The gravure printing technique is currently under investigation as a possible method for the roll-to-roll production of OLEDs in the 6th framework EU funded project entitled ROLLED - "Roll-to-roll manufacturing technology for flexible OLED devices and arbitrary size and shape displays". The objective in the project is to fabricate an entire OLED structure by using roll-to-roll manufacturing methods and to examine, how the commercial production could be set up and integrated into an existing printing process. In order to attain a roll-to-roll compatibility, all the materials, inks and device structures need to be suitable for printing. Since, such OLED device structures are very sensitive to moisture and oxygen, high barrier materials to be applied as wet chemical coatings on transparent polymer films such as PET by common roll-to-roll coating techniques have been investigated. The barrier films on their respective substrates act as front and back side encapsulation materials, where the front side encapsulation material is to be used as a transparent and flexible substrate for OLED fabrication. The transmission rates to be achieved for both front and back side encapsulation for oxygen and water vapour are 5 mg m-2day-1 (corresponding to 7 cm3m-2day-1 for O2). In this paper, we show how light-emitting devices manufactured by gravure printing operate compared to the ones manufactured by traditional methods. Furthermore, we present recent results on the development of ITO nanoparticle coatings, cathode inks and flexible barrier materials.
Transparent conducting coatings and patterns of ITO (indium tin oxide) were deposited by a direct gravure printing on PET foils using nanoparticle-based UV-curable inks. Solid areas with thicknesses ranging between 300 and >1000 nm were obtained by varying the ink composition (e.g. ITO content, solvents) and fundamental parameters of the printing plate such as the line density. The best ITO coating patterns showed a sheet resistance of 3 to 10 kΩ□ and a transmission of up to 88 % with a haze of less than 1 %. One of the most crucial steps during film formation is the drying of the wet film as it changes the rheology and polarity of the ink and in consequence decisively influences the film formation. Typical fields of application of the gravure-printed ITO patterned electrodes include smart windows, flexible displays and printed electronics.
The impedance spectra of non-sensitized and Ru(II)-sensitized Nb2O5 nanoporous coatings have been measured in the dark and under solar illumination using an electroactive electrolyte. All the Nyquist plots consist of a high and a low frequency depressed semicircle. The results have been modeled and fitted by an equivalent electric circuit consisting of a resistor Rs in the series with two parallel RC circuits containing both a constant phase element (CPE). The resistor Rs describes the total resistance of the electrolyte and conducting electrodes (SnO2:F). The high frequency semicircule (f<1kHz) describes the capacitance and resistance of the semiconducting materials (grain boundaries and interfaces). The low frequency cycle (f < 1 kHz) is related to the formation of a double charge layer capacitance at the nanoparticle/electrolyte interface and a charge transfer resistance. Both values are strongly dependant of the experimental conditions, in particular of the applied potential and the state of illumination. The evolution of the electric elements is presented and discussed. It is shown in particular that the measurements in the dark cannot be directly compared to those under illumination as in teh latter all the Nb2O5-film is accessed.
The conventional dip coating techinques, as it is used for flat surfaces, cannot be applied to deposit homogeneous coatings in optical quality inside tubes. The resulting coatings exhibit large variations in thickness and roughness over the length of the tube and show a morphology with a network of cracks. The main reason for these problems seem to be a delayed and restricted solvent evaporation due to the impediment of a laminar flow and the progressive saturation of the atmosphere in the tube. A modified dip coating technique was therefore developed to allow forced flow conditions inside the tubes by an additional exhausting tube. By means of this modification transparent conducting coatings of sol-gel SnO2:Sb (antimony-doped tin oxide - ATO) could be deposited on both sides of borosilicate glass tubes (300 mm, inner diamters down to 11 mm) with excellent thickness uniformity and low roughness (Ra≈1nm). After a heat treatment at temperatures up to 500°C, the prepared ATO coatings are mechanically stable and highly transparent (>85% transmission) with a sheet resistance down to 10 kΩ. The coatings can be used as electrodes for electronic devices and electrical heaters or to give antistatic properties to the substrate.
CeO2-TiO2 sol-gel coatings are well known as Li-ion storage electrode in electrochromic (EC) devices of the form glass/ TE /WO3/ electrolyte/ CeO2-TiO2/ TE/ glass (TE: transparent electrode, e.g. SnO2:F, FTO). The charge capacity of the CeO2-TiO2 coating is a limiting factor to get a high coloration intensity of such devices. In order to improve the charge capacity of these electrodes, new routes for the preparation of thick porous CeO2-TiO2 sol-gel layers were tested.
One route was the preparation of thick porous TiO2 coatings on a conducting glass support (FTO) using a solution of colloidal TiO2 particles. After heat treatment at temperatures up to 550°C the coatings were soaked in a solution of a cerium-IV (Ce(NH4)2(NO3)6) or a cerium-III salt (Ce(NO3)3 6H2O) and heat treated again. Another route was the preparation of sols by mixing a solution of the cerium-IV or cerium-III salt or a colloidal CeO2-sol with the colloidal solution of TiO2. After dip coating on FTO-glass the coatings were also heat treated at temperatures up to 500°C. ALl these coatings were studied electrochemically in 1 M LiC1O4 in propylene carbonate electrolyte. Although thick porous single coatings could be obtained, typically 450 nm for TiO2 and 600 nm for cerium-titanium oxide, the intercalated and deintercalcated Li+ charges remain small and lie in the range of 2 mC/cm2 to 3 mC/cm2. The reasons for such low charge capacity is discussed.
The paper focuses on a systematic study of the influence of water on the electrochemical and optical properties of CeO2-TiO2 amd WO3 sol-gel coatings as well as devices made with these layers. The coatings were studied electrochemically in 1 M LiC1O4 in propylene carbonate electrolyte with water content up to 3 wt%. The intercalculated and deintercalated charge was measured during Cyclic Voltammetry (CV) and Chronoamperometric (CA) cycles up to 500 cycles (TiO2-CeO2) and 7000 cycles (WO3). For CeO2-TiO2 it was found to increase from 3mC/cm2 (dry electrolyte) up to 11 mC/cm2 (3 wt% water). This increase is important for the coloration of EC-devices because the charge capacity of this counter electrode is known to be a limiting factor for the transmission change of the EC-devices. For WO3 coatings, the transmission change (Tcolored-Tbleached)is higher in wet electrolytes (1 wt% water) than dry electrolyte and above all remains constant (74%). These improvements are essentially due to an increase of the kinetics of the intercalation and deintercalation of Li+ ions. The electro-optical behavior of solid state EC-devices with and without incorporation of water in the solid electrolyte measured up to 500000 CA cycles is also presented and discussed.
In this work we present result on the preparation of planar waveguides based on HfO2 and HfO2-SiO$2). Stable sols containing europium and erbium doped HfO2 nanoparticles have been prepared and characterized. The nanosized sol was either deposited on quartz substrates or embedded in (3-glycidoxipropil)trimethoxisilane used as a hybrid host for posterior deposition. The refractive index dispersion and luminescence characteristics were determined for the resulting HfO2 films. The optical parameters of the waveguides such as refractive index, thickness and propagation losses were measured for the hybrid composite. The planar waveguides present thickness of a few micra and support well confined propagating modes.
Electrochromic windows using Nb2O5, Nb2O5- Li2O, Nb2O5-TiO2 and Nb2O5-MoO3 sol-gel layers as cathodic coloring electrodes and CeO2- TiO2 as anodic ion-storage electrode have been assembled and tested. In agreement with a theoretical estimation, the switching behavior of these devices is strongly dependent on the difference between the internal potential of the electrochromic and the ion-storage electrodes. Only the configuration using a Nb2O5-MoO3 layer enters in consideration for long-time switching within the safe voltage range -2.5 V < V < +2.5 V. All other cells require larger negative potentials. The 8 X 4 cm2 devices built with Nb2O5:Mo are transparent with a slightly yellow color in the bleached state (+2.5 V) and dark gray in the colored state (-2.5 V). For a fixed thickness of the EC electrode (180 nm) the photopic transmittance increases with the thickness of the ion storage layer dIS in agreement with the model prediction. It reaches a maximum of ca. 0.3, corresponding to a Li+ intercalated charge density of 18 mC/cm2. For dIS is congruent to 950 nm, such devices are stable up to at least 12,000 cycles under potentiostatic cycling between +/- 2.5 V, 120 s.
Two-layer coatings of Nb2O5 and niobia doped with tin (up to 15 at%), zirconium (up to 15 at%), lithium (up to 15 at%), titanium (up to 30 at%) and molybdenum (up to 30 at%) have been prepared by the sol-gel process and deposited by dip-coating on conductive ITO coated glass (R(open square) equals 10 (Omega) (open square)). They have been sintered in air between 450 degree(s)C and 600 degree(s)C. Their structure, morphology, electrochemical and optical properties (measured in a liquid electrolyte (1 M LiClO4 in PC)) are reported. All systems exhibit electrochromic properties. Those are essentially determined by the structure and the morphology of the layers which can be changed in a controlled manner by the amount of doping and the sintering temperature. After Li+-insertion all amorphous coatings present a brown color, a blue one when the structure is hexagonal and a gray one when the structure is orthorhombic (pure, titanium or molybdenum doped niobia) or monoclinic (Li-doped niobia). Samples with two different structures color either blue or gray.
Single and multilayer sol-gel coatings of transparent antimony-doped tin oxide (SnO2:Sb) have been prepared on borosilicate and fused silica substrates using either a 5 mole% SbCl3 doped 0.5 M solution of SnCl2(OAc)2 in ethanol or a water suspension of crystalline Sb-doped tin oxide nanoparticles. The nanoscale morphology and the electrical parameters of the layers have been determined after different firing procedures and heating rates varying from 0.2 to 4300 K/s obtained either in a furnace or by cw carbon- dioxide laser irradiation. For a given sintering temperature (approximately 540 degrees Celsius) a slow heating process in furnace leads to porous, homogeneous single and multilayers consisting of small crystallites. They present a high resistivity of about (rho) equals 4 multiplied by 10-2 (Omega) cm. With increasing heating rate the layers become denser with larger crystallites and the resistivity value decrease down to approximately 7 X 10-3 (Omega) cm for 4300 K/s (carbon-dioxide laser sintering). It is proposed that the densification of the coatings is determined by a competition between nucleation at low temperatures and the growth of the crystallites at high temperatures.
Nb2O5 prepared by a sol-gel process in the form of coatings are new materials which have outstanding properties: (1) The coatings present electrochromic properties and exhibit a blue coloration under Li+ insertion with 100% reversible variation of the optical transmission in the visible and near infrared range between 80% and 20% and have a high chemical stability (tested up to 2000 cycles). (2) They are semiconductors and present a photoelectric effect when illuminating in the UV region ((lambda) < 360 nm). These films are therefore very promising for use in electrochromic devices and as electrodes for photoelectrochemical purposes and the development of nanocrystalline solar cells.
Nb2O5 prepared by a sol-gel process in form of coatings and aerogels are new materials which present interesting properties: (a) the coatings present electrochromic properties and exhibit a blue coloration under Li+ insertion with 100% reversible variation of the optical transmission in the visible and near infrared range between 80% and 20% and have a high chemical stability (tested up to 2000 cycles), (b) they are semiconductor and present a photoelectric effect when illuminating in the UV region ((lambda) < 360 nm). These films are therefore very promising to be used in electrochromic devices, as electrodes for photoelectrochemical purpose and the development of nanocrystalline solar cell, and (c) when prepared in aerogel form, the high BET surface area of the powders is a promising asset to use these new materials for catalytic purposes.
Electrochromic devices have potential applications in architectural and automotive fields to regulate the transmission and reflection of the radiant energy. Preliminary characteristics of an all sol-gel window with the configuration glass\ITO\WO3\TiO2\TiO2-CeO2\ITO\glass is presented, and compared with another window where WO3 was deposited by evaporation. We also present preliminary data of a sol-gel Nb2O5 electrochromic layer as well as a thorough evaluation of the properties of a sol-gel TiO2-CeO2 ion storage coating where electrochemical properties are studied as a function of various parameters (thickness, heat treatment, etc.).
Thin layers of mixed oxides CeO2 - TiO2 and CeO2 - ZrO2 with molar ratio 1:1 have been prepared by sol-gel process using the dip-coating technique. The precursor sols consist of a mixture of nitrate ammonium cerium salt [Ce(NH4)2 (NO3)6], zirconium propoxide or tetraisopropyl orthotitanate, and ethanol. The thickness of the multilayer films is typically 300 nm after densification at 450 degree(s)C. Their use as transparent H+ and Li+ ions storage electrode (counter electrode) for electrochromic devices is studied by electrochemical voltammetry, impedance, and optical spectroscopies. They typically have an optical transmission of 80% in the visible range, do not color after ions insertion, and show good electrochemical stability. These layers also have been tested in complete, all solid state cell having the configuration glass/ITO/EC/electrolyte/CeO2 - TiO2/ITO/glass. The electrochromic layer (EC) was either WO3 or polytoluidine. The protonic electrolyte was a cellulose-polyacetate polymer and the lithium electrolyte was PEO-LiN(SO2CF3)2. Their electrochemical, optical, and lifetime performances are reported.
Single and multilayer sol-gel thin films of Ti02-PbO, Ti02-Bi203 and Ti02-CeO2
composition were deposited on glasses using the dip coating technique. The precursors
included Ti(OPri)4 chemically modified by acetyl acetone and diluted in PriOH and
sols of Pb(OAc)2, Bi(NO3)3 5H20 diluted in acetic acid. The Ti02-Ce02 sol was prepared by mixing Ce NH2 (NO3 )6 in ethanol and then adding Ti (O-iso-C3H7 )4. Structure
texture and homogeneity of their main constituants was established by XRD, XPS,SIMS
and SEM-EDX techniques as a function of heat treatments.
Sol-gel cerium - titanium oxide layers present potential application as transparent
counter-electrode (ion storage layer) in electrochromic windows and mirrors
using lithium conducting electrolyte and W03 electrochromic coating. The precursor
sol, prepared by mixing Ti(OPri)4 and Ce(N03)6 (NH4)2 in ethanol, is initially dark
red and becomes transparent after a few days aging indicating the presence of Ce3
complexes. The layers have been obtained by dip coating technique and heat treated
at 4509C during 15 minutes.They have been characterized by XRD, SIMS, optical absorption
and electrochemical techniques; it is shown that the electrochemical reaction
corresponds to a reversible insertion-extraction oflithium ions within a Ti02
amorphous film containing small Ce02 crystallites. At low sweep frequencies the
process is controlled by a diffusion mechanism (DLi ≊ 6.4 lOl2cm2/s at 259C). Characterizations
of an all solid state electrochromic window/glass/JTO/ W03/ POE-Li N
(502 CF3)2 1 Ti02 - Ce02 I ITO I glass I are also presented.
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