A new model of recombination of carriers at grain boundaries in polycrystalline silicon under optical illumination is presented by considering the monoenergetic density of grain boundary states. Calculations have been performed on the grain boundary barrier heights (Vg), and on the interface and effective recombination velocities of minority carriers as a function of illumination level, grain size (d) and bulk diffusion length of the minority carriers (Lb). These computations show that if the grain size lies in the range Wg < d < Lb (where Wg is the depletion width) and the illumination level is high, the dependence of Vg on grain size and illumination level will be much higher than that in the small and large grain size ranges. It is also found that, in the small grain size range, the dependence of interface and effective recombination velocities on the grain size is quite different, especially when illumination level is low. The dependence of the bending of the minority-carrier quasi-Fermi level in the grain boundary space-charge and quasi-neutral regions on the grain size and on the illumination level is studied. Calculations show that the validity of the quasi-equilibrium assumption decreases as grain size decreases and Vg increases. This model predicts that the existing experimental studies cannot be used to calculate the effective recombination velocity of the minority carriers for polysilicon with small grain sizes. A number of experimental results have been fitted well on the basis of the theory.
The quality of CDS cast polycrystalline silicon has steadily improved over the past decade. Further improvement will require a more detailed understanding and modelling of interactions between the silicon and its crucible, modelling of the solidification process, and characterization of defect and impurity distributions and their effect on carrier transport.
This paper presents the surface analysis of a black copper selective coating using AES and XPS techniques. This ???? indicates that the textured black copper selective coating is a multiphase system. These surfaces are stable up to 250°C in air and under vacuum conditions. Subsequent annealing of these surfaces to temperatures > 370°C shows significant changes. The external environment exposed surface has indicated no significant change.
Perspectives d'utilisation des energies renouvelables en 2020, aux USA et de developpement des centrales heliothermiques
A water/steam receiver system was designed, fabricated, assembled and tested at the solar central receiver facility at the Weizmann Institute.The system is designed for 2 MW(th) input (March 21st, noon time) and generates steam at 15 atg. Nominal maximum energy flux on the evaporating panels is 300 kw/sq.m.The receiver is built as a "C" shaped cavity with three active panels made of 1 inch vertical tubes. Construction details are given. The water is forced circulated at a ratio of 1:25. Steam is separated in a steam drum.The codes used for design and evaluation are: MIRVAL, CAVEG, SHAPEFACTOR, RADSOLVER and SAPPHIR-WIS. These codes were originally developed at Sandia National Laboratories, USA, with the exception of CAVEG which was developed at the Weizmann Institute of Science (WIS), and were modified for cavity conditions. This package proved to be useful and convenient for a parametric study in the assessment of the behavior of the system under extreme conditions (i.e. burnout) and in the evaluation of the test results.A summary of design data, including energy fluxes, temperature profiles and steam generation along the panels is given. Different flow regions in the tubes and their respective transitions under various operating conditions are indicated. Typical test results of the first year are analyzed and compared to computated results. Start-up time and system stability are shown. The minimization of parasitic water volume in the system proved to be helpful in reducing cold start-up time to 50 min and warm start-up time to 35 min.
A new spin cast process to mass-produce polycrytalline silicon sheets for low-cast and reasonable efficiency solar cells has been developed, and new equipment for mass-production has been constructed. In the preliminary process, it was difficult to achieve high throughput and reasonable efficiency at the same time. Using the new process high quality silicon sheets can be produced with a production rate of 15 s/sheet, because a low physical growth rate was maintained in each sheet in spite of high throughput. An average solar cell efficiency of over 13% was obtained.
The survey contains information regarding the history, the objectives, the concept and the contents of the 5th IEA Symposium on Solar High-Temperature Technologies, held at Davos in August 1990. It ends with a short introduction to the basics of solar energy utilization in general and solar high-temperature technology in particular.
We present a detailed study of the electronic transport parameters in amorphous silicon based single junction solar cells. Employing the time-of-flight technique we determine the drift mobility and mobility-lifetime product for electrons and holes, as well as the electric field profile in optimized devices having conversion efficiencies between 10% and 12%. We have also studied the trends in these parameters due to alterations in the device design and material. Here we discuss the importance of buffer layers, and the changes induced by light-soaking. Finally, we discuss our results with respect to future improvements in the conversion efficiency.
Fabrication technologies are presented for large area tandem solar cells. We have attained a total-area efficiency of 10.05% for a 30 × 40 cm2 tandem submodule. For further improvement of conversion efficiency of the tandem cell, we tried to reduce the p-layer thickness. Using ZnO coated SnO2 transparent electrode, we have reduced the p-layer thickness from 12 to 4 nm without decreasing open-circuit voltage and attained a conversion efficiency of 12.0% for a 1 cm2 tandem cell.
The surfaces of selective solar absorbers prepared with an original conversion treatment of ferritic steel are characterized with different methods and more particulary with electrochemical ones (voltammetric and impedance diagram determinations). The coating is a microporous and rough medium presenting a fractal structure. Roughness parameters, fractal dimensions and porosity characteristics are determined and allow the complex material on a rather simple basis to be described.
New constructions for catalytic reactor-receivers of concentrated solar radiation are considered. Theoretical and experimental investigations of heat losses from the cavity of the reactor-receiver and the optimization of regimes were carried out. A peculiarity of solar energy flux conversion was investigated in catalytic reactor-receiver with transparent walls.
The p layer is one of the key factors for improving conversion efficiency of amorphous silicon solar cells. This paper summarizes various approaches for achieving high quality p layers which are used in window layers of solar cells. Theoretical considerations, preparation methods, doping gas, and doping methods are reviewed in conjunction with solar cell structures and characteristics.
A luminiscent solar energy concentrator based on uranyl-ion-doped in poly(methyl methacrylate) (PMMA) has been suggested. A qualitative study shows that alkaline species of uranyl ion with high fluorescence intensity can be a good activator for LSC.
A review is given on recent progress in solar photovoltaic science and engineering in Japan. Firstly, some statistics on the solar cell annual production and related cost transitions are introduced, and key issues to achieve PV utility power generation are discussed. Secondary, recent R&D efforts to improve solar cell efficiencies, and their topics are demonstrated. Finally, the present state of the art in the PV system developments and industrializations are reviewed including the new role for contribution to the environmental issue.
Within a research programme devoted to the study of photoelectrochemical and photovoltaic properties of natural and synthetic FeS2 (pyrite) we have measured the open circuit photopotentials of n-FeS2 single-crystal/aqueous electrolyte junctions. The influence of the electrolyte composition (with or without I− species) and of the polychromatic light intensity (from 20 to 150 mW/cm2) has been investigated. Maximum photopotential output, about 100 mV, was obtained in concentrated KI electrolytes. The variation of photopotential with illumination time has been analyzed in the light of the high impurity content of the natural samples.
Absorption coefficient, α(λ), and energy gap, Eg, of CdSe1−xTex thin films were determined from the measured transmittance and reflectance at normal incidence of light in the wavelength range 450–2500 nm. The thin films were electrochemically prepared on glass plates coated with conducting thin films of SnO2. A combined method from Goodman and Lubberts was used to determine the absorption coefficient and its dependence on the wavelength. The evolution of the optical gap versus the composition of Te in CdSe1−xTex was made and a value of 1.4 eV of the optical gap was obtained for the composition of CdSe0.65Te0.35.
Hall-effect measurements carried-out on n-type InSe thin films obtained by vacuum evaporation and thermally annealed above the onset of crystallization are reported. Annealing is performed at different temperatures and for different periods of time. The temperature dependence of Hall mobility and conductivity is tentatively explained in terms of Petritz' model where the scattering mechanism is due to a potential barrier introduced by the grain boundaries. The effect of grain sizes on the mobility and the barrier height is also observed.
In this paper we describe the preparation of black zincated steel plates and zinc powders. Infrared reflection/absorption (6°, 78°), reflection (6°) and transmission spectroscopy have been used for identification of layers formed on the corresponding substrates. It was found that modification of the existing method proposed by Telkes is necessary in order to obtain zinc powder with sufficient solar absorptance. Colour strengths of the layers and powders formed have been correlated with the existence of various copper(I) and copper(II) oxides in the layers. A negative reflectance peak at 595 cm−1 has been detected in the reflectance spectrum of black zinc powder and its origine was attributed to the coupled νCuO phonon mode with free electron oscillations of metallic zinc.
A new electrolyte has been proposed for the deposition of black cobalt selective absorber coatings. These coatings are used in solar collectors for photothermal conversion of solar energy. We have studied the influence of electrolyte composition and operating parameters on the properties of the black cobalt coatings including optical (α, ϵ) and electcical properties. Thermal stability and corrosion resistance tests showed good durability of black cobalt selective coatings for high temperature applications.
