Showing papers in "Solar Energy Materials in 1979"

Transition-metal dopants for extending the response of titanate photoelectrolysis anodes

Abstract: Photoelectrolysis of water with doped TiO2 and SrTiO3 electrodes is reported. Dopants used are transition metals V, Cr, Mn, Fe, Co, Ni, as well as Al. The response to visible light is greatest at Cr, then decreases across this period of elements, being absent in Co, Ni and Al. Hydrogen and oxygen generation are observed with Cr-doped TiO2 under visible light excitation. Ultraviolet light response decreases with the dopants in reverse order, being highest for Al and lowest for V. An explanation is given in terms of the minority carrier diffusion length,1which is calculated for samples with each of the dopants.

The effect of fluorescent wavelength shifting on solar cell spectral response

Abstract: Fluorescent wavelength shifting has been used to enhance the spectral response and AM0 conversion efficiency of several types of solar cells. Plastic fluorescent materials are useful for devices with a sharp cut-off in response, while ruby is suitable for devices with more gradual cut-offs. Efficiency improvements of 0.5 to 2 percentage points were measured on some cells, and greater improvements can be expected for optimized optical components. The optical efficiencies (light transmitted into the solar cell compared to light incident on the fluorescent sheet) exceeded 50% for the plastic sheets and 75% for ruby.

Coatings for enhanced photothermal energy collection I. Selective absorbers

Abstract: For economical and efficient utilization of solar energy various types of absorber coatings and preparations can be used for solar collectors. In this study several varieties of commercial and research selective absorbers are reviewed and tabulated for application. For many of these coatings, detailed reflectance, emittance and thermal stability data are presented. Selective coatings can take advantage of various optical absorption methods including light trapping, particulate coatings, semiconductor-metallic layers, multilayer films, quantum size effects and intrinsic absorption. The results of this study revealed many promising low temperature (150–300° C) absorber coatings along with a few highly engineered coatings which can withstand temperatures above 500° C.

Effect of annealing on the optical properties of plasma deposited amorphous hydrogenated silicon

Abstract: Amorphous silicon films prepared by radiofrequency plasma decomposition of silane contain between 10 and 25 at % hydrogen depending on the potential and temperature of the substrate and plasma parameters. The way hydrogen is bonded in these films has been determined from the infrared stretching, bending, and rocking or wagging modes of the Si-H complexes. Monohydrides SiH, dihydrides SiH2 and short chains of polysilane ( SiH 2 ) n an be identified. The potential of the substrate with respect to the plasma and to a lesser degree the substrate temperature determine the prevalent Si-H complex. At substrate potentials close to that of the plasma ( SiH 2 )n complexes are favored whereas isolated SiH and SiH2 groups are found in films prepared on strongly negative potential substrates. The films can be dehydrogenated by annealing. Substrate potential and temperature influence strongly the refractive index and the onset of interband optical transitions. The optical gap of these films lies between 1.6 and 1.85 eV which is appreciably higher than the values 1.2–1.5 eV of sputtered and evaporated amorphous Si films. As the hydrogen is driven out by annealing the optical gap decreases to 1.6–1.7 eV. At photon energies below the optical gap one observes a preparation sensitive absorption tail between α=10 2 and10 3 cm −1 which cannot be removed by annealing. The films crystallize between 700 and 780°C with a crystallization energy of (2±0.3) kcal/mol.

Optical properties and structure of amorphous silicon films prepared by CVD

Abstract: Silicon films were deposited by pyrolytic decomposition of silane on substrates held at various temperatures, T s , in the range 550 to 800°C The absorption coefficient, refractive index, anf X-ray diffraction pattern of these films were determined The films deposited at temperatures, T s ≤660°C are amorphous, and their absorption profile resembles that reported in the literature for sputtered or evaporated amorphous films after long-time anneal Films deposited on substrates at or above 670°C are partially crystallized, with particle size increasing gradually with substrate temperature When the amorphous films are annealed, the resulting changes depend on length and temperature of the anneal After a temperature-dependent induction period, the samples crystallize rapidly The volume shrinks by ≈3% as determined from the decrease in film thickness The onset of crystallization is indicated first by a red shift of the absorption edge, which after further anneal is overcompensated by a blue shift The results demonstrate that the superior solar absorptance of amorphous silicon can be utilized in photothermal solar energy converters of sufficient stability without sacrificing the advantages of CVD fabrication

Electrochemical solar cells based on layer-type transition metal compounds: Performance of electrode material

Abstract: The effect of doping of MoSe2 (band gap 14 eV) as well as the influence of the electrochemical potential of the redox-couple in the electrolyte have been investigated in an effort to optimize solar cells based on layer compounds It is shown that in the case of I−/I2 the mechanism of photopotential generation is assisted by a favourable chemical modification of the electrode surface which introduces both negative charges into the double layer and traps for holes into the space charge layer The effect is traced to the formation and reaction behaviour of a five-valent Mo(V)-I complex The MoSe2:I−/I2-cell could be improved to an energy conversion efficiency of 8% for monochromatic and 4% for solar radiation Its good stability was established by continued operation at 10 mA/cm2 for 10 months This system cannot compete with solid state solar cells, but it could possibly be developed as hybride cell for the simultaneous production of heat and electricity

Metal/insulator composite selective absorbers

Abstract: The optical properties of metal/insulator composite films have been examined, and their utilization as solar selective absorbers studied both theoretically and experimentally. Calculations have been made on the solar selective performance of graded composite films, demonstrating the performance and limitations to be expected for the general class of composite film absorbers. Design guidelines for the fabrication of optimum selective absorbers have been developed and subsequently implemented in the construction of selective absorbers using two promising composite materials, Ni/Al2O3 and Pt/Al2O3. Measurements of reflectance, solar absorptivity α, thermal emissivity e and high temperature durability have been made. The results show values of α⩾0.94 for both Ni and Pt composite films. Values of α/e 1 3 at 150°C were obtained for both composite materials when deposited on Cu substrates. A selectively absorbing film has been produced which shows no change in performance parameters after extended heating at 600°C in air.

Microstructure of a black chrome solar selective absorber

Abstract: The structure of Harshaw Chemicals' “CHROM-ONYX” type of black chrome/metal selective absorber was studied to gain a better understanding of its influence upon the mechanism of wavelength selectivity. Spectral reflectance measurements were performed on seven samples. In this study, the best selectivity was found by these measurements to be 1.0 μm of black chrome on copper and 0.7 μm of black chrome on nickel. Both transmission and scanning electron microscopy were employed to study microstructure and chemical composition. As a result of the combined studies, some effects of black chrome thickness and the metallic substrate were determined. It was found that black chrome consisted of a very fine metallic distribution of particles of chromium, possibly suspended within a matrix of an oxide of chromium. This combination was, inturn, agglomerated into larger particles within the 0.05–0.3 μm size range. These larger particles formed a network which constituted the surface coating.

Photovoltaic efficiency of InSe solar cells

Abstract: Indium selenide n-type substrates made from ordinary-grade elements, are suitable to make heterojunctions with semitransparent platinum layers Those low-cost devices have photovoltaic efficiencies for solar energy conversion under 6%, at the present time They can be readily improved to reach external efficiencies in the 10% range

Physics of doped tin dioxide films for spectral-selective surfaces

Abstract: The spectral selectivity of tin dioxide layers has been optimised in an experimental study using Drude's theory. This theory gives a free electron model which makes it possible to compute the spectral reflectance as a function of electrical properties of a conducting material. By measuring the reflectance, on the one hand, and by measuring the Hall coefficient and electrical conductivity, on the other, the validity of this model has been checked. For an optimum spectral selectivity it is shown that, besides a minimum layer thickness of 0.5 μm, a high electron mobility of at least 4.0 × 10 −3 m 2 / Vs and an electron number density of about 3 × 10 26 m −3 is favourable. We could obtain these conditions with a fluorine-doped tin oxide coating. Applying this coating on a black-enamelled steel gives a good spectral-selective surface for solar collectors with a solar absorptance of 0.92 and a thermal emittance of 0.15.

The surface microstructure optical properties relationship in solar absorbers - Black chrome

Abstract: The relationship between the surface microstructure and the optical properties of solar absorbing electrodeposited black chrome films has been studied by scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS) and sputter depth-profiling techniques. The black chrome films have been determined to consist of a top layer of small (≈400 A) Cr 2 O 3 particles with one or two sublayers of larger (≈1000 A) closely packed chromium particles. The optical properties of the solar absorping films have been measured and analyzed via the spheroid model and it has been shown that the optical response of the particulate black chrome films is significantly determined by their microstructure.

Efficiency of tandem solar cell systems as a function of temperature and solar energy concentration ratio

Abstract: This paper presents the results of a comprehensive theoretical analysis of tandem photovoltaic solar cells as a function of temperature and solar concentration ratio The I–V characteristics of the solar cells were assumed to be governed by the relation I = I 0 ( e qV/AkT − 1) with I 0 = K e −E G /BkT and A = B The overall efficiencies of tandem cell stacks consisting of as many as 24 cells having energy gaps in the 07 to 36 eV range were calculated for temperatures of 200, 300, 400 and 500 K and for illumination by an AMO solar spectrum having concentration ratios C = 1, 100, 500 and 1000 suns For ideal diodes (A = B = 1), the calculations show that the optimized overall efficiency has a limiting value ηopt ≈ 70% for T = 200 K and C = 1000 For T = 300 K and C = 1000, this limiting efficiency approaches 60% Most of the gain in efficiency occurs with the number of semiconductors in the tandem system between six and ten, eg for T = 300 K and C = 1000, an optimized six cell system has a theoretical limit efficiency of about 53% Calculations were also conducted for the A = B = 2 case (non-ideal diode behavior); in this case the limiting value of η for a 24 cell system is about 65% at 200 K and 55% at 300 K

Coatings for enhanced photothermal energy collection. II - Non-selective and energy control films

Abstract: Several types of coatings and surface preparations, other than selective absorbers, can be utilized for economical collection and control of solar energy. These films can be used for both solar thermal collectors and for window systems in buildings. Numerous non-selective, hot and cold mirror, and antireflective coatings are reviewed and tabulated. Detailed reflectance, emittance and thermal stability data are presented for these various coatings. Both moderately selective and non-selective absorbers consist of black paint, chemical conversion finishes, electroplated and anodized coatings. Both hot and cold mirror coatings are selective transmitters of energy. They are considered for applications where light and heat need to be separated and trapped. Antireflective films are evaluated for use on glass surfaces. The findings of this study reveal many different types of inexpensive and promising coatings for efficient utilization of solar energy.

Hydrogen content, electrical properties and stability of glow discharge amorphous silicon

Abstract: The hydrogen content, C H , the room temperature photoconductivity and the dark conductivity and its activation energy have been investigated in a-Si specimens produced in three different preparation units by the glow discharge technique. Hydrogen evolution measurements were made on films deposited on glass and aluminium substrates and checked by nuclear resonance experiments. C H values range between 3 and 18 at%, indicating a definite dependence on the preparation unit. Annealing experiments show that in all specimens the incorporated hydrogen is stable up to 400°C, irrespective of the initial C H value. Photoconductivity, like C H , decreases rapidly at temperatures above 400°C, showing that additional defect states are introduced. Changes in room temperature dark conductivity and activation energy take place above 500°C and indicate a change in transport mechanism. The results on as-deposited specimens provide no evidence for any direct connection between C H and photoconductive or conductive properties. Photostability experiments showed that 2 h exposure to 20 mW cm −2 illumination had a negligible effect on the electrical properties of some of the specimens, suggesting that the large photostructural changes recently reported in the literature are not an inherent property of glow discharge a-Si.

Porosity and oxidation of amorphous silicon films prepared by evaporation, sputtering and plasma-deposition

Abstract: The amount of water adsorbed by amorphous silicon films upon exposure to normal humidity levels has been measured with a quartz-crystal microbalance. Films electron-beam evaporated on room temperature substrates absorb about 11 mol% H 2 O on the internal surfaces of an interconnected void structure. We estimate that nearly 15% of the silicon atoms lie at internal void surfaces. The water causes internal oxidation and cannot be removed by drying. It produces an increase in resistance and a shift of the optical absorption edge to higher photon energies. Hydrogenated amorphous silicon prepared by sputtering at 200°C or by rf plasma deposition at 25 and 150°C, on the other hand, is not porous. Exposure to 60% relative humidity at 300 K produces a 5 A thick oxide layer in about 20 h. Water adsorption strongly affects the conduction in the space charge layer near the surface of plasma-deposited films but has little or no effect on the conductivity of sputtered films. Long term relaxation effects are observed in the conductivity of freshly plasma-deposited amorphous silicon-hydrogen films prepared at room temperature. Based on the very low hydrogen content found in chemical-vapor-deposited (CVD) amorphous silicon we conclude that the CVD films are impervious to water.

Stabilized CVD amorphous silicon for high temperature photothermal solar energy conversion

Abstract: By pyrolytic decomposition of silane in the presence of dopant gases, a set of amorphous silicon films was prepared that contains various concentrations of carbon, nitrogen, boron or germanium. The effect of these dopants on the crystallization process and the optical properties is investigated. Films containing about 18 at % carbon show the properties most favorable for solar absorbers. The crystallization is retarded to temperatures near 1000°C, and the solar absorptance is greater than that of non-intentionally doped CVD amorphous silicon. From the experimentally determined activation energy of crystallization, the structural lifetime for such absorber films is extrapolated to be in excess of several decades for continuous operation at 700°C. For identical thicknesses of absorber layers, spectrally selective stacks of stabilized amorphous silicon deposited on top of a molybdenum reflector have higher solar absorptance than stacks composed of polycrystalline silicon on a silver reflector, amorphous silicon on molybdenum having been tested at temperatures in excess of 500°C.

Possibilities of new materials for solar photovoltaic cells

Abstract: The requirements for desirable technological properties of semiconductors for photovoltaic conversion are discussed and two criteria are suggested for the selection of materials that could guide the search towards low melting compounds made of cheap component elements: (1) the use of a deviation from ideality vs. energy gap diagram for the prediction of energy gaps from the knowledge of melting temperatures or viceversa; (2) the use of the Parthe-Goryunova-Mooser-Pearson criteria for compounds of which only their chemical formula is known but not their structure, in order to guess the structure and therefore be able to use the first criterion. For materials for Schottky devices a third criterion is necessary, derived by Nethercott that relates the Mulliken electronegativities of the component elements of a compound to its electron affinity. This can also be applied in the search of metallic alloys, semimetals or narrow band gap semiconductors that might replace the metal in a Schottky barrier. It is shown that a number of compounds exists which might meet these criteria.

Crystal growth and properties of CuGaxIn1−xSe2 chalcopyrite compound

Abstract: The preparation of single crystals of CuGa x In 1−x Se 2 by a chemical transport method in a closed tube is described. Electrical and optical properties, measured as a function of CuGa x In 1−x Se 2 composition, indicate that this material, when coupled with zinc and cadmium chalcogenide windows, can be used as an absorber in heterojunction solar cells with near zero lattice mismatch.

Composite semiconductors - Selective absorbers of solar energy

Abstract: We have produced composite semiconductors by cosputtering CaF2 with either Ge or Si and measured their optical constants. Their optical behavior can be used described as being similar to that of the parent semiconductor, with the same energy gap but with a reduced, concentration dependent index of refraction. The normal specular reflectance of films sputtered on mirrored surfaces was measured. These data were used to compute the solar absorptance αs and thermal emittance eth. It was found that α s ≈ 0.7 and e th ≈ 0.06 with a weak dependence on composition, thickness and operating temperatures. Thus at T = 500° C conversion efficiencies of 50% are currently possible at solar concentration ratios C of 7–8 and about 70% for C ≈ 40 and improved performance can be expected with continuing research.

CuInS2 films prepared by spray pyrolysis

Abstract: The electrical, optical and structural properties of thin films of CuInS 2 prepared by spraying a solution of Cu 2 Cl 2 , InCl 3 and thiourea onto heated glass and alumina substrates are described. The polycrystalline films produced at prescribed spraying rates and substrate temperatures were nearly stoichiometric as shown by EDAX analysis, homogeneous, and had the sphalerite structure The films were p -type and their resistivity could be adjusted over the range 0.1 ω cm to very high values by controlling the temperature of the substrate during spraying. Heterojunction devices which exhibited a weak photovoltaic effect were prepared by combining these films with either single crystals of CdS or with sprayed films of CdS.

A study of oxide-based heterostructure photoelectrodes

Abstract: A review of the literature on solar-driven water photoelectrolysis cells employing semiconductor electrodes revealed that all devices tested suffered from three major problem areas: corrosion, poor sunlight absorption and external bias requirement. Only certain oxides are stable, but they are not good absorbers of sunlight and require biasing arrangements. The correlation between the external bias requirement and electron affinity of oxides was identified. A series of composite electrodes comprised of a stable wide band gap oxide (TiO2, SrTiO3, Al2O3) and a corrosion prone sunlight absorber (CdS, CdSe, ZnTe, Si) were fabricated and tested. Performance of heterostructure electrodes was shown to be limited either by pin hole problems or by potential barriers between the valence bands. These potential barriers can be minimized by using insulating oxide samples, with high work functions, and the photovoltaic properties of insulating SrTiO3 are presented.

Cobalt oxide as a spectrally selective material for use in solar collectors

Abstract: Cobalt oxide on bright nickel-electroplated steel was investigated for its use as a spectrally selective material in solar collectors. Cobalt was formed electrochemically in a high efficiency cobalt sulphate bath. Oxidation was done in air for two hours at 400°C. Of all bath parameters the pH-value had the greatest influence on the optical properties of the oxide. The best results were obtained at a pH of 2.3. Also the influence of iron(3) sulphate additions in the bath was investigated. We can present three types of cobalt oxide on bright nickel-plated steel, produced under optimal process conditions. At a temperature of 55°C and a current density of 400–800 A/m2 we found for the ratio of the normal solar absorptance and the normal total emittance at 100°C, α n /ϵ n = 0.76/0.05 ( pH =4) ; 0.95/0.11 ( pH =2.3 ); 0.90/0.07 ( pH =2.3 with iron(3) addition). The high absorptance found at pH =2.3 is mainly due to the structure of the upper part of the surface, which is needle-like without and rod-like with an iron(3) sulphate addition. Heating tests at different temperatures both in air and in vacuum showed that of the two coatings with a high solar absorptance, the one made with an iron(3) addition to the bath seemed to be stable up to 300°C.

Electrical and optical properties of CuGaSe2

Abstract: Electrical conductivity, Hall effect and photoluminescence have been measured in CuGaSe 2 single crystals grown by chemical transport. It was found that the electrical and optical properties of this material depend on thermal treatment. Schottky barriers Cu/CuGaSe 2 also have been investigated. Our results show that CuGaSe 2 is promising as a material for solar cells.

Cathodoluminescence characteristics of CuξS films produced by different methods

Abstract: This paper shows how the cathodoluminescence (CL) spectrum of chalcocite (Cu 2,00 S) films like those in CuCdS solar cells can be used to develop information about the opto-electronic properties of the material. Comparison of the CL spectra of bulk polycrystalline samples known to be chalcocite with those of thin films produced by sulfurizing thin films of copper and of thin films produced by the chemical substitution processes commonly used to fabricate CuCdS cells, leads to the conclusion that the CL spectra of the latter may be affected by the presence of Cd as a dopant. This hypothesis was tested by observing the CL spectra of chalcocite samples which were doped with various amounts of cadmium. Comparison of these CL spectra suggest that chalcocite films derived from CdS contain 0.25 and 1.0 wt% Cd. The CL spectra of chalcocite films produced by dipping CdS crystals into aqueous solutions of cuprous chloride exhibit an “ageing” process whose cause has not been identified. Cathodoluminescence was also observed from thin film and sintered samples containing a tetragonal phase (not djurleite) having an approximate composition Cu 1,96 S.

Electric and photovoltaic properties of CdTe pn homojunctions

Abstract: Shallow CdTe pn junctions were prepared by vapour phase epitaxy (CSVT method). Electric conduction is dominated by generation recombination in the space charge region. Low values of saturation current are measured (10−13−10−10 A/cm2) indicating a high quality junction interface. Recombination parameters are also determined: L n = 0.51 μ m , L p = 0.28 μ m and surface recombination velocity S = 10 6 cm/s . The conversion efficiency measured, 8%, corrected for reflexion losses, is mainly limited by surface recombination.

Evaporated layers of cuprous sulfides: Technology and methods of characterization

Abstract: This paper shows how the composition of the CuxS films can be optimized by only monitoring the resistance of the layers during evaporation. This presumed composition x is confirmed by different methods of characterization.

Schottky barrier height, photovoltage and photocurrent in liquid-junction solar cells

Abstract: The height of the Schottky barrier at a CdS electrode in contact with various redox electrolytes has been measured at electronic equilibrium and the open-circuit photovoltage, V oc , has been found being linearly related to this quantity at constant illumination intensity. The photocurrent-voltage curves of such redox electrolyte junctions with CdS, n-GaAs and n-MoSe2 electrodes have been studied as a function of the light intensity which was varied over three orders of magnitude. The photocurrent of such electrochemical solar cells is at the open-circuit voltage compensated by the forward current through the junction. Its magnitude in comparison to the saturation current gives information on whether the recombination or the forward current controls V oc . The application of such studies for the evaluation of electrochemical solar cells is discussed.

CVD molybdenum thin films in photothermal solar converters

Abstract: Molybdenum thin films deposited by pyrolytic decomposition of molybdenum carbonyl attain, after anneal in a reducing atmosphere at 1000°C, infrared reflectance values within 0.7% of the reflectance of supersmooth bulk molybdenum. This result combines the refractory nature of molybdenum with the high infrared reflectance generally associated with conventional mirror materials. The entire production sequence proceeds at one atmosphere pressure and requires less than one hour. Films deposited under oxidizing conditions exhibit a solar absorptance of a = 0.77 and a thermal emittance of e = 0.44 . Absorptance of a = 0.82 with e = 0.08 are obtained by first annealing and then overcoating these black molybdenum films with an antireflecting layer. Changes in the optical properties during annealing are related to compositional and structural properties of the films, as determined by X-ray diffraction, electron microscopy and Auger spectroscopy.

Chromium black coatings for photothermal conversion of solar energy, part I: Preparation and structural characterization

Abstract: The preparation of selective surfaces on steel and copper substrates by black chrome electroplating is described. The structural characterization of these black chrome surfaces by X-ray diffraction and photoelectron spectrometry is reported. Thermal and chemical stability tests have also been carried out on the surfaces. Finally the conversion efficiencies of black solar collectors which use black chrome and black paint absorbers are compared.

Property-composition relationships in sputter-deposited a-Si:H alloys

Abstract: Property-composition relationships in a-Si:H have been determined for films made over a wide range of sputter deposition conditions, including rf-induced dc bias to the substrate. The optical band gap increases from 1.3 to 2.1 eV for H content in the film increasing from 8 to 36 at%, while the index of refraction at 2500 nm shows a corresponding decrease from 3.45 to 2.10. The electrical resistivity at 300 K shows an exponential increase with optical band gap in the range 1.3 to 2.1 eV. The dependence on band gap suggests the onset of intrinsic semiconductor behavior of electrical and optical properties, due to reduction of dangling bond defects to a minimally significant level. Only SiH bonding is observed for the lowest hydrogen concentrations. For hydrogen concentrations of 15–20 at%, mixtures of SiH + SiH2 predominate. For hydrogen concentrations greater than 20 at%, SiH2 or SiH3 or mixtures of SiH2 + SiH3 occur most frequently.