Towards lightweight and flexible electrochemical devices

Resume : SMART-EC aims at the development of self powered (energy harvesting and storage) electrochromic (EC) devices on a flexible substrate. Its purpose is to save energy, and to enhance comfort and security in automotive, ID-card and smart packaging applications SMART-EC is a Large-Scale Project bringing together 13 partners. It is supported by funding under the Seventh Research Framework Programme of the European Union. The project is financed under the Work Programme ICT-2009-3.9

Resume : While flexible electronics share many of the same challenges as flexible energy storage and electricity conversion, there are significant differences. For example, many energy storage components are already available in printable format, and some devices demand a liquid component for optimal performance. Areal resolution is not critical but, device thickness must be on the order of ~10 microns or higher for area conservation. Mechanical changes over both a cycle and the lifetime of the device must be considered with the flexible/stretchable parameters. Finally, energy storage demands mass, thus the cost per unit mass of the energy storage components must be significantly less than that of the electronic components. These constraints and opportunities demand not only new materials, but understanding of how to benefit from existing materials while systematically addressing their shortcomings, what we have dubbed “system sympathetic” approaches.

Resume : With the emerging demand for large-scale, energy-storing batteries, concerns have been raised regarding the consumption of a large volume of material resources in the fabrication of batteries mostly based on transition metals. As such, requests for greener and naturally abundant materials in energy storage have been escalating recently in society. In this respect, organic chemicals available in natural resources are promising alternatives. The minimal environmental footprint as well as distinctive material properties such as light weight, flexibility, and chemical tunability makes them beneficial as an electrode material in large-scale batteries. In particular, the use of bio-inspired organic electrodes that imitate energy metabolisms, such as respiration and photosynthesis, will enable a design of more sustainable batteries. For example, the electro-active carbonyl compounds mimicking biological quinone cofactors that can be obtained from biomass through eco-friendly processes are intriguing candidates for such electrode materials. Also, flavin-based electrodes that function through the imitation of the cellular energy transduction mechanism are promising candidates we recently introduced. Despite the recent advances in organic-based electrode materials, critical obstacles still exist for their practical use in energy storage. Most organic-based electrodes suffer from rapid capacity fading upon cycling due to the dissolution of active organic chemicals into organic electrolytes and show poor power capability because of the low electronic conductivity. Several strategies have been suggested to resolve the dissolution issue, such as covalent attachment of redox molecules to substrates, polymerization of active compound, optimization of molecular structure, and the use of solid electrolytes. Some recent works used organic composites as high power electrodes, but active material loaded in the electrode was quite low (less than 10%). While these recent achievements bring the use of naturally benign organic materials a step closer to the practical battery systems, a strategy to enhance cyclic stability and power capability simultaneously in a simple way that can be generally applied to organic-based electrode materials is still lacking. In this talk, we present a novel and general approach for the development of organic electrodes in which active organic materials with aromatic redox centers are immobilized to conductive scaffolds through non-covalent bonding yielding a self-standing and flexible nanohybrid organic electrode. The nanohybrid organic electrodes exhibited surprisingly high capacity retention of nearly no capacity reduction after 100 cycles with unprecedentedly high power capability. The mechanical flexibility of the hybrid paper enables the fabrication of the high performance and low cost flexible batteries.

Resume : TiO2 nanotube arrays have been widely used as the photoelectrode in flexible dye-sensitized solar cells (DSSCs)because of their high ordered structure with tunable shape and diameter [1]. Among them, the conical TiO2 nanotube arrays (TNTs) with dual-diameter structure may promote the light refraction, as the small diameter tip is used for minimal reflectance while the large diameter base possesses maximal effective absorption [2]. Therefore, the conical TNTs used as the photoanodes in flexible dye-sensitized solar cells are expected to maximize light capture and then have high energy conversion efficiency. However, there is no work on the use of the conical TiO2 nanotube arrays as the photoanodes in flexible dye-sensitized solar cells. In the work, conical TiO2 nanotube arrays were prepared Ti foil by adjusting the applied voltage during the two-step anodic oxidation process. Then the as-grown nanotube arrays can be transferred to the flexible FTO/PEN substrates by using titania slurries without any organic binder. It is used as the photoanodes to fabricate the front-illuminated flexible DSSCs. SEM image showed that the conical titania nanotube arrays were successfully transferred to the FTO/PEN substrates. UV-Vis spectra were used to explore the enhanced optical absorption. The photovoltaic properties for incident photon to current efficiency (IPCE) and photovoltaic conversion efficiency is also studied under AM 1.5 illumination, it shows that the conical TiO2 nanotube arrays have the higher photovoltaic conversion efficiency (5.6%) than that of the regular TiO2 nanotube arrays (3.4%), which is due to the enhanced light-harvesting ability of the conical TiO2 nanotube arrays. References: [1] Kim J Y, Noh J H, Zhu K. ACS Nano, 2011, 5(4): 2647. [2] Fan Z, Kapadia R, Leu P W. Nano Lett., 2010, 10(10): 3823.

Resume : Development of smart self-powered systems on flexible foils responds to an increase demand for energy saving, comfort and security in automotive, e-cards and smart packaging sectors. CEA-LETI is involved in different European projet (SMART-EC and INTERFLEX) to develop flexible energy storage associated with electrochromic component. Thin film solid state inorganic battery and electrochromic technologies will be presented as well as their compatibility with flexible foil heterogeneous integration techniques. Special attention will be put on 2D/3D interconnections of different components on foil. Results on compatibility of the batteries layers and their associated encapsulation with the temperature, chemistry, pressure, bendability constraints will be discussed.

Resume : Multimetal oxides have drawn great attention in the display industry over the last few years. Atmospheric spatial-ALD is emerging as a new industrially-scalable technique which combines the advantages of conventional ALD with high growth rates (~ nm/s). We have used atmospheric spatial-ALD to grow amorphous InGaZnO and ZnSnO. The films are deposited by sequentially exposing a substrate to the pre-mixed metal precursors vapors (i.e., DEZ, TMIn, TEGa or TDMA-Sn) and to water vapor, which are spatially separated in the gas injector, so that a purge step is no longer needed as in conventional ALD. The composition of InGaZnO and ZnSnO is accurately controlled in the range of 0-In/Zn-0.30, 0-Ga/Zn-0.06 and 0-Sn/Zn-0.30, by varying the partial pressure of TMIn, TEGa, TDMA-Sn and DEZ precursors in the deposition zone up to 0.38, 0.20 mbar, 0.05 and 1.3 mbar, respectively, and the exposure time from 10 to 500 ms at a deposition temperature of 200C. XRD-diffraction shows that ZnO films are polycrystalline, while In- or Sn-rich films have an amorphous structure. The resistivity is varied from 4 to 10^6 mOhm cm in both InGaZnO and ZnSnO by controlling the metal composition or applying a post-deposition annealing (400C, 1 hours in atm). InGaZnO has been tested as active channel in thin film transistors, achieving a maximum device mobility of 10 cm2/Vs and an optimum threshold voltage of 0.3 V. Atmospheric plasma-enhanced spatial-ALD of InGaZnO is currently under investigation.

Resume : Undoped and Mo-doped ZnO (2% Mo) [ZMO] films were deposited by radio-frequency magnetron sputtering on Si(100) and glass substrates at 30 and 300°C. X-ray diffraction patterns show that all films exhibit the hexagonal wurtzite crystal structure with a preferred orientation of the crystallites along the [002] direction. Plane view and cross-section transmission electron microscopy observations showed that the films present a columnar growth. Rutherford backscattering spectrometry indicates that Mo is homogeneously distributed inside the films. Scanning electron microscopy and atomic force microscopy show that Mo doping lead to a reduction of the grain size and surface roughness. According to X-ray photoelectron spectroscopy measurements, the valence of the Mo ions in the ZnO matrix is +5 and +6. Optical measurements in the UV-Visible range show a transmittance of about 80% for large wavelengths. A sharp absorption onset is observed at about 375 nm corresponding to the fundamental absorption edge of ZnO at 3.26 eV. This gap value decreases slightly down to 3.23 eV upon Mo doping. The Hall effect measurements carried out at room temperature show that both undoped and Mo-doped ZnO films present an n-type conduction. The 2% Mo doping increases the carrier concentration and decreases the resistivity measured in pure ZnO by about three orders of magnitude.

Resume : Quantum dot sensitized solar cells (QDSSCs) have received much attention due to the narrow band gap, multiple photo-induced electrons generation and quantum size effect [1-3]. In recent report, the CuInS2/Mn-CdS quantum dot co-sensitized solar cells have the good photovoltaic efficiency. However, most of the researches were focused on the hard substrate [4]. In order to fabricate the lightweight and flexible solar cell devices, the soft substrates (such as PEN, PET, et al.) have gradually become the investigation hotspot. Recently, there is a growing interesting in making fibrous photovoltaic device, which has obtained some considered photovoltaic performance [5, 6]. Moreover, TiO2 nanowire arrays have been widely researched in dye-sensitized solar cells [7]. Therefore, it is very important to investigate the flexible fibrous CuInS2/Mn-CdS sensitized TiO2 nanowire array solar cells. In the present work, the TiO2 nanowire arrays are prepared by hydrothermal process using single Ti wire as the substrate. The CuInS2 and Mn-CdS quantum dots are deposited on the TiO2 nanowire arrays by assembly linking and successive ionic-layer adsorption and reaction process, respectively. The optical absorption has an obvious red-shift after the QDs co-sensitization. The flexible QDSSCs are fabricated by using Cu2S/Cu wire as the counter electrode, which obtain the incident photon to current efficiency (IPCE) of 55% and the photovoltaic conversion efficiency of 3.5% under AM 1.5 illumination. References: [1] Kongkanand A, Tvrdy K, Takechi K, et al. J. Am. Chem. Soc., 2007, 130 : 4007. [2] Chang C H, Lee Y L. Appl. Phys. Lett., 2007, 91: 053503. [3] Huang S Q, Zhang Q X, Huang X M, et al. Nanotech., 2010, 21: 375201. [4] Santra P K, Nari P V, Kamat P V, et al. J. Phys. Chem. Lett., 2013, 4: 722. [5] Lee M R, Eckert R D, Forberich K, et al. Science, 2009, 324: 232. [6] Zhang S, Ji C Y, Cao A Y, et al. Nano Lett., 2011, 11: 3383. [7] Liao J Y, Lei B X, Chen H Y, et al. Energy Environ. Sci., 2012, 5: 5750.

Resume : One-dimensional nanostructures are attracting considerable attention because of their subwavelength size and unique optical properties including light confinement, guiding, and amplification. These properties mean they are promising materials for small optical devices such as light sources and waveguides. Electrospun polymer fibers have diameters in the nanometer range and very high aspect ratios, so are suited for use in optical devices. Ionic transition metal complexes (iTMCs) are used in organic light-emitting devices because they enable efficient operation and simple device structure. Although several groups have reported guiding of emitted light from doped organic dyes in electrospun polymer nanofibers, iTMCs have not been used as a dopant. In this paper, we fabricate a single electrospun polymer nanofiber of poly(methyl methacrylate) containing the iTMC tris(2,2’-bipyridine)ruthenium(II) hexafluorophosphate, and characterize the guiding properties of the emitted light in the fiber. The mean diameter of the fiber is 550±20 nm (mean±standard deviation). The fiber is covered by Cytop cladding, and emission spectra at the end face of the fiber are measured following scanning with an excitation laser beam. The emission intensity decreases exponentially with increasing the distance between the end face of the fiber and irradiating point of the laser beam. Curve fitting reveals the propagation loss is 30 dB•cm-1 at a wavelength of 700 nm.

Resume : Adhesion at the interfaces between different layers is critical to reliability and lifetime of flexible electronic devices. Poor adhesive characteristics of polymer make it hard to adopt cost effective processes such as roll-to-roll based inkjet or transfer printing to manufacture the flexible devices. To solve these issues and realize device manufacturing processes under open-air atmospheric-pressure and room temperature conditions, we present novel process for adhesion control of polymer substrate by plasmas. Developed plasma reactor generated uniform cold plasmas without auxiliary vacuum systems. After being exposed to the plasmas, polyimide exhibited hydrophilic characteristics with creation of functional groups over surface under continuous treatment with process speed of maximum 2 m/min. Optical emission spectroscopic measurement results showed that the wettability/adhesive surface characteristics is closely related to the specific plasma conditions. We also demonstrated large-area plasmas by reactor with 500 mm width for capable of cost-effective mass production. At last, combination of plasma process and other post process will be discussed in roll-to-roll pilot manufacturing process.

Resume : Vanadium pentoxide exhibits distinctive physical and chemical properties that make it suitable for significant technological applications in electrochromic windows, capacitors and batteries. Electrochromic windows are windows that can be darkened or lightened via the application of voltage. This capability allows the automatic control of the amount of light and heat that passes through the windows, thereby presenting an opportunity for the windows to be used as energy-saving devices. In this work, the hydrothermal growth was chosen for the deposition of V2O5 microstructured coatings because it is an environmental friendly and cost-effective technique, allowing the growth of coatings at relatively low temperature. The influence of the pH solution on the electrochemical properties of the samples is highlighted. This study aims to put forward growth strategies to be developed for “smart windows” and electrochemical devices in order to decrease their production cost and permit their successful commercialisation.

Resume : Vanadium oxide coatings were deposited on fluorine doped tin dioxide glass substrates varying the amount of VOSO4 in the solution. The structural and morphological properties of the samples were evaluated by X-ray diffraction and scanning electron microscopy respectively. The electrochemical measurements were accomplished in 1 M LiClO4/propylene carbonate solution, which acted as the electrolyte, using a scan rate of 10 mV s-1 through the voltage range of –1000 mV to +1000 mV. All samples were scanned for 1, 100 and 250 times. The effect of the different vanadium precursor present in the solution to modify the properties of the final products and consequently their electrochemical characteristics is discussed.

Resume : Stretchable electronic system on elastomeric substrates is a new emerging class of electronics. In recent years, this field has gained widespread interest as stretchable circuits enable conformability of electronic systems to more complex shapes in comparison with conventional flexible systems. They allow for improved user comport and reliability by enhanced dynamical shaping and matched mechanical properties of the electronic system to its environment. To date, a various strategies towards the realization of stretchable electronic systems have been reported. One strategy, a supporting elastomeric substrate is first stretched, then circuit materials are deposited onto the pre-strained substrate, and finally releasing the pre-strain. This relaxation leads to the spontaneous formation of wrinkled wavy structures. However, basically this pre-strain wavy method is susceptible to reliability because it is difficult to precisely control the pre-strain over a very large substrate area. Here, we developed a new Polyimide(PI)/ Polydimethylsiloxane(PDMS) hybrid stretchable substrates on the wavy silicon mold without pre-stretching. Thinned PI is spin-coated on the silicon wavy mold substrate and then thick rigid islands of PI is produced by conventional photolithography on the thin PI. Then, by casting PDMS and thin/thick islands of PI is transferred on the PDMS. In the repetitive stretching test, thick rigid islands area of PI is not deformed and only thin wavy membrane area is deformed.(at 70% applied overall tensile strain, membrane area showed more than 70% tensile strain) In this technique, we can provide various types of wavy profiles for stretchable substrates.

Resume : Despite the high potential impact applications for implantable devices, the problems with their fabrication costs, materials development (substrates, electrodes,...), reliability and the possibility to integrate them in lightweight and flexible microsystems, remain a clear challenge for the scientific community. For the latter point, study of organic and inorganic nanomaterials functionalized flexible substrates is an important issue. In this work, surface properties monitoring of different flexible substrates (glass: commercial Soda- lime, polymers: kapton, PEN, PET and metal foil: flexible copper) with different preparation protocols (ultrasonication, organic solvents and plasma). The surface quality has been investigated by means of contact angle measurements for evaluation of his surface energy and atomic force microscopy (AFM) for morphology study. The same methodology has been adopted for the study of these flexible substrates functionalized with inorganic nanomaterials (ZnO nanoparticles, Cu) with different deposition techniques. Results show that the surface energy is improved (and reach a maximum value) and roughness value are optimized for a specific preparation procedures and fabrication parameters of the microsystem. Acknowledgments: This work is supported by the tunisian-french project CMCU14 “µSESAME”.

Resume : We report an-solid-state-supercapacitor with closely matching performance with a liquid counterpart. Conceived strategy here is controlled electro-deposition of polyethylenedioxythiophene (PEDOT) on to individual carbon fibers of porous carbon substrate and followed by intercalating the matrix with polyvinyl alcohol-sulphuric acid (PVA-H2SO4) gel electrolyte. SEM imaging shows highly intercalated gel electrolyte inside the porous substrate in which fibers are coated with PEDOT having 3-D flower morphology. Due to the 1-Dimensional growth of PEDOT over fibers even with a high mass loading of 3.78 mg/cm2 enough pores are available of gel electrolyte. Thus strategy adopted here can be used to replace liquid electrolyte from convention supercapacitors for the development of lighter, thinner, safer and cheaper electric devices. The established electrode-electrolyte interface nearly mimics the interface of the counterpart based on the liquid electrolyte. Consequently, the solid device attained a high specific capacitance (181 F g-1) for PEDOT at a discharge current density of 0.5 A g-1. This value is highest among the reported capacitance of PEDOT. Even with a high volumetric capacitance of 28 F cm-3, the solid device retained a mass-specific capacitance of 111 F g-1 for PEDOT. At 10 A /g current density, PEDOT in solid device shows 75% of its initial capacitance which is exactly matching with that experiment done in 0.5M H2SO4. Detailed Impedance analysis is used to measure the ESR, time constant and phase angle of solid device to get deep insight of the advantage of the strategy adopted here. The device also showed excellent charge-discharge stability for 12000 cycles at 5 A g-1. Performance of the device was consistent even under wide range of humidity (30 to 80 %) and temperature (-10 to 80oC) conditions. A device is fabricated by increasing the electrode area by four times was used to light an LED, which validated the scalability of the process

Resume : Printed logics which encompass a large fraction of activities in the field of printed electronics are attracting increasing interest in recent years. While traditionally organic field-effect transistors (OFETs) have been studied, solution-processed FETs from inorganic materials (mostly involving inexpensive and non-toxic metal oxides) are introduced relatively recently. Typically, two distinct ways for printing FET’s from inorganic materials are considered. When solution-processed inorganic oxide semiconductors are prepared from metal precursors satisfactory performance has only been achieved for technically unacceptable process temperatures, i.e., beyond the glass transition temperature of commonly used, inexpensive (polymer, cellulose) substrates. Conversely, devices made from inorganic nanoparticles have suffered from inefficient gating due to the large interface roughness between the nanoparticulate film and the gate insulator. In contrast, we present a novel approach involving electrolyte-gating that can overcome some of the above mentioned shortcomings. The advantage of electrolyte gating for the printed oxide FETs will be demonstrated for both the relatively porous nanoparticulate channel and the precursor-based dense channel devices. While, in the first approach, for the first time, the process temperature of solution processed/printed inorganic FETs has been successfully reduced down to room temperature, simultaneously demonstrating one order of magnitude superior device mobility compared to the printed OFETs, the second approach yields unprecedented device performance and large signal gain for the oxide CMOS logics. The excellent electrical characteristics of electrolyte-gated transistors results from an atomically smooth and highly conformal interface between the oxide semiconductor and the electrolytic insulators. Furthermore, it will be shown that the speed of the electrolyte-gated FETs may not be limited by the ionic conductivity of the composite solid polymer electrolyte (CSPE) used in our study, it is rather the printing resolution that determines the channel length and alongside the maximum attainable switching speed. High environmental stability and a large temperature range of operation will also be demonstrated.

Resume : Past studies have documented various implementations of direct laser writing on graphite oxide thin films, accounting for the latest developments in printing and lithographic techniques for patterning microscale supercapacitors. Further improvements in the technology of laser-scribed supercapacitors need to focus on scalability and form factor, if we are to scale down to the nanoscale. The present work expands on the theme of electrode geometry by investigating fractal, pseudo-fractal and spiral patterns that capitalize on scaling parameters to account for vastly increased internal lengths over finite surfaces. Fractals are generated by simple iterative algorithms that can result in space-filling curves, whose length-to-surface ratio increases with the number of iterations, pertaining to a fractal dimension that converges at two at the upper bound - for a unit Euclidean dimension of the fractal curve. We examine the implications of design (stacking and orientation) and the effects of resolution (dictated by the laser operating parameters) on the geometrical properties of the imprinted electrodes, the electrochemical properties of the supercapacitor and the scaling potential of the device towards the nanoscale.

Resume : Among the energy storage devices, tremendous research on Electrochemical Capacitors has been conducted since they combine both high energy and power density. The problem is the relatively low energy density compared to batteries and increasing electrode’s specific capacitance is the best way to enhance energy density. As the active material of the electrode, manganese oxide was chosen because of its low cost, environmentally friendliness and natural abundance. In this study, electrodeposition was used to synthesize manganese oxides onto two different substrates (nickel mesh and ITO coated glass) under hydrothermal conditions. These electrodes were characterized by XRD, FE-SEM, Raman, XPS, Cyclic Voltammetry (CV), Galvanostatic Charge Discharge (CD), and Electrochemical Impedance Spectroscopy. The electrodes cathodically electrodeposited onto nickel mesh in hydrothermal conditions showed excellent pseudocapacitative behavior in CV and CD by reaching 386.7F/g at scan rate of 2mV/s, and 438.6F/g at current density of 0.1mA/cm2, respectively. Whereas the electrodes anodically electrodeposited onto ITO substrates attained 95.5F/g at scan rate of 2mV/s in CV and 115F/g at current density of 0.2mA/cm2 in CD, respectively. The films deposited onto nickel mesh and ITO had 67.0% and 90.5% capacitance retention after 1000 cycles of CV, respectively. Herein, we have demonstrated the pseudocapacitative performance due to 3D porous network obtained in hydrothermal conditions.

Resume : Highly flexible, conducting thin polyethylenedioxythiophene (PEDOT) is made from in-situ polymerizing them on a paper. The sheet resistance obtained is 3 Ω for 40µm thick paper in which PEDOT shows a conductivity of 390 S/cm. Uniform and conducting PEDOT is formed due the slow polymerization on the hydrophilic paper substrate. The paper is used for making highly flexible supercapacitor and showing 140 F/g for PEDOT in flexible as well as twisted conditions. Excellent stability also found during continues charge-discharge 1000 cycles. 3 cells are patterned in single paper to form in series to achieve a potential of 3V for higher energy requirements. The derived paper also used as counter electrode in DSSC’s instead of TCO and Pt showing excellent conversion efficiency of 4.5 %.