Output-increasing, protective cover for a solar cell

A flexible cover (14) for a flexible solar cell (12) protects the cell from the ambient and increases the cell's efficiency. The cell(12)includes silicon spheres (16) held in a flexible aluminum sheet matrix (20,22). The cover (14) is a flexible, protective layer (60) of light-transparent material having a relatively flat upper, free surface (64) and an irregular opposed surface (66). The irregular surface (66) includes first portions (68) which conform to the polar regions (31R) of the spheres (16) and second convex (72) or concave (90) portions (72 or 90) which define spaces (78) in conjunction with the reflective surface (20T) of one aluminum sheet (20). Without the cover (14) light (50) falling on the surface (20T) between the spheres (16) is wasted, that is, it does not fall on a sphere (16). The surfaces of the second portions are non-parallel to the direction of the otherwise wasted light (50), which fact, together with a selected relationship between the refractive indices of the cover and the spaces, result in sufficient diffraction of the otherwise wasted light (50) so that about 25% of it is reflected from the surface (20T) onto a sphere (16).

BACKGROUND OF THE INVENTION 
The present invention relates to a protective cover for a solar cell, and, 
more particularly, to an output-increasing, protective cover for a solar 
cell. 
Various types of photovoltaic ("PV") devices such as solar cells, for 
converting radiant energy, such as sunlight, into electricity are known. 
One type of solar cell which is of particular current interest comprises a 
plurality of spaced members, typically spherical or spheroidal particles, 
supported by a conductive sandwich which includes first and second 
conductive sheets separated by an insulative coating. Each sphere is a 
semiconductor, for example silicon having a P-type interior and an N-type 
exterior or skin. 
The first sheet is a thin, flexible metallic foil, typically aluminum, with 
a plurality of spaced cell-retaining apertures formed therethrough, for 
example, by an emboss-then-etch or stamping process. The apertures 
typically define a regular geometrical pattern. Preferably, the pattern 
comprises overlapping hexagons which permits maximum packing of the 
apertures, and, hence, of the spheres. The spheres are placed on a top 
surface of the first foil and, by the use of negative pressure and 
doctoring or other techniques, each aperture ultimately has one sphere 
nested therein. Thereafter, heat and pressure are applied to the cell 
sandwich to move the nested spheres partially into and through the 
apertures. This movement effects the interaction of the aperture walls 
with the spheres to locally remove the native aluminum and silicon oxides 
so that the abutting aluminum mechanically bonds directly to, and forms an 
electrical contact with, the N-type exterior of the silicon spheres, 
thereby affixing the spheres to the first foil. 
The affixing of the spheres to the first foil results in an upper 
light-gathering portion of each sphere protruding or extending above the 
top surface of the first foil and a lower portion of each sphere 
protruding below a lower surface of the first foil. The N-type exterior is 
removed from the cells below the first foil's lower surface. The lower 
foil surface and the exposed P-type interior of the lower sphere portions 
are then coated with a flexible, electrically insulative coating, 
typically a polyimide. The insulative coating on the spheres is then 
treated to remove some of the coating to thereby expose the P-type 
interior of each sphere through so-called vias. 
Following via formation, the second foil, preferably a flexible aluminum, 
is electrically connected to the P-type interiors of the spheres through 
the vias. The flexible solar cell so formed may power utilization devices 
connected between the foils. 
The foregoing and similar solar cells and techniques for producing them are 
disclosed in the following commonly assigned U.S. Pat. Nos.: 4,407,320; 
4,521,640; 4,581,103; 4,582,588; 4,806,495; 4,872,607; 4,917,752; 
4,957,601; 5,028,546; 5,192,400; 5,091,319; and 5,086,003. 
The above-described solar cells comprise a plurality of miniature PV 
devices--the spaced silicon or other semiconductive members, spheres, 
spheroids or other particles--connected in electrical parallel via the 
first and second foils. The foils, therefore, are connectable to a 
utilization device or circuit for electrical energization thereof when the 
cells are exposed to radiant energy. The cells are flexible and may be 
formed into various non-planar configurations, either free-standing or 
conforming to an irregular underlying surface. 
While solar cells constructed as set forth above are mechanically robust, 
protecting them from the deleterious effects of the environment and 
ambient conditions is generally thought to be prudent. For example, water 
in the form of rain or other precipitation, in prolonged direct contact 
with the spheres or other particles or the foils can cause oxidation and 
corrosion, giving rise to mechanical and/or electrical degradation of the 
cell. Pollutants may also deleteriously affect the cell, such as by 
attacking the spheres or foil of the cell or by decreasing or preventing 
radiant energy from reaching the particles or spheres. 
For the foregoing and other reasons, it is typical to cover, encapsulate or 
otherwise house solar cells to protect them against ambient-caused 
degradation. Such protective measures viewed in the context of prior art 
non-flexible solar cells have often taken the form of rigid "picture 
frames" having a transparent cover which surrounds the solar cell to 
isolate it from the ambient. The cover, of course, permits sunlight and 
other radiation to reach the cells where it is converted to electricity. 
Such picture frame covers are not flexible and limit the range of uses to 
which the flexible cells of the above patents may be put. 
The upper portion of each particle or sphere--typically an N-type silicon 
hemisphere--functions as a spherical lens. That is, this upper portion 
gathers light incident on the particle or sphere and directs this light 
onto the particle's or sphere's P-N junction. These spherical lenses are 
able to direct to the P-N junction only that light which is directly 
incident on the particles spheres. Some of the light which is incident on 
the top surface of the first foil between the particles or spheres--that 
is, light which "misses" the particles or spheres--is, in effect, "wasted" 
and is not effective to produce electricity, because it does not reach the 
P-N junction of the spheres, and is, instead, back-reflected to the 
ambient. 
Commonly assigned U.S. patent application, Ser. No. 08/060,773, filed May 
11, 1993 discloses a flexible protective cover with an undulating free 
surface which comprises a coating having particle-conformal portions which 
extend into the spaces between adjacent particles. The portions of the 
coating conforming to each particle act as lenses, directing otherwise 
"wasted" light--light which would fall on the first foil between adjacent 
particles and be reflected back along its incoming path--onto the 
underlying particle. While this cover achieves solar cell efficiency 
increases of about 10%-20% (for a given amount of radiant energy incident 
on the cell), dirt and pollution-borne contaminants can be difficult to 
remove from its undulating irregular free surface. 
The present invention contemplates the provision of a cover for solar cells 
which encapsulates and protects the cells from the ambient and which 
directs onto the particles or spheres a significant amount of the 
otherwise "wasted" light incident on the solar cells. The present 
invention also contemplates a flexible cover having the foregoing 
characteristics which permits the forming of flexible solar cells capable 
of assuming non-planar configurations, as well as methods for producing 
the aforenoted cover. The present invention further contemplates a cover 
which is easier to clean and which achieves a comparable efficiency 
increase to that of the '773 application. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention contemplates a cover for a photovoltaic 
cell. The cell includes a number of spaced semiconductor particles. 
Portions of the particles extend away from a reflective surface, and the 
particle portions are exposed to radiant energy directed at the cell. 
Preferably, the particles are spheres or spheroids of a semiconductor 
material such as silicon and the reflective surface is one surface of a 
flexible sheet or foil, which may be a metal such as aluminum, by which 
the particles are held. Preferred cells according to the foregoing 
construction are flexible. 
The particle portions are hemispherical or nearly hemispherical portions of 
the spheres or spheroids which extend away from or protrude above the 
reflective surface of the sheet. A significant amount of the radiant 
energy falling on the cell is perpendicular or nearly perpendicular to the 
reflective surface between adjacent particles. Such energy is reflected in 
such a way that it does not fall on the particle portions. Usually this 
radiant energy is reflected back virtually along its incident path and is 
"wasted," that is, is not used by the particle to produce electricity. 
Where about 20% of the cell's area is the reflective surface, about 20% of 
the energy incident on the cell is "wasted." 
The cover of the invention includes a layer of radiant-energy transparent 
material having a free or upper surface and an opposed surface. The 
opposed surface is irregular. This irregularity results from the opposed 
surface conforming to the polar or upper regions of the particle portions 
and being configured into prism-like cusps, extensions, projections or 
deformations between adjacent particles. The cusps approach, but do not 
contact the reflective surface. The resulting spaces, gaps or volumes 
between the cusps and the reflective surface may contain a substance, such 
as air, other gases, a liquid, a polymer of a different index of 
refraction, or may be evacuated. The material between adjacent particles 
may be concave as viewed form the reflective surface. The resulting spaces 
are otherwise similar to those previously noted. 
The configuration of the cusps or concavities and the refractive indices of 
the layer and the resulting spaces are related and cooperate so that a 
significant amount of the radiant energy which passes through the layer 
between adjacent particles--which radiant energy, as noted above, would be 
otherwise "wasted"--is reflected by the reflective surface onto the 
particle portions. 
Specifically, the otherwise "wasted" radiant energy is refracted as it 
passes through the cusp-space or concavity-space interfaces. The 
refraction causes the radiant energy to impinge on and to be reflected 
from the reflective surface in a non-perpendicular manner. The 
non-perpendicularity of the reflection directs the energy onto the 
particle portions. The path taken by the otherwise "wasted" energy is 
generally along the cusp proximate to one of the adjacent particles and 
then onto and away from the reflective surface and along the cusp 
proximate the other adjacent particle, in the case of the cusps, or onto 
the one particle, in the case of the concavities. 
Preferably the free surface of the cover is smooth and easy to keep clean 
and the cover is flexible so that it can be shaped with the flexible cell. 
The cover may increase cell efficiency up to 25% as it protects the cell 
from harm caused by the ambient. 
According to the method aspects of the present invention, the cover is 
produced by contacting the poles of the particles with a layer of a 
radiant energy-transparent, compressible material and then applying forces 
to the free surface of the layer in the direction of the reflective 
surface so that the opposed surface of the layer assumes the irregular 
configuration which functions in the manner described above.

DETAILED DESCRIPTION 
Referring to FIGS. 1 and 2 there are shown sectioned elevational views of 
fragments 10 of a solar cell 12. In FIG. 2 the solar cell 12 is protected 
by a cover 14 according to the present invention, while the solar cell 12 
of FIG. 1 is not so protected. 
The cell 12 may include a plurality of spaced semiconductive particles or 
members 16, which are preferably spheres or spheroids having diameters of 
about 25-45 mils and typically constituted of silicon, affixed to a 
conductive sandwich 18. Adjacent particles or spheres 16 are typically 
spaced apart by 1-4 mils. The conductive sandwich 18 may include a first 
or upper metallic foil 20 and a lower or second metallic foil 22 separated 
by an intervening insulative coating 24. Both foils 20 and 22 are 
preferably flexible and may be fabricated from aluminum. The coating 24 is 
also preferably flexible and may be a polyimide. The flexibility of the 
foils 20,22 and the coating 24 results in the cell 12 being flexible and 
conformally or otherwise shapable. 
The particles or spheres 16 include an interior portion 26 of one 
conductivity type, e.g., P-type, and an outer portion 28 of the opposite 
conductivity type, e.g., N-type. The particles or spheres 16 are affixed 
or mounted to the first foil 20 by locating them in apertures 30 formed 
therein, with the walls of the apertures 30 being mechanically and 
electrically connected to the N-type outer portions 28. An extending or 
upper portion 31 of each particle or sphere 16 extends away or protrudes 
above a first or top surface 20T of the first foil 20. The first surface 
20T of the first foil 20 is typically reflective. 
The extending portion 31 of each particle 16, whether or not it is a 
sphere, may be viewed as having a pole 31P and an upper, surrounding polar 
region 31R. The second foil 22 is mechanically and electrically connected 
to the interior portion 26 of each particle or sphere 16. To that end the 
outer portion 28 of each particle or sphere 16 below the first foil 20 is 
removed and the resulting exposed interior portion 26 of each sphere 16 
and a second or lower surface of the first foil 20 are covered with the 
insulative coating 24. The second foil 22 electrically contacts metallic 
contacts 32 which engage the interior portions 26 of the spheres 16 and 
are positioned in vias or holes 34 formed through the insulative coating 
24. 
When light or other appropriate radiant energy 50 is perpendicularly or 
nearly perpendicularly--or, vertically or nearly vertically--incident on 
the cell 12, some of it 50a, as shown in FIG. 1, is directly incident on 
the extending or upper portions 31 of the particles or spheres 16, 
including the pole 31P and polar region 31R thereof. Some of the light 50b 
is incident on the first or top surface 20T of the first foil 20 between 
the adjacent particles or spheres 16. Since a substantial amount of the 
light 50b perpendicularly or vertically incident on the top foil surface 
20T is perpendicularly reflected back away from the foil 20, little, if 
any, of this light 50b, ever reaches the particles or spheres 16. In 
typical cells 12, about 20% of the cell area comprises the regions of the 
first surface 20T between the adjacent particles or spheres 16. Thus, 
about 20% of the incident light 50 is "wasted." The upper portions 31 of 
the spheres 16, comprising the N-type silicon 28, function as spherical 
lenses to gather and direct the light 50a incident thereon onto the P-N 
junctions 26-28 of the spheres 16. Light incident on this junction 26-28 
produces electricity which flows in the foils 20 and 22 for consumption by 
a utilization device or circuit. 
As noted, a majority of the light 50 collected by the upper sphere portions 
31 is the perpendicular/vertical or nealy perpendicular/vertical light 50a 
directly incident thereon. Some small amount of the light 50c 
non-perpendicularly incident on or reflected from the top foil surface 20T 
will, depending on its angle of incidence and proximity to a sphere 16, be 
collected by the spherical lens 31 thereof and directed to the P-N 
junctions 26-28, as shown to the left of the leftwardmost sphere 16. Most 
of the light 50 incident directly on the top foil surface 20T is either 
reflected away therefrom along its incident path, as shown at 50b for 
perpendicularly incident light, or is reflected away without striking a 
sphere 16, as shown at 50d to the right of the leftwardmost sphere 16 in 
FIG. 1. In effect, most of the light 50b, 50c, 50d not falling directly on 
the spheres 16 is "wasted," that is, it is not effective in the production 
of electricity by the particles or spheres. 
Various elements and portions of the cell 12 may be damaged or otherwise 
adversely affected by the environment. Rain, other precipitation, 
particulate matter, pollutants and other contents of the ambient can 
corrode, etch, render opaque and otherwise damage the cell 12. In view of 
this, the prior art typically resorted to enclosing rigid solar cells in a 
picture frame-like structure with glass or other optically clear material 
overlying the upper sphere portions for protecting the cell 12 against 
environmental damage. The cover 14 according to the present invention 
protects the cell 12 against such environmental damage and increases the 
output of the cell 12 for a given amount of light 50 incident thereon. The 
cover 14 is also flexible, permitting the flexible cell 12 to be formed 
into various non-planar configurations. 
The cover 14 shown in FIG. 2, directs much of this otherwise "wasted" light 
50b, etc. onto the particles or spheres 16 and protects the cell 12 
against the deleterious effects of the environment. 
The cover 14 includes a radiant energy-transparent or optically clear layer 
60 which absorbs little if any of radiation in the solar spectrum, defined 
herein to mean radiation having a wavelength within the approximate range 
of 0.4-1.1 micron. Radiation within this wavelength range is particularly 
effective in producing electricity at the P-N junction 26-28. Preferably, 
the layer 60 also resists the deleterious effects--such as embrittlement, 
cracking, warping or other deformation, loss of flexibility and 
clouding--of UV and high temperatures. 
The layer 60 is flexible and is compressible and conformable, that is, it 
can be extruded or deformed into small gaps and can be conformed to 
irregular surfaces. In preferred embodiments, the layer 60 is a foamed 
polymer or plastic tape 62 having a thickness of about 10-20 mils. Polymer 
materials such as acrylic, ethylene vinyl acetate, and fluoropolymers have 
been found suitable, with acrylic being preferred. For purposes of the 
present description, the layer 60 will be viewed as having a free or upper 
surface 64 and an opposed or second surface 66. 
The tape 62 or other layer 60 is first placed on top of the particles or 
spheres 16 with the opposed surface 66 in contact with the poles 31P 
thereof. Next, forces are applied to the free surface 64 of the tape 62. 
These forces are directed toward the reflective surface 20T of the first 
foil 20, effect the adherence of the cover to the cell 12, and render the 
second surface 66 of the tape 62 irregular to achieve two results. 
First, the second surface 66 is urged against the poles 31P and the polar 
regions 31R of the extending or upper portions 31 of the particles 16. In 
this manner the second surface 66 becomes conformal with the polar regions 
31R, as indicated at 68. Second, the tape 62 is extruded or forced into 
the separation 70 between adjacent particles 16 so that, within each 
separation 70, the second surface 66 is configured as a convexity 
configured as a prism-like cusp, projection or similarly shaped extension 
72. 
Each cusp or prism 72 is continuous with the adjacent conformal areas 68 of 
the second surface 66. The walls or sides 73 of the cusps or prisms 72 are 
spaced away from the adjacent particles 16 by gaps 74 which increase in 
the direction of the reflective surface 20T. Stated differently, the walls 
73 of the cusps or prisms 72 are highly inclined relative to each other 
and to the adjacent particles or spheres 16, are highly inclined relative 
to and are not perpendicular to the reflective surface 20T, and are highly 
inclined and are not parallel to light 50b directed at the cell 12 
generally perpendicularly to the reflective surface 20T. 
The terminus of each cusp or prism 72 approaches, but is spaced from, the 
reflective surface 20T. A space or chamber-like volume 78 is therefore 
defined by the walls 73 of each cusp or prism 72, the particles 16 
adjacent thereto and the reflective surface 20T. 
Preferably, the free surface 64 of the tape 62 is and remains flat, meaning 
smooth and non-rough, regardless of the non-planar configuration assumed 
by the cell 12 and the cover 14. In this manner, the cover 14 is less 
likely to retain dirt or pollutants. Dirt and pollutants which adhere to 
the flat free surface 64 are easy to remove therefrom. In forming the 
conformal areas 68 and the cusps or prisms 72, as described above, and/or 
in order to maintain the free surface 64 of the tape 62 flat, the forces 
applied to the free surface 64 of the tape 62 may be accompanied by the 
application of heat. Such heat or an appropriate adhesive may also be 
employed to adhere the cover to the cell 12. 
A flat outer layer 80 may be applied to the free surface 64 of the tape 62 
to ensure that the outermost surface of the cover is flat. The 
characteristics of the outer layer 80 may be selected to enhance or ensure 
the ambient-protective function of the cover, and to that end, the outer 
layer 80 has optical and protective properties similar to those of the 
tape 62. 
The cover 14, like the cell 12, is flexible. As the cell 12 assumes various 
configurations, the free surface 64 of the tape 62 remains essentially 
parallel to the reflective surface 20T of the first foil 20. Moreover, the 
walls 73 of the cusps or prisms 72 maintain the above-described 
relationships with the adjacent particles or spheres 16, the reflective 
surface 20T and the light 50b, etc. during flexing of the cell 12 and its 
cover 14. As a consequence, whether or not the cell 12 assumes a 
non-planar configuration, the layer 60, and particularly the cusps 72 of 
the irregular surface 66 thereof, effect an increase in the efficiency of 
the cell 12. 
Specifically, and referring to FIG. 2, the irregular surface 66 of the tape 
62 and the cusps or prisms 72 thereof function to direct otherwise 
"wasted" light 50b, etc. which would fall on the reflective surface 20T 
between adjacent particles or spheres 16 if the cover 14 was not present, 
onto the extending or upper portions of the adjacent particles or spheres 
16. 
The vertical or nearly vertical light 50b, etc. enters the tape 62 through 
its free surface 64 from the ambient or after passing through the outer 
layer 80, if such is present. The light 50b, etc. then vertically or 
nearly vertically traverses the cusps or prisms 72 and passes through the 
interfaces between the walls 73 of the cusps 72 and the space 78, as 
indicated at 82. The indices of refraction of the material of the tape 62 
and of the space 78 are selected to cooperate with the highly inclined 
orientation of the walls 73 so that the light 50b, etc. is highly 
refracted at the interfaces 82 as shown in FIG. 2. 
The highly refracted light 50b, etc. passes through the gap 74 along the 
wall 73 of the cusp or prism 72 proximate to one of the adjacent particles 
16 and impinges non-perpendicularly on the reflective surface 20T. From 
the reflective surface 20T, the light 50b, etc. is non-perpendicularly 
reflected through the gap 74 along the wall 73 of the cusp or prism 72 
proximate to the other adjacent particle 16 until it impinges on the 
portion 31 of such other adjacent particle 16. 
The amount of refraction which occurs at the interfaces 82 depends in part 
on the index of refraction of the spaces 78. The spaces 78 may contain 
air. Because constant exposure of the cell 12 to moisture may be 
deleterious to its operation, the air in the spaces 78 may be dry, or 
appropriate moisture getters may be included in the spaces 78. Other 
gaseous or liquid substances, or polymers, which will not deleteriously 
affect the cell 12 may be included in the spaces 78 to selectively affect 
the refraction of the light 50b, etc. The spaces 78 may also be partially 
evacuated. 
The efficiency increase effected by the cover 14 has been measured to be 
about 25% for a given amount of light 50 incident on the cell 12. The 
cover 14 of FIG. 2 causes an apparent increase of more than 10% in the 
diameters of the particles or spheres 16 when viewed from its free surface 
64, which translates into an increase of more than 21% in the projected 
area of the particles 16 leading to a concomitant increase in electrical 
output. This apparent increase results in the increased efficiency of the 
cell 12 due to the increased amount of light 50b, etc. entering the 
spheres 16. 
Depending on factors such as the flexibility and deformability of the tape 
62 and the relative adherence between the surfaces of the particles 16 and 
the lower surface 66 of the tape 62, the placement of the tape 62 on the 
cell 12 may result in the formation of concavities 90 rather than the 
convex cusps or prisms 72. Such concavities 90 are depicted in FIG. 3. In 
the event of concavity 90 formation, the spaces 78 are bounded by the 
reflective surface 20T and the walls 92 of the concavities 90. As with the 
previous embodiment, if the spaces 78 contain a vacuum or a 
light-transparent gaseous, liquid or solid substance, the refractive index 
of which has an appropriate relationship to the refractive index of the 
tape 62, some of the light 50b, etc. will be prevented from being wasted 
and will enter the particles 16. 
Specifically, light 50b, etc. generally perpendicular to the separation 
between adjacent particles 16 will, upon passing through a concavity wall 
92-space 78 interface 94, change direction from nearly vertical to a path 
generally aimed at the particle 16 to which the light 50b, etc. was 
closer. The directionally altered light then is reflected from the surface 
20T and into the closer particle 16. As will be recalled, the cusps or 
prisms 72 effected direction of light 50b, etc. entering a space 78 nearer 
to one particle 16 into another, adjacent particle 16. Thus, the cusps or 
prisms 72 and the concavities 90 are functional equivalents, even though 
each directs the otherwise wasted light 50b, etc. along different paths. 
In both embodiments, the under surface 66 of the cover 14 between adjacent 
particles 16 is not perpendicular to the vertical or nearly vertical light 
50b, etc. which is, accordingly, diffracted within the space 78 for 
non-normal reflection by the surface 20T into a particle 16. 
Those skilled in the art will appreciate that numerous other embodiments 
and equivalents of those disclosed are within the purview of the foregoing 
description and are covered by the following claims. Such persons will 
also appreciate that both the angular relationship of the walls 73,92 to 
the direction of the wasted light 50b, etc. and the refractive indices of 
the cusps 72 or concavities 90 may be adjusted to achieve various 
increases in the efficiency of the cell 12.