Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Patent
1994-02-14
1995-12-26
Monin, Jr., Donald L.
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
257440, 257656, H01L 3104
Patent
active
054790433
DESCRIPTION:
BRIEF SUMMARY
The invention relates, in general, to converting light energy, in particular solar energy, into electrical energy by the photovoltaic effect produced in semiconductors.
Components implementing this effect and generally called "solar cells" normally make use of one species of semiconductor material only, generally silicon or gallium arsenide GaAs.
Nevertheless, it is known that under such circumstances, the complete transformation into electrical energy of the light energy contained in solar radiation cannot be obtained because of the spread in the solar spectrum. Any given semiconductor material has a forbidden band of determined width such that photons of energy below said band width are never absorbed and therefore cannot generate the electron-hole pairs necessary for producing photocurrent. FIG. 1 shows the spectral characteristic of sunlight (ignoring atmospheric absorption), as a plot of radiant flux density as a function of the wavelength or (and this is equivalent) as a function of photon energy. When gallium arsenide is used as the material, the width of the forbidden band is 1.43 eV, corresponding to a wavelength .lambda..sub.0 of 867 nm. Photons of energy below 1.43 eV (i.e. of wavelength greater than 8.67 nm) produce no photovoltaic effect, such that the light energy corresponding to the area referenced I is never transformed in any way into electrical energy. In the case of GaAs, this loss of light energy by "transparency" represents 40% of the total available energy in the case of GaAs.
Photons of energy greater than the width of the forbidden band (i.e. of wavelength less than .lambda..sub.0) will give rise to electron-hole pairs, but with excess energy relative to the energy of the forbidden band, which excess is converted into heat and not into electrical energy. Such losses by excess energy, corresponding to area II in FIG. 1, may be as much as 20% of the total energy. The use of only one species of semiconductor in a solar cell thus makes it impossible to take advantage of the entire solar spectrum; the intrinsic limit on conversion efficiency for GaAs is thus of the order of 40% (area III in FIG. 1), whereas if it is illuminated with monochromatic light at an energy of (1.43+0.1) eV, the intrinsic efficiency of the material is close to 95%.
To improve the conversion of solar energy, several solutions have been proposed, in which a plurality of different semiconductors having different forbidden bandwidths are associated. Such components are referred to as "multispectral solar cells" and they can be subdivided into three families corresponding to three different basic configurations.
In a so-called "dichroic" first configuration, an optical separator system having dichroic mirrors splits the incident solar spectrum into a plurality of portions corresponding to sub-bands of the spectrum. Each of these portions is applied to a solar cell of a different type, optimized for a given photon energy.
That configuration is effective, but it requires a complex optical system to be implemented that is bulky, fragile, and expensive.
A so-called "monolithic" second configuration consists in providing a stack of solar cells made up of successive layers grown epitaxially on a common substrate, the various cells being electrically coupled together in series by tunnel junctions. The first cell captures the most energetic photons of the incident flux, while passing the others that are absorbed by the next cell down, and so on.
Because of its monolithic character, said second configuration is extremely compact and robust, but it nevertheless suffers from several drawbacks.
A first drawback stems from the fact that it is not possible to associate silicon cells whose advantages and ease of manufacture are well known with GaAs cells since it is not known at present how to make a tunnel junction between silicon and gallium arsenide.
A second drawback stems from the fact that although it is known how to make tunnel junctions between III-V semiconductors by epitaxy, providing said materials have crystal lattice c
REFERENCES:
patent: 4846931 (1989-07-01), Gmitter et al.
Charles Kittel, "Introduction to Solid State Physics" 1971 pp. 100-104.
Y. Matsumoto, "A New Type of High Efficiency With a Low-Cost Solar Cell Having the Structure of a uc-SiC/Polycrystalline . . . ", J. Applied Physics, vol. 67, No. 10, May 15, 1990, pp. 6538-6543.
Y. Kishi et al., "Ultralight Flexible Amorphous Silicon Solar Cell & Its Application for an Airplane", 5th In'l Photovoltaic Science & Eng. Conf., Nov. 26, 1990, pp. 645-648.
B. J. Stanbery et al., "Lightweight Tandem GaAs/CuInSe2 Solar Cells", IEEE Transactions on Electron Devices, vol. 37, No. 2, Feb. 1990, pp. 438-442.
E. Yablonovitch et al., "Van Der Waals Bonding of GaAs Epitaxial Liftoff Films onto Arbitrary Substrates", Applied Physics Letters, vol. 56, No. 24, Jun. 11, 1990, pp. 2419-2421.
E. Yablonovitch et al., "Van Der Waals Bonding of GaAs on Pd Leads to a Permanent, Solid-Phase-Topotaxial, Metallurgical Bond", Applied Physics Letters, vol. 59, No. 24, Dec. 9, 1991, pp. 3159-3161.
Monin, Jr. Donald L.
Picogiga Societe Anonyme
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