method requiring no lithographic mask. Also the process works in ambient atmosphere, so it can be used for temperature or pressure sensitive applications.

Laser induced forward transfer (LIFT) is a freeform, additive patterning technique capable of depositing high resolution metal structures. A laser pulse is used to generate small droplets from the donor material, defined by the spot size and energy of the pulse. Metallic as well as non-metallic materials can be patterned using this method. Being a contactless, additive and high resolution patterning technique, this method enables fabrication of multi-layer circuits, enabling bridge printing, thereby decreasing component spacing. Here we demonstrate copper droplet formation from a thin film donor. The investigation of the LIFT process is done via shadowgraphy and provides detailed insight on the droplet formation. Of particular importance is the interplay of the droplet jetting mechanism and the spacing between donor and receiving substrate on a stable printing process. Parameters such as the influence of laser fluence and donor thickness on the formation of droplets are discussed. An angle deviation analysis of the copper droplets during flight is carried out to estimate the pointing accuracy of the transfer. The possibility of understanding the droplet formation, could allow for stable droplets transferred with large gaps, simplifying the process for patterning continuous high-resolution conductive lines. © 2014 SPIE.

This paper presents a study of selective ablation of thin organic films (LEP- Light Emitting Polymer, PEDOT:PSS- Poly 3,4-ethylenedioxythiophene: polystyrene sulfonate) by using 248 nm Excimer laser, on various kinds of multilayered SiN barrier foils for the development of Organic Light Emitting Diodes (OLED). Different Silicon Nitride (SiN) barrier foils with dedicated absorption spectra are taken into account for this purpose. The drive for looking into different types of SiN originates from the fact that the laser selective removal of a polymer without damage to the barrier layer underneath is challenging in the dynamic laser processing of thin films. The barrier is solely responsible for the proper encapsulation of the OLED stack. The main limitation of current OLED design is its shorter life span, which is directly related to the moisture or water permeation into the stack, leading to black spots. An optimization of laser parameters like fluence and number of shots has been carried out for the various types of SiN barrier foils. We are able to obtain a wider working process window for the selective removal of LEP and PEDOT:PSS from SiN barrier, by variation of the different types of SiN.

This paper describes the implementation of a low-cost technology platform for fluorescence-based optochemical sensors made up of arrays of multimode waveguides and coupling structures integrated onto a flexible substrate. Such aconfiguration is ideal for multi-analyte detection owing to a possibility of future integration of different dyes in each waveguides. The presence of light sources, fluorescent sensing elements and photodetectors in a foil platform makes it a compact optochemical sensor, which has wide-range of applications in medical, biochemical, and environmentaldiagnostics. Flexible lightguides fabricated using soft-lithography based replication techniques, are used in combination with 45° micromirror coupling structures, having a loss of 0.5dB. Fluorescent dyes are incorporated with the lightguides enabling a detection of shift in fluorescence-peaks in contact with gases, which are read-out at the detection. Initial measurements yielded promising results of the waveguides mixed with fluorescent dyes showing response to toluene.

Laser-assisted chemical vapor deposition growth is an attractive mask-less process for growing locally aligned nanotubes in selected places on temperature sensitive substrates. An essential parameter for a successful and reproducible synthesis of nanotubes is the temperature during growth. Here, we demonstrate a temperature feedback control mechanism based on the dynamic, in situ monitoring of the infrared radiation coupled with reflectivity information. With the information provided by these sensors, an infrared laser, focused on a silicon substrate covered with aluminum-oxide and iron catalyst layers, can be controlled. The growth takes place in a gaseous mixture of argon (carrier gas), hydrogen (process gas), and ethylene (carbon-containing gas). Scanning electron microscopy and Raman spectroscopy analysis demonstrate the excellent reproducibility of the closed-loop control process over multiple experiments. Furthermore, we developed a unique method to identify the onset for catalyst formation and activation by monitoring the fluctuation of the reflected laser beam. © 2012 American Institute of Physics.

This paper describes the development of a low-cost technology platform for fluorescence-based optochemical sensors. These sensors were constructed by incorporating fluorescent sensing elements in the core of multimode waveguides or lightguides, and have applications in medical, biochemical and environmental diagnostics. Flexible lightguides were fabricated either with silsesquioxane-based or PDMS-type optical polymers using photolithography or soft-lithography based replication techniques respectively. Spectral transmission characteristics were measured along with loss values obtained by cut-back measurements for several wavelengths from visible to mid-IR. Propagation losses as low as 0.14dB/cm were measured for 50 x 50 μm2 waveguides. For coupling light in and out of the waveguides, different types of coupling structures, e.g. 45° micromirror plugs were investigated. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

A picosecond pulsed laser has demonstrated successful selective ablation of different thin film layers from multilayer stacks on flexible substrates allowing structuring of large area flexible organic electronic devices. These thin film layers are extremely thin and sensitive to thermal damage, so the ultrashort pulse duration combined with the flexibility to use infrared, green and ultraviolet wavelengths allows clean scribing of a range of thin film materials used in organic, electronic devices. This capability combined with the high repetition rate allows high scribe speeds of several metres/second with little or no surface debris or heat affected zone. Such laser patterning is well suited for reel-to-reel processes offering the opportunity for cost effective production of flexible devices.A picosecond pulsed laser has demonstrated successful selective ablation of different thin film layers from multilayer stacks on flexible substrates allowing structuring of large area flexible organic electronic devices. These thin film layers are extremely thin and sensitive to thermal damage, so the ultrashort pulse duration combined with the flexibility to use infrared, green and ultraviolet wavelengths allows clean scribing of a range of thin film materials used in organic, electronic devices. This capability combined with the high repetition rate allows high scribe speeds of several metres/second with little or no surface debris or heat affected zone. Such laser patterning is well suited for reel-to-reel processes offering the opportunity for cost effective production of flexible devices.

A feedback control mechanism based on infrared radiation monitoring coupled with reflectivity information was developed to control the temperature of a laser assisted chemical vapor deposition process for the growth of carbon nanotube forests. An infrared laser operating at 808 nm is focused on a silicon substrate containing a 20 nm-aluminum-oxide layer and a 1.5 nm-iron catalyst layer. The growth takes place in an argon/ hydrogen/ ethylene gaseous environment. SEM and Raman spectroscopy analysis show that good controllability and reproducibility is achieved over multiple experiments.

A study on via drilling and channel scribing on PEN and PET substrates for flexible electronic application is discussed in this paper. For the experiments, both KIF excimer laser (248 nm) and frequency tripled Nd:YAG (355 nm) laser are used. Different measurement techniques like optical microscopy, Dektak profilometer, Confocal microscopy and scanning electron microscopy (SEM) were employed to characterize the quality of the channels and vias. The patterned structures were filled by three different methods using a conductive paste or ink that is cured in an oven at an elevated temperature. The cross-sectional measurements of channels and vias were carried out using SEM to study the uniformity of filling.

Multimode laser emission is observed in a polymer optical fiber doped with a mixture of rhodamine 6G and rhodamine B dyes. Tuning of laser emission is achieved by using the mixture of dyes due to the energy transfer occurring from donor molecule (rhodamine 6G) to acceptor molecule (rhodamine B). The dye doped polymethylmethacrylate (PMMA) based polymer optical fiber is pumped axially at one end of the fiber using 532nm pulsed laser beam from an Nd: YAG laser and the fluorescence emission is collected from the other end. At low pump energy levels, fluorescence emission is observed. When the energy is increased beyond a threshold value, laser emission occurs with a multimode structure. The optical feedback for the gain medium is provided by the cylindrical surface of the optical fiber which acts as a cavity. This fact is confirmed by the mode spacing dependence on the diameter of the fiber.

The fast-growing market of organic electronics stimulates the development of versatile technologies for structuring thin-film materials. Ultraviolet lasers have proven their full potential for patterning organic thin films, but only a few studies report on interaction with thin-film barrier layers. In this paper, we present an approach in which the laser patterning process is optimized together with the barrier film, leading to a highly selective patterning technology without introducing barrier damage. This optimization is crucial, as the barrier damage would lead to moisture and oxygen ingress, with accelerated device degradation as a result. Following process optimization, a laser processed flexible organic LED has been fabricated and thin-film encapsulated and its operation is shown for the first time in atmospheric conditions.

Laser induced forward transfer is an emerging material deposition technology. We investigated the feasibility of this technique for printing thermally sensitive, electrically conductive adhesives with and without using an intermediate dynamic release layer. A 248nm KrF-excimer laser was used to print the epoxy-based conductive adhesives containing silver flakes down to 75μm dot size. The process is particularly relevant for realizing electrical connections to surface mount devices in the microelectronics industry. Characterization of the printed materials was analyzed by Fourier transform infrared spectroscopy, four-point electrical measurements, die-shear testing and temperature shock testing, to establish that the properties of the adhesive were not affected by direct or indirect laser irradiation. The lack of degradation by the laser onto the adhesives confirms the potential of this technique for interconnection applications. cop. 2014 Astro Ltd.

A femtosecond laser has been successfully utilized for patterning thin Flexible Organic Light Emitting Diode (FOLED) structures of individual layer thickness around 100nm. The authors report in this paper a step-like ablation behavior at the layer interfaces which accounts for a local removal of entire layers. Various surface analyzing techniques are used to investigate the morphologies and chemical compositions within and in the vicinity of the ablation areas. This study opens a new avenue in selectively ablating different layers from a multilayer stack on flexible substrates using fs lasers allowing post deposition structuring of large area flexible organic electronic devices. (C)2010 Optical Society of America