U.S. patent application number 09/964095 was filed with the patent office on 2003-03-27 for photocell with fluorescent conversion layer.
Invention is credited to MacDonald, Stuart G..
Application Number | 20030056820 09/964095 |
Document ID | / |
Family ID | 25508122 |
Filed Date | 2003-03-27 |
United States Patent
Application |
20030056820 |
Kind Code |
A1 |
MacDonald, Stuart G. |
March 27, 2003 |
PHOTOCELL WITH FLUORESCENT CONVERSION LAYER
Abstract
A photocell device having enhanced conversion efficiency. The
device is adapted to convert light energy to electrical energy in a
semiconductor layer, to convert a first, monochromatic light, to a
second light of different wavelength, in a fluorescent layer, to
transmit the first light through a hot mirror layer, to reflect
light from the hot mirror layer, and to reflect light from a mirror
layer disposed at a second surface of the device.
Inventors: |
MacDonald, Stuart G.;
(Pultneyville, NY) |
Correspondence
Address: |
GREENWALD & BASCH, LLP
349 WEST COMMERCIAL STREET, SUITE 2490
EAST ROCHESTER
NY
14445
US
|
Family ID: |
25508122 |
Appl. No.: |
09/964095 |
Filed: |
September 26, 2001 |
Current U.S.
Class: |
136/247 ;
136/256; 257/E31.129 |
Current CPC
Class: |
H01L 31/02322
20130101 |
Class at
Publication: |
136/247 ;
136/256 |
International
Class: |
H01L 031/00 |
Claims
I claim:
1. A photocell device having enhanced conversion efficiency,
comprising means for converting light energy to electrical energy
in a semiconductor layer, means for converting a first,
monochromatic light, to a second light of different wavelength, in
a fluorescent layer, means for transmitting said first light
through a hot mirror layer at a first surface of said device, means
internally for reflecting said second light from said hot mirror
layer, and means for internally reflecting said first and said
second light from a mirror layer disposed at a second surface of
said device.
2. The photocell device as recited in claim 1, wherein said
photocell device further comprises a source of said light
energy.
3. The photocell device as recited in claim 2, wherein said source
of light energy emits monochromatic light.
4. The photocell device as recited in claim 3, wherein said
monochromatic light has a half-power bandwidth of less than about
50 nanometers.
5. The photocell device as recited in claim 4, wherein said
monochromatic light has a half-power bandwidth of less than about
25 nanometers.
6. The photocell device as recited in claim 3, wherein said
monochromatic light has a wavelength of form about 400 to about 575
nanometers.
7. The photocell device as recited in claim 2, wherein said source
of light energy is a laser diode.
8. The photocell device as recited in claim 2, wherein said source
of light energy is a light-emitting diode.
9. The photocell device as recited in claim 1, wherein said hot
mirror preferentially transmits at least 80 percent of light with a
wavelength of from about 400 to about 700 nanometers and
preferentially reflects at least 80 percent of light with a
wavelength of from about 750 to about 1500 nanometers.
10. The photocell device as recited in claim 1, wherein said hot
mirror preferentially transmits at least 90 percent of light with a
wavelength below 675 nanometers and preferentially reflects at
least 90 percent of light with a wavelength above 750
nanometers.
11. The photocell device as recited in claim 1, wherein said hot
mirror preferentially transmits at least 95 percent of light with a
wavelength less than 675 nanometers and preferentially reflects at
least 97 percent of light with a wavelength above 750
nanometers.
12. The photocell device as recited in claim 1, wherein said hot
mirror preferentially transmits at least 95 percent of light with a
wavelength less than about 550 nanometers and preferentially
reflects at least 95 percent of light with a wavelength above 550
nanometers.
13. The photocell device as recited in claim 2, wherein said
fluorescent layer is comprised of a fluorescent dye with a peak
excitation wavelength and a peak emission wavelengh.
14. The photocell device as recited in claim 13, wherein said light
source emits light at a wavelength which is about the same as the
peak excitation wavelength of said fluorescent dye.
15. The photocell device as recited in claim 2, wherein said device
is comprised of said hot mirror layer and, disposed beneath said
hot mirror layer, said semiconductor layer.
16. The photocell device as recited in claim 15, wherein said
semiconductor layer is photovoltaic layer.
17. The photocell device as recited in claim 16, wherein said
photovoltaic layer is contiguous with said hot mirror layer.
18. The photocell device as recited in claim 17, wherein said
fluorescent layer is disposed beneath said photovoltaic layer.
19. The photocell device as recited in claim 16, wherein said
fluorescent layer is disposed between said hot mirror and said
photovoltaic layer.
20. The photocell device as recited in claim 19, wherein said
fluorescent layer is contiguous with said hot mirror layer.
Description
FIELD OF THE INVENTION
[0001] A process for converting light to electricity at an
efficiency of at least about fifty percent.
BACKGROUND OF THE INVENTION
[0002] Commercially available prior art devices for converting
light to electricity are relatively inefficient. These commercially
available devices include photocells, which are characterized by
the efficiency with which they can convert incident light energy to
useful electrical energy. In general, such efficiency does not
exceed about 23 percent. Thus, as is disclosed in U.S. Pat. No.
6,278,055, " . . . photosensitive optoelectronic devices have been
constructed of a number of inorganic semiconductors. . . . Solar
cells are characterized by the efficiency with which they can
convert incident solar power to useful electric power. Devices
utilizing crystalline or amorphous silicon dominate commercial
applications and some have achieved efficiencies of 23% or greater.
However, efficient crystalline-based devices, especially of large
surface area, are difficult and expensive to produce due to the
problems inherent in producing large crystals without significant
efficiency-degrading defects."
[0003] It is an object of this invention to provide a process for
converting light to electricity which is substantially more
efficient and reliable than prior art processes.
SUMMARY OF THE INVENTION
[0004] In accordance with this invention, there is provided a
process comprising the steps of: transmitting light at a first
wavelength through a first optical layer, transmitting said light
with said first wavelength through a photocell element and a second
optical layer, converting said light with said first wavelength
while it is within said second optical layer to light with a second
wavelength, reflecting said light with a second wavelength from a
mirror, transmitting said light with a second wavelength which has
been internally reflected by said mirror through said photocell
element and said second optical layer to said first optical layer,
and internally reflecting said light with a second wavelength which
has been internally reflected by said mirror through said photocell
element from said first optical layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The invention will be described by reference to the
following drawings, in which like numerals refer to like elements,
and in which:
[0006] FIG. 1 is a schematic diagram illustrating one preferred
process of the invention, and
[0007] FIG. 2 is a schematic diagram illustrating a second
preferred process of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0008] The process of this invention provides increased conversion
of light to electricity. This efficiency, often referred to as
"conversion efficiency," refers to the efficiency of the process of
converting light energy to electrical energy within a photovoltaic
device.
[0009] Photocells are characterized by the efficiency with which
they can convert incident light energy to useful electrical energy.
Devices utilizing crystalline or amorphous silicon dominate
commercial applications, and some have achieved efficiencies of 23%
or greater. However, efficient crystalline-based devices,
especially of large surface area, are difficult and expensive to
produce due to the problems inherent in producing large crystals
without significant efficiency-degrading defects. More recent
efforts have focused on the use of organic photovoltaic cells to
achieve acceptable photovoltaic conversion efficiencies with
economical production costs, but in general the materials used in
these devices are not reliable over a long period of time.
[0010] Semiconductors that convert photons into electrical energy
do so due to their having a conduction band and a valence band that
are separated by an energy gap, E.sub.g, that varies with material
composition and temperature. When a photon is absorbed by a
semiconductor, an electron is promoted from the valence band into
the conduction band, thereby creating a hole in the valence band.
Since a hole represents the absence of an electron, it can be
regarded as a positive charge carrier. When donor or n-type
impurities (which increase the number of electrons available to
carry current) are added to one side of a semiconductor crystal and
acceptor or p-type impurities (which increase the number of holes)
to the other, a p-n junction is formed that permits current flow in
one direction but restrains it in the opposite direction. Thus, p-n
junctions are ideal for converting light into electricity.
[0011] It will be known to those skilled in the art that a photon
of wavelength .lambda. (as measured in a vacuum) and frequency .nu.
has an energy h.nu.=hc/.lambda. and can be absorbed by a
semiconductor when h.nu..gtoreq.E.sub.g. However, any extra energy
in the photon (h.nu.-E.sub.g) is converted into thermal rather than
electrical energy, since only one electron-hole pair can be created
for each absorption event. On the other hand, a semiconductor is
more transparent to wavelengths corresponding to energies less than
E.sub.g, since in this case the photons are not energetic enough to
promote electrons from the valence band into the conduction band.
Thus, no single semiconducting material can convert the entire
solar spectrum into electrical energy, since the most energetic
photons produce largely thermal energy and are therefore
inefficiently utilized, while the least energetic photons cannot be
absorbed. However, by cascading the p-n junctions of different
materials, the overall conversion efficiency can be improved
somewhat. In so-called "tandem" solar cells, a top cell having a
p-n junction of a high-energy band gap semiconductor intercepts the
most energetic photons. Lower energy photons pass through the top
cell before they enter another cell having a p-n junction of a
lower energy band gap semiconductor. In this way, an additional
portion of the solar spectrum is used. Prior art devices, such as
the device disclosed in U.S. Pat. No. 5,853,497 make use of
multiple photovoltaic layers to take advantage of the above
factors, but they typically are restricted to a small number of
layers due to the complexity of manufacture and the lower
manufacturing yields associated with more complex structures. The
entire disclosure of this United States patent is hereby
incorporated by reference into this specification.
[0012] Other prior art devices relate to use of fluorescent
materials to convert solar radiation to higher wavelengths that are
more efficiently converted to electricity by photovoltaic devices.
One such device, disclosed in U.S. Pat. No. 4,367,367, makes use of
large glass sheets that are doped with a fluorescent material and
utilize a photovoltaic device affixed to one edge, the other edges
being coated with a reflective mirror. As can be seen from a study
of this U.S. Pat. No. 4,367,367, this configuration results in a
high degree of attenuation of light in the glass sheet, and the
resulting low conversion efficiency cited in the patent (under 5%)
is not unexpected. The entire disclosure of this United States
patent is hereby incorporated by reference into this
specification.
[0013] In one embodiment of the instant invention, there is
provided an enhanced photocell device for converting optical energy
to electrical energy, said photocell device having one or more
optical layers and/or thin-film coatings which improve conversion
efficiency by means of light reflection, wavelength translation,
and light trapping within the photocell device.
[0014] In one embodiment, the device of this invention is
incorporated into an energy transmission system wherein 1) the
coupling efficiency from a light transmission means into a
photocell, and 2) the conversion efficiency of light energy to
electrical energy within the photocell, are of primary importance.
However, this invention may be applied to any system wherein light
energy is being coupled into a conversion device, including
non-semiconductor photovoltaic devices, photoresistive devices,
photodiode devices, semiconductor communications devices, and the
like. Thus, by way of illustration, one may use this invention with
any of the photovoltaic devices disclosed in U.S. Pat. Nos.
5,424,223, 5,341,008, 5,330,918 (method of forming a high voltage
silicon-on-saphire photocell array), U.S. Pat. No. 5,330,585
(gallium arsenide/aluminum gallium arsenide photocell including
environmentally sealed ohmic contact grid interface and method of
fabricating the cell), U.S. Pat. Nos. 5,206,534, 5,072,109
(photocell array with multi-spectral filter), U.S. Pat. Nos.
6,281,428, 6,268,223, 6,265,653 (high voltage photovoltaic power
converter), U.S. Pat. No. 6,261,862 (process for producing
photovoltaic element), U.S. Pat. Nos. 6,252,158, 6,252,157
(amorphous silicon-based thin film photovoltaic device), U.S. Pat.
Nos. 6,242,686, 6,211,454 (photovoltaic element), U.S. Pat. Nos.
6,184,456, 6,180,870, 6,166,319 (multi-junction photovoltaic device
with monocrystalline I-layer), U.S. Pat. Nos. 6,162,988, 6,159,736,
and the like. The entire disclosure of each of these United States
patents is hereby incorporated by reference into this
specification.
[0015] FIG. 1 is a schematic representation of one preferred
embodiment of the invention. Referring to FIG. 1, a light source 10
emits lights rays 12 through a first optical layer 14, a photocell
assembly 16, and a fluorescent layer 18. The light rays 12 passing
through fluorescent layer 18 are divided into two light rays 20 and
22, each of which have a different wavelength than light rays 12.
The light rays 20 and 22 reflect off of mirror 24 and also pass
through photocell assembly 16 within which at least part of their
photonic energy is converted into electrical energy. Those portions
of the light rays 20 and 22 which are not absorbed by the photocell
assembly 16 are then internally reflected from first optical layer
14.
[0016] The light source 10 preferably emits monochromatic light. As
used herein the term monochromatic light refers to light which
consists of a single wavelength or a very narrow band of
wavelengths. Reference may be had, e.g., to U.S. Pat. Nos.
6,291,184, 6,291,151, 6,290,382, 6,288,815, 6,285,345, 6,283,597,
6,282,438, 6,282,013, 6,281,491, 6,276,798, 6,275,251, 6,274,860,
6,272,440, 6,271,913, 6,266,167, 6,264,470, 6,263,291, 6,262,710,
6,256,530, and the like. The entire disclosure of each of these
United States patents is hereby incorporated by reference into this
specification.
[0017] By way of illustration and not limitation, the monochromatic
light used in the process of this invention has a half-power
bandwidth of less than about 50 nanometers and, more preferably,
less than about 25 nanometers. As is known to those skilled in the
art, half power bandwidth refers to the range of wavelengths where
the energy of the light source is above 50 percent of its peak
value. Reference may be had, e.g., to U.S. Pat. Nos. 6,061,140,
5,959,773, 5,703,364, 5,233,197, 5,218,207, 5,173,274, 5,086,229,
5,028,787, 4,990,772, 4,967,090, 4,959,551, and the like. The
entire disclosure of each of these United States patents is hereby
incorporated by reference into this specification.
[0018] In one embodiment, the monochromatic light has a wavelength
of less than about 600 nanometers and, preferably, has wavelengths
ranging from about 400 nanometers to about 575 nanometers.
[0019] One may use any of the prior art devices adapted to produce
such monochromatic light. Thus, by way of illustration and not
limitation, one may use a Melles Griot DPSS Laser, model number
58GIL, which is sold by the Melles Griot Company of Carlsband,
Calif. and emits monochromatic light with a wavelength of 523
nanometers.
[0020] Thus, by way of further illustration, one may use a
light-emitting diode to produce the monochromatic light 12.
Alternatively, one may use a filtered incandescent source to
produce the light 12. Other sources of such monochromatic light are
well known to those skilled in the art.
[0021] Referring again to FIG. 1, the monochromatic light 12 is
caused to contact and pass through hot mirror 14. As is known to
those skilled in the art, a hot mirror is a mirror that
preferentially transmits at least 80 percent light below a certain
wavelength, and preferentially reflects at least 80 percent of
light above a certain wavelength. Thus, e.g. the hot mirror 14 may
preferentially transmit at least 80 percent of light with a
wavelength from from about 400 to about 700 nanometers and
preferentially reflects at least 80 percent of light with a
wavelength of from about 750 nanometers to about 1500 nanometers
and above. Reference may be had, e.g., to U.S. Pat. Nos. 6,275,602,
6,249,348, 6,246,479, 6,220,713, 6,185,041, 6,117,530, 5,982,078,
5,962,114, 5,923,471, 5,882,774, 5,857,768, 5,795,708, and the like
The entire disclosure of each of these United States patents is
hereby incorporated by reference into this specification.
[0022] By way of further illustration, hot mirror 14 may be, e.g.,
the "hot mirror" sold as catalog number HM-25.4 by the Quantum
Optics Company of Pomfret, Conn. This "hot mirror" transmits 90
percent at most wavelengths of light below 675 nanometers, and it
reflects 90 percent of light above 750 nanometers.
[0023] By way of further illustration, hot mirror 14 may be, e.g.,
the "hot mirror" sold by the Melles Griot Company as catalog number
03 MHG007.
[0024] In one preferred embodiment, the hot mirror 14 transmits at
least about 95 percent of light in a band below 675 nanometers and
reflects at least about 97 percent of light in a band above 750
nanometers, wherein each of said bands has a half power bandwidth
of less than 100 nanometers. In one preferred embodiment, each of
said bands has a half power bandwidth of less than about 50
nanometers.
[0025] By way of yet further illustration, and in one preferred
embodiment, the hot mirror transmits at least about 95 percent of
light in a band below 550 nanometers and reflects at least about 95
percent of light in a band above 550 nanometers.
[0026] Referring again to FIG. 1, and in the preferred embodiment
depicted therein, after the light 12 passes through the hot mirror
14, it then passes through photocell element 16. As is known to
those skilled in the art, a photocell is a device designed to
provide an electric output that corresponds to the radiation that
is incident upon the device. Reference may be had, e.g., to U.S.
Pat. Nos. 6,278,055, 6,198,092, 6,198,091, 6,091,017, 5,868,869,
5,853,497, 5,720,827, 5,714,012, 5,674,325, 4,629,821, and the
like. The entire disclosure of each of these United States patents
is hereby incorporated by reference into this specification.
[0027] As is know to those skilled in the art, one can custom
design a hot mirror with specified optical properties. The hot
mirror utilizes a short-wavelength pass filter stack design of the
basic form 1.6 (0.5L H 0.5L).sup.n wherein: H=QWOT of high index
material, L=QWOT of low index material, and n is equal to the
number of layers of material in the hot mirror.
[0028] Commercially available hot mirrors typically have a
transition between near total transmission and near total
reflection at a wavelength of about 700 nanometers. However, these
characteristics can be custom modified using the aforementioned
formula to effect a transition wavelength of, e.g., 600
nanometers.
[0029] By way of illustration, one may use the photocell depicted
in U.S. Pat. No. 5,342,451; the entire disclosure of this United
States patent is hereby incorporated by reference into this
specification. The figures of this patent depict a series of
semiconductor layers contained in a photovoltaic cell. In the
device of this patent, there is depicted a substrate which is
covered by a buffer layer, and a two-part active layer comprising
an `n` base and a `p` emitter. These layers are in turn covered by
a window. In such device, there is an additional conducting layer
that is not required for the photocell operation but which improves
the efficiency of the photocell by providing a path for electron
flow that has substantially reduced lateral resistivity than the
emitter layer in combination with the window. Thus the photocell
element may be considered as an individual functional element
rather than a complex multilayer construct.
[0030] Referring again to FIG. 1, it will be apparent that some of
the light 12 is absorbed by the photocell element 16 and converted
into electricity. However, some of the light 12 is transmitted
through the photocell element 16 and contacts a fluorescent layer
18.
[0031] Fluorescent layer 18 preferably contains a fluorescent dye
which effects a "Stokes shift" on light 12. As is known to those
skilled in the art, a Stokes shift is the displacement of spectal
lines or bands of luminescent radiation toward longer wavelengths
than those of the absorption lines or bands. Reference may be had,
e.g., to U.S. Pat. Nos. 6,287,765, 6,277,984, 6,274,065, 6,259,104,
6,251,687, 6,242,430, 6,238,931, 6,211,954, 6,207,464, 6,198,107,
6,174,424, 6,166,385, 6,140,051, 6,138,800, 6,130,094, and the
like. The entire disclosure of each of these United States patents
is hereby incorporated by reference into this specification.
[0032] As is known to those skilled in the art, fluorescent dyes
often effect a Stokes shift upon incident radiation. Without
wishing to be bound to any particular theory, applicant believes
that this phenomenon occurs because the excitation light dissipates
its energy by raising an electron in the fluorescent material to a
higher orbital as such light is absorbed. However, there is excess
energy that is dissipated thermally. Upon relaxation from the
higher orbital state, the electron release a photon having a lower
energy state. This released photon with a lower energy state
exhibits itself as a wave with longer wavelength. The Stokes shift
is the difference between the wavelength of the incident light and
the emitted light.
[0033] In one aspect of the preferred embodiment illustrated in
FIG. 1, the incident light 12 has a wavelength of about 530
nanometers, and the emitted has a wavelength of about 570
nanometers. As will be apparent, by choosing different fluorescent
materials and/or different thicknesses of fluorescent materials,
one can effect different Stokes shifts.
[0034] Fluorescent materials which effect different Stokes shifts
are well known to those skilled in the art. Reference may be had,
e.g., to U.S. Pat. Nos. 6,214,563, 6,210,910, 6,207,831, 6,205,281,
6,200,762, 6,192,261, 6,180,354, 6,141,096, and the like. The
entire disclosure, of each of these United States patents is hereby
incorporated by reference into this specification.
[0035] As is known to those skilled in the art, fluorescent
materials are characterized by having both a peak excitation
wavelength and a peak emission wavelength. Reference may be had,
e.g., to The "Handbook of Fluorescent Probes and Research
Chemicals" which is published by Molecular Probes, Inc. of Eugene
Oreg. Much information about flurochromes also will be found in a
book edited by Taylor, Wang, and Yu-Li entitled "Fluorescence:
Miscrocopy of Living Cells in Culture, Parts A and B (Academic
Press, New York, 1989). Reference also may be had to publications
by the Omega Optical Company and the Chroma Technology Company.
[0036] Thus, by way of illustration, one may use one or more of the
following fluorescent materials: (1) 7-aminoactinomycin D-AAD, with
an excitation wavelength of 546 nanometers and an emission
wavelength of 647 nanometers, (2) Astrazon Red 6B, with an
excitation wavelength of 520 nanometers and an emission wavelength
of 595 nanometers, (3) Genacryl Brilliant Red B, with an excitation
wavelength of 520 nanometers and an emission wavelength of 590
nanometers, (4) Magdala Red, with an excitation wavelength of: 524
nanometers and an emission wavelength of 600 nanometers, and the
like. These fluorescent materials are well known to those skilled
in the art and are presented in a table entitled "Flurochrome Data
Tables: Exitation/Emission Wavelenghts Listed by Flurochrome,"
which was prepared by Michael W. Davidson, Mortimer Abramovitz,
Olympus America Inc., and The Florida State University.
[0037] As will be apparent to those skilled in the art, depending
upon the excitation energy of the fluorescent material used, one
will preferably use a light source will effectively excite the
fluorescent material and cause it to emit at its emission
wavelength. Thus, by way of illustration and not limitation, for a
fluorescent material which is excited at a wavelength of about 532
nanometers, one may advantageously use a laser sold as the "Green
Diode-Pumped Solid State Laser Module," as part number L54-550, by
the Edmund Industrial Optics Company of 101 East Gloucester Pike,
Barrington, N.J. 08007; this laser light source emits light at a
wavelength of 532 nanometers, plus or minus 5 nanomters.
Alternatively, or additionally, one may use a light emitting diode
which contains gallium phosphide and which emits radiation at a
wavelength of about 0.55 microns; see, e.g., page 106 of Harry
Dutton's "Understanding Optical Communications" (Prentice Hall PTR,
Upper Saddle River, N.J., 1998). Alternatively, or additionally,
one may use a light emitting diode which consists essentially of
gallium arsenide, that has a spectral emission of 540 nanometers, a
half power bandwidth of 3 nanometers, and a green color; see, e.g.,
page11-10 of Donald G. Fink's "Electronics Engineers' Handbook,"
Second Edition (McGraw-Hill Book Company, New York, N.Y., 1982).
Other suitable light sources, and/or combinations of light sources,
will be apparent to those skilled in the art.
[0038] It is preferred that the Stokes shift effected by optical
layer 18 be at least 100 nanometers.
[0039] As will be apparent to those skilled in the art, the device
of FIG. 1 (and of FIG. 2) acts as a light trap, shifting some or
all of the incident light to a wavelength wherein it will be
totally internally reflected until complete conversion of such
light is effected by the conversion of such light to electricity.
This light trap feature substantially increases the efficiency of
the utilization of the incident light.
[0040] FIG. 2 is a schematic diagram of another preferred process
of the invention which is similar to the process depicted in FIG. 1
but differs therefrom in that layer 18 is disposed immediately
adjacent to hot mirror 14 instead of being disposed adjacent to
mirror 24.
[0041] It is to be understood that the aforementioned description
is illustrative only and that changes can be made in the apparatus,
in the sequence and combination of elements of said apparatus, as
well as in other aspects of thee invention discussed herein,
without departing from the scope of the invention as defined in the
following claims.
* * * * *