Concrete solar cell

An inexpensive, robust concrete solar cell (10) comprises a photovoltaic material embedded in and extending beyond the major surfaces (16 and 18) of a matrix layer (14). The matrix layer typically comprises a high strength, cementitious material, such as a macrodefect free cement. The photovoltaic material comprises particles (12) of high-resistivity single crystal silicon, typically ball milled from ingot sections unsuitable for slicing into silicon wafers. The ingot sections include unprecipitated dissolved oxygen that is electrically activated by a low temperature annealing process to produce n-type silicon, even in silicon crystals that include a p-type dopant. An aluminum sheet (28), positioned on the backside of the matrix layer, is briefly melted together with the silicon particles to produce a p-type aluminum-doped silicon region (22) that forms a pn junction with the n-type region (24) of the particle. The aluminum sheet also provides the electrical contact to the p-type regions. The front surface of the matrix layer, from which the n-portion of the silicon particle protrudes, is covered with a translucent indium tin oxide conductive layer (30) that provides electrical contacts to the n-portion of the pn junction and digitated electrode (32) for conducting current off the cell. A voltage is generated between the two conductive layers when light incident on the photovoltaic particle through the indium tin oxide conductive layer creates charge carriers.

TECHNICAL FIELD 
This invention relates to an economical, robust solar cell. 
BACKGROUND OF THE INVENTION 
Solar cells convert light into useful energy, such as electricity or 
chemical energy. The high cost of solar cells, however, has prevented them 
from competing with conventional devices for generating power. Solar cells 
have typically been limited to low power applications, such as 
calculators, or niche applications, such as powering spacecraft, buoys, or 
other remote equipment. 
Solar cells are typically constructed by forming a pn junction on a wafer 
of single crystal, electronic grade semiconductor silicon. The pn junction 
is typically formed parallel to the major surfaces of the silicon wafer. 
One side of the pn junction is electrically contacted by a conductor on 
the back surface of the solar cell, while the other side of the pn 
junction is contacted by a metallic grid on the front surface of the solar 
cell. Light incident on the cell creates electron-hole pairs that cause a 
voltage difference between the conductor on the back surface of the cell 
and the conductive grid on the front surface of the cell. Because such 
cells require electronic grade semiconductor silicon, they are expensive 
to manufacture. Such cells are relatively fragile and typically require 
mounting in a protective enclosure having a cover of a translucent 
material, i.e., a material that transmits a portion of the incident light. 
Another type of solar cell that is constructed from spheres of metallic 
grade silicon is described in Levine, et al., "Basic Properties of the 
Spheral Solar.TM. Cell," Proceedings of the Twenty Second IEEE 
Photovoltaic Conference, Vol. 2, pp. 1045-48 (1991). Spheres of metallic 
grade silicon somewhat smaller than 1.0 mm in diameter and including a 
p-type dopant are purified, and an outer shell of each sphere is doped 
with an n-type material to form a pn junction. The spheres are bonded to a 
flexible aluminum foil, and electrical contacts are formed between the 
aluminum and the outer n-type shell. The spheres are etched to allow 
formation of an electrical contact to the inner p-type material. Although 
such cells are purportedly cheaper to produce than cells using wafers of 
electronic grade semiconductor silicon, the manufacture of such cells is 
complex. Furthermore, such cells, like previous cells, are relatively 
fragile and must be mounted in a protective module having a translucent 
glass or polymer superstrate. 
Another type of solar cell uses silicon crystals embedded in a frit glass 
insulator, surrounded by clear hydrobromic acid. The voltage across the 
silicon crystals causes an electrochemical reaction that produces gaseous 
hydrogen, liquid bromine, and heat. The solar energy is thus stored as 
chemical energy in the hydrogen and bromine, which can be used in a fuel 
cell. McKee, et al., "Development and Evaluation of the Texas Instruments 
Solar Energy System," 16th IEEE PVSC Proceedings, p. 257 (1982). 
SUMMARY OF THE INVENTION 
An object of the present invention is, therefore, to inexpensively convert 
light into a useful energy source. 
Another object of this invention is to produce an economical, large surface 
area solar cell for converting solar radiation into electricity. 
A further object of this invention is to produce such a solar cell that is 
sufficiently robust to function with little or no maintenance for extended 
periods in outdoor environments. 
Yet another object of this invention is to reduce the cost of disposing of 
scrap silicon produced during the manufacturing of silicon wafers. 
The present invention comprises an apparatus for converting light into 
useable energy and a method for manufacturing the apparatus. Particles of 
a photovoltaic material are embedded in and extend beyond the major 
surfaces of a dielectric matrix, such as a high strength cementitious 
material. Light incident on the photovoltaic material generates charge 
carriers that travel to the portion of the photovoltaic particles 
extending beyond the dielectric matrix layer, where a voltage is produced. 
Applicants refer to the combination of photovoltaic particles with a 
cementitious matrix as a "concrete solar cell." 
In a preferred embodiment, the photovoltaic particles comprise single 
crystal silicon. The silicon particles can be provided, for example, by 
comminuting high-resistivity by-products of silicon wafer production, such 
as ingot ends that are unsuitable for wafer production. Silicon particles 
that are not composed of an n-type material are annealed to electrically 
activate dissolved oxygen to convert the silicon to an n-type material. 
The dielectric matrix material is typically a rigid, weather-resistant 
material, such as a high strength cement, with a macrodefect free cement 
("MDFC") being the preferred material. 
An aluminum sheet is positioned on one side of the dielectric matrix layer 
and contacts the silicon particles that extend beyond the layer. The 
aluminum and n-type silicon are melted together at the interface using the 
577.degree. C. eutectic process and resolidified, leaving a portion of the 
silicon doped with aluminum atoms to create a p-type region in each n-type 
particle, thereby forming a pn junction and electrical contacts between 
the aluminum sheet and the p-type region. The front side of the dielectric 
matrix layer, from which the n-type portion of the silicon particles 
protrude, is covered with a translucent conductive layer, such as an 
indium tin oxide ("ITO") layer, that provides electrical contacts to the 
n-type portion. A digitated, metallic grid may be added to reduce the 
sheet resistance of the front surface. A protective layer, such as a 
translucent MDFC layer may be added over the ITO layer and metallic grid. 
The invention can also be made using other junction types, such as a 
Schottky barrier junction, instead of a pn junction. The invention can 
also be used as part of an electrochemical cell. 
Large area, robust photovoltaic panels can be constructed, for example, as 
shingles for placement on the roofs or sides of structures or on concrete 
railroad ties. 
Additional objects and advantages of the present invention will be apparent 
from the following detailed description of a preferred embodiment thereof, 
which proceeds with reference to the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIGS. 1-4 show a photovoltaic cell 10 that represents a preferred 
embodiment of a solar cell of the present invention. Photovoltaic cell 10 
comprises photovoltaic particles 12 embedded in a dielectric matrix layer 
14 having first and second major surfaces 16 and 18. Photovoltaic 
particles 12 and matrix layer 14 together form a concrete layer 20. 
Each photovoltaic particle 12 includes a portion 22 of a p-type material 
and a portion 24 of an n-type material that together form a pn junction 26 
at their interface and extend beyond major surfaces 16 and 18, 
respectively, to electrically contact a conductive layer 28 and a 
translucent conductive layer 30, respectively. A digitated electrode 32 
positioned on translucent conductive layer 30 reduces the effective 
electrical sheet resistance of translucent conductive layer 30. 
Photovoltaic particles 12 and the various layers shown in the figures are 
exaggerated for clarity. 
Light 34 incident on photovoltaic particle 12 and having energy greater 
than the band gap energy of the photovoltaic material comprising particle 
12 passes through translucent conductive layer 30 and creates in 
photovoltaic particle 12 charge carriers, i.e., a conduction band electron 
and a valence band hole. An electric field within the pn junction, known 
in the art as the "built-in field," causes the electrons to move toward 
translucent conductive layer 30 and causes the holes to move toward 
conductive layer 28, thereby producing a voltage between conductive layers 
28 and 30 that can be used to do work such as driving an electrical load 
or an electrochemical cell. 
Matrix layer 14 is preferably composed of a high flexural strength, i.e., 
greater than 10 MPa, cement, such as a macrodefect free cement ("MDFC"). 
An MDFC is a cement that is exceptionally strong because, unlike ordinary 
cement, it contains essentially no large voids. For example, an MDFC may 
contain less than 2 percent voids by volume, with essentially no voids 
larger than 15 .mu.m. Such large voids considerably weaken normal cement. 
The flexural strength of MDFC is, therefore, two orders of magnitude 
greater than that of normal cement, and its fracture energy is five orders 
of magnitude greater than that of ordinary cement. MDFC can be formed from 
many cementitious materials by carefully controlling the grain size, using 
a high shear mixer that defloculates the grains, lubricating the particles 
with a water soluble organic polymer, and casting or calendering the 
cement at moderate pressures of between approximately 5 MPa and 50 MPa. 
The proportion of water in the MDFC composition is typically less than 25 
percent, and preferably less than 12 percent, by weight, although the 
amount of water should not be so low that a plastic dough-like shapeable 
cementitious composition cannot be formed. 
A preferred MDFC, as described in "Microstructural and Microchemical 
Characterization of a Calcium Aluminate-Polymer Composite (MDF Cement)," 
Popoola, et al., 74 J. Am. Ceramic Soc., pp. 1928-33 (1991), includes 
calcium aluminate cement, poly(vinyl alcohol/acetate), glycerine 
plasticizer, and distilled water. The poly(vinyl alcohol/acetate) may be 
79.3 mol % hydrolyzed with a 1,700 unit degree of polymerization and a 
medium particle size of 12 .mu.m. Photovoltaic particles 12 may be added 
to the cement dough before calendering into a layer preferably between 50 
.mu.m and 100 .mu.m in thickness, depending upon the average dimensions of 
the photovoltaic particles 12. Thicker layers, such as layers of 400 .mu.m 
in thickness, are sturdier and easier to produce but produce a less 
efficient photovoltaic cell 10. Other high strength cements, such as a 
portland- or pozzolanic-type cement, can also be used to embed 
photovoltaic particles 12 in the construction of photovoltaic cell 10. 
Adding photovoltaic particles 12 to the cement before it is processed in 
the high shear mixer that defloculates the cement grains produces a cement 
having good mechanical strength but may damage the photovoltaic particles 
12. Adding photovoltaic particles 12 to the cement in a low force mixer 
after it has been defloculated is less likely to damage photovoltaic 
particles 12 but may introduce voids, which produces a weaker cement, and 
adversely affects the setting time. Alternatively, photovoltaic particles 
12 can be distributed onto conductive layer 28 and then an uncured MDFC 
layer 14 can be calendered onto photovoltaic particles 12 and conductive 
layer 28. 
Calendering is performed preferably using hard rubber rollers that compress 
the cement paste and leave the ends of photovoltaic particles 12 extending 
slightly beyond the cement paste. Calendering may also be performed using 
pliable sheets of plastic or rubber. The calendering scrubs the top 
surface of photovoltaic particles 12 free of both oxide and cement and 
drives photovoltaic particles 12 into conducting layer 28 with sufficient 
force to break the insulating oxide layers on the photovoltaic particles 
12 and conductive layer 28, which may comprise, for example, an aluminum 
foil. In some cases, however, it may be desirable to improve the 
electrical connection between particles 12 and conductive layer 28 by 
performing an additional cleaning step to remove an insulating layer of 
cement from the ends of photovoltaic particles 12. 
An efficient photovoltaic cell 10 has a large proportion of the volume of 
concrete layer 20 comprised of photovoltaic particles 12. Too large a 
volume proportion of photovoltaic particles 12 would, however, reduce the 
mechanical strength of photovoltaic cell 10 and increase the probability 
of undesirable electrical contacts between different photovoltaic 
particles 12. A surface area of 35 percent photovoltaic particles 12 has 
been attained while maintaining a sufficiently strong photovoltaic cell 
10. 
The characteristics of MDFC make it very suitable for use in photovoltaic 
cell 10. MDFC is translucent, electrically insulating, and can be cast 
into sheets as thin as 20 .mu.m. It bonds with silicon photovoltaic 
particles and with aluminum conductive layers. It is tough, strong, 
essentially nonporous, and water resistant and can withstand a wide range 
of environmental temperatures. The relative dielectric constant of calcium 
aluminum-based MDFC is typically between 7 and 9, which is less than the 
11.8 relative dielectric constant of silicon. This difference in 
dielectric constants results in light being refracted from the MDFC into 
the silicon and light from within the silicon being refracted back into 
the silicon. The MDFC thus acts as an antireflection coating to enhance 
the efficiency of photovoltaic cell 10. Other types of MDFC can have lower 
dielectric constants, and thus would perform this function even better. 
A photovoltaic cell 10 composed of MDFC has sufficient structural strength 
and is sufficiently robust that it can be used on the sides or roofs of 
buildings without a protective structure and cover glass. However, an 
optional translucent protective layer 42 of thin MDFC can be applied as a 
protective, antireflective coating over digitated electrodes 32 and 
translucent conductive layer 30 to provide further environmental 
protection for photovoltaic cell 10. An additional protective layer (not 
shown) can also be applied over conductive layer 28. 
Photovoltaic particles 12 are high-resistivity (greater than 25 
m.OMEGA.-cm) n-type semiconductors when they are embedded into MDFC layer 
14. The particles can be made from high-resistivity semiconductor silicon 
doped with n-type electron donor impurities, such as phosphorus, arsenic, 
or antimony. The particles can also be made from semiconductor silicon 
that is undoped or doped with p-type, electron acceptor impurities by 
electrically activating dissolved oxygen in the silicon to change the 
silicon to n-type. The oxygen is electrically activated by annealing, 
typically between 425.degree. C. and 475.degree. C., to move oxygen atoms 
from interstitial positions to lattice sites where they can donate valence 
band electrons. In a typical Czochralski-grown silicon crystal, annealing 
can activate approximately 3.times.10.sup.16 atoms per cm.sup.3 of oxygen, 
which is a sufficient concentration to change high-resistivity p-type 
silicon into high-resistivity n-type silicon. 
Photovoltaic particles 12 can be formed by comminuting scrap sections of 
ingots grown for producing silicon wafers to be used in the manufacture of 
integrated circuits and from silicon remaining in the growing crucible 
after an ingot is grown. Ingot sections such as the seed and tail ends 
that are unsuitable for slicing into wafers are suitable for use in 
photovoltaic cell 10 of the present invention. Photovoltaic particles 12 
are preferably milled to an octahedral shape having an average particle 
size of 50 .mu.m to 100 .mu.m using a ball mill and sieves. An average 
grain size of 50 .mu.m would produce a more efficient photovoltaic cell 
10, but such a cell would be more difficult to produce than a photovoltaic 
cell 10 having a larger grain size. The use of ball mills and sieves for 
producing particles of uniform size is well known in the powder metallurgy 
art. Approximately 65 percent of the silicon produced in the United States 
is of a high-resistivity type suitable for such use. High resistivity 
p-type scrap silicon is usable but must be procured before the dissolved 
oxygen is precipitated, so the oxygen is available to be electrically 
activated by annealing to convert the p-type scrap to an n-type material 
as described above. The present invention thus provides a beneficial use 
for scrap material that is a currently a costly waste disposal problem for 
the silicon industry. 
Milling photovoltaic particles 12 from larger crystals of semiconductor 
silicon can cause crystal defects in photovoltaic particles 12. Such 
defects cause high surface recombination velocities and low minority 
carrier lifetimes that reduce the efficiency of photovoltaic cell 10. 
The amount of crystal structure damage can be reduced by adding a lubricant 
during the milling process to reduce the energy of comminution. Suitable 
lubricants include those typically used in the slicing operation of 
silicon wafer manufacturing. Other methods also believed to be useable for 
reducing the crystal structure damage include annealing the photovoltaic 
particles 12 prior to casting them into MDFC layer 14 and etching 
photovoltaic particles 12 to enhance stable crystal facets and passivate 
dislocations. 
Another technique that may be useful for improving minority carrier 
lifetimes and surface recombination velocity includes growing an oxide 
layer on photovoltaic particles 12 and then heating them to approximately 
1,000.degree. C. in the presence of lime to convert a portion of the oxide 
to calciated silica, thereby passivating dislocations. This method may 
also enhance the mechanical properties of cell 10 by increasing the 
adhesive between photovoltaic cell 10 and the MDFC material. The surface 
recombination velocity and minority carrier lifetime may also be improved 
by forming an n-type layer on photovoltaic particles 12 using chemical 
vapor deposition or organo-metallic chemical vapor deposition. The surface 
of the photovoltaic particles 12 may also be passivated by reacting with 
the cement, with the degree of passivation being determined by the type of 
cement used. 
Photovoltaic particles 12 can also be formed from electronic grade 
polycrystalline silicon or from metallurgical grade silicon as described 
by Jules D. Levine, et al., in "Basic Properties of the Spheral Solar.TM. 
Cell," Proceedings of the Twenty Second IEEE Photovoltaic Conference, Vol. 
2, pp. 1045-48 (1991) and U.S. Pat. No. 5,069,740 for "Production of 
Semiconductor Grade Silicon Spheres from Metallurgical Grade Silicon 
Particles." Such spheres have diameters on the order of a millimeter and 
require, therefore, a correspondingly thicker MDFC layer 14. Other 
photovoltaic materials, such as silicon carbide and gallium phosphide, can 
be used in photovoltaic particles 12. The starting material for creating 
photovoltaic particles 12 can be varied depending on the price and 
availability of the various raw materials. 
P-type portions 22 of photovoltaic particles 12 are preferably formed 
simultaneously with the formation of electrical connections between 
photovoltaic particles 12 and conductive layer 28. Conductive layer 28 
typically comprises an aluminum foil. The aluminum foil and the silicon of 
photovoltaic particles 12 are briefly melted together at their interface 
by using, for example, a rapid thermal annealer or by applying a high 
voltage between the aluminum sheet and a tin-coated calendering roller on 
the opposite side of the MDFC layer 14. The eutectic reaction at 
577.degree. C. results in a concentration of approximately 
3.times.10.sup.18 aluminum atoms per cm.sup.3 in the resolidified silicon. 
Because aluminum is an electron acceptor, the aluminum doped silicon is a 
p-type semiconductor. The interface between aluminum-doped p-type region 
22 and the remaining n-type region 24 of photovoltaic particles 12 results 
in a pn junction 26 that provides an internal electric field that drives 
the photo-induced charge carriers to conductors 28 and 30. 
The depth of pn junction 26 is controlled by controlling the energy applied 
to melt the silicon-aluminum interface. Alternatively, the depth of the pn 
junction 26 can be controlled by limiting the quantity of aluminum 
available, for example, by vacuum depositing a thin layer of aluminum onto 
MDFC layer 14 before melting the aluminum-silicon interface. After the pn 
junction is formed from the thin aluminum layer, an additional conductor, 
such as an aluminum foil, is applied onto major surface 16 to increase the 
cross-sectional area and reduce the sheet resistance of conductive layer 
28. The aluminum foil can be bonded to the deposited aluminum by heating 
both layers above the 577.degree. C. aluminum-silicon eutectic 
temperature. A preferred conductive layer 28 has a thickness of 
approximately 100-150 .mu.m. P-type region 22 extends into photovoltaic 
particle 12 a distance 44, preferably equal to the lesser of approximately 
10 .mu.m or half of the diameter of photovoltaic particle 12. 
Another method of controlling the depth of pn junction 26 entails 
depositing a layer of aluminum approximately 2 .mu.m in thickness onto a 
conductive substrate, such as a steel backing sheet, that has a melting 
temperature significantly higher than the 577.degree. C. aluminum-silicon 
eutectic temperature. Upon heating to 577.degree. C., the aluminum at the 
silicon interface melts to form the pn junctions and electrical contacts. 
The steel remains solid, essentially soldered to the silicon by the 
aluminum. Because of the detrimental effect of heavy metal atoms, such as 
iron, on minority carrier lifetimes in silicon, temperatures during 
formation of the pn junctions should remain low to preclude significant 
diffusion of atoms from the metallic backing sheet into the silicon. 
If photovoltaic particles 12 are not mixed into the matrix material before 
calendering, they can be deposited directly onto conductive layer 28. The 
matrix material can then be deposited onto the photovoltaic particles 12 
and conductive layer 28 to form concrete layer 20. For example, silicon 
particles can be distributed on an aluminum foil, and then a MDFC can be 
calendered onto the aluminum foil using a conductive, e.g., tin-coated, 
roller having a sufficiently high voltage applied between the tin roller 
and the aluminum foil to weld the photovoltaic particles 12 to the 
aluminum foil. During the weld, the p-type region would be formed by the 
eutectic process and the electrical contact between the aluminum and the 
silicon would be established. Any tin deposited by the roller onto the 
MDFC or the silicon would simply become a part of an indium tin oxide 
conductive layer 30. 
Alternatively, the photovoltaic particles 12 can be deposited onto 
conductive layer 28 and pn junctions 26 could be formed, for example, by 
rapid thermal annealing. The matrix material is then deposited onto 
conductive layer 28, for example, by calendering with hard rubber rollers 
and cured to form matrix layer 14. Whether matrix layer 14 is deposited 
before or after pn junctions 26 are formed depends in part upon the 
ability of the matrix material to withstand without damage the short 
period at 577.degree. C. required to form pn junctions 26. 
To reduce deterioration of the matrix material during junction formation, 
the temperature required for junction formation can be reduced by adding 
one or more additional elements at the photovoltaic particle-conductive 
layer interface. For example, an additional alloying material, such as 
tin, gallium, or zinc, can be added to aluminum at the interface between 
conductive layer 28 and photovoltaic particles 12. Such three-component 
systems can have a melting temperature lower than the silicon-aluminum 
binary system. The alloying material can be added, for example, by 
evaporating it onto major surface 16 or by depositing it onto or 
incorporating it into a metallic foil used to form conductive layer 28. An 
alloying material can be chosen for its effects as a dopant, as well as 
its ability to lower junction formation temperature. Binary systems other 
than the silicon aluminum system could also be used to form the junctions. 
For example, antimony can be used to form np junctions in p-type silicon 
particles. 
If matrix layer 14 is formed from a cementitious material other than a 
MDFC, it will typically have pores that can fill with a conductive 
material during processing and cause short circuits between conductive 
layers 28 and 30. Such short circuits can be prevented by filling the 
pores using an electrodeposition process and then oxidizing or anodizing 
the exposed top surface of the deposited material to form an insulating 
layer. Short circuits through voids in concrete layer 20 caused by 
imperfect deposition of the matrix material can be similarly prevented. 
Shorted photovoltaic particles 12, i.e., those in which both p-type region 
22 and n-type region 24 contact the same conductive layer 28 or 30, can be 
isolated by an anodization process, similar to that described in U.S. Pat. 
No. 5,192,400 to Parker et al. for "Method of Isolating Shorted Silicon 
Spheres." The resistivity between conductive layers 28 and 30 across 
concrete layer 20 is preferably greater than 250 .OMEGA./cm.sup.2 to 
ensure a sufficiently small leakage current. 
After the pn junctions are formed, translucent conductive layer 30 is 
formed, for example, by depositing a layer of indium tin oxide, preferably 
approximately 5 .mu.m in thickness, onto major surface 18. Metallic 
digitated electrode 32, deposited on translucent conductive layer 30 by 
known techniques, exhibits lower resistivity than the indium tin oxide of 
translucent conductive layer 30 and, therefore, reduces the electrical 
resistance between photovoltaic particles 12 and an electrical load (not 
shown) driven by photovoltaic cell 10 by effectively reducing the sheet 
resistance of translucent conductive layer 30. The area covered by 
digitated electrodes 32 is sufficiently small so that the increase in 
efficiency caused by the decreased electrical resistivity is greater than 
the decrease in efficiency caused by blocking some of the incident light 
34. Optionally, protective MDFC layer 42 can be applied over digitated 
electrode 32 and translucent conductive layer 30, and a second protective 
MDFC layer can be applied over conductive layer 28. 
Although photovoltaic cell 10 could be produced as wide strips with 
arbitrary lengths, such a configuration would result in a low voltage, 
large current device. It would be preferable to configure multiple panels 
of photovoltaic cell 10 in a series to increase the voltage output. One 
method of series connecting photovoltaic cell 10 would be to assemble them 
as roofing shingles, with conductive layer 28 of each course electrically 
connected to the conductive layer 30 of the subsequent course, so that the 
voltage difference between each course and the first course increases with 
each subsequent course. 
Photovoltaic cell 10 has sufficient structural strength and is sufficiently 
robust that it can be used on the outside of structures, such as concrete 
buildings, concrete railroad ties, and roofs, with no cover glass or other 
support structures. The rigidity of a typical photovoltaic cell 10 
enhances its usefulness as a building material when compared to prior art, 
such as aluminum foil-matrix cells. Photovoltaic cell 10 is, therefore, 
inexpensive to install and requires little or no maintenance. Care must be 
taken, however, when photovoltaic cell 10 is installed onto structures of 
conventional concrete that the water content of the conventional concrete 
does not corrode the aluminum of photovoltaic cell 10. This can be 
accomplished by placing a layer of MDFC between aluminum and the 
conventional concrete. 
It will be obvious that many changes may be made to the above-described 
details of the invention without departing from the underlying principles 
thereof. For example, pn junction 26 can be formed by methods other than 
those described. A photovoltaic cell can also be constructed using other 
junction types, such as a Schottky barrier junction, a heterojunction, or 
a metal-insulator-semiconductor junction, in place of a pn junction. A 
Schottky barrier junction can be formed, for example, at the interface 
between the silicon of a photovoltaic particle 12 and the indium tin oxide 
of a translucent conductive layer 30. An electrochemical or galvanic cell 
can also be constructed using the principles of the present invention. In 
such a cell, concrete layer 20 is immersed in a fluid that 
electrochemically stores energy from incident light 34. The scope of the 
present invention should, therefore, be determined only by the following 
claims.