Solar Cells and Collector Structures

This invention in an important aspect concerns the art of exposing semiconductors to sunlight to convert at least a part thereof to electrical energy. Such means for converting electromagnetic energy to electricity are known as photovoltaic cells or photocells, and common examples of such photocells are silicon or gallium arsenide semiconductors having P-N junctions. Commonly, an electrical lead is connected on either side of the semiconductor across the P-N junction.

Semiconductor photocells are very expensive; in consequence, it has been the practice to gather and concentrate the sunlight reaching a given semiconductor photocell so that extremely large areas of semiconductor material need not be employed as would be necessary without such a gathering system. The common gathering systems in the past were optical systems, wherein lens systems concentrated the light and focused same on a given photocell.

However, such a lens system was and is relatively expensive and is not useful in diffuse light on a cloudy day. More recently, however, there has been conceived a different type of collector and concentrator of radiation to be impinged on a semiconductor photocell. For instance, Weber and Lambe in Applied Optics, Vol. 15, pages 2299-2300, October 1976, disclose a system whereby a large area sheet of material, such as a rigid plastic or a glass doped with a luminescent material is exposed to solar radiation. The luminescent medium ideally has a strong absorption for the sun's rays, especially in the visible where the solar spectrum peaks, and it emits electromagnetic radiation of a longer wavelength suitable for activating the semiconductor photocell. A large portion of the light emitted from the luminescent species is in effect trapped in the collector with essentially total internal reflection until the light reaches the area where a photocell, such as a silicon photocell, is optically coupled to a small area, for instance an edge, of the collector. In this way, the light from the sun is not only converted to more suitable wavelengths for activation of the photocell but is concentrated since the light received by the large area of the collector escapes only in the small area where the photocell is optically connected to the collector.

Another article, by Levitt and Weber, appears in Applied Optics, Vol. 16, No. 10, pages 2684-2689, October 1977, should be read with the article first mentioned.

Other publications aiding in the understanding of the setting of the present invention include Goetzberger, Applied Physics, 14, 123-139, 1977, German patent application No. 2620115, published Nov. 10, 1977 (not published before my conception, however) and, referred to in the latter patent application, German patent application No. 2554226, published June 8, 1977, which is of some peripheral interest.

Also, numerous patents deal with the conversion of solar energy to different wave lengths by means of luminescent or fluorescent layers and impinging emitted light on a photocell; examples are U.S. Pat. Nos. 3,426,212 and 3,484,606, which patents, however, do not have the concept of concentrating the light from a large area and collecting it over a much smaller area by optical coupling to a relatively small area semiconductor photocell.

In the recent prior art structure of the type disclosed in the aforesaid Weber and Lambe publication, which describes what is there termed a "luminescent greenhouse collector", the luminescent medium or layer for practical reasons needs to be self-supporting. Therefore, the luminescent layer obviously must be at least one-half millimeter thick if it is as small as, say, 10 centimeters on a side, or it must be much thicker if it is, say, a meter on a side.

It is an object of the present invention to provide a luminescent collector-concentrator of solar energy of increased efficiency.

It is a further object of the invention to provide such a luminescent solar collector-concentrator optically coupled at a small fraction of its surface to a semiconductor photovoltaic cell.

Other objects, as well as aspects and advantages, of the invention will become apparent from the present specification, the drawings and the claims.

According to the present invention, a very thin, or at least a relatively very thin luminescent layer is supported in good optical contact on a radiation conducting support layer ("in optical contact" meaning that there is minimum reflection at the interface of the support layer and the luminescent layer), where the support layer is thick enough for its area to be self-supporting. Thus, the support layer is generally at least one-half millimeter thick, usually thicker, and has a thickness ratio to the thin luminescent layer of more than 4:1 and usually very much higher. It is usually preferable that the refractive index of the luminescent layer be essentially the same as or somewhat less than the refractive index of the support layer, in order to minimize reflection at the interface, although for reasons to be discussed later the reverse situation might be tolerated in certain instances.

As an aid to understanding basic principles of the invention, reference is made to FIGS. 1 to 4.

In the drawings

FIG. 1 is a hypothetical representation of the absorption and emission bands of a given luminescent species of a given concentration in the luminescent host layer.

FIG. 2 depicts schematically a side view of a single thick layer containing throughout a luminescing species.

FIG. 3 depicts a thinner layer containing throughout the same luminescing species at a higher concentration.

FIG. 4 depicts a composite layer of the invention wherein the luminescing species in the bottom layer has the same concentration as in FIG. 3 and wherein the top layer is the support layer.

FIG. 5 shows a luminescent solar collector of the invention.

FIG. 6 is similar to FIG. 5 but shows a solar cell coupled to the collector .

The principles of the invention can be seen more clearly by examining these drawings. Many luminescing materials such as fluorescent organic dyes have some overlap between their absorption and emission bands, as is shown in FIG. 1. In the type of collector or concentrator of the present invention, much of the energy emitted in the region of overlap would be reabsorbed and lost, as will be understood by those skilled in the art. This effect becomes more severe as the concentration of the dye is increased. For one thing, the wavelength area of overlap in the absorption and emission spectra as in FIG. 1 would be much greater. Furthermore, since the dye concentration is increased in a thinner film to maintain the same absorption, loss by reabsorption would increase for this reason alone. Thus, referring to FIGS. 2, 3 and 4, 1 is a thick self-supporting luminescent material of thickness t.sub.1 comprising a solid host containing a luminescent species in concentration c.sub.1 ; 2 is a much thinner luminescent material of thickness t.sub.2 comprising the same host containing the same luminescent species in higher concentration c.sub.2. In FIG. 4 the luminescent material 5 identical to 2 of FIG. 3 is in optical contact with and supported by thicker, self-supporting material 4 of high optical quality which in this instance contains no luminescent species. The numeral 6 in each figure represents a single ray in each figure being totally internally reflected through distance d.sub.1 or d.sub.2, at a given angle .alpha.. In order for the above plates to have the same absorption efficiency for the incident solar light, it must be true that c.sub.1 t.sub.1 =c.sub.2 t.sub.2 where c.sub.1 and c.sub.2 are the dye concentrations in the plates 1, 2 and 5. Also, since t.sub.1 /t.sub.2 =d.sub.1 /d.sub.2, it can easily be seen that the same reabsorption loss will occur in the total internally reflected luminescence when traveling distances l.sub.1, l.sub.2 and l.sub.1 transversely across the plates of FIGS. 2, 3 and 4 respectively. Therefore, it can be seen from the above that the same low reabsorption can be accomplished from a very thin layer of host material doped at high concentration, supported on a thick clear plate as can be accomplished from the thick plate at lower concentration. Of course, in FIG. 4 there is depicted the idealized situation in which the index of refraction of material 4 is the same as that of material 5. If the index is somewhat different in the two materials, it will be understood that there will be some change in the angle when the ray in FIG. 4 enters 4 or 5 as the case may be, and thus the distance l.sub.1 in FIG. 4 will vary somewhat from l.sub.1 in FIG. 2, but the principle and advantages of the invention structure still obtain. Moreover, the refractive index, n, of the thin layer should ideally be equal to or less than n of the thick layer, but the reverse is possible if a particular combination of the substrate layer and thin layer with its luminescent species is particularly advantageous in quantum efficiency of light conversion and thus makes it worthwhile to suffer some light loss by reason of the disadvantageous relationship of the indices.

In FIG. 5 is depicted a luminescent solar collector of the invention. FIG. 6 shows a luminescent solar collector with a semiconductor silicon solar photovoltaic cell covering one edge thereof. Electrical leads are connected to the silicon cell but are not shown. Like parts have the same reference numbers in FIGS. 5 and 6.

In FIG. 5 collector 10 comprises thick, self-supporting layer 12 of material capable of conducting electromagnetic radiation and thin layer 14 of plastic, glass, a gel or other suitable host material containing at least one luminescent species capable of absorbing incident solar light and emitting electromagnetic radiation in another wavelength, usually a longer wavelength. Three edges of the collector are silvered or otherwise coated to reflect light while edge 16 is not. In operation, light from the sun hitting either face of extended area is absorbed in layer 14 and excites the luminescent species which emits radiation of desired wave lengths, as before described with respect to FIG. 4.

A large portion of the emitted radiation reflects back and forth by the process of total internal reflection, and reflection from the mirrored edges, until the concentrated light reaches the window of edge 16 where it escapes and is put to use as for instance in FIG. 6.

In FIG. 6, identical to FIG. 5, except that the edge (which is numbered 16 in FIG. 5) has semiconductor photovoltaic cell 18, such as a silicon cell having a P-N junction, optically coupled thereto. This can be accomplished for instance by placing an anti-reflective coating on the facing surface of the silicon cell and interposing an oil film of intermediate refractive index (usually about 1.5) between the silicon cell and the edge of the luminescent solar collector or concentrator.

While in connection with the description of FIGS. 5 and 6, it is explained that three edges have a reflective coating, in its broadest aspect the edges need not be coated with a reflective coating. The concentrator works perfectly without such coating, but of course the concentrated light will be emitted through all unmirrored edges. As will be noted hereafter, it is possible and in some cases definitely contemplated that the thick supporting layer of my invention such as in FIGS. 4, 5 and 6 will have a luminescent species, but it will be understood from the explanation of the basic advantage of my structure as explained in connection with FIGS. 1-4, that (1) the thick support layer 1 is mainly a radiation conducting layer particularly for radiant energy emitted from the luminescent species in the thin luminescent layer and (2) in its usually preferred embodiment is less absorptive of said radiant energy than said thin layer, but not always. For instance, thick layer 12 of FIGS. 5 and 6 can contain luminescent species A that absorbs and is activated by wavelengths emitted by luminescent species B in thin layer 14, species A then emitting even longer wavelengths.

Several advantages of the structure of the present invention are apparent on examination. Some of these are as follows:

1. For practical reasons, some form of acrylic resin or plastic, such as poly(methyl methacrylate), was before my invention the only commercially suitable host for organic luminescent materials. It was the only material that could withstand weather and UV light, had the right transmission properties and was compatible with common fluorescent materials at a low enough price to be practical in an economic sense. In the present invention my thin luminescent layer preferably has a thickness of no more than 0.25 mm, whether or not it contains more than one luminescent species, and therefore layer 14 can be chosen among exotic (expensive) materials to best match the luminescent materials. Thus, it is well-known that the host material can synergistically enhance or shift the properties of the luminescent material, such as a dye, contained therein. For instance, the separation of the peaks of the absorption and emission spectra is influenced by the properties of the solvent, such as the static dielectric constant, for instance.

2. While the thick or supporting layer 12 can still be the fairly inexpensive acrylic resin of high quality, it is a distinct advantage that the support layer can be glass with its high optical quality, extremely low cost and low absorption characteristics. Yet heat sensitive organic luminescent materials can nevertheless be used in the collector, but in the thin layer comprising an organic medium such as a plastic.

3. The possibility of using glass as the thick support layer allows it to be doped if desired with an ion such as the uranyl ion or Mn.sup.II that absorb in low wave lengths and emit in visible wave lengths that are efficiently absorbed by a suitable dye or chelate that is the luminescent species in the underlying plastic layer. Or Ce.sup.III can be incorporated in the glass, since it absorbs in the UV and thus protects the plastic and the organic luminescent material in the plastic layer from degradation by UV. Moreover, the Ce.sup.III is luminescent and emits in the visible spectrum and is thus converted to light that can be used to excite the organic luminescent dye or other luminescent species in the thin plastic layer.

4. A marked advantage is that when two or more luminescent species are used in the thin plastic layer in the manner disclosed in the Swartz et al. paper in Optics Letters, Vol. 1, No. 2, August 1977, pp. 73-75, they can be used at a high enough concentration that radiationless (intermolecular) transfer of energy from one luminescent species to the next results. Such radiationless transfer is much more energy efficient. See Th. Forster, Discus. Faraday Soc. 27, 7(1959).

Thus, in one particularly advantageous embodiment of the invention, the thick support layer is glass and the thin layer is a plastic containing a luminescent material. In another, related embodiment, the glass layer contains a UV absorber luminescent material that emits in the visible spectrum.

As luminescent materials useful in the present invention there can be mentioned by way of example fluorescent chelates, fluorescent dyes, inorganic ions and even finely divided solid phosphors, all dispersed in a suitable host as previously discussed.

In the following examples the dye or chelate was uniformly mixed with a 50 volume percent solution in butyl alcohol of partially cured but further curable polysiloxane resin, prepared as described in Example 1 of U.S. Pat. No. 3,395,117 (incorporated herein by reference). The coatings on the substrate, in each case clear poly(methyl methacrylate) squares 1/8 inch thick and four inches on a side, were made to achieve a final cured thickness of doped polysiloxane film of about 0.5 mil and of the concentration noted in the respective examples. The solutions were applied to one face of the clean poly(methyl methacrylate) plates and allowed to drain off. The final curing was effected in a drying oven held at 60.degree. C. for two days.

Table 1 shows the three test plates, the luminescent species used and its concentration in the thin polysiloxane film coating.

In the tests of the solar concentrators, plus the control, a silicon semiconductor cell having a P-N junction was used. The plates were polished on only one edge and the other edges had no reflective coating. A flat surface of the silicon cell was optically coupled to part of the polished edge. That surface of the cell carried the usual antireflective oxide coating and an index oil of n.sub.D of 1.457 was interposed between the cell and the edge of the luminescent solar concentrator. The silicon cell covered only 8.5 cm. of the length of the edge of the luminescent solar concentrator plate in each instance. The same cell was used in each example so that the results are comparable. The cell was connected through electrical leads to test instruments by which the short circuit current was measured with the sun on a bright day shining on the acrylic surface of the concentrator. The solar cell was shielded from the sun's direct rays. The results in milliamperes is shown in Table 2, together with the brightness of the sunlight as measured by a lightmeter.

As will be evident to those skilled in the art, various modifications of this invention can be made or followed in the light of the foregoing disclosure and discussion without departing from the spirit and scope of the disclosure or from the scope of the claims.