Proceedings Volume 1016

Optical Materials Technology for Energy Efficiency and Solar Energy Conversion VII

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Proceedings Volume 1016

Optical Materials Technology for Energy Efficiency and Solar Energy Conversion VII

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Volume Details

Date Published: 6 March 1989
Contents: 1 Sessions, 40 Papers, 0 Presentations
Conference: 1988 International Congress on Optical Science and Engineering 1988
Volume Number: 1016

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Table of Contents

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"Nonlinear Optical Effects In Organic Molecules And Polymers -Theory, Measurements And Devices"
Paras N. Prasad
Nonlinear optical processes have received international attention because of their importance in optical signal processing and computing. Organic molecules and polymers form an important class of nonlinear optical material. This paper discusses the basic relevant concepts in the newly emerging area of nonlinear optical processes in organic molecules and polymers. It provides the results of a comprehensive study in this area being conducted in my laboratory. This study includes theoretical calculation of microscopic optical nonlinearity; design, molecular engineering and synthesis of novel organic structures; experimental investigation of nonlinear processes using picosecond and femtosecond laser pulses; and the study of device phenomena. At the end, the current status of this field and future directions of research are discussed.
Characterization And Performances Of WO3 Anodic Films In Electrochromic Devices
Pierre Delichere, Polycarpos Falaras, Anne Hugot-Le Goff
Anodic oxydation of tungsten in sulfuric solutions gives thin films with the uncommon hexagonal structure. When the films have grown at low voltages, the hexagonal channels are distorted with a lack of long-distance order. These films present very interesting electrochromic properties, over all when the coloration is done in an organic medium with insertion of Li+. The coloration absorption band is then centered in the visible wavelengths range. The optical indices in the bleached and colored states are obtained by a sophisticated statistical method of reflectance curves fitting. The kinetics of coloration/bleaching are studied by Multichannel Optical Analysis (OMA), and the lattice perturbations created by the insertion are followed by Raman spectroscopy. The electrical and optical reversibilities are perfect after more than 2x106 coloration/ bleaching cycles and, in reason of the position of the coloration band in the visible wavelengths range, the coloration yield is high.
Temperature Dependent Optical Absorption In Electrochromic Tungsten And Molybdenum Oxides
J. S. E. M. Svensson, D. K. Benson, C. E. Tracy
The optical absorption in thin films of electrochromic oxides of tungsten and molybdenum has over the solar spectral range at temperatures from about 100 K to 365 K. The films were depos at room temperature by RF plasma enhanced chemical vapor deposition and then converted to th injection compounds LixWO3 and LixMoO3 by reaction with n-butyl lithium in a water-free an environment to duplicate their colored elctrochromic state. The optical absorption in the LixW03 films consists of a broad peak centered near 1.7 eV. The Absorption in the LixMo03 films consists of a broad band centered near 1.3 eV, a temperature dependent Urbach band edge near 3 eV plus a very broad tailing absorption connecting these two features. The optical absorption in these materials in interpreted in terms of a small polaron absorption process and temperature dependent structual disorder.
Optical And Electrical Properties Of Amorphous LixWO3 Films
Junichi Nagai
Amorphous thin films of Li.W03 are investigated to de,termine optical and electrical properties together with their photoelectrochemical behaviors. Optical spectra of the colorations in amorphous LixW03 films were consistent with EMT (Effective Medium Theory) on the assumption that spheres of Drude-like metallic tungsten bronzes (LiW03) and tungsten oxides(WO3) are randomly mixed. EMT also predicts the percolation threshold at x>xc where insulator-metal transition of dc-conductivity occurs at x>xc. These features give new insight on the nature of optical and electrical properties of amorphous tungsten bronzes. DC-conductivity of LixW03 was strongly dependent on the deposition conditions of the W03 films. Dense W03 film deposited under higher vacuum showed higher dc-conductivity and was less sensitive to UV light. Photoelectrochemical coloring of dense WO3 was lower than porous( and highly disordered) W03. This phenomena could be primarily ascribed to the high electron mobility so that it can be easy to recombine the photogenarated electrons and holes. Consequently it was proved that the electronic states in disordered amorphous WO3 structure played important roles on photoelectrochemistry of W03/electrolyte interface.
Electrochromic Behavior Of Hydrated Nickel-Oxide Films Prepared By Rf Sputtering
Shoji Yamada, Tatsuo Yoshioka, Masatoshi Miyashita, et al.
Nickel-oxide films are prepared by rf sputtering, and their electrochromic properties are investigated. Sputtering atmosphere used is an Ar-02 mixture. The films obtained can be electrochemically colored and bleached reversibly in an 1-N NaOH solution, and are colored anodically. Colored film is dark brown, and bleached one is colorless and transparent. Maximum coloration efficiency obtained is 36 cm2/C at a wavelength of 640 nm. Coloration efficiency depends on total pressure of sputtering atmosphere, but is independent of oxygen content in the atmosphere.
Towards An All-Solid-State Smart Window: Electrochromic Coatings And Polymer Ion Conductors
A. M. Andersson, C. G. Granqvist, J. R. Stevens
We introduce a laminate two-layer approach to electrochromic smart windows. It incorporates two glass panes, each having a two-layer coating, laminated by a transparent adhesive solid polymer electrolyte. Each coating has a transparent conducting base layer and a top layer of an ion insertion compound. We made ion-insertion layers of W03, V205 and NiOx by sputtering and evaporation. These layers were characterized by cyclic voltammetry in liquid electrolytes and by spectrophotometry. Our novel electrolyte comprised a methyl methacrylate network incorporating poly (propylene glycol) complexed with lithium perchlorate. Initial optical data are reported for laminated devices having an "active" electrochromic LixWO3 layer and a "passive" LixV205 counter electrode.
Investigation To The Electrochromism Of a-W03 In Single Films And All-Solid-State Devices
Klaus Bange, Reimund Keller, Wolfgang Wagner, et al.
Amorphous films of W03, colored and bleached in electrochemical and in H2-containing gas experiments, were characterized by optical and electrical methods, and the hydrogen content CH was measured by nuclear reaction analysis (NRA) . In the gas phase experiments the kinetics of the coloration process and the optical density show a strong dependence on the H2 partial pressure. The change in CH connected with the electrochromic effect of W03 could be determined by comparing NRA results with optical data. Distinct differences between samples colored electrochemically and by gas phase experiments were observed. The electrochromism of all-solid-state devices was studied also and is compared with the findings on single a-W03 layers.
Electrochromism In Sputtered Vanadium Pentoxide
Stuart F. Cogan, Nguyet M. Nguyen, Stephen J. Perrotti, et al.
Optical characterization from 300-1600 nm shows that lithium intercalated V205 exhibits cathodic coloration in the near-infrared and anodic coloration on the blue and near-UV (<450 nm). The anodic coloration is associated with a shift in the optical gap to shorter wavelengths. The spectral coloration efficiency in the near-IR (600-1600 nm) is quite low, typically less than 15 cm2/C. Absorption coefficients and solar transmittance of LixV2O5 are reported as a function of Li concentration (x).
Characterization Of Nickel Oxide Electrochromic Films
Niall R Lynam, Hamid R. Habibi
NicNickel oxide (Ni0) thin films deposited by electron-beam evaporation were studied using Fourier Transform Infrared Spectroscopy(FTIR), Rutherford Backscattering Spectroscopy (RBS), and spectrophotometry. Evaporated nickel oxide films ehibited good electrochromic behavior and a coloring efficiency of 30.1 cm2/C coating was 4.41 g/cc and stoichiometry was Nio1.4; both established by RBS. The infrared spectral properties of the as-deposited film was consistent with the structure being Ni0x. After repetitive electrochromic cycling in 1N KOH, the film changed to a structure more similiar to the hydrated nickel hydoxide films previously reported in the literature.
'First-Generation' Model Of The Indium Tin Oxide-Indium Phosphide Solar Cell.
S. H. Pulko, N. M. Pearsall, J. Winckler
Solar cells based on ITO-InP are currently being studied extensively for application to space satellite power systems. The enthusiam for the ITO-InP cell results fromtheir low degradation under exposure to radiation typical of space environments. It is believed that the performance of the cell is profoundly influenced by the precise nature of the various layers and interfaces involved. A simple numerical model has been developed for prediction of performance of the ITO-InP cell. The iterative routine developed to date is based on the Transmission Line Matrix (TLM) method of numerical analysis. This method has the advantages of being physical in its concepts, one-step, explicit, and unconditionally stable. The model deals with the creation, diffusion and recombination of excess carriers on both sides of the junction, together with separationof carriers in and near the junction region. The ultimate aim of the model is to predict the optimum precise cell structure. Here the model has been used to predict the short circuit output of a cell in reponce to incident light of various spectral compositions. The results of the simulations are compared with experimentally derived values.
Dependence Of The Sprayed Cds Photoelectrode Behaviour Upon The Pyrolysis Atmosphere
Jean Ebothe
In dark conditions as well in light , the behaviour of CdS films sprayed under the same conditions varies with the atmosphere used during the process of pyrAolys,ls . The study of the current-voltage ( J-E ) characteristics , drawn from a CdS / (Sx-2/S-2) liquid schottky barrier reveals some changes in the potential value related to the onset dark-current . Analysis of the film's photoresponse shows that a specific behaviour of the photocurrent Jp parameter plotted against E is obtained with each the pyrolyzed samples. The film composition and its surface texture appear to provide the most likely explanation for this phenomenon . Regenerative cell made of CdS photoanode is highly efficient with N2 -treated samples . The lower power conversion corresponds to films pyrolyzed in He atmosphere . The use of H2 as described in this paper , leads to an interfacial instability
Radiative Cooling With Pigmented Polyethylene Foils
Gunnar A. Niklasson, Tord S. Eriksson
Polyethylene foils containing a nonabsorbing pigment can be suitable for radiative cooling because of their high reflectance of solar radiation combined with a high transmittance in the atmospheric window region in the thermal infrared. We have studied the optical properties in the wavelength range 0.3-50 μm of extruded polyethylene foils containing various amounts of 0.23 μm diameter Ti02 particles. It appears that the foils can prevent heating of an underlying material, even when directly illuminated by the sun. The total transmittance and reflectance of the Ti02-polyethylene foils were compared with multiple scattering calculations. Lorenz-Mie theory was used to model the scattering and absorption of a single TiO2 sphere. The single scattering parameters were then introduced into the four flux theory, by which the transmittance and reflectance were calculated. We find a satisfactory agreement between theory and experiments in most cases.
Effects Of Deposition Conditions On The Conductivity And Photoconductivity Of rf Sputtered a-Ge:H and a-Gel_xCx:H
J. Shinar, H.-S. Wu
a-Ge:H and a-Ge1_xCx:H films were prepared by rf sputtering of 4 polycrystalline Ge target in a background vacuum ~4x10-7 at rf power 5O<P<600W (0.27-3.3W/cm2), target-to-substrate distances 1≤d≤7", varying partial pressures of Ar, H2, and propane, and flow rates f. The dark conductivity σd and photoconductivity σph increase with increasing d up to 6", apparently due to thermalization of the ions and Ge atoms bombarding the growing film. σd and σph also increase at f<lsccm, due to the incorporation of oxygen and other impurities from the background residual atmosphere. At the optimal values of f and d, σph and ad increase with P. Although all increases in aph are significant relative to those bf previously reported rf sputtered a-Ge:H, the values arb still much lower than those of glow discharge deposited films, which are in turn lower than those of rf magnetron sputtered films grown in high vacuum (<5x10-9 torr). The similar behavior of σd and σph, the relatively long lifetimes of the carriers ι - 0.8ms but very low mobility μ 5x10-7 cm4/Vsec and low subgap absorption suggest that the transport is dominated by defects near the mobility edge. The failure of a biased screen placed above the substrate to affect σph suggests that the high values of aph reported using rf magnetron sputtering are at least in part due to the improved background vacuum. Although incorporation of significant amounts of carbon sharply reduces σph, a small content, sufficient to increase the optical gap to 1.3-1.4eV, has little effect on σph.
On Plasma-Film Interactions In rf Sputtered a-Si:H and a-Ge:H
J. Shinar, S. Mitra, H.-S. Wu
The extension of a previous investigation of plasma-film interactions in a-Si:H rf sputtered in He/H2, Ar/H2, and Xe/H2, at a target-to-substrate gap d = 1", to include higher gaps and rf sputtered a-Ge:H, is described and discussed. As expected, the interactions are weakened at d≥2", and the resulting films appear to be more relaxed than at d = 1". The analysis of the IR stretch and bending mode bands in annealed films sheds new light on relaxation processes and the relation between the stretch mode structure and the optoelectronic quality of a-Si:H films. In a-Ge:H, the deposition rate increases with increasing rf power in a manner similar to that in a-Si:H, and decreases exponentially with increasing d. The dihydride and trihydride bonding configuration densities decrease and the photoconductivity increases with increasing rf power (as in a-Si:H) and increasing d. These phenomena are discussed in relation to the dependence of the plasma-film interactions on these parameters.
A SIMS And IR Study Of Hydrogen Motion And Relaxation Processes In rf Sputtered a-Si:H and a-Ge:H
J. Shinar, R. Shinar, S. Mitra, et al.
The motion of hydrogen and deuterium in undoped multilayers of rf sputter deposited a-Si:H, was recently studied by IR absorption and SIMS profiling of the annealed films. It was shown that in films containing significant SiH2 and SiH3 bonding configurations, as determined by IR absorption spectra and usually associated with microvoids, the connected network providing the H diffusion path is disrupted below the percolation threshold. Measurements of the index of refraction of these films were shown to be indeed consistent with this assignment of SiH2 and SiH3 configurations. In films containing essentially only isolated monohydride type bonding, the smearing of the interfaces exhibited a power law time dependence of the diffusion constant, D ∝ t-∝. Yet the value of ∝, 0.75±0.1, is considerably higher than that found by Kakalios, Street, and coworkers on p- and n-doped a-Si:H films deposited by rf glow discharge decomposition of silane and correlated with electronic transport decay processes. The results thus introduce a complicating factor in the suggested correlation between the hydrogen motion and the formation and annealing of light induced defects which degrade the photoconductivity of these films. The diffusion in a-Ge:H appears to be qualitatively similar, but considerably faster. Yet annealing processes in a-Ge:H apparently introduce additional complicating factors which may be related to the poor device quality of a-Ge:H based alloys.
Optical And Electrical Studies Of Electrochromic Polycrystalline WO3 And MoO3 Thin Films Prepared By CVD
Albert Donnadieu, Dimitrios Davazoglou, Ahmed Abdellaoui
The electrical and optical properties of polycrystalline WO3 and MoO3, thin films prepared by Chemical Vapor Deposition were determined. The electrochromic characteristics of these films were investigated using protons as inserting ions. The influence of preparative conditions on optical, electrical and electrochromic properties is presented. An experimental study of optical spectra of colored or bleached WO3 samples is shown and discussed using a free electron scattering model.
Optical Properties Of Electrochromic Nickel Oxide Devices Utilizing A Polymeric Electrolyte
A. Pennisi, C. M. Lampert
We describe the preparation and optical analysis of experimental electrochromic switching devices. These devices utilize hydrated nickel oxide as the electrochromic material, with fluorine-doped tin oxide as the counter-electrode layer and polymeric quarternary ammonium hydroxide as the electrolyte. The polymer electrolyte was synthesized from an quaternary ammonium chloride polymer by ion exchange between and OH- ions. The exchange was accomplished by using either an ion exchange resin or membrane dialysis. Voltammetric and optical measurements are compared for devices using the two polymeric electrolytes. The best device performance was achieved using the dialysis-hydroxylated polymer. We investigated the optical switching properties of devices under static and dynamic conditions. We also observed that adding lithium atoms in the nickel oxide layer improves the performance of the devices by reducing the switching time and increasing the transmittance modulation range. Our best device to date (not yet optimized) showed the integrated photopic transmittance (Tp) to be Tp(bleached)=0.70 and Tp(colored)=0.21. The corresponding solar transmittance (Ts) was Ts(bleached)=0.50, Ts(colored)=0.23. A device without the polymer electrolyte had optical properties of Ts(empty)=0.44 and Tp(empty)=0.57.
Metal Oxyfluoride Coatings For Energy-Efficient Windows
K. A. Khan, S. J. Jiang, Z.-C. Jin, et al.
Indium-tin-oxyfluoride films were made by dc magnetron sputtering in Ar + 02 + CF4 at a rate of 1.2 nms-1. A wide-bandgap material, with refractive index ~1.5 and good durability, was produced. It is very well suited for antireflecting indium-tin-oxide coatings. Vanadium-oxyfluoride films were made by rf magnetron sputtering in Ar + 02 + CF4. Thermo-chromism was studied by electrical and spectral optical measurements. Fluorination was found to yield a significant decrease of the luminous absorptance.
Fatigue-Resistant Photochromic Plastics
Nori Y. C. Chu
The optical switching properties of a photochromic spirooxazine compound in retrofit polyester film and in poly (vinyl butyral) have been measured. The light fatigue resistance of these two optical switching elements were tested by an accelerated method. The length of photochromic activity of the optical switching elements can be improved by various organonickel and hindered amine light stabilizers. The effectiveness and optimal concentration for each light stabilizer in these host materials has been determined. These two types of light stabilizers act synergistically in improving the light fatigue resistance of the optical switching elements significantly.
Evaporated VOx Thin Films
T. Stapinski, E. Leja
VOx thin films on glass were obtained by thermal evaporation of V205, powder. The structural investigations were carried out with the use of X-ray diffractometer. The electrical properties of the film were examined by means of temperature measurements of resistivity for the samples heat-treated in various conditions. Optical transmission and reflection spectra of VOX films of various composition showed the influence of the heat treatment.
Electrochromism In Nickel Oxide Films
Pierre Delichere, Suzanne Joiret, Anne Hugot-Le Goff, et al.
Electrochromism effect in the nickel compounds has excited a recent interest. Here, the properties of films obtained by two different preparation techniques : anodic oxidation and e- beam evaporation are studied and compared. The kinetics of colour changes are studied by multichannel optical analysis (OMA), in view to determine if the optical response of this new system can really become usable. Raman spectroscopy is used to determine the chemical and crystallographic natures of the colored and bleached states. In particular, this technique has proved that, contrarily to the hypothesis generally done up to now, the bleached state is Ni0x and not an hydrated state as Ni(OH)2.
Properties Of Electrochromic Nickel Oxide Coatings Produced By Reactive Evaporation
Klaus Bange, Friedrich G.K. Baucke, Bernd Metz
Single films of nickel oxide deposited by reactive evaporation and all-solid-state devices (ASSDs) containing such films have been investigated. The as-deposited nickel oxide films were analysed by standard surface and thin film-sensitive methods (AES, ESCA, RBS, NRA), and the findings were correlated with deposition parameters. The electrochromism of single layers was characterized by cyclic voltammetry and photospectrometry and compared with optical and electrical data of electrochromic all-solid-state devices.
The Potential Waveform In The Optical Switching Of Hydrated Nickel Oxide
Agostino Pennisi, Francesca Simone
In this work it has been analyzed the transient of the current flowing in an electrochemical cell in which the working electrode is constituted by a hydrated nickel oxide layer. The analysis of the current, utilizing a model for the diffusion mechanism of protons into the electrochromic material, allows to determine the diffusion coefficient for the oxidation and reduction processes. It ha5been observed that the diffusion coefficient for the bleaching process is larger than that one of the oxidation process. This fact could explain why the transition to the coloured phase occurs more rapidly than the inverse transition and with an apparently more simple function. Attempts have been made to put in correlation the eventual formation of metastable cristallographyc phases, observed by the X-rays diffraction technique, submitting the sample to a periodic potential, with a square waveform, for several cycles.
Evaluation And Control Of The Optical And Thermal Properties Of Low-Emissivity Coatings
Joachim Szczyrbowski, Anton Dietrich
The optical and thermal characteristics of silver based interference type low-emissivity coatings for architectural applications are evaluated using basic theoretical thin film relations. It is concluded that in order to characterize the films it is sufficient to measure the sheet resistivity and the light transmission of either the partial layer system glass/oxide/Ag or even the complete layer stack glass/oxide/Ag/oxide in the visible and near infrared region. For our investigation a layer system, consisting of glass/Sn02/Ag/Sn02, was used. The transmission T measured in the visible and near infrared region can be described by the relation 1/T = S/w2 + CT, where the slope S is a known function of the plasma frequency and of the thickness d of the silver layer. The dependence of S on both the optical constants and the thickness of the used oxide films is very weak and may be neglected. Thus, from the measured value of S the thickness of the silver .Layer can be obtained immediately. It is further shown that the so-called normal emissivity en of the considered film is given by en = 4*εo*c*Rm, thus, it is completely given by the measured sheet resistivity. The aesthetic appearance of the coated substrates is defined mainly by the thickness of the oxide films. Finally, it is shown how these investigations can be used in a continuous on-line quality control of the layer properties on large scale production machines.
System Design For High Rate Deposition Of Indium Oxide Solar Coatings
Alaric G. Spencer, Ronald P. Howson
Indium oxide solar coatings have been extensively investigated and the conditions for their small scale production well characterized. For widespread use of these solar coatings they must be produced at high rate over large areas. Reactive magnetron sputtering is a suitable process and problems in extending this process to larger areas and higher rates are discussed. This is illustrated by results from a 0.5m wide coater. In a typical commercial coater the process is unstable and so process control by plasma emission monitoring (PEM) is required to give a stable deposition process. To get good film uniformity the gas must be distributed by a gas manifold and there is some interplay between the gas manifold and the PEM control. Experimental results are given to adjust the manifold design for optimum control. On a small scale coater with inherent stability we have achieved a deposition rate of 8 nm/s for indium oxide (with a resistivity of 4 x 10-6ohm.m). In a larger unstable coater with PEM control we have produced deposition rates of 6 nm/s with a slightly degraded resistivity of 6 x 10-6 ohm.m.
Uncertainty Of Multiple Determination Of Optical Constants And Layer Thicknesses Of Thin Films Case Of TiO2/Ag/TiO2 Coating On Glass
Valery A. Koss, Abraham Belkind, David E. Aspnes, et al.
Multiply determined thicknesses of constituent layers and optical constants of Ag layer in Tio2/Ag/TiO2 coatings on glass are compared using regions of uncertainties for evaluated parameters. The measurements were done using spectroscopic ellipsometry and spectrophotometry. The regions of uncertainty for each evaluated parameter were obtained using a separately determined accuracy of measurements. It is shown that the actual structure of the coating is more complex than a three or five layer model.
Optical Properties Of A Silver Layer In ZnO/Ag/ZnO and TiO2/Ag/TiO2 Systems
Abraham Belkind, Valery A. Koss, Kazem Memarzadeh, et al.
Effective dielectric constants of the silver layers in three ZnO/Ag/ZnO and two Ti02/Ag/Ti02 coatings are determined using spectroscopic ellipsometry assuming a three-layer model for the coatings. Drude analysis of the data shows that the effective dielectric constants of silver in the red and near infrared regions are close to that of bulk silver, and both, bulk plasmon energy and core polarizability, depend on the layer thickness. Three peaks, at 3.8-3.9, 3.5-3.7 and 2.9-3.3 eV, are observed in the spectra of the imaginary part of the effective dielectric constant. Generation of surface plasmons, indicated by these peaks, is considered as a consequence of surface roughness which is not included in the original three-layer model. Analysis of data on transmittance and reflectance, measured at incidence angles of 45° and 60° , confirms surface plasmon generation.
Optical and electrical properties and long term stability of transparent, infrared-reflecting coatings on polymer substrates
V. Neveling, W. Muller, H. Pytlik
Transparent, infrared (IR) - reflective coatings are commenly used for energy saving because they protect against heat loss due to radiation through windows. Basic physics tells us that the degree of IR-reflection and the electrical conductance are positively correlated , therefore this type of films is also of interest in the field of electro-magnetic shielding, e.g. for data security or for the protection of operators at work-stations against irritation by ejected dust particles. We have prepared such films consisting of the coating sequence dielectric / silver / dielectric by magnetron sputtering on polymer substrates. We investigated their optical and electrical data after preparation and under the influence of corrosive atmosphere. As a further optional element of protection against film-degradation some of the coatings where overcoated with an organic lacquer of low IR-absorbtance. The details of these investigations will be presented in this paper.
Optimization Procedure For A Holographic Lens Solar Concentrator
C. G. Stojanoff, R. Kubitzek, St. Tropartz
The subject matter of this report is the presentation of an optimization technique developed for the design and manufacturing of a holographic lens solar concentrator. The emphasis is placed on the realization of a homogeneous diffraction efficiency distribution across the aperture of the holographic lens. The nature of the holographic process requires precise control of the mechanical properties of the gelatin layer such as moisture content, shrinking, swelling, hardness and elasticity, which specify the slant angle, the mean index of refraction, the spatial frequency and the amplitude of the refractive index variatrion. These parameters determine then uniquely the Bragg condition and the diffraction efficiency. The range of parameter variation is obtained from a large number of experimentally analyzed transmission gratings with spatial frequencies that are comparable to those expected across the aperture of the holographic lens
Performance And Bandwidth Analysis Of Holographic Solar Reflectors
H. D. Tholl, C. G. Stojanoff
Inexpensive manufacturing of holographic optical elements (HOE) in dichromated gelatin may be achieved by means of replication using a master hologram. Such manufacturing will render large format holograms useful for the collection and concentration of solar radiation provided they cover large bandwidth. This paper discusses holographic mirrors that exhibit a bandwidth in excess of 100 nm.
Laser And Spectroscopic Characterization Of Thin Films
Renata Reisfelda, Valery Chernyak, Marek Eyal, et al.
Transparent glass or polymer films doped by luminescent species are new promising materials for luminescent solar concentrat-ors, wave guides, two dimensional lasers and nonlinear materials. Polymethylmethacrylate (PMMA) and Polyvinylacetate (PVA) films containing Rhodamine 6G (Rh6G) and glass films containing Rh6G and/or inorganic ions were prepared. Chemical and spectroscopic properties of the doped films were determined by means of absorption and polarized emission spectroscopy and lifetimes of the dopants. Thicknesses of the films were determined by laser interferometry and absorption spectroscopy in the visible and infrared. Simultaneously absorption spectra of the dyes uniformly dispersed in the film in a monomeric form were measured and the film thickness and homogeneity determined from the molar extinction coefficient of the dye. The films may be prepared with controlled and predetermined thickness. The nonlinear properties of the doped films may be enhanced during their formation by orienting the dopants either in an electric field or by stretching the films uni-axially. Fluorescence depolarization allows energy transfer measurements.
High Efficiency Collectors For Solar Energy Applications: Analysis And Preliminary Experimental Results.
Antonello Cutolo, Donato Curtotti, Mario Pirro, et al.
Non metallic concentrators with tapered input sections are carefully investigated for solar energy applications. In addition, preliminary experimental results are compared with the numerical analysis in order to derive the basic criteria for their optimum design.
Possibilities Analysis Of Increasing Efficiency Of Solar Energy Conversion By Use Of Manyfocal Manyspectral Concentrators
V. L. Fara, R. Grigorescu, I. A. Dorobantu
The fundamental idea of this new type of solar concentrator consists in the use of con-vergent lens, which on concave face (exposed toward the Sun) has an infrared reflecting layer, what determines the formation of a spherical mirror for this spectral range; a visible range is concentrated by lens in classical manner. Thermal or photovoltaic receivers with optimum performances, for these spectral bands can be placed in respective foci. In the case of thermal receivers, selective surfaces for narrow spectral bands could be achieved by a less sophisticated technology. Use of these concentrators for photovoltaic receivers would allow manufacturing of manyspectral photovoltaic converters. A mathematical model which takes into account spectral and geometrical characteristics of concentrators and receivers has been elaborated for performances evaluation by computing simulation.
Fluorescent Quantum Yields And Long-Term Stability Of Silver Island/Polymer/Dye Systems, With Applications To Longer Wavelength Fluorescent Solar Concentrators.
Helen Rose Wilson
With the aim of extending the spectral range of fluorescent solar concentrators to longer wavelengths, the electromagnetic interaction between the fluorescent dyes and evaporated silver island films was investigated theoretically and experimentally. Mathematical expressions and spectra for the radiative and non-radiative decay rates, true and "apparent" quantum yields and the fluorescent decay lifetimes of the silver/dye complexes are obtained in dependence on the form of the silver islands, modelled as oblate spheroids. The expected improvement in long-term stability and a suggestion for a collector incorporating these principles are presented.
Methods For The Comparison Of Accelerated And Natural Ageing Tests
Michael Kohl, Ulrich Frei, Karlhanns Gindele
The performance ofmaterials used for solar energy conversion should be maintained for some decades of years. During the development of new products their durability must be investigated in short-term tests, in which the applied loads should be as similar as possible to the natural stresses under working conditions, but the tests should accelerate the degradation considerably. For these purposes the validation of accelerated indoor tests by a comparison with the results of natural ageing is essential. The naturally aged selective solar absorber coatings were only exposed to a part of the required lifetime. Therefore the observed degradations often are very small and a reliable comparison with the degradation achieved in accelerated temperature and moisture tests demands very sensitive analytical methods. Optical spectroscopy (0,3 - 15 μm) has also been used for the investigation of specific absorption bands as well as for the evaluation of differential spectra. Depending on the sample type, the analysis of the spectra has yielded optical thicknesses and refractive indices, or surface roughness parameters. The methods used for lifetime estimation can also be helpful for the comparison of the degradation of indoor and outdoor ageing.
Service Lifetime Prediction Of Solar Absorber Surfaces From Results Of Accelerated Ageing Tests
M. G. Hutchins, P. R. Dolley, K. Gindele, et al.
A review of an ongoing case study which aims to develop a methodology for the prediction of the service lifetime of solar absorber coatings is presented. The work forms part of Task X of the International Energy Agency Solar Heating and Cooling Programme which is investigating materials demand and performance in solar energy systems. The case study encompasses the identification of performance criteria for specific applications, e.g. domestic hot water systems, the ageing of absorber materials by accelerated testing of both individual samples and complete collectors, the identification and analysis of degradation mechanisms and failure modes and the development of mathematical models, which input data from accelerated ageing tests, to predict the service lifetime of an absorber coating in a particular end-use application. Commercially available black chrome and nickel-pigmented aluminium oxide selective absorbers are being used as test samples for development of the methodology.
Preparation And Properties Of Black Zinc Selective Absorbers Formed By Reactive rf Sputtering
Michael G. Hutchins, Christopher C. Figgures, Geoffrey N. Childs
Black zinc selective solar absorber coatings with solar absorptance 0.94 and thermal emittance 0.21 have been prepared by the reactive rf sputtering of Zn targets in Ar-02 atmospheres. For these films the zinc to oxygen ratio is greater than one and the films are composed of both zinc and zinc oxide. The surface microstructure of the films considerably enhances the short wavelength absorptance properties. The coatings represent a possible low-cost selective absorber for flat plate and evacuated tube solar collector applications.
Influence Of Surface Roughness On The Optical Properties Of Cermet Coatings
Wolfgang Graf, Michael Kohl, Karlhanns Gindele
Metal-dielectric composite materials (cermets) have been often investigated as solar thermal absorber coatings. The cermet, deposited on an infrared-reflecting metal substrate, is used as interference layer absorbing in the wavelength range of the solar irradiation. The attainable solar absorptance is limited by reflections at the cermet surface. A reduction of these reflection losses can be achieved by additional dielectric interference layers, grading of the filling factor of the metal in the cermet (decreasing from the substrate to the surface of the layer), or a suitable microroughness of the substrate-cermet interface and/or the surface of the coating. The latter way is described for microrough aluminium reflectors, deposited by physical vapour deposition on heated substrates, in combination with Ni/MgF2 cermet layers. The roughness parameters are evaluated by means of spectral reflectance measurements and scanning electron microscopy in order to investigate their dependence on the production parameters. The obtained optical properties are compared with those of absorber coatings produced according to the other methods mentioned above for achieving an enhanced absorptance of cermet absorber coating. The durability at temperature loads in air and in vacuum, respectively, is another topic of this paper.
Optical And Mechanical Properties Of The Zinc Chromate Absorbing Selective Surfaces
Alia H. Musa, F. Y. M. Al-Shaikley
Selectively absorbing coatings were prepared on zinc electroplated-steel substrates by chromate conversion method after addition of either AgNO3 or CuSO4.5H20 to the chromate bath supplied by Electrical Industry Company (EIC) in Baghdad, Iraq. The values of the absorptance, ∝, integrated over the solar spectrum and emittance, ε, integrated over the blackbody, radiation at 100°C were calculated from the measured reflectance values for various times an at different concentration of AgN03 of conversion. The results indicate that addition 0.2 g/l of either AgNO3 or CuSO4.5H2O to EIC bath resulted in an absorbing selective coatings with ∝ = 0.88 and ε(100°C) = 0.16 for an immersion time of 25 sec (surface 1) and with ∝= 0.85 and 6 (100°C) = 0.10 for an immersion time of 20 sec respectively. Both surfaces shower a little decrease in total absorptance with increasing angle of incidence. The surface analysis (SEM, EDXA) showed cracks of the coatings on surfaces 1 and 2 and the main constituents were zinc and chromium. However, the Zn/Cr ratio was greater for the former. Finally mechanical properties, thermal stability, humidity and outdoor field tests were also carried out for both surfaces.
Sputtered Aluminium Composite Selective Absorbing Surfaces
Yin Zhiqiang, Shi Yuayan
Deposition parameters and optical constants n and k have been determined for A1-N-F and A1-O-F dielectric films which have a refractive index n~1.4 and a low k prepared in D.C. magnetron sputtering system. A new solar selective surface of graded Al-N and A1 base layer with A1-N-F antireflection film has been fabricated, and good agreement is obtained between computed and experimentally measured reflectances and absorptances for the surface incorporating A1-N-F, solar absorptance increases to~0.97(AD) and ~.0.95(HT). The solar selective surface have been analyzed by Auger and ESCA.