Proceedings Volume 2017

Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XII

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Proceedings Volume 2017

Optical Materials Technology for Energy Efficiency and Solar Energy Conversion XII

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Volume Details

Date Published: 22 October 1993
Contents: 5 Sessions, 37 Papers, 0 Presentations
Conference: SPIE's 1993 International Symposium on Optics, Imaging, and Instrumentation 1993
Volume Number: 2017

Table of Contents

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Table of Contents

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  • Transparent Insulation and Super Window Materials
  • Chromogenics for Smart Windows
  • Photovoltaics, Photochemistry, and Photoelectrochemistry
  • Poster Presentations
  • Selective Absorbers, Solar Concentrators, and Reflectors
  • Photovoltaics, Photochemistry, and Photoelectrochemistry
  • Selective Absorbers, Solar Concentrators, and Reflectors
Transparent Insulation and Super Window Materials
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Optical and thermal characterization of advanced windows with low-e coatings
Arne Roos, Bjorn Karlsson
Optical characterization of windows with advanced glazing materials is discussed. Measurements at oblique incidence introduce errors, which make it difficult to properly estimate the solar energy performance of a window unit with several panes. The influence of experimental errors of the individual glazing products on the total performance of a window is discussed. A new formula for the evaluation of the annual energy balance of a window is presented. The total heat flow through the window during the heating season, or the total cooling load during the summer can be estimated with this formula. The method is useful for the comparison of different window configurations and for the evaluation of their annual energy performance.
Angular-dependent spectral optical properties of architectural glazings: results of an interlaboratory comparison of measurements
Michael G. Hutchins, Philippe Ageorges
In 1991 the Bureau Commune de Reference (BCR) of the Commission of the European Communities funded an inter-laboratory comparison of angular dependent optical properties of architectural glazings to establish the state-of-the-art of such measurements in European laboratories. The inter-comparison was performed amongst 12 laboratories in 8 countries and the European glass industry was represented by 4 major companies. Measurements were made of the spectral transmittance, T(lambda) , and spectral reflectance, R(lambda) , on three samples of architectural glazing at 10 degree(s) and 60 degree(s) angle of incidence for both s- and p- polarization of the incident beam at wavelengths in the visible and near infrared regions of the solar spectral range. The technical objectives of the intercomparison were to assess the applicability of the different measurement techniques employed by the participants, to establish confidence levels for angular dependent transmittance and reflectance measurements, to identify sources of measurement error and to evaluate potential calibration and reference materials.
Optical constants of sputtered ZrN films for heat mirror applications
Monica Veszelei, Kent E. Andersson, Arne Roos, et al.
Opaque and semi-transmitting film of ZrN have been reactively magnetron sputtered and their optical performance for solar control coating applications evaluated. The optical constants of the opaque films were determined from near normal reflectance measurements and Kramers- Kronig calculations over the interval 0.23 - 3.0 micrometers . The thin film optical constants were determined with combined R- and T-measurements. Systematic variations with growth conditions were observed. Thin films were observed to have higher n-values in the visible, a change that could be partly compensated by substrate heating. For the triple layers: glass / ZrO2 / ZrN / Zro2 it was noted that the crystallinity of the first oxide did not influence the refractive index of this oxide, but improved significantly the selectivity of the nitride growing upon it. Triple layers for solar control applications with 57% luminous transmittance and close to 1/3 each of solar transmittance, reflectance and absorption have been realized.
Design optimization and manufacturing of holographic windows for daylighting applications in buildings
Hans Dieter Tholl, Christo G. Stojanoff, Ruediger Kubitzek, et al.
The function of holographic optical elements in daylighting applications is to redirect sunlight from the immediate window area into the rear of a room in order to illuminate the darker regions and to reduce glare. A prerequisite for the successful application of holograms in daylighting systems is the solution of the problems of white light diffraction and of uniform holographic properties across a large aperture. This paper presents theoretical and experimental investigations of these two problems. It will be shown that white light diffraction is possible and that uniform diffraction efficiencies over large apertures are attainable.
In-situ measurement of reflectance and DC-resistivity of thin silver films in multilayer interference filters
Thomas Eisenhammer, F. Muggenthaler, R. Sizmann
Heat mirrors based on multilayers of silver and anti-reflective dielectrics can be used in solar process heat applications at elevated temperatures (approximately equals 250 degree(s)C). The optical properties of silver films depend on preparation conditions and differ from bulk material properties; especially in multilayer systems they are at present not well understood. An apparatus for simultaneous measurement of reflectance at one wavelength in the range 600 - 2000 nm and DC-resistivity during ion-beam sputtering of thin films is presented. The device is sensitive for changes in reflectance smaller than 0.001. Si3N4 / Ag / Si3N4 heat mirrors were produced on float glass. During growth of the silver film, reflectance initially decreases and then increases sharply. An interpretation with effective medium theories is presented. During deposition of the second dielectric layer, a sharp wavelength independent initial decrease in reflectance of approximately equals 2.5% is observed. After an increase of approximately equals 0.5% the reflectance decreases as predicted by Fresnel's equations. DC-resistivity closely follows this behavior. Different mechanisms for the observed effects are discussed.
Fine tuning the spectral response of metal insulator composites for specific solar applications
Geoffrey B. Smith, M. W. Ng, A. J. Reuben, et al.
Theories developed to understand angular selective films, recent new deposition procedures , the structural information on the resulting films, and new advances in effective medium theories have together provided the insights into controlling the spectral optical response of cermets to a degree not previously realized. The key is understanding the resonance characteristics of cermets. Several factors play a role, but three have a prime influence. They are the spatial distribution of the metal particles (even more important than total content), the dielectric constant of the host material and the metal itself.
Optimization of the electrical and optical properties of ITO layers deposited by magnetron sputtering
Geoff Cochrane, Zhitao Zheng, Geoffrey B. Smith
A comprehensive investigation of the deposition parameters of In2O3:Sn films on Corning 7059 glass substrates using r.f. planar magnetron sputtering has lead to the production of films with sheet resistance < 1.5 ohms per square, an integrated luminous transmittance of approximately 77% and an integrated solar transmittance of approximately 54%. Films characterized by measurement of their resistivity and Hall coefficient show a distinct minimum in film resistivity with deposition rate, substrate temperature, oxygen partial pressure, total deposition pressure and target composition. Results are presented showing the variation in carrier concentration and carrier mobility as a function of the deposition parameters together with the optical properties and microstructure of films with a resistivity approaching 10-4 (Omega) cm. Potentially, films with these properties can be used for energy efficient window applications. The results indicate that it should be possible to industrially produce large area ITO for electrochromic windows with sufficiently low sheet resistance coupled with high visible transmittance.
Effect of high-temperature treatment in different ambient on the properties of SnO2:F
Georgi D. Beshkov, Kroum M. Kolentsov, D. B. Dimitrov, et al.
The influence of an additional high-temperature treatment of SnO2 : F thin films prepared on silicon substrates is studied. The films are deposited by a spray pyrolytic technique at various substrate temperatures and deposition times. As-deposited SnO2 : F thin films are further annealed at temperature of 1000 degree(s)C in oxygen environment for times of 15 to 90 min as well as by rapid thermal annealing in vacuum of 6, 7 X 10-3 Pa for 1 min at the same annealing temperature. The changes of the film characteristics after the treatment are studied. Some peculiarities of the morphology film surface both on the deposition conditions and the various thermal treatment are investigated. Possible mechanisms explaining the obtained experimental results are discussed.
Chromogenics for Smart Windows
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Electrochromic oxides: microstructures and optical properties
This paper attempts to lay down a foundation for a comprehensive theory of electrochromism among oxides. It is pointed out that crystals of all the well known electrochromic metal (Me) oxides are composed of MeO6 octahedra in various corner-sharing and edge-sharing arrangements. In electrochromic thin films there are cluster-type and columnar microstructures based on the MeO6 units. The coordination of the ions leads to schematic electronic bandstructures that, at least for the (defect) perovskite and rutile lattices, are able to explain the presence or absence of cathodic and anodic electrochromism. Small polaron absorption prevails in disordered oxides.
Fractal behavior of thin-film tungsten oxide electrodes
Kathleen A. MacDonald, John M. Bell, Joanna Barczynska, et al.
Tungsten oxide thin films prepared by a dip-coating process show great promise as the electrochromic layer in optically switchable windows. Current models describe the electrochemical response of electrochromic evaporated or anodized WO3 thin films, using conventional chronoamperometric theory applied to a two-dimensional electrode. Here, we place emphasis on electrochemical ion diffusion in and through a non-two-dimensional thin- film electrode using a liquid electrolyte, in order to characterize the current-time (i-t) and current-voltage response. In particular, these characteristics can be related to the thickness, surface roughness and microstructure of the film using fractal theory. Complimentary microstructural analyses of these films are carried out using electron and atomic force microscopic techniques, with particular attention focussed on the effects of ion intercalation on the microstructure of sol-gel deposited thin films.
Self-bleaching mechanism of electrochromic WO3 films
Ji-Guang Zhang, David K. Benson, C. Ed Tracy, et al.
We report the first investigation of the self-bleaching behavior of electrochemically colored WO3 films. These films exhibit accelerated self-bleaching behavior over an 8-hour period when exposed to ambient air and show negligible or limited transmittance changes in other environments. The rate of self-bleaching in these films also depends on their preparation conditions, the electrolytes used in the coloring process, and film thickness and roughness. Self-bleaching in a WO3 film colored by lithium ions can be attributed mainly to the reaction between Li+ ions and water vapor, but self-bleaching in WO3 films colored by protons may be mainly due to the reaction between H+ and oxygen. The fast self-bleaching of the films in their early stages is dominated by the reaction of surface Li+ or H+ ions which are in direct contact with the reactive gases. The rate of self-bleaching increases with increasing film surface roughness but decreases with film thickness. The latter stages of self-bleaching exhibit a slower transmittance increase and are limited by diffusion of the reaction species.
Sol-gel deposition of electrochromic copper oxide films
Nilgun Ozer, Fatma Tepehan
This paper discusses the optical and electrochromic properties of CuxO films deposited by sol-gel process. UV-VIS-NIR spectroscopy, SEM, X-ray Diffraction and X-ray Photoelectron spectroscopy have been employed in a study of structure and optical properties of coatings. The sol-gel process offers new approaches to optical properties of coatings prepared by the acid catalyzed reactions of alkoxides. CuxO polymeric solutions are formed upon hydrolysis-condensation of copper ethoxide. Transparent amorphous CuxO coatings ranging in the thickness from 43 to 615 nm, were deposited on Corning 7059 and transparent conducting plates from polymeric solutions. These films exhibited reversible electrochromism that their color turn from transparent to pale brown. The combined effect of firing temperature (in the range of 70 - 450 degree(s)C) on optical properties and response time of these films are investigated. Significant changes in optical properties and electrochromic properties have been observed with firing temperatures.
Assessment of electrochromic devices based on sol-gel deposited films
John M. Bell, Geoffrey B. Smith, David C. Green, et al.
We discuss the preparation and performance of various electrochromic devices based on sol- gel deposited thin films. The films are deposited by dip-coating from alcohol-based sol-gel precursor solutions and the process is well suited to scaling up to large areas for window production. The coloration and bleaching performance of the devices is compared with the performance of individual films colored in liquid electrolytes, as well as with devices deposited using sputtered and evaporated films. Devices are based on sol-gel deposited tungsten oxide thin films which forms the active electrode. A range of counter-electrode materials have been tested, including sol-gel deposited vanadium pentoxide and titania films. Various ion conductors have been tested with these films, including simple, doped PEO and hartolyte, as well as a range of other polymers and inorganic ion conductors. Results will be presented showing the optical switching performance of the devices and comparing the performance of the devices with other device configurations. Initial results show that the sol- gel-based devices exhibit good switching performance, but the devices are sensitive to the details of the precursor solution used. Long term switching performance is an important issue in window applications, and the behavior of the sol-gel deposited films under repeated cycling will also be discussed, in particular a significant difference between the performance in liquid electrolytes and solid-state devices.
Characteristics of laminated electrochromic devices using polyorganodisulfide electrodes
Carl M. Lampert, Steven J. Visco, Marca M. Doeff, et al.
The use of polyorganodisulfides as optically passive counter-electrodes in a variety of electrochromic devices are discussed. Characteristic data is presented for electrochromic devices using proton, and lithium coloration ions with polyethylene oxide electrolyte and polydimercaptothiadiazole positive electrodes. Solid state devices consisting of molybdenum doped WO3, emorphous polyethylene oxide electrolyte (a-PEO), and a polyorganodisulfide counter-electrode colored rapidly from a pale yellow to a deep blue-green, upon application of 1.2 V d.c. The photopic transmittance changed from 61 to 9%, and the solar transmittance from 45 to 5% during the coloration process. Also, our experiments with polyimidazole are detailed. This family of compounds due to its unique electrical and ion conduction properties allow a single composite ion storage and ion conductor electrode to be made, simplifying the device construction. Devices made from this family of compounds color to deep blue-gray upon application of 1.2 - 1.5 V. Bleaching occurs at -0.4 to -0.5V. The photopic transmittance changed from 55 to 9%, and the solar transmittance from 34 to 4% during coloration. Both coloration and bleaching are quite rapid.
Electrochromic device with a polymer ionic conductor
Yueming Zhou, Peifu Gu, Jinfa Tang
The optical and electrochromic properties of tungsten oxide films deposited by e-beam evaporation were investigated. A laminated structure device was prepared. The as-deposited tungsten oxide film was used as the electrochromic film. A solid polymer electrolyte PPG- PMMA-LiClO4 was used as an ionic conductor. Spectrophotometric measurements show that the luminous transmittance of the device can be modulated between about 60% and 15%.
WO3-based quasi-symmetric smart window
Le-quang Nguyen, Pandurang V. Ashrit, George Bader, et al.
In the present work we have studied in detail the lithiation behavior of reactively sputtered crystalline tungsten oxide (WO3) films. These films, depending on their thickness and other deposition conditions, exhibit a wide range of changes under lithium insertion from deep coloration to no coloration at all. This gives the possibility of using the WO3 films either as electrochromic (EC) layer or as counter electrode (CE) layer for lithium ion storage, respectively. From these results a quasi- symmetric electrochromic system encompassing a- WO3 as EC layer and c-WO3 as the CE layer is fabricated and studied for its electrochromic behavior.
Structure study on electrochromic films of nickel oxide
Xingfang Hu, Xiaofeng Chen, Xiangyun Song
Using HREM, the relationship between structure and electrochromic properties of rf diode sputtered nickel oxide films with good and poor electrochromic performance has been investigated. The experimental results indicate that all two kinds of films consist of cubic nickel oxide with nano-crystal structure. For the films having good electrochromic properties, the grain size ranges about 5 - 10 nm. In the films exhibiting poor performance, an amorphous phase of nickel oxide as a continuous phase existing in the film has been observed and the cubic nickel oxide grains appear as isolate island existing in the amorphous phase. From the structural features of the films, it may be concluded that the grain boundary of nano- polycrystalline structure plays an important role in the electrochromic reaction and the grain boundary would act as channel for the injection and extraction of alkali metal ions and electrons during the coloring and bleaching process. So, it is important to control the structure of films in the deposition process to prepare the film with good electrochromic performance.
Electrochromic properties of manganese oxide (MnOx) thin films made by electron beam deposition
Olof Erlandsson, Johannes Lindvall, Ngoc Toan Nguyen, et al.
Electrochromic MnOx thin films were prepared by using an electron beam technique followed by annealing post-treatment. Electrochromic properties of the films were studied in three different solutions: ll1 LiC104 in propylene carbonate, KOH (pH= 10.5 ) and natrium borate ( pH= 9.2 )_ The transmittance spectra of the coloured films combained with their cyclic voltammograms have showed that the enhancement of the electrochromic behaviour of these films can be attributed to the insertion (or extraction) of the OH- anions into (or from) the MnOx films. The best electrochromic efficiency of the films was obtained in the borate electrolyte.
Photovoltaics, Photochemistry, and Photoelectrochemistry
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Development of thin-film polycrystalline silicon solar cells by a solid-phase crystallization (SPC) method
Takao Matsuyama, Toshiaki Baba, Makoto Tanaka, et al.
Polycrystalline silicon (poly-Si) thin films prepared by the solid phase crystallization (SPC) method were studied for photovoltaic materials. To improve the properties of the poly-Si thin film, a-Si films suited to the solid phase crystallization were investigated. It was found that TA/TO (peak height ratio of TA peak and TO peak) in Raman spectra of a-Si films had the good correlation with the average grain size, and a factor for the enlargement of grain size was an increase of distortion energy stored in a-Si films. The first adoption of a textured substrate was also performed, which had effects on the enlargement of grain size in poly-Si thin films be the SPC method. By applying the a-Si films with large TA/TO value on textured substrate, the n-type poly-Si thin-film with the grain size of 6 micrometers was fabricated and this film showed the Hall mobility of 623 cm2/Vs (electron density: 3.0 X 1015 cm-3). A new heterojunction technology, which was called by 'Artificially Constructed Junction (ACJ)', was developed by depositions of thin a-Si films on single- crystalline silicon (c-Si). In a solar cell using this technology, a high conversion efficiency of 18.7% was achieved. This is the highest value ever reported for solar cells in which the junctions were fabricated at a low temperature of less than 200 degree(s)C. In a thin-film poly- Si solar cell (thickness: 10 micrometers ) applying this technology, a conversion efficiency of 6.3% was also obtained and a collection efficiency of 51% was achieved at a wavelength of 900 nm. This high value attributes to the hole diffusion length of 11 micrometers , which is longer than the poly-Si thickness.
Multiphoton photochemical reactions during laser-induced deposition of silicon thin films
Shoshana Tamir, Yigal Komem, Moshe Eizenberg, et al.
Silicon deposition via decomposition of SiH4 molecules induced by excimer laser irradiation has been previously reported to be controlled by a two photon absorption process. This behavior was found at a total pressure lower than 5 Torr. At higher pressures the absorbed intensity depended linearly on the incident intensity. However, a multiphoton chemical reaction is indicated by a non-linear dependence of the deposition rate on laser energy. A different non-linear dependence of the deposition rate on laser energy was found in our work during the investigation of the mechanism of silicon thin films deposition by laser irradiation of a silane gas diluted with argon (15% SiH4/Ar). The Si films were deposited by a focused laser beam irradiating in parallel to silicon and silicon dioxide substrates at a gas flow rate of 20 SCCM, total pressure of 60 Torr and a repetition rate of 15 Hz. At laser energies higher than 160 mJ/cm2 the deposition rate was almost independent of the incident laser energy, while at a lower energy the deposition rate depended strongly on the laser energy. A 3/2 power was found in absorption measurements carried out at the same pressure under flow conditions and for several repetition rates at average laser intensity above 300 mW. This kind of behavior is typical of a multiphoton absorption process involving saturation effects caused by focusing of the laser beam. Below 300 mW the power dependence indicated a two-photon absorption process. From the observed photochemical yield we found the value of 5.7 X 10-44 cm4 sec. molec.-1 for the two- photon absorption cross section.
Correlation between material properties and photovoltaic performance in amorphous silicon solar cells
Masao Isomura, Yoshihiro Hishikawa, Toshihiro Kinoshita, et al.
We have focused on the i-layer material in our efforts to improve in the conversion efficiency of a-Si:H solar cells. Reductions in the defect density has been also investigated from the viewpoints of extrinsic (impurities) and intrinsic effects. The main incorporated impurity in a- Si:H is oxygen, which affects the conversion efficiency of a-Si:H solar cells by increasing the defect density and its donorlike behavior. A unified relationship can be observed among the properties of intrinsic (pure) a-Si:H. The film deposition rate plays an essential role in controlling the properties. A lower or higher deposition rate results in a narrower or wider bandgap, respectively. Therefore, the properties of a-Si:H can be controlled independent of the substrate temperature in a certain range by varying the film deposition rate. The controllability of the a-Si:H properties can be improved by applying vibrational / rotational energy to SiH4 molecules or related radicals by heating the source gas, and a-Si:H with the same properties as the best conventional one can be deposited at a lower substrate temperature and/or a higher film deposition rate. The highest conversion efficiency of 12% for an integrated a-Si solar cell submodule of 100 cm2 has been achieved by combining the high-quality i-layer and other technologies.
Verification of the current voltage characteristics and efficiencies in dye-sensitized TiO2 solar cells
Recently, a paper was published by the Lausanne Group headed by Dr. M. Gratzel which claimed to obtain a low cost 7% efficient photoelectrochemical solar cell from a Ru carboxylated bipyridyl charge transfer dye adsorbed on the very rough surface of a colloidal TiO2 film. In the current paper, a verification of this result is presented as well as the requirements for efficient collection in this new type of cell. Measurements are reported in simulated and natural sunlight which confirm that the efficiency is indeed in the range previously reported. In addition, a comparison will be made with detailed balance type calculations which support that the cell operates within the limits defined by thermodynamics and the optical absorption of the adsorbed dye. A discussion is made as to the use of the sensitization concept with other materials as well as to the basic economics of such a device.
Colloidal films from TiO2, an electrode material for dye-sensitized solar cells
Sten-Eric Lindquist, Anders Hagfeldt, Henrik Lindstrom, et al.
Some recent results from our laboratory on the properties of microporous colloidal TiO2 film electrodes are summarized. The photoelectrochemical properties of the colloidal film electrodes are compared with solid transparent thin films of TiO2. In particular the information that can be derived from analysis of the action spectra for front and back side illumination of the films is given. The mechanism of charge separation in the colloidal film is qualitatively discussed in terms of the kinetics at the electrode-electrolyte interface, and some effects of the presence of O2 in the electrolyte are demonstrated. Finally we also briefly report the solar energy efficiency from measurements on dye-sensitized TiO2 film electrodes in solar cells.
Design and optimization of a holographic concentrator for two-color PV operation
Ernst Ulrich Wagemann, Klaus Froehlich, Jochen Schulat, et al.
The subject matter of this research project is to develop, manufacture and field test a spectrally dispersing solar collector system using a holographic solar concentrator in conjunction with spectrally matched advanced solar cells for photovoltaic power generation. The advantage of a holographic solar concentrator as compared to a conventional one is seen in the overall reduction of investment cost and in the possibility to generate inexpensive solar electric power. In this paper we present the techniques specifically developed for the design and manufacturing of efficient holographic optical elements and holographic lens stacks that are used in the fabrication of bandwidth matched solar concentrators for VIS and NIR photovoltaic operation. The lens stack separates the white light radiation into several spectral ranges that are focussed onto photocells possessing corresponding spectral characteristics. Contrary to previously published arrangements, we present here the concept and the design characteristics of a holographic concentrator that allows positioning of the cell in a plane parallel to the lens aperture. The initial idea of using two lenses recorded in the same aperture or same holographic layer focussing onto two off-axis foci proved to be of limited value due to the off-axis focussing that introduces strong reflection and aberration. Here we present a new concept in which the two lenses are shifted in the plane of the aperture so that each lens-cell configuration exhibits axial geometry. Both lenses are designed as axially corrected holographic stacks that include a lens and a correction grating. The design minimizes the cross coupling between the two holographic systems. Stack layouts for AlGaAs/GaAs and GaAs/Si combinations are discussed. Cross-coupling effects and aberrations involving the IR lens are minimized. Experimental diffraction efficiencies are fitted with non-cosinusoidal refractive index modulation showing best performance for 100 by 100 mm2 aperture. The theoretical predictions are compared with the first experimental results.
Properties of vacuum-evaporated CdS and CdSe double-layer-films doping with copper and chlorine
Peifu Gu, Haifeng Li, Zhenchai Zhu, et al.
The effect of doping with copper and chlorine on various properties of vacuum-evaporated CdS and CdSe double layers has been studied. The properties specifically studied were: (1) the dark conductivity and photoconductivity as functions of the doping concentration and the ratio of copper to chlorine, (2) the response time of various photoconductive films and (3) the optical absorption and spectral response. It was found that the dark conductivity decreases and the photoconductivity increases significantly if the ratio of Cu to Cl is suitable. The response time was about 5 - 10 ms for doping films and more than 100 ms for undoping ones. The optical absorption increases and the spectral response moves to a longer wavelength as the concentration of CdSe increases. The response wavelength can be changed by adjusting the thickness ratio of CdS to CdSe.
Poster Presentations
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Multilayer coatings for solar energy control applications
R. T. Kivaisi, G. Mbise
This work presents some results for window coatings that are suitable for solar control applications. Selected research results are given for metal-dielectric based coatings optimized for normal incidence as well as for oblique incidence. These coatings can be used to improve the performance of windows both for architectural and automobile sectors. Surface coatings which are transparent at 0.3 < (lambda) < 0.7 micrometers can be used to solar control windows. A thin homogeneous noble metal film (e.g., Ag) can combine short wavelength transmittance with high lone wavelength reflectance. By embedding the metal film between high refractive index dielectric layers one can optimize the transmittance in the desired spectral region. Transmittance data for multilayer stacks designed for normal and non-normal incidence to the coating are presented.
Window coatings for energy efficiency
ShengJun Huang, Peifu Gu, Jinfa Tang
Coatings for energy efficiency are one of functional thin films which are developing rapidly. In this paper, essential design considerations are given in details and design method of the coatings is also presented. The paper shows spectral properties of bilayer system Ti/TiO2 and demonstrates that attractive reflection colors are available. The result of the experiment reveals that anti-reflection layer TiO2 work well to improve visible transmittance and introduce attractive colors.
Some optoelectric properties of chemically deposited thin films of nickel and cobalt oxides
A. Joseph Varkey, A. F. Fort
For solar energy devices to become affordable to everyone, the need for developing modest techniques should be given priority attention. In our laboratory we have developed very simple methods for preparation of cobalt oxides, CoOOH and Co3O4, and nickel oxides NiO and NiOOH. They were deposited on glass substrates from solutions containing ammine complexes of the respective metal ions. For cobalt oxides, the initial deposition was of brown CoOOH, which was converted into black Co3O4 by annealing it at 590 K in air. The films were characterized by measuring their optical absorbance, transmittance, and reflectance in the spectral range of 300 - 900 nm. The data were used to calculate the spectral variation of their optical constants. Absorption spectra of the films were analyzed to obtain their optical band gaps. Some possible uses of these films are mentioned, and the prospects of this method for deposition of other oxide films are also discussed.
Development and fabrication of a hybrid holographic solar concentrator for concurrent generation of electricity and thermal utilization
Klaus Froehlich, Ernst Ulrich Wagemann, B. Frohn, et al.
The efficiency of photovoltaic generators that are based on different semiconductor materials with optimized band-gaps can achieve considerably higher values than those obtained from single junction devices, e.g. Si-based solar cells. Hence, the splitting of the solar spectrum for use with the different band-gap cells is a desired characteristic of the solar collector. An enhanced efficiency is realized with the concentration of the incident solar radiation onto the corresponding solar cell. The optical characteristics of the holographic solar concentrator satisfy these requirements. The undiffracted solar radiation should be collected by an absorber that also cools the solar cells. This is the concept of the hybrid collector for electricity and thermal utilization that is presented in this paper. It can achieve an electrical efficiency above 22% and a thermal efficiency of 35% with a temperature of 100 degree(s)C.
Optical and structural studies of Cu-SiO cermet thin films
Mohammad Sayeedur R Khan, E. J. Beynon
Optical absorption measurements have been carried out on Cu-SiO cermet films of the thickness range 200 - 400 nm, prepared by thermal co-evaporation in vacuo approximately 1mPa. The optical band gap Eopt varied between 2.58 and 2.05 eV over the composition range 5 - 15 vol% Cu of the cermet. The addition of Cu to the cermet causes a systematic reduction of Eopt. The values of Ee were found to increase as the copper concentration increased. Electron micrographs indicated a two-phase structure consisting of discrete metallic particles, 1.5 - 3.0 nm in size, in a continuous amorphous phase. The mean particle size increased with increasing Cu content and substrate temperature.
Selective Absorbers, Solar Concentrators, and Reflectors
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Operation and efficiency of large-scale solar thermal power plants
Gilbert E. Cohen
Nine large solar thermal power plants, 15 - 80 MWe in scale, located in Southern California's Mojave Desert, are proving since 1985 that power solar technology is a working reality. These plants produce over 90% of solar electric generation in the world. This paper provides summary information, more particularly on five of the SEGS (Solar Electric Generating Systems) plants located at Kramer Junction, and operated by KJC Operating Company. General description of the plants and the technologies involved are discussed with an overview of performance after five years of full operation.
Oxidation of small metal particles
Rafi Gatt, Gunnar A. Niklasson, Maiken Heim, et al.
Selective absorbers utilize in many cases the enhanced solar absorptance of small metal particles embedded in a layered ceramic medium. A crucial factor in determining the performance of such an absorber is the degradation rate of this layer. The oxidation of ultrafine gas evaporated metal particles is examined here. Several metal develop a stable oxide film around the surface of small particles. It is shown that there is a linear dependence between the size of the metal core and the particle radius for Al, Mg, Be and Sn in room temperature and for Si at high temperatures. Charge quantization is proposed as a possible mechanism responsible for this phenomenon. Thermogravimetric measurements on Ni particles show a rapid oxidation for intermediate temperatures.
Optimization of SiO2-TiNxOy-Cu interference absorbers: numerical and experimental results
Michel P. Lazarov, R. Sizmann, Ulrich Frei
SiO2 - TiNxOy-Cu absorbers were prepared with activated reactive evaporation (ARE). The deposition parameters for the ARE process were adjusted according to the results of the numerical optimizations by a genetic algorithm. We present spectral reflectance, calorimetric and grazing incidence X-ray reflection (GXR) measurements. Best coatings for applications as selective absorber in the range of T equals 100 (DOT)(DOT)(DOT) 200 degree(s)C exhibit a solar absorptance of 0.94 and a near normal emittance of 0.044 at 100 degree(s)C. This emittance is correlated with the hemispherical emittance of 0.061 obtained from calorimetric measurements at 200 degree(s)C. First results on lifetime studies show that the coatings are thermally stable under vacuum up to 400 degree(s)C. The SiO2 film passivates the absorber, a substantial slow down of degradation in dry air is observed. Our tests demonstrate that the coating will withstand break down in cooling fluid and vacuum if mounted in an evacuated collector.
Nitrogenation of amorphous carbon layers as a method for improving their performance as a spectrally selective absorber coating
Olaf Stenzel, Ralf Petrich, Till Wallendorf, et al.
Nitrogen-containing amorphous hydrogenated carbon layers have been deposited on silicon and aluminum by a dc plasma technique. The optical constants of the layers (refractive index and absorption coefficient), their solar absorptance and the thermal emittance at 373 K have been determined from spectrophotometric measurements. It is demonstrated that the solar absorptance of nitrogenated layers is higher than that of corresponding pure amorphous hydrogenated carbon layers, while the emittance values are comparable.
Thermal and chemical metallic-dielectric transitions of TiNxOy-Cu absorber tandems
H. Schellinger, Michel P. Lazarov, H. Klank, et al.
Reliability and structural properties of selective TiNxOy-Cu tandem absorbers for solar applications at high temperatures were investigated. A tempering process which improves thermal emittance of the coatings was observed. Degradation is monitored by directional total thermal emittance. Four steps of degradation have been separated and confirmed by XRD, EDAX, REM and optical spectroscopy. Coatings show crystalline phases and transitions to amorphous due to degradation and tempering. They are susceptible to oxidation and to reactions with the substrate.
Photovoltaics, Photochemistry, and Photoelectrochemistry
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Morphology of nanocrystalline titanium dioxide films
Valery Shklover, Paul Liska, Mohammad Nazeeruddin, et al.
The further morphological investigation of nanocrystalline TiO2 films, which sensitized by suitable transition metal complexes forming a new type of molecular photovoltaic systems, the studies of new films and a new Co complexes are presented herein. Scanning electron microscopy, X-ray single crystal and powder diffraction as well as X-ray diffraction at low glancing angles using synchrotron radiation have been used. The morphology of electro- and thermodeposited Pt-layers on TCO (a), polymorphic composition of films (b), possible geometry of location of complex dye molecules on the exposed most faces of TiO2 microcrystals (c) and the crystal structure of a Co(II) complex solvated by electrolyte 4- methyl-1,3-dioxolan-2-on as a precursor of the corresponding redox-system (d) are discussed.
Selective Absorbers, Solar Concentrators, and Reflectors
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Diffractive daylighting: ways to obtain wide-angular-range, large-efficiency, near-achromatic operation
Hendrik J. Gerritsen
'Daylighting' is the name for any technique which brings as much daylight as possible deep into an architectural space. This paper describes how this can be done with the use of diffractive structures, it points out the design advantages and disadvantages of various structures and gives results obtained with some prototypes. The prototypes are highly efficient over a wide angular range, produce near achromatic light and are relief structures, which allow for inexpensive reproduction by embossing.