DE2806881A1 - PHOTOCELL WITH LIQUID/SEMICONDUCTOR TRANSITION WITH GAAS ELECTRODE - Google Patents

Description

BLUMBACH. WESER. BERC

ZWSRNER. DEER . BREHSVS

O ρ rj r p Q 1 PATENT LAWYERS IN MUNICH AND WIESBADEN -DOO!

• 3-

Patentconsult Radeckestraße 43 8000 Munich 60 Telephone (089) 883603/883604 Telex 05-212313 Telegrams Patentconsult Patentconsult Sonnenberger Straße 43 6200 Wiesbaden Telephone (06121) 562943/561998 Telex 04-186237 Telegrams Patentconsult

Western Electric Company, Incorporated K "C Chang 2-5-10

Broadway, New York, NY 10038,
United States

Photo cell with liquid / semiconductor transition with GaAs electrode

Description:

The invention relates generally to liquid / semi-conductor photocells ter-transition.

In view of the expected shortage of fossil fuels As energy sources, extensive research has been carried out in recent years into alternative energy sources find and develop. Such alternative energy sources also include solar energy, which is based on either direct way by means of photovoltaic working devices

Munich: R. Kramer Dipl.-Ing. · W. Weser Dipl.-Phys. Dr. rer. nat. · P. Hirsch Dipl.-Ing. . H. P. Brehm Dipl.-Chem. Dr. phil. nat. Wiesbaden: P. G. Blumbach Dipl.-Ing. - P. Bergen Dipl.-Ing. Dr. jur. · G. Zwirner Dipl.-Ing. Dipl.-W.-Ing.

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is formed, since the production of such transitions is relatively complex Require measures such as epitaxy or diffusion processes; in contrast, with the cells under consideration The liquid / semiconductor interface forms the transition spontaneously.

So far, the full success of these cells has been hampered by various difficulties. This includes the problem that some semiconductor materials that are useful for an effective conversion of solar energy into electrical energy Bandat »stand, and therefore as material for the fixed electrode are aimed at are not stable under the working conditions of such a cell. This leads to, that the efficiency of such a cell decreases with increasing operating time for various reasons. For example The photo-excitation can create holes in the surface of the solid, which react chemically with the electrolyte. These and other other reactions cause corrosion and / or passivation of the semiconductor surface, which leads to deterioration of the efficiency of the cell, which can be seen from the fact that with increasing operating time the output of the cell Photocurrent decreases. Chemical etching of the semiconductor surface can be added to the other reactions acting in this direction or the deposition of contaminants on that surface.

Gallium arsenide has a band gap of approximately 1.4 eV

. ο '

on. It is theoretically expected that a band gap of approx this order of magnitude has the greatest efficiency in the photovoltaic "conversion of solar energy into electrical energy Energy guaranteed; therefore, for economic reasons, a cell with an electrode made of this material would be the over provides a steady photocurrent for extended periods of time, extraordinary he wishes. So far, however, cells with GaAs electrodes have proven to be particularly sensitive to one Electrode degradation or electrode deterioration proven; the photochemical reactions of the GaAs electrode with the redox electrolyte have led to a decrease in the photocurrent emitted by the cell. The ones described in the specialist literature Photocells with liquid / semiconductor transition with GaAs electrodes, as described by Gerischer in Journal Electroanalytical Chemistry and Interfacial Electro Chemistry, 5.8,263 (1975) and by Brighton in Bulletin of the American Physical Society, 22, 60 (1977) had too short a lifespan for such cells to be seriously considered a promising commercial Application would have come into question; furthermore, these cells were not capable of generating a photocurrent in a useful Order of magnitude.

Based on this, the object of the present invention is to provide photocells with a photovoltaic junction between a gallium arsenide electrode and a liquid, a redox couple containing electrolyte to provide, which over longer Periods of time have a high degree of efficiency.

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The inventive solution to this problem is with the characterizing Features of claim 1 reproduced. Advantageous embodiments of the invention emerge from the Subclaims.

With the invention is thus a photocell with a photovoltaic Transition between gallium arsenide and a liquid electrolyte containing a redox couple provided, wherein the potential of the redox couple is more negative than -0.5 volts, and the concentration of the redox couple is greater than 0.1 molar concentration.

It has been found that under these conditions gallium arsenide as a semiconductor electrode in a photo cell with a liquid / semiconductor transition has a significantly increased service life.

A photocell according to the invention with a G-aAs electrode has a high efficiency of approximately 3% when converting solar energy into electrical energy and supplies a reasonably constant photocurrent. Be the lifespan Rann more than 1 year, below is the form of a photoelectric cell according to the invention in detail with reference to Pig an exemplary execution. 1 to 4 described; show in detail:

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Pig. 1 shows the theoretically expected efficiency in the form of a graph Energy conversion of light of the solar spectrum by means of a device with photovoltaic Transition as a function of the band gaps of the various semiconductor materials;

Pig. 2 a schematic representation of a photocell with liquid / semiconductor transition;

Mg. 3 graphically shows the ratio of corrosion current to total current in Dependence on the selenide concentration; and

Fig. 4- in the form of a graphical representation of the photocurrent occurring during operation of a photocell -. density depending on the operating time.

With Fig. 1, the efficiency in the conversion of solar energy into electrical energy is dependent in an idealized form from the band gap in semiconductor materials, starting from the solar spectrum. For each value of the Band gap results in the efficiency range from the various atmospheric conditions and certain assumptions about expected voltage losses. As can be seen from the illustration is corresponds to gallium arsenide with a band gap

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of approximately 1.42 eV to a large extent the material which should theoretically have the greatest efficiency.

To the one with Pig. The cell shown in FIG. 2 includes a container 20 in which the electrolyte 21 is located, the counter electrode 22 (which consists of carbon in the "cells under consideration", however can also consist of another inert material ") and the active electrode 25. An aqueous electrolyte is usually provided, although non-aqueous electrolyte is also provided. Electrolytes such as Propylene carbonate or tetrahydrofuran can be used. At the points that are not irradiated or activated, the active electrode 23 is insulated with a coating of epoxy resin 24. The container 20 may be of any conventionally accessible type Consist of glass or plastic. The bottom of the cell opposite the electrode 23 is made of transparent material, so that the incident light can pass through, as indicated by the arrow.

When irradiated in a suitable electrolyte, typically an aqueous electrolyte, holes occur in the surface of the η-conductive gallium arsenide, which leads to an oxidative Dissolution of GaAs according to the following reaction

GaAs + 6h ⁺ - ^ Ga ³⁺ + As ³⁺

leads. If this is the only reaction that occurs, the material is photoetched. The photoetch reaction

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can be suppressed if a competing reaction can be provided which leads to a rinsing or cleaning of the holes and competes directly with the photoetching reaction; although even under such conditions it will not be possible to completely suppress photoetching. It has been found that more efficient flushing of the holes occurs as the redox potential of the electrolyte becomes more negative. However, unless the redox potential is more negative than -0.5 volts, it is impossible to flush the holes sufficiently to reduce the photocorrosion on the GaAs to an acceptable fourth. The redox pairs Se "/ Se ^ ^- and

O O mm

Te " / ^ι Σβ2 ~ or a mixture of these fulfills this condition and suppresses the photoetching in GaAs cells sufficiently so that such cells can be used in practice. The selenium takes up charges on the irradiated electrode, for example in accordance with the reaction

2Se ² "" + 2h ⁺

The reaction takes place at the dark electrode

Se ² "+ 2e ·" 2Se ² "

so that there is no chemical change in the cell. Suitable concentrations for the redox electrolyte are between one Maximum value for a saturated solution and a minimum value at approximately 0.1 molar concentration; this minimal, approximated

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0.1 molar concentration is required in aqueous solution to sufficiently "consume" the holes when exposed to sunlight "is irradiated, so that as a result, excessive photoetching is prevented will. Instead of aqueous electrolytes, other electrolytes can also be used; however, since this is usually a lower one Have electrical conductivity, this also reduces the efficiency of the cell. The "at high concentrations Light absorption that occurs in the electrolyte can be compensated for by only providing a thin layer of liquid will.

The diselenide ions and polyselenide ions can be dissolved in the solution by introducing HpSe into a basic solution such as aqueous KOH solution produces v / earth, allowing air oxidation,

2— 2—
thus, to a certain extent, Se is oxidized _{to Se 2;} alternatively, metallic selenium can be dissolved directly in the "basic solution. Instead of KOH lye, other bases can also be used, such as NaOH lye or NH.OH solution.

In the photocells just described, the active electrode 23 consists of an η-conductive GaAs single crystal with a Thickness of 600 µm; the electrolyte contains the redox couple Se / Se ^. The cells were given different selenide concentrations

2— 2—

operated, with an Se / Se ^ ratio of approximated in each case 8 has been complied with; the research was weight loss measured by the GaAs electrode. One was used for lighting Quartz halogen lamp used, which was operated with the proviso

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that an approximately equivalent degree of illumination of 3 suns was obtained under air mass-2 conditions. Here correspond the air mass 2 conditions approximated the solar radiation in the afternoon in mid-latitudes. To the GaAs electrode a controlled potential of -0.4 Y was applied to a saturated calomel electrode; this corresponds to a positive potential of 0.56 V of the solution "when using Fermi units. This value is 1 V more oxidizing than the value measured at maximum energy output from the cell will; furthermore, under these conditions, the rate of photocorrosion should be significantly greater than when operating the cell at its maximum efficiency, although the interface potential that actually occurs is not known. 3 shows, in the form of a graph, the dependence of the ratio of the corrosion current to the total current represented by the selenide concentration. The one shown in dashed lines Curve corresponds to the following equation

- £ ■ = (1 + 35OOcJ _e r ¹

where C «for the total molar selenium concentration in the solution stands. Based on the stoichiometry of the dissolution reactions, according to which 6 electrons are released when 1 molecule of GaAs is converted, the weight loss measured at the electrode was converted into corrosion current; the results were due to the heating speed at a selenide concentration of O confirmed.

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The ordinate values correspond to that part of the photoetrome, which does not take the desired path to regenerate the Solar cell goes off and therefore causes the etching. How out As can be seen from the illustration, high concentrations of selenide cause a relatively small corrosion current, with it fails to completely suppress the corrosion current.

For the current and voltage of a typical cell with an n-conducting GaAs electrode and an aqueous electrolyte (KOH lye, about 1 molar total selenium concentration), the following “values are obtained when operated under sunlight 69 m ^T .i / cm a short-circuit current of 16.5 mA / cm is obtained, which corresponds to a quantum yield of approximately 65a> for the solar spectrum under air in-mass conditions 2. The maximum power is 0.45 volts and 13.5 mA / cm, which leads to a fill factor of approximately 57 ° ό; this results in an efficiency of approximately $ 8.8. The crystal used as an active electrode in this cell was doped with tin (Sn) and had a concentration of free Electrons of approximately 2 × 10 / cm. Instead of this special dopant, other materials with good n-conductivity, such as silicon (Si) and tellurium (Te), can be used, since the dopant is obviously of no particular importance ommt. It will be a

upper limit for the content of dopant of less than 5 x 1 Pt "* i

Aimed at 10 / cm. The space charge layer is above this value too thin so that it is essentially not possible to absorb all of the light within the space charge layer;

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the resulting recombination of charge carriers reduces the efficiency of the cell,

With Pig. 4 shows in the form of a graphic representation for this cell the dependence of the photocurrent on the time during a longer test; For operation, a quartz-halogen lamp was irradiated, the degree of illumination of which corresponds to 3 suns under air mass 2 conditions. “Here, an essentially constant photocurrent was obtained over a period of more than 400 hours. Thus, for this approximately 1 molar total selenium concentration, an i / i ratio of approximately 0.001 is obtained, which corresponds to an average corrosion depth of less than 12 μm over the test, which lasted more than 400 hours . The constancy of the output current is not only based on the suppression of the photoetching reaction, but also on the fact that the properties of the semiconductor surface are continuously maintained during cell operation. As long as the final life of the cell is guaranteed, a limited and controlled removal of semiconductor materials can remove the deposition of impurities on or near the semiconductor surface , such impurities can be deposited from the electrolyte, or such impurities can be produced as a product a chemical reaction between GaAs and the electrolyte occurs; As a result, the controlled removal of semiconductor material ensures that the original properties and the quality of the transition are retained.

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In the cells described, an electrode made of a GaAs single crystal has been used; the basics, the one constant photocurrent in a cell with a GaAs single crystal electrode ensure, but can also be used on cells with polycrystalline GaAs electrodes.

Furthermore, the usefulness of the cells as solar cells has been described; however, it is evident that these cells are not only useful for converting sunlight but are also able to convert other radiant energy into electrical energy; therefore it is appropriate to this Cells as photocells rather than solar cells.

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e r s e 11 e

Claims (4)

ZWIRNER. HIRSCH BREHSVl oonroo-j / oüboo f PATENTANWÄLTE IN MUNICH AND WIESBADEN Patentconsult Radeckestrasse 43 8000 Munich 60 Telephone (089) 883603/883604 Telex 05-212313 Telegrams Patentconsult Patentconsult Sonnenberger Strasse 43 6200 Wiesbaden Tel Telegrams Patentconsult Western Electric Companys Incorporated K “C Chang 2-3-10 Broadway, New York, NoYo 10038, USA Photo cell with ITliquid / semiconductor junction with GaAs electrode 1. \ photocell with a photovoltaic junction between gallium arsenide (GaAs) and a liquid electrolyte containing a redox couple, characterized in that the potential of the redox couple is more negative than -0.5 volts; and the concentration of the redox couple is greater than 0.1 molar concentration. 745 Munich: R. Kramer Dipl.-Ing. · W. Weser Dipl.-Phys. Dr. rer. nat. · P. Hirsch Dipl.-Ing. · H. P. ßrehm Dipl.-Chem. Or. Phil. nat. Wiesbaden: P. G. Blumbach Dipl.-mg. - P. Bergen Dipl.-Ing. Dr. jur. · G. Zwirner Dipl.-Ing. Dipl.-W.-Ing. 2. photocell according to claim 1,
characterized in that the electrolyte (21) consists of a solution that either Selenium anions or telluran ions or a mixture of these anions contains. 3. Photocell according to claim 1 or 2, characterized in that da3 the electrolyte (21) as a solvent or propylene carbonate Contains tetrahydrofuran. 4. Photocell according to one of claims 1 to 3, characterized in that the gallium arsenide of the active electrode (23) in the form of a η-conducting GaAs sinking crystal is present, which with tin (Sn), Silicon (3i) or tellurium (Te) is doped, 18 3 where the dopant concentration is below 5 × 10 / cm.