Defect reduction in electrochemically deposited CdS thin films by annealing in 02

Using the electrochemical deposition method, CdS thin films were deposited from acid solutions (pH = 2.5) containing CdS0₄ and Na₂S₂0₃ on indium-oxide coated glass substrates. These films were annealed in N₂, air, or O₂ atmosphere at 200–500°C for 30 min. Photoluminescence spectra were measured at 77 K. For the films annealed in N₂, the band edge emission became weaker and the luminescence due to defects shifted to longer wavelengths as the annealing temperature was raised above 300°C. However, for the films annealed in air or O₂, the band edge emission was observed strongly irrespective of the annealing temperature and the luminescence due to defects was weak. Thus the O₂ annealing is useful for the defects reduction.     

Phase change stability of sodium acetate trihydrate and its mixtures

Mixtures of NaCH₃COO·3H₂O with NaBr·2H₂O (10 wt%) or NaHCOO·3H₂O were found very stable on repeated phase change (over 1000 cycles) in a heat cycle test (60 30°C), using vertical glass tubes of 950 mm height. On the other hand, NaCH₃COO·3H₂O deteriorated rapidly under similar experimental conditions (70 40°C), settling anhydrous NaCH₃COO. Pressed mixture of anhydrous NaCH₃COO, Na₂HPO₄ and polyethylene powder was found effective as a nucleating agent. A brief discussion on the heat of fusion of a binary mixture is presented in the appendix.     

Preparation and characterisation of a spectrally selective black chrome coating for solar energy applications

Investigations have been carried out on spectrally selective solar absorbing coatings of black chrome prepared by spraying an aqueous solution of CrO₃ on to chemically brightened aluminium substrates heated to 250–300°C. The advantages are good stability up to 250°C, low cost and the fact that the coatings can be used on collectors of any desired size. When subjected to severe humidity conditions the coatings showed signs of degradation. A thin polymer (polystyrene) overcoat prevented such degradation. The coatings have high absorptivities in the solar spectral region (_s ≈ 0·93) and low emissivities in the infra-red region ((100°C) ≈ 0·16). In this paper the coating properties and problems encountered with available selective surfaces are discussed.     

Angle-resolved optical characterisation of an electrochromic device

An electrochromic prototype with WO₃ and NiO as electrochromic layers was analysed in an absolute spectrophotometer. The electrochromic glazing was measured in combination with a clear float glass and a low-e glass in order to simulate a ‘real’ window. Similar measurements were performed on a commercial electrochromic product, i.e., a Gentex Night Vision Safety™ (NVS^®) mirror from Gentex Corporation, and the results were compared. The spectral transmittance was measured, in bleached and coloured state, over the solar wavelength range at the angles of incidence, φ=0, 40, 60 and 70°. The direct solar transmittance, T_sol, the visual transmittance, T_vis, and the angular dependence for these parameters were calculated.     

Concentration of the solar radiation in a solar furnace

Energy concentration in a solar furnace is greatly influenced by the optical accuracy of the reflecting surface of the mirror. The highest density of a heat flux on the heating surface of a specimen is determined by its concentration and by the reflectivity of the mirror.

Since the apparent diameter of the moon is almost equal to that of the sun and the illumination of the moon may be adapted to the photographic determination of flux density, the rate of energy concentration in the authors' solar furnace was studied by the moon's image projected upon the heating zone in place of that of the sun.

As a result, about 300 watts per cm was obtained as the highest possible density of the heat flux in this solar furnace. This figure also agreed with the results of an optical analysis based on the practical finishing of the present mirror surface. With a heat flux of 300 watts per sq cm, in accordance with Stefan-Boltzmann's law of radiation, the highest temperature attainable is estimated to be approximately 2700°K. Besides the above, in actual experiment, approximately 2300°C was obtained in observing the melting points of binary mixtures, e.g., MgO---CoO and MgO---Cr₂O₃. Thus, the above-mentioned three approaches to the problem are in fairly good agreement.     

Novel device structure for Cu(In,Ga)Se2 thin film solar cells using transparent conducting oxide back and front contacts

Cu(In_1−xGa_x)Se₂ (CIGS)-based thin film solar cells fabricated using transparent conducting oxide (TCO) front and back contacts were investigated. The cell performance of substrate-type CIGS devices using TCO back contacts was almost the same as that of conventional CIGS solar cells with metallic Mo back contacts when the CIGS deposition temperatures were below 500 °C for SnO₂:F and 520 °C for ITO. CIGS thin film solar cells fabricated with ITO back contacts had an efficiency of 15.2% without anti-reflection coatings. However, the cell performance deteriorated at deposition temperatures above 520 °C. This is attributed to the increased resistivity of the TCO’s due to the removal of fluorine from SnO₂ or undesirable formation of a Ga₂O₃ thin layer at the CIGS/ITO interface. The formation of Ga₂O₃ was eliminated by inserting an intermediate layer such as Mo between ITO and CIGS. Furthermore, bifacial CIGS thin film solar cells were demonstrated as being one of the applications of semi-transparent CIGS devices. The cell performance of bifacial devices was improved by controlling the thickness of the CIGS absorber layer. Superstrate-type CIGS thin film solar cells with an efficiency of 12.8% were fabricated using a ZnO:Al front contact. Key techniques include the use of a graded band gap Cu(In,Ga)₃Se₅ phase absorber layer and a ZnO buffer layer along with the inclusion of Na₂S during CIGS deposition.     

Low temperature μc-Si film growth using a CaF2 seed layer

This paper describes low temperature thin film Si growth by remote plasma chemical vapor deposition system for photovoltaic device applications. Using CaF₂/glass substrate, we were able to achieve an improved μc-Si film at a low process temperature of 300°C. The μc-Si film on CaF₂/glass substrate shows that a crystalline volume fraction of 65% and dark conductivity of 1.65×10⁻⁸ S/cm with the growth conditions of 50 W, 300°C, 88 mTorr, and SiH₄/H₂=1.2%. XRD analysis on μc-Si/CaF₂/glass showed crystalline film growth in (1 1 1) and (2 2 0) planes. Grain size was enlarged as large as 700 Å for a μc-Si/CaF₂/glass structure. Activation energy of μc-Si film was given as 0.49 eV. The μc-Si films exhibited dark- and photo-conductivity ratio of 124.     

Phase change stability of CaCl2·6H2O

A CaCl₂·6H₂O composition with a slight excess of water was found very stable on repeated phase change (over 1000 cycles) in a heat cycle test (35 18°C), using vertical glass tubes of 950 mm height. The mole ratio of water/CaCl₂, n, was 6.11, less than that of the peritectic composition (n = 6.14). Dissolved NaCl had superior nucleating ability to that of common barium salts under certain experimental conditions. This result was attributed to a “memory effect.” The dissolved NaCl decreased the heat of vaporization of water about 15 per cent. NaF had a similar effect to that of NaCl, because NaF powder reacted with CaCl₂ in solution yielding NaCl and CaF₂. A brief discussion of CaCl₂·4H₂O formation is presented.     

Superstrate-type CuInSe2-based thin film solar cells by a low-temperature process using sodium compounds

Superstrate-type solar cells with a Au/CuInSe₂(CIS)/In_xSe_y,/ZnO : Al/glass structure were investigated. The CIS films were deposited by coevaporation method with intentionally incorporated Na₂S at a substrate temperature of 350°C. Even at relatively low substrate temperatures, sodium compounds enhanced the (1 1 2) preferred orientation of the chalcopyrite structure, and also improved the cell performance. The In_xSe_y buffer layers disappeared after CIS deposition by interdiffusion. Preliminary cells yielded an efficiency of 7.5% with V_oc, = 430 mV, J_sc = 29.4 mA/cm² and FF = 0.60. The light soaking and forward bias effects were observed for these cells.     

Investigation of evacuated tube heated by solar trough concentrating system

Two types of solar evacuated tube have been used to measure their heating efficiency and temperature with fluids of water and N₂ respectively with a parabolic trough concentrator. Experiments demonstrate that both evacuated tubes present a good heat transfer with the fluid of water, the heating efficiency is about 70–80%, and the water is easy to boil when liquid rate is less than 0.0046 kg/s. However, the efficiency of solar concentrating system with evacuated tube for heating N₂ gas is less than 40% when the temperature of N₂ gas reaches 320–460 °C. A model for evacuated tube heated by solar trough concentrating system has been built in order to further analyze the characteristics of fluid which flow evacuated tube. It is found that the model agrees with the experiments to within 5.2% accuracy. The characteristics of fluid via evacuated tube heated by solar concentrated system are analyzed under the varying conditions of solar radiation and trough aperture area. This study supports research work on using a solar trough concentrating system to perform ammonia thermo-chemical energy storage for 24 h power generation. The current research work also has application to solar refrigeration.     

Investigation of a eutectic mixture of sodium acetate trihydrate and urea as latent heat storage

In this paper, the pseudobinary system CH₃COONa·3H₂O---CO(NH₂)₂ is studied by means of differential scanning calorimetry (DSC). Its eutectic mixture is found to melt congruently at 30°C, its heat of fusion is 200.5 J/g, considering the temperature of phase change and its heat storage capacity. This eutectic mixture is an excellent material for latent heat storage of solar energy.     

Fertilizer solar ponds as a clean source of energy: Some observations from small scale experiments

Commercially available NH₂CONH₂ is used to establish a salinity gradient solar pond in a small 1 m² outdoor tank. With a salinity difference of 35% between the upper and lower zone, a temperature difference of 23°C was obtained without any instabilities in the gradient zone. The difference in concentration of solution required to sustain a temperature difference of 40°C across the gradient zone is 520 kg/m³. By economically using runoff into the fertilizer cycle of an agricultural system the estimated cost of fertilizer solar pond generated heat is Rs. 1.10/kWh.     

Heat loss factor of evacuated tubular receivers

Tubular receivers with an evacuated space between the absorber and concentric glass cover to suppress convection heat loss are employed as absorbers of linear concentrators in the intermediate temperature range. A knowledge of their heat loss factor is important for a study of the thermal performance of such solar concentrating systems. The heat loss factor of a collector can be calculated by solving the governing heat transfer equations or estimated from an empirical equation, if available. The governing equations must be solved simultaneously by iterations, but this is tedious and cumbersome. Although several correlations exist for determining the heat loss factor for flat-plate collectors and non-evacuated tubular absorbers of linear solar collectors, there is no available correlation for predicting the heat loss factor of evacuated receivers.

A correlation to calculate the heat loss factor (U_L) of evacuated tubular receivers as a function of variables involved (absorber temperature, emittance, diameter and wind loss coefficient) has been obtained. The correlation developed by a least square regression analysis predicts the heat loss factor to within ±1.5% of the value obtained by exact solution of the simultaneous equations in the following range of variables: wind loss coefficient, 10–60 W/m²°C; emittance, 0.1–0.95; and absorber temperature, 50–200°C.     

Electrical properties of CuInSe2 films prepared by evaporation of Cu2Se and In2Se3 compounds

We prepared CuInSe₂ films by evaporating In₂Se₃ and Cu₂Se compounds instead of elemental sources. The resulting CuInSe₂ film grown at 680°C had a smooth and dense microstructure with the grain size of 2 3 μm. But the CuInSe₂ films were Cu-rich, with a low resistivity of about 0.1 Ω cm. So we conducted H₂ post annealing to control the electrical resistivity and composition of CuInSe₂ films. In a H₂ atmosphere, the resistivity increased to about 100 Ω cm by annealing at 350°C for 1 h. The resistivity decreased again when the annealing temperature was above 350°C. This resistivity change might be related to the contents of Cu, In, Se atoms and the valency states of Cu and In ions in the films. We discussed the reason of resistivity change caused by H₂ post annealing in this paper.     

Optical and electrochemical characteristics of niobium oxide films prepared by sol-gel process and magnetron sputtering A comparison

Electrochromic niobia (Nb₂0₅) coatings were prepared by the sot-gel spin-coating and d.c. magnetron sputtering techniques. Parameters were investigated for the process fabrication of sol-gel spin coated Nb₂0₅ films exhibiting high coloration efficiency comparable with that d.c. magnetron sputtered niobia films. X-ray diffraction studies (XRD) showed that the sot-gel deposited and magnetron sputtered films heat treated at temperatures below 450°C, were amorphous, whereas those heat treated at higher temperatures were slightly crystalline. X-ray photoelectron spectroscopy (XPS) studies showed that the stoichiometry of the films was Nb₂0₅. The refractive index and electrochromic coloration were found to depend on the preparation technique. Both films showed low absorption and high transparency in the visible range. We found that the n, k values of the sot-gel deposited films to be lower than for the sputtered films. The n and k values were n = 1.82 and k = 3 × 10⁻³, and n = 2.28 and K = 4 × 10⁻³ at 530 urn for sot-gel deposited and sputtered films, respectively. The electrochemical behavior and structural changes were investigated in 1 M LiC10₄/propylene carbonate solution. Using the electrochemical measurements and X-ray photoelectron spectroscopy, the probable electrode reaction with the lithiation and delithiation is Nb₂O₅ + x Li⁺ + x e⁻ ↔ Li_xNb₂0₅. Cyclic voltametric (CV) measurements showed that both Nb₂0₅ films exhibits electrochemical reversibility beyond 1200 cycles without change in performance. “In situ” optical measurement revealed that those films exhibit an electrochromic effect in the spectral range 300 < λ < 2100 nm but remain unchanged in the infrared spectral range. The change in visible transmittance was 40% for 250 nm thick electrodes. Spectroelectrochemical measurements showed that spin coated films were essentially electrochemically equivalent to those prepared by d.c. magnetron sputter deposition.     

Heat loss coefficients for box-type solar cookers

The top and overall heat loss coefficients for the entire feasible operating range of box-type solar cookers are evaluated experimentally and presented in a graphical form as a function of the difference between mean plate temperature and ambient temperature with wind velocity and number of glass covers as parameters.

The range of plate temperatures considered is from 50°C to 180°C. While the wind velocity is varied from 0 to 3.33 m/s, and the number of glass covers considered are from 1 to 4.

Based on these experimental results, a correlation for the determination of top loss coefficient in terms of optical properties of cooker, the spacing between glass cover and absorber plate, wind velocity and number of glass covers, is derived and presented in the paper.     

Formation of CuInSe2 and Cu(In,Ga)Se2 films by electrodeposition and vacuum annealing treatment

Polycrystalline thin films of CuInSe₂ and Cu(In,Ga)Se₂ (CIGS) were grown on both polished Mo substrates and Mo-coated glass substrates by one-step electrodeposition. All the as-deposited films have been annealed in vacuum at 450°C for a short time to improve the crystalline properties. The films have been characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. The results indicate that the crystallization of the films was greatly improved after annealing. Further more, a CIGS film with 23 at% Ga was obtained.     

Growth kinetics and structural characterization of polycrystalline CdTe films grown by hot-wall vacuum evaporation

A systematic growth of CdTe films on two kinds of substrates, Corning 7059 glass and SnO₂-coated glass, has been performed by hot-wall vacuum evaporation (HWVE). From the dependence of deposition rate on source temperature, the growth in the HWVE system is limited by the transport of CdTe vapor from source to substrate. The deposition rate of CdTe films grown on either 7059 glass or SnO₂-coated glass gradually decreases with increasing substrate temperature, while the grain sizes increase with substrate temperature. X-ray diffraction showed that all of the CdTe films on either Corning 7059 glass or SnO₂-coated glass deposited at substrate temperature below 275°C have a preferential orientation of the (111) planes parallel to the substrate. However, the degree of the preferential orientation in the films at substrate temperature of 300°C reduced remarkably. High-energy shift of absorption edge in the grown CdTe films were also observed in transmittance spectra.     

Cadmium telluride films prepared by pulsed electrodeposition

Cadmium telluride polycrystalline films were deposited on various transparent semiconductors on glass using periodic pulse electrolysis from an aqueous solution of Cd²⁺ and HTeO₂⁺ ions. Substrates included fluorine doped tin oxide/glass, tin oxide/indium tin oxide/glass and on those substrates with an electrodeposited cadmium sulphide film on the oxide. The properties of the deposited films were determined as a function of variables, viz. initial cathodic voltage V₁, on-time t₁, second cathodic voltage V₂, on-time t₂, solution concentration and type of substrate. Film quality was judged by adherence, continuity, optical quality, composition and morphology. The preferred deposition conditions (versus SCE) were V₁ = −0.76 V, t₁ = 1 s, V₂ = −0.60 V, t₂ = 0.1 s using a stirred 90°C solution with composition 2.5 M Cd²⁺, 160 ppm HTeO₂⁺ and pH of 1.7. Films deposited under those conditions were cubic polycrystals. X-ray diffraction spectra showed reflections from the (111) (220) and (311) planes with the most intense being the (111) reflection. As the concentration of species in the solution decreased, the reflection intensities from the (220) and (311) planes decreased relative to the (111) reflection. After annealing under conditions to type convert the n-CdTe to p-CdTe, the crystallinity improved and if a CdS layer was present, the (220) and (311) planes were further developed relative to the (111) plane. Resistivity through the film was (3.0±0.8)×10⁸) Ω cm but reduced to (1.0±0.3) × 10³ Ω cm after annealing. The band gap was 1.48±0.03 eV for both deposited and annealed films. There was a limited range over which the deposition variables could be altered. The pulse duration for the more cathodic phase needed to be longer than the less cathodic phase for adherent films. Better adherence was achieved when pulse durations were greater than 0.1 s, especially for the more cathodic phase. The magnitude of the pulse duration and potential in each phase of a particular cycle determined whether the deposited film was rich or deficient in cadmium and whether the film adhered.     

Monitoring the global climatic impact of direct heat addition associated with escalating energy use

Using Budyko's overall heat-balance equation, we estimate that direct heat addition associated with worldwide energy use in the year 2050 will be responsible for a mean global temperature rise of 0.27 °C at a 20 kw_t per capita energy consumption for a world population of ten billion people. The corresponding temperature rise between 15 and 60 °N is estimated to be 0.44 °C. If per capita energy consumption during the year 2050 is reduced to 5 kw, (i.e. about one half of U.S. consumption in the year 1970), the estimated temperature rise for the 15–60 °N latitudinal belt will be about 0.11 °C and therefore still not negligibly small.

A program for monitoring the global climatic impact of escalating energy use involves precise monitoring of the following quantities:

1. (a) the solar constant

2. (b) the effective earth-atmosphere albedo

3. (c) the net (long-wavelength) radiant energy emitted from the earth-atmosphere system.

Both the effective albedo and the (long-wavelength) radiant energy emitted from the earth-atmosphere system will depend on the nature and size of particulate concentrations in the atmosphere, on molecular emitters (especially CO₂ and H₂O), cloud cover, and on the radiative-convective circulation pattern. A satellite observation program that is closely integrated with ground-based and atmospheric measurements and with a detailed program of theoretical analysis will be needed for more precise predictions of inadvertent climate changes and for developing the means to effect desirable global climate controls.