U.S. patent application number 12/144548 was filed with the patent office on 2009-03-05 for solar modules with enhanced efficiencies via use of spectral concentrators.
Invention is credited to John Kenney, John Midgley, William Matthew PFENNINGER, Nemanja Vockic, Jian Jim Wang.
Application Number | 20090056791 12/144548 |
Document ID | / |
Family ID | 40186260 |
Filed Date | 2009-03-05 |
United States Patent
Application |
20090056791 |
Kind Code |
A1 |
PFENNINGER; William Matthew ;
et al. |
March 5, 2009 |
SOLAR MODULES WITH ENHANCED EFFICIENCIES VIA USE OF SPECTRAL
CONCENTRATORS
Abstract
Described herein are solar modules including spectral
concentrators. In one embodiment, a solar module includes a set of
photovoltaic cells and a spectral concentrator optically coupled to
the set of photovoltaic cells. The spectral concentrator is
configured to: (1) collect incident solar radiation; (2) convert
the incident solar radiation into substantially monochromatic,
emitted radiation; and (3) convey the substantially monochromatic,
emitted radiation to the set of photovoltaic cells.
Inventors: |
PFENNINGER; William Matthew;
(Fremont, CA) ; Midgley; John; (San Carlos,
CA) ; Vockic; Nemanja; (San Jose, CA) ;
Kenney; John; (Palo Alto, CA) ; Wang; Jian Jim;
(Orefield, PA) |
Correspondence
Address: |
COOLEY GODWARD KRONISH LLP;ATTN: Patent Group
Suite 1100, 777 - 6th Street, NW
Washington
DC
20001
US
|
Family ID: |
40186260 |
Appl. No.: |
12/144548 |
Filed: |
June 23, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60945869 |
Jun 22, 2007 |
|
|
|
60977067 |
Oct 2, 2007 |
|
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Current U.S.
Class: |
136/247 |
Current CPC
Class: |
H01L 31/055 20130101;
Y02E 10/52 20130101 |
Class at
Publication: |
136/247 |
International
Class: |
H01L 31/042 20060101
H01L031/042 |
Claims
1. A solar module comprising: a set of photovoltaic cells; and a
spectral concentrator optically coupled to the set of photovoltaic
cells, wherein the spectral concentrator is configured to: collect
incident solar radiation; convert the incident solar radiation into
substantially monochromatic, emitted radiation; and convey the
substantially monochromatic, emitted radiation to the set of
photovoltaic cells.
2. The solar module of claim 1, wherein the spectral concentrator
includes a luminescent stack including: a luminescent layer having
a first surface and a second surface; a first reflector adjacent to
the first surface; and a second reflector adjacent to the second
surface.
3. The solar module of claim 2, wherein the luminescent layer
includes a luminescent material that exhibits photoluminescence
having: (a) an internal quantum efficiency of at least 50 percent;
(b) a spectral width no greater than 100 nm at Full Width at Half
Maximum; and (c) a peak emission wavelength in the near infrared
range.
4. The solar module of claim 2, wherein the luminescent layer
includes a luminescent material having the formula:
[A.sub.aB.sub.bX.sub.x], wherein A is selected from elements of
Group IA; B is selected from elements of Group VA; X is selected
from elements of Group VIIB; a is in the range of 1 to 9; b is in
the range of 1 to 5; and x is in the range of 1 to 9.
5. The solar module of claim 2, wherein the luminescent layer
includes a luminescent material selected from InP, Zn.sub.3P.sub.2,
Cu.sub.2O, CuO, CuInGaS, and CuInGaSe.
6. The solar module of claim 2, wherein at least one of the first
reflector and the second reflector includes a dielectric stack.
7. The solar module of claim 1, wherein at least one of the set of
photovoltaic cells has a vertical junction orientation with respect
to the spectral concentrator.
8. The solar module of claim 1, wherein at least two of the set of
photovoltaic cells are connected in series.
9. The solar module of claim 1, wherein the spectral concentrator
defines multiple grooves, and multiple ones of the set of
photovoltaic cells are positioned in respective ones of the
grooves.
10. The solar module of claim 1, wherein the spectral concentrator
includes a resonant cavity structure.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 60/945,869, filed on Jun. 22, 2007, and U.S.
Provisional Application Ser. No. 60/977,067, filed on Oct. 2, 2007,
the disclosures of which are incorporated herein by reference in
their entireties.
FIELD OF THE INVENTION
[0002] The invention relates generally to solar modules. More
particularly, the invention relates to solar modules including
spectral concentrators.
BACKGROUND
[0003] A solar module operates to convert energy from solar
radiation into electricity, which is delivered to an external load
to perform useful work. A solar module typically includes a set of
photovoltaic ("PV") cells, which can be connected in parallel, in
series, or a combination thereof. The most common type of PV cell
is a p-n junction device based on crystalline silicon. Other types
of PV cells can be based on amorphous silicon, polycrystalline
silicon, germanium, organic materials, and Group III-V
semiconductor materials, such as gallium arsenide.
[0004] During operation of an existing solar module, incident solar
radiation penetrates below a surface of the PV cell and is absorbed
within the PV cell. A depth at which the solar radiation penetrates
below the surface can depend upon an absorption coefficient of the
PV cell. In the case of a PV cell based on silicon, an absorption
coefficient of silicon varies with wavelength of solar radiation.
For example, for solar radiation at 900 nm, silicon has an
absorption coefficient of about 100 cm.sup.-1, and the solar
radiation can penetrate to a depth of about 100 .mu.m. In contrast,
for solar radiation at 450 nm, the absorption coefficient is
greater at about 10.sup.4 cm.sup.-1, and the solar radiation can
penetrate to a depth of about 1 .mu.m. At a particular depth within
the PV cell, absorption of solar radiation produces charge carriers
in the form of electron-hole pairs. Electrons exit the PV cell
through one electrode, while holes exit the PV cell through another
electrode. The net effect is a flow of an electric current through
the PV cell driven by incident solar radiation. The inability to
convert the total incident solar radiation to useful electrical
energy represents a loss or inefficiency of the solar module.
[0005] Current solar modules typically suffer a number of technical
limitations on the ability to efficiently convert incident solar
radiation to useful electrical energy. One significant loss
mechanism typically derives from a mismatch between an incident
solar spectrum and an absorption spectrum of PV cells. In the case
of a PV cell based on silicon, photons with energy greater than a
bandgap energy of silicon can lead to the production of
photo-excited electron-hole pairs with excess energy. Such excess
energy is typically not converted into electrical energy but is
rather typically lost as heat through hot charge carrier relaxation
or thermalization. This heat can raise the temperature of the PV
cell and, as result, can reduce the efficiency of the PV cell in
terms of its ability to produce electron-hole pairs. In some
instances, the efficiency of the PV cell can decrease by about 0.5
percent for every 1.degree. C. rise in temperature. In conjunction
with these thermalization losses, photons with energy less than the
bandgap energy of silicon are typically not absorbed and, thus,
typically do not contribute to the conversion into electrical
energy. As a result, a small range of the incident solar spectrum
near the bandgap energy of silicon can be efficiently converted
into useful electrical energy.
[0006] Also, in accordance with a junction design of a PV cell,
charge separation of electron-hole pairs is typically confined to a
depletion region, which can be limited to a thickness of about 1
.mu.m. Electron-hole pairs that are produced further than a
diffusion or drift length from the depletion region typically do
not charge separate and, thus, typically do not contribute to the
conversion into electrical energy. The depletion region is
typically positioned within the PV cell at a particular depth below
a surface of the PV cell. The variation of the absorption
coefficient of silicon across an incident solar spectrum can impose
a compromise with respect to the depth and other characteristics of
the depletion region that reduces the efficiency of the PV cell.
For example, while a particular depth of the depletion region can
be desirable for solar radiation at one wavelength, the same depth
can be undesirable for solar radiation at a shorter wavelength. In
particular, since the shorter wavelength solar radiation can
penetrate below the surface to a lesser degree, electron-hole pairs
that are produced can be too far from the depletion region to
contribute to an electric current.
[0007] It is against this background that a need arose to develop
the solar modules and related methods described herein.
SUMMARY
[0008] Embodiments of the invention relate to solar modules having
enhanced efficiencies with respect to conversion of incident solar
radiation to useful electrical energy. In one embodiment, a solar
module includes a set of photovoltaic cells and a spectral
concentrator optically coupled to the set of photovoltaic cells.
The spectral concentrator is configured to: (1) collect incident
solar radiation; (2) convert the incident solar radiation into
substantially monochromatic, emitted radiation; and (3) convey the
substantially monochromatic, emitted radiation to the set of
photovoltaic cells.
[0009] Other aspects and embodiments of the invention are also
contemplated. The foregoing summary and the following detailed
description are not meant to restrict the invention to any
particular embodiment but are merely meant to describe some
embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] For a better understanding of the nature and objects of some
embodiments of the invention, reference should be made to the
following detailed description taken in conjunction with the
accompanying drawings.
[0011] FIG. 1 illustrates solar modules implemented in accordance
with various embodiments of the invention.
[0012] FIG. 2 illustrates a solar module implemented in accordance
with another embodiment of the invention.
[0013] FIG. 3 illustrates a solar module implemented in accordance
with another embodiment of the invention.
[0014] FIG. 4 illustrates aspects and potential loss mechanisms of
a luminescent stack implemented in accordance with another
embodiment of the invention.
[0015] FIG. 5, FIG. 6, and FIG. 7 illustrate features related to
optical, electrical, and mechanical structures for solar modules,
according to various embodiments of the invention.
[0016] FIG. 8 illustrates a combined representation of an incident
solar spectrum and measured absorption and emission spectra of
UD-930 in accordance with an embodiment of the invention.
DETAILED DESCRIPTION
Overview
[0017] Embodiments of the invention relate to solar modules having
enhanced efficiencies with respect to conversion of incident solar
radiation to useful electrical energy. For some embodiments, a
solar module includes a spectral concentrator and a set of PV cells
that are optically coupled to the spectral concentrator. The
spectral concentrator can perform a number of operations,
including: (1) collecting incident solar radiation; (2) converting
the incident solar radiation to substantially monochromatic
radiation near a bandgap energy of the PV cells; and (3) conveying
the converted radiation to the PV cells, where the converted
radiation can be converted to useful electrical energy. By
converting a wide range of energies of the incident solar radiation
to a narrow band of energies matched to the bandgap energy of the
PV cells, significant improvements in efficiency can be achieved.
In addition, the design of the PV cells can be optimized or
otherwise tailored based on this narrow band of energies.
[0018] As described herein, further improvements in efficiency of
solar modules can be achieved by incorporating suitable luminescent
materials within spectral concentrators and by exploiting resonant
cavity effects in the design of the spectral concentrators. Also
described herein are specific features related to optical,
electrical, and mechanical structures for solar modules. These
features include vertical junction PV cells with bifacial or
two-sided illumination as well as integrated diodes and other
electrical circuitry. These features can be combined into easily
assembled solar modules, along with electrical interconnect for low
resistive loss and low thermal load on PV cells.
DEFINITIONS
[0019] The following definitions apply to some of the elements
described with regard to some embodiments of the invention. These
definitions may likewise be expanded upon herein.
[0020] As used herein, the singular terms "a," "an," and "the"
include plural referents unless the context clearly dictates
otherwise. Thus, for example, reference to a luminescent material
can include multiple luminescent materials unless the context
clearly dictates otherwise.
[0021] As used herein, the term "set" refers to a collection of one
or more elements. Thus, for example, a set of layers can include a
single layer or multiple layers. Elements of a set can also be
referred to as members of the set. Elements of a set can be the
same or different. In some instances, elements of a set can share
one or more common characteristics.
[0022] As used herein, the terms "optional" and "optionally" mean
that the subsequently described event or circumstance may or may
not occur and that the description includes instances where the
event or circumstance occurs and instances in which it does
not.
[0023] As used herein, the term "ultraviolet range" refers to a
range of wavelengths from about 5 nanometer ("nm") to about 400
nm.
[0024] As used herein, the term "visible range" refers to a range
of wavelengths from about 400 nm to about 700 nm.
[0025] As used herein, the term "infrared range" refers to a range
of wavelengths from about 700 nm to about 2 millimeter ("mm"). The
infrared range includes the "near infrared range," which refers to
a range of wavelengths from about 700 nm to about 5 micrometer
(".mu.m"), the "middle infrared range," which refers to a range of
wavelengths from about 5 .mu.m to about 30 .mu.m, and the "far
infrared range," which refers to a range of wavelengths from about
30 .mu.m to about 2 mm.
[0026] As used herein, the terms "reflection," "reflect," and
"reflective" refer to a bending or a deflection of light, and the
term "reflector" refers to an object that causes, induces, or is
otherwise involved in such bending or deflection. A bending or a
deflection of light can be substantially in a single direction,
such as in the case of specular reflection, or can be in multiple
directions, such as in the case of diffuse reflection or
scattering. In general, light incident upon a material and light
reflected from the material can have wavelengths that are the same
or different.
[0027] As used herein, the terms "luminescence," "luminesce," and
"luminescent" refer to an emission of light in response to an
energy excitation. Luminescence can occur based on relaxation from
excited electronic states of atoms or molecules and can include,
for example, chemiluminescence, electroluminescence,
photoluminescence, thermoluminescence, triboluminescence, and
combinations thereof. For example, in the case of
photoluminescence, which can include fluorescence and
phosphorescence, an excited electronic state can be produced based
on a light excitation, such as absorption of light. In general,
light incident upon a material and light emitted by the material
can have wavelengths that are the same or different.
[0028] As used herein with respect to photoluminescence, the term
"quantum efficiency" refers to a ratio of the number of output
photons to the number of input photons. Quantum efficiency of a
photoluminescent material can be characterized with respect to its
"internal quantum efficiency," which refers to a ratio of the
number of photons emitted by the photoluminescent material to the
number of photons absorbed by the photoluminescent material. In
some instances, a photoluminescent material can be included within
a structure that is exposed to solar radiation, and the structure
can direct, guide, or propagate emitted light towards a PV cell. In
such instances, another characterization of quantum efficiency can
be an "external quantum efficiency" of the structure, which refers
to a ratio of the number of photons that reach the PV cell to the
number of solar photons that are absorbed by the photoluminescent
material within the structure. Alternatively, quantum efficiency of
the structure can be characterized with respect to its "overall
external quantum efficiency," which refers to a ratio of the number
of photons that reach the PV cell to the number of solar photons
that are incident upon the structure. As can be appreciated, an
overall external quantum efficiency of a structure can account for
potential losses, such as reflection, that reduce the fraction of
incident solar photons that can reach a photoluminescent material.
A further characterization of quantum efficiency can be an "energy
quantum efficiency," in which the various ratios discussed above
can be expressed in terms of ratios of energies, rather than ratios
of numbers of photons. An energy-based quantum efficiency can be
less than its corresponding photon number-based quantum efficiency
in the event of down-conversion, namely if a higher energy photon
is absorbed and converted to a lower energy emitted photon.
[0029] As used herein, the term "absorption spectrum" refers to a
representation of absorption of light over a range of wavelengths.
In some instances, an absorption spectrum can refer to a plot of
absorbance (or transmittance) of a material as a function of
wavelength of light incident upon the material.
[0030] As used herein, the term "emission spectrum" refers to a
representation of emission of light over a range of wavelengths. In
some instances, an emission spectrum can refer to a plot of
intensity of light emitted by a material as a function of
wavelength of the emitted light.
[0031] As used herein, the term "excitation spectrum" refers to
another representation of emission of light over a range of
wavelengths. In some instances, an excitation spectrum can refer to
a plot of intensity of light emitted by a material as a function of
wavelength of light incident upon the material.
[0032] As used herein, the term "Full Width at Half Maximum" or
"FWHM" refers to a measure of spectral width. In the case of an
emission spectrum, a FWHM can refer to a width of the emission
spectrum at half of a peak intensity value.
[0033] As used herein with respect to an absorption spectrum or an
excitation spectrum, the term "substantially flat" refers to being
substantially invariant with respect to a change in wavelength. In
some instances, a spectrum can be referred to as being
substantially flat over a range of wavelengths if absorbance or
intensity values within that range of wavelengths exhibit a
standard deviation of less than 20 percent with respect to an
average intensity value, such as less than 10 percent or less than
5 percent.
[0034] As used herein with respect to an emission spectrum, the
term "substantially monochromatic" refers to emission of light over
a narrow range of wavelengths. In some instances, an emission
spectrum can be referred to as being substantially monochromatic if
a spectral width is no greater than 120 nm at FWHM, such as no
greater than 100 nm at FWHM, no greater than 80 nm at FWHM, or no
greater than 50 nm at FWHM.
[0035] As used herein, the term "nanometer range" or "nm range"
refers to a range of dimensions from about 1 nm to about 1 .mu.m.
The nm range includes the "lower nm range," which refers to a range
of dimensions from about 1 nm to about 10 nm, the "middle nm
range," which refers to a range of dimensions from about 10 nm to
about 100 nm, and the "upper nM range," which refers to a range of
dimensions from about 100 nm to about 1 .mu.m.
[0036] As used herein, the term "micrometer range" or ".mu.m range"
refers to a range of dimensions from about 1 .mu.m to about 1 mm.
The .mu.m range includes the "lower .mu.nm range," which refers to
a range of dimensions from about 1 .mu.m to about 10 .mu.m, the
"middle .mu.m range," which refers to a range of dimensions from
about 10 .mu.m to about 100 .mu.m, and the "upper .mu.m range,"
which refers to a range of dimensions from about 100 .mu.m to about
1 mm.
[0037] As used herein, the term "size" refers to a characteristic
dimension of an object. In the case of a particle that is
spherical, a size of the particle can refer to a diameter of the
particle. In the case of a particle that is non-spherical, a size
of the particle can refer to an average of various orthogonal
dimensions of the particle. Thus, for example, a size of a particle
that is a spheroidal can refer to an average of a major axis and a
minor axis of the particle. When referring to a set of particles as
having a particular size, it is contemplated that the particles can
have a distribution of sizes around that size. Thus, as used
herein, a size of a set of particles can refer to a typical size of
a distribution of sizes, such as an average size, a median size, or
a peak size.
[0038] As used herein, the term "nanoparticle" refers to a particle
that has a size in the nm range. A nanoparticle can have any of a
variety of shapes, such as box-shaped, cube-shaped, cylindrical,
disk-shaped, spherical, spheroidal, tetrahedral, tripodal,
tube-shaped, pyramid-shaped, or any other regular or irregular
shape, and can be formed from any of a variety of materials. In
some instances, a nanoparticle can include a core formed from a
first material, which core can be optionally surrounded by a
coating or a shell formed from a second material. The first
material and the second material can be the same or different.
Depending on the configuration of a nanoparticle, the nanoparticle
can exhibit size dependent characteristics associated with quantum
confinement. However, it is also contemplated that a nanoparticle
can substantially lack size dependent characteristics associated
with quantum confinement or can exhibit such size dependent
characteristics to a low degree.
[0039] As used herein, the term "dopant" refers to a chemical
entity that is present in a material as an additive or an impurity.
In some instances, the presence of a dopant in a material can alter
a set of characteristics of the material, such as its chemical,
magnetic, electronic, or optical characteristics.
Solar Modules
[0040] FIG. 1 illustrates solar modules 100, 102, and 104
implemented in accordance with various embodiments of the
invention.
[0041] As illustrated in FIG. 1, the solar module 100 includes a PV
cell 106, which is a p-n junction device formed from crystalline
silicon. However, the PV cell 106 can also be formed from another
suitable photoactive material. As illustrated in FIG. 1, the PV
cell 106 is configured to accept and absorb radiation incident upon
a side surface 108 of the PV cell 106, although other surfaces of
the PV cell 106 can also be involved.
[0042] In the illustrated embodiment, the solar module 100 also
includes a spectral concentrator 110, which is formed as a slab
having a side surface 112 that is adjacent to the side surface 108
of the PV cell 106. The spectral concentrator 110 performs spectral
concentration by converting a relatively wide range of energies of
solar radiation into a narrow band of energies close to the bandgap
energy of silicon, or another photoactive material forming the PV
cell 106. In turn, the narrow band radiation emitted from the
spectral concentrator 110 can be efficiently absorbed within a
depletion region of the PV cell 106. By matching the energy of the
emitted radiation with the bandgap energy of the PV cell 106, much
higher solar energy conversion efficiencies can be achieved,
including efficiencies of 90 percent or more.
[0043] During operation of the solar module 100, incident solar
radiation strikes the spectral concentrator 110, which absorbs this
solar radiation and emits radiation in a substantially
monochromatic energy band. In particular, the spectral concentrator
110 is configured to perform down-conversion with a bandgap energy
E.sub.g close to a bandgap energy of the PV cell 106. Solar
radiation with energies at or higher than the bandgap energy
E.sub.g arc absorbed and converted into emitted radiation with
lower energies that match the bandgap energy of the PV cell 106. In
this manner, thermalization can mostly occur within the spectral
concentrator 110, rather than within the PV cell 106. Emitted
radiation from the spectral concentrator 110 is guided within the
spectral concentrator 110 and is directed towards the side surface
108 of the PV cell 106, which absorbs and converts this emitted
radiation into electricity. In the illustrated embodiment, the PV
cell 106 is optimized to operate with respect to the substantially
monochromatic, emitted radiation, but can also operate efficiently
with respect to incident solar radiation.
[0044] Still referring to FIG. 1, the solar module 102 includes a
PV cell 114, which is a p-n junction device formed from crystalline
silicon. However, the PV cell 114 can also be formed from another
suitable photoactive material. As illustrated in FIG. 1, the PV
cell 114 is configured to accept and absorb radiation incident upon
a top surface 116 of the PV cell 114, although other surfaces of
the PV cell 114 can also be involved. In the illustrated
embodiment, the solar module 102 also includes a spectral
concentrator 118, which is formed as a coating, film, or layer
adjacent to the top surface 116 of the PV cell 114. Certain aspects
of the solar module 102 can be implemented in a similar manner as
described above for the solar module 100, and, therefore, are not
further described herein.
[0045] During operation of the solar module 102, incident solar
radiation strikes the spectral concentrator 118, which absorbs this
solar radiation and emits radiation in a substantially
monochromatic energy band. In particular, the spectral concentrator
118 is configured to perform down-conversion with a bandgap energy
E.sub.g close to a bandgap energy of the PV cell 114. Solar
radiation with energies at or higher than the bandgap energy
E.sub.g are absorbed and converted into emitted radiation with
lower energies that match the bandgap energy of the PV cell 114. In
this manner, thermalization can mostly occur within the spectral
concentrator 118, rather than within the PV cell 114. Emitted
radiation from the spectral concentrator 118 is directed downwards
to the top surface 116 of the PV cell 114, which absorbs and
converts this emitted radiation into electricity. In the
illustrated embodiment, the PV cell 114 is optimized to operate
with respect to the substantially monochromatic, emitted radiation,
but can also operate efficiently with respect to incident solar
radiation.
[0046] As illustrated in FIG. 1, the solar module 104 is a multi
junction device, including multiple layers of spectral
concentrators 120A, 120B, and 120C that are optically coupled to
respective PV cells 122A, 122B, and 122C having different bandgap
energies. For example, the PV cells 122A, 122B, and 122C can be
formed from Group III materials, Group IV materials, Group V
materials, or combinations thereof, with bandgap energies in the
range of about 2.5 eV to about 1.3 eV or in the range of about 2.5
eV to about 0.7 eV. For example, silicon has a bandgap energy of
about 1.1 eV, and germanium has a bandgap energy of about 0.7 eV.
Certain aspects of the solar module 104 can be implemented in a
similar manner as described above for the solar module 100, and,
therefore, are not further described herein.
[0047] During operation of the solar module 104, incident solar
radiation strikes the spectral concentrator 120A, which is
configured to perform down-conversion with a bandgap energy
E.sub.gA close to a bandgap energy of the PV cell 122A. Solar
radiation with energies at or higher than the bandgap energy
E.sub.gA are absorbed and converted into substantially
monochromatic, emitted radiation that is guided towards the PV cell
122A, which absorbs and converts this emitted radiation into
electricity. Solar radiation with energies lower than the bandgap
energy E.sub.gA passes through the spectral concentrator 120A and
strikes the spectral concentrator 120B, which is configured to
perform down-conversion with a bandgap energy E.sub.gB close to a
bandgap energy of the PV cell 122B. Solar radiation with energies
at or higher than the bandgap energy E.sub.gB (and lower than the
bandgap energy E.sub.gA) are absorbed and converted into
substantially monochromatic, emitted radiation that is guided
towards the PV cell 122B, which absorbs and converts this emitted
radiation into electricity. Solar radiation with energies lower
than the bandgap energy E.sub.gB passes through the spectral
concentrator 120B and strikes the spectral concentrator 120C, which
is configured to perform down-conversion with a bandgap energy
E.sub.gC close to a bandgap energy of the PV cell 122C. Solar
radiation with energies at or higher than the bandgap energy
E.sub.gC (and lower than the bandgap energy E.sub.gB) are absorbed
and converted into substantially monochromatic, emitted radiation
that is guided towards the PV cell 122C, which absorbs and converts
this emitted radiation into electricity. In the illustrated
embodiment, bandgap energies E.sub.gA, E.sub.gB, and E.sub.gC are
related as follows: E.sub.gA>E.sub.gB>E.sub.gC.
[0048] By operating in such manner, the solar module 104 provides
enhanced utilization of a solar spectrum, by allowing different
energy bands within the solar spectrum to be collected and
converted into electricity. While three layers are illustrated in
FIG. 1, the solar module 104 can include more or less layers
depending upon the particular implementation. In some instances,
solar energy conversion efficiency can be increased from a value of
about 31 percent when one PV cell is used to a value of about 50
percent when three PV cells are used and towards a value of about
85 percent when a virtually unlimited number of PV cells are
used.
[0049] Additional aspects and benefits of spectral concentration
can be appreciated with reference to FIG. 2, which illustrates a
solar module 200 implemented in accordance with another embodiment
of the invention. The solar module 200 includes a PV cell 202,
which is a p-n junction device formed from a thin slice or strip of
crystalline silicon. The use of thin slices of silicon allows a
reduction in silicon consumption, which, in turn, allows a
reduction in manufacturing costs. Micromachining operations can be
performed on a silicon wafer to form numerous silicon slices, and
each of the silicon slices can be further processed to form PV
cells, such as the PV cell 202. The PV cell 202 can have dimensions
of about 300 .mu.m by about 300 .mu.m by a few centimeters in
length, or dimensions of about 250 .mu.m by about 250 .mu.m by
about 3 inches in length. As illustrated in FIG. 2, the PV cell 202
is configured to accept and absorb radiation incident upon a side
surface 204 of the PV cell 202, although other surfaces of the PV
cell 202 can also be involved.
[0050] In the illustrated embodiment, the solar module 200 also
includes a spectral concentrator 206, which is formed as a slab
having a side surface 208 that is adjacent to the side surface 204
of the PV cell 202. The spectral concentrator 206 includes a set of
luminescent materials that convert a relatively wide range of
energies of solar radiation into a relatively narrow, substantially
monochromatic energy band that is matched to an absorption spectrum
of the PV cell 202. During operation of the solar module 200,
incident solar radiation strikes a top surface 210 of the spectral
concentrator 206, and a certain fraction of this incident solar
radiation penetrates below the top surface 210 and is absorbed and
converted into substantially monochromatic, emitted radiation. This
emitted radiation is guided laterally within the spectral
concentrator 206, and a certain fraction of this emitted radiation
reaches the side surface 204 of the PV cell 202, which absorbs and
converts this emitted radiation into electricity.
[0051] In effect, the spectral concentrator 206 performs a set of
operations, including: (1) collecting incident solar radiation; (2)
converting the incident solar radiation into substantially
monochromatic, emitted radiation near a bandgap energy of the PV
cell 202; and (3) conveying the emitted radiation to the PV cell
202, where the emitted radiation can be converted to useful
electrical energy. The spectral concentrator 206 can include
distinct structures that are optimized or otherwise tailored
towards respective ones of the collection, conversion, and
conveyance operations. Alternatively, certain of these operations
can be implemented within a common structure. These operations that
are performed by the spectral concentrator 206 are further
described below.
[0052] Collection refers to capturing or intercepting incident
solar radiation in preparation for conversion to emitted radiation.
Collection efficiency of the spectral concentrator 206 can depend
upon the amount and distribution of a luminescent material within
the spectral concentrator 206. In some instances, the luminescent
material can be viewed as a set of luminescent centers that can
intercept incident solar radiation, and a greater number of
luminescent centers typically increases the collection efficiency.
Depending upon the distribution of the luminescent centers,
collection of incident solar radiation can occur in a distributed
fashion throughout the spectral concentrator 206, or can occur
within one or more regions of the spectral concentrator 206. The
collection efficiency can also depend upon other aspects of the
spectral concentrator 206, including the ability of incident solar
radiation to reach the luminescent material. In particular, the
collection efficiency is typically improved by suitable optical
coupling of incident solar radiation to the luminescent material,
such as via an anti-reflection coating to reduce reflection of
incident solar radiation.
[0053] Conversion refers to emitting radiation in response to
incident solar radiation, and the efficiency of such conversion
refers to the probability that an absorbed solar photon is
converted into an emitted photon. Conversion efficiency of the
spectral concentrator 206 can depend upon photoluminescent
characteristics of a luminescent material, including its internal
quantum efficiency, but can also depend upon interaction of
luminescent centers with their local optical environment, including
via resonant cavity effects as further described herein. Depending
upon the distribution of the luminescent centers, conversion of
incident solar radiation can occur in a distributed fashion
throughout the spectral concentrator 206, or can occur within one
or more regions of the spectral concentrator 206. Also, depending
upon the particular luminescent material used, the conversion
efficiency can depend upon wavelengths of incident solar radiation
that are absorbed by the luminescent material.
[0054] Conveyance refers to guiding or propagation of emitted
radiation towards the PV cell 202, and the efficiency of such
conveyance refers to the probability that an emitted photon reaches
the PV cell 202. Conveyance efficiency of the spectral concentrator
206 can depend upon photoluminescent characteristics of a
luminescent material, including a degree of overlap between
emission and absorption spectra, but can also depend upon
interaction of luminescent centers with their local optical
environment, including via resonant cavity effects as further
described herein.
[0055] By performing these operations, the spectral concentrator
206 provides a number of benefits. In particular, by performing the
collection operation in place of the PV cell 202, the spectral
concentrator 206 allows a significant reduction in silicon
consumption, which, in turn, allows a significant reduction in
manufacturing costs. In some instances, the amount of silicon
consumption can be reduced by a factor of about 10 to about 1000.
Also, the spectral concentrator 206 enhances solar energy
conversion efficiency based on at least two effects: (1)
concentration effect; and (2) monochromatic effect.
[0056] In terms of the concentration effect, the spectral
concentrator 206 performs spectral concentration by converting a
relatively wide range of energies of incident solar radiation into
a narrow band of energies close to the bandgap energy of the PV
cell 202. Incident solar radiation is collected via the top surface
210 of the spectral concentrator 206, and emitted radiation is
guided towards the side surface 204 of the PV cell 202. A solar
radiation collection area, as represented by, for example, an area
of the top surface 210 of the spectral concentrator 206, can be
significantly greater than an area of the PV cell 202, as
represented by, for example, an area of the side surface 204 of the
PV cell 202. A resulting concentration factor onto the PV cell 202
can be in the range of about 10 to about 100 and up to about 1000
or more. In turn, the concentration factor can increase the open
circuit voltage or V.sub.oc of the solar module 200, and can yield
an increase in solar energy conversion efficiency of about 2
percent (absolute), or 10 percent (relative), for each
concentration factor of 10 in emitted radiation reaching the PV
cell 202. A typical solar radiation energy flux is about 100 mW
cm.sup.-2, and, in some instances, a concentration factor of up to
10.sup.6 (or more) can be achieved by optimizing the spectral
concentrator 206 with respect to the collection, conversion, and
conveyance operations.
[0057] In terms of the monochromatic effect, the narrow band
radiation emitted from the spectral concentrator 206 can be
efficiently absorbed by the PV cell 202, which can be optimized in
terms of its junction design to operate on this narrow band,
emitted radiation. In addition, by matching the energy of the
emitted radiation with the bandgap energy of the PV cell 202,
thermalization can mostly occur within the spectral concentrator
206, rather than within the PV cell 202.
[0058] Attention next turns to FIG. 3, which illustrates a solar
module 300 implemented in accordance with another embodiment of the
invention. The solar module 300 includes multiple rows 302A, 302B,
and 302C of PV cells, which are spaced with respect to one another
by about 2 cm to about 10 cm along the x direction. While three
rows 302A, 302B, and 302C are illustrated in FIG. 3, more or less
rows can be included depending upon the particular implementation.
In the illustrated embodiment, each of the rows 302A, 302B, and
302C includes multiple PV cells that are connected in series,
although a parallel connection is also contemplated. Connection of
PV cells in series can serve to increase output voltage, while
connection of PV cells in parallel can serve to increase output
current. PV cells within a particular row, such as the row 302A,
can be p-n junction devices, and the p-n orientation of the PV
cells can alternate along the y direction for that row. As
illustrated in FIG. 3, each of the rows 302A, 302B, and 302C is
bifacial and, thus, is able to accept and absorb radiation incident
upon two side surfaces.
[0059] In the illustrated embodiment, the solar module 300 also
includes a spectral concentrator 304, which includes multiple
structures that allow the spectral concentrator 304 to perform
collection, conversion, and conveyance operations. In particular,
the spectral concentrator 304 includes a substrate 306, which is
formed from a glass, a polymer, or another suitable material that
is optically transparent or translucent. An anti-reflection layer
308 is formed at a top surface of the substrate 306 to reduce
reflection of incident solar radiation. As illustrated in FIG. 3,
the spectral concentrator 304 also includes a luminescent stack
310, which includes a set of luminescent materials that convert
incident solar radiation into a relatively narrow, substantially
monochromatic energy band that is matched to an absorption spectrum
of the rows 302A, 302B, and 302C of PV cells. The luminescent stack
310 is sandwiched by an adhesive layer 312 and a protective layer
314, which are adjacent to a top surface and a bottom surface of
the luminescent stack 310, respectively. The adhesive layer 312,
which is formed from a polymer or another suitable adhesive
material that is optically transparent or translucent, serves to
couple the luminescent stack 310 to the substrate 306, and to
provide proper optical alignment of the luminescent stack 310 with
respect to the rows 302A, 302B, and 302C of PV cells. In some
instances, the adhesive layer 312 can also serve to thermally
isolate the rows 302A, 302B, and 302C of PV cells from the spectral
concentrator 304, and, thus, reduce efficiency losses as a result
of heating. It is also contemplated that the adhesive layer 312 can
be omitted for certain implementations, such that the luminescent
stack 310 is formed at a bottom surface of the substrate 306. In
the illustrated embodiment, the protective layer 314 serves to
protect the luminescent stack 310 from environmental conditions,
and is formed from a polymer or another suitable material. While
not illustrated in FIG. 3, side edges and surfaces of the spectral
concentrator 304, which are not involved in conveyance of
radiation, can have a Lambertian reflector formed thereon, such as
white paint or another suitable reflective material.
[0060] Still referring to FIG. 3, the spectral concentrator 304
includes multiple grooves 316A, 316B, and 316C to accommodate
respective ones of the rows 302A, 302B, and 302C of PV cells.
During manufacturing of the spectral concentrator 304, various
layers can be formed adjacent to the top and bottom surfaces of the
substrate 306, and certain portions of the substrate 306 and the
layers adjacent to its bottom surface can be removed to form the
grooves 316A, 316B, and 316C. Alternatively, a selective coating,
patterning, or deposition technique can be implemented to form the
grooves 316A, 316B, and 316C.
[0061] During operation of the solar module 300, incident solar
radiation strikes a top surface of the spectral concentrator 304,
and a certain fraction of this incident solar radiation passes
through the substrate 306 and the adhesive layer 312 and reaches
the luminescent stack 310. In turn, the luminescent stack 310
absorbs and converts this solar radiation into substantially
monochromatic, emitted radiation. This emitted radiation is then
guided within the luminescent stack 310, and a certain fraction of
this emitted radiation reaches the rows 302A, 302B, and 302C of PV
cells, which absorb and convert this emitted radiation into
electricity. As can be appreciated with reference to FIG. 3,
guiding of emitted radiation is such that each of the rows 302A,
302B, and 302C of PV cells is illuminated from two sides, thereby
enhancing solar energy conversion efficiency.
[0062] FIG. 4 illustrates aspects and potential loss mechanisms of
a luminescent stack 400 implemented in accordance with another
embodiment of the invention. The luminescent stack 400 includes a
luminescent layer 402, which includes a set of luminescent
materials. By selecting a luminescent material having a high
absorption coefficient for solar radiation, a thickness of the
luminescent layer 402 can be reduced, such as in the range of about
0.1 .mu.m to about 2 .mu.m, in the range of about 0.2 .mu.m to
about 1 .mu.m, or in the range of about 0.2 .mu.m to about 0.5
.mu.m.
[0063] As illustrated in FIG. 4, the luminescent layer 402 is
sandwiched by a top reflector 404 and a bottom reflector 406, which
are adjacent to a top surface and a bottom surface of the
luminescent layer 402, respectively. This pair of reflectors 404
and 406 serve to reduce loss of emitted radiation out of the
luminescent stack 400 as the emitted radiation is guided towards a
PV cell. The top reflector 404 is omni-reflective over emission
wavelengths, while allowing relevant wavelengths of incident solar
radiation to pass through and strike the luminescent layer 402. The
bottom reflector 406 is omni-reflective over substantially all
wavelengths and, thus, allow for two-pass solar irradiation. In
particular, any remaining fraction of the solar radiation, which
passes through the luminescent layer 402, strikes the bottom
reflector 406, which reflects the solar radiation. Reflected
radiation is directed upwards and strikes the luminescent layer
402, which can absorb and convert this reflected radiation into
emitted radiation.
[0064] In the illustrated embodiment, each of the top reflector 404
and the bottom reflector 406 is implemented as a dielectric stack,
including multiple dielectric layers and with the number of
dielectric layers in the range of 2 to 10, such as in the range of
4 to 8. Each dielectric layer can have a thickness in the range of
about 0.1 .mu.m to about 0.2 .mu.m, such as in the range of about
0.1 .mu.m to about 0.15 .mu.m. For certain implementations, a
dielectric stack can include alternating layers formed from
different dielectric materials. Examples of dielectric materials
that can be used to form the top reflector 404 and the bottom
reflector 406 include silica (e.g., SiO.sub.2 or
.alpha.-SiO.sub.2), alumina (e.g., Al.sub.2O.sub.3), TiO.sub.2,
SiO.sub.xN.sub.2-x, and other suitable thin-film dielectric
materials. The top reflector 404 and the bottom reflector 406 can
have tolerances (in terms of a Q value) that are up to 10.sup.8 or
more, such as in the range of about 5 to about 100 or in the range
of about 5 to about 10.
[0065] Depending on the number of dielectric layers forming the top
reflector 404 and the bottom reflector 406, a total thickness of
the luminescent stack 400 can be in the range of about 0.4 .mu.m to
about 4 .mu.m, such as in the range of about 1 .mu.m to about 2
.mu.m or in the range of about 1 .mu.m to about 1.5 .mu.m. While a
single luminescent layer 402 is illustrated in FIG. 4, it is
contemplated that multiple luminescent layers can be included in
other implementations. These multiple luminescent layers can be
formed on top of one another or can be interspersed among multiple
dielectric layers.
[0066] The luminescent stack 400 can be formed in accordance with
any of a number of manufacturing techniques. For example,
roll-to-roll techniques can be used to deposit a luminescent
material along with multiple dielectric layers, with the layers
deposited sequentially. Alternatively, various layers can be
laminated onto a substrate, rather than deposited onto the
substrate. For example, the luminescent stack 400 can be formed
from polymer films, with a luminescent material coated onto one set
of films, and a second set of films laminated to form the full
luminescent stack 400. The films can be formed from
ultraviolet-compatible polymers, and can be loaded with
nanoparticles, such as formed from TiO.sub.2, to adjust a
refractive index. The films can be extruded in a multi-layer form
and then laminated, or can be coated sequentially. The luminescent
material can also be loaded into one or more of the films when
forming the luminescent stack 400.
[0067] As a further example, Atomic Layer Deposition ("ALD") can be
used to form the luminescent stack 400 in a single deposition run.
ALD typically uses a set of chemicals to form alternate, saturated,
chemical reactions on a surface, resulting in self-limited growth
with desirable features such as conformity, uniformity,
repeatability, and precise control over thickness. ALD typically
involves sequentially introducing reactants to a surface in a gas
phase to form successive monolayers. For example, ALD can be used
to deposit CdS or UD-930 (e.g., using cesium acetate (or formate),
carbon tetraiodide (or iodoform), methylene iodide (or methyl
iodide), and combined with tin dichloride or organo-tin compounds).
Certain aspects of ALD are described in Nanu et al.,
"CuInS.sub.2-TiO.sub.2 Heterojunctions Solar Cells Obtained by
Atomic Layer Deposition," Thin Solid Films, Vol. 431-432, pp.
492-496 (2003); Spiering et al., "Stability Behaviour of Cd-free
Cu(In,Ga)Se.sub.2 Solar Modules with In.sub.2S.sub.3 Buffer Layer
Prepared by Atomic Layer Deposition," Thin Solid Films, Vol.
480-481, pp. 195-198 (2005); and Klepper et al., "Growth of Thin
Films of CO.sub.3O.sub.4 by Atomic Layer Deposition," Thin Solid
Films (2007), available online at www.sciencedirect.com; the
disclosures of which are incorporated herein by reference in their
entireties.
[0068] Still referring to FIG. 4, the operation of the luminescent
stack 400 is illustrated in the case of "normal" performance. In
the case of "normal" performance and in the absence of resonant
cavity effects, incident solar radiation is intercepted by a
luminescent center, and the resulting emission of radiation is
substantially isotropic. Emitted radiation in directions above a
total internal reflection angle remains in the luminescent stack
400, and is guided laterally along the luminescent stack 400.
[0069] Also illustrated in FIG. 4 are cases of potential losses
during operation of the luminescent stack 400:
[0070] (1) Nonradiative recombination: This refers to an instance
in which incident solar radiation is intercepted by a luminescent
center, but does not lead to emitted radiation. Nonradiative
recombination losses can be reduced by selecting a luminescent
material having a high internal quantum efficiency and by reducing
defects or interface recombination sites.
[0071] (2) Residual loss cone: While emitted radiation in
directions above a total internal reflection angle remains in the
luminescent stack 400, emitted radiation in directions below the
total internal reflection angle leaves the luminescent stack 400
and defines a loss cone of emitted radiation. Emission losses in
the case of omni-directional or isotropic emission can be up to 20
percent via the loss cone. Increasing the number of dielectric
layers on top or below the luminescent layer 402 or increasing an
index of refraction contrast between the luminescent layer 402 with
respect to its surroundings can decrease the loss cone. Moreover,
as further described herein, resonant cavity effects can be
exploited to control the direction of emitted radiation and, thus,
reduce the fraction of emitted radiation leaving the luminescent
stack 400.
[0072] (3) Scattering: This refers to an instance in which emitted
radiation strikes a scattering center and leaves the luminescent
stack 400. Scattering losses can be reduced by increasing material
uniformity within the luminescent stack 400 and by reducing defects
or scattering sites.
[0073] (4) Self-absorption: Emitted radiation that remains in the
luminescent stack 400 can be subject to self-absorption losses. In
particular, emitted radiation can be at longer wavelengths than
absorbed radiation, namely it is Stokes shifted. If the extent of
the Stokes shift is relatively small, there can be an overlap of
emission spectrum and absorption spectrum, which can lead to
self-absorption. Self-absorption losses can be reduced by selecting
a luminescent material having a large Stokes shift. Also, as
further described herein, resonant cavity effects can be exploited
to control photoluminescence characteristics and enhance the
inherent Stokes shift of the luminescent material.
[0074] Additional features related to optical, electrical, and
mechanical structures for solar modules are described with
reference to FIG. 5, FIG. 6, and FIG. 7, according to various
embodiments of the invention.
[0075] FIG. 5 illustrates two different orientations of a PV cell
500 with respect to a spectral concentrator 502 including a
luminescent stack 504. One orientation is referred to as a
horizontal junction orientation, in which a depletion region of the
PV cell 500 is substantially aligned with respect to solar
radiation that is incident vertically on the spectral concentrator
502, but is substantially perpendicular with respect to emitted
radiation guided along the luminescent stack 504. In the case of
the PV cell 500 having the horizontal junction orientation, a pair
of electrodes 506 and 508 are coupled to respective sides of the
depletion region to extract charge carriers produced by the PV cell
500. As illustrated in FIG. 5, the electrodes 506 and 508 are both
positioned adjacent to a bottom surface of the PV cell 500, and
have a finned structure to dissipate heat from the PV cell 500.
[0076] Another orientation of the PV cell 500 is referred to as a
vertical junction orientation, in which the depletion region of the
PV cell 500 is substantially perpendicular with respect to solar
radiation that is incident vertically on the spectral concentrator
502, but is substantially aligned with respect to emitted radiation
guided along the luminescent stack 504. In the case of the PV cell
500 having the vertical junction orientation, a pair of electrodes
510 and 512 are coupled to respective sides of the depletion region
to extract charge carriers produced by the PV cell 500. As
illustrated in FIG. 5, the electrode 510 is positioned adjacent to
a top surface of the PV cell 500, while the electrode 512 is
positioned adjacent to a bottom surface of the PV cell 500. The
electrode 512 has a finned structure to dissipate heat from the PV
cell 500. The vertical junction orientation of the PV cell 500 can
provide a number of benefits. In particular, the alignment of the
depletion region with respect to emitted radiation can enhance
uniformity of optical excitation across the depletion region and
enhance solar energy conversion efficiencies. Also, the positioning
of the electrodes 510 and 512 with respect to the PV cell 500 can
reduce potential blockage with respect to optical excitation of the
depletion region. Moreover, by allowing the electrodes 510 and 512
to be spaced further apart from one another, the vertical junction
orientation can allow at least one of the electrodes 510 and 512 to
have a larger cross-sectional area for improved heat dissipation as
well as low-loss conduction to external circuitry. Furthermore, the
vertical junction orientation can facilitate connection of PV cells
within a row such that the p-n orientation of the PV cells
alternates from one PV cell to the next PV cell along that row.
[0077] For improved electrical performance, integrated circuit
techniques can be used to integrate additional electrical circuitry
into PV cells. Examples of such additional electrical circuitry
include voltage step-up circuits, voltage control circuits,
inverters, Direct Current ("DC")-to-Alternating Current ("AC")
converters, voltage regulators, bypass diodes, blocking diodes,
shunt diodes, and other electrical protection and electrical
processing circuits.
[0078] For example, a voltage step-up circuit can be integrated
into a PV cell to reduce electrical contact and conduction losses.
Also, a higher output voltage from a solar module can translate
into simpler and lower cost implementations for an external
inverter and interface circuit to an AC line voltage. The external
inverter and associated interface circuit can represent up to about
10 percent to about 20 percent of the total system cost, and can
dissipate about 10 percent of an output power. Accordingly, for
certain implementations, an DC-to-AC converter and a voltage
regulator can be integrated into a PV cell, thereby eliminating the
need for an external inverter. As another example, a PV cell can
include a voltage control circuit for battery charging during
change of sunlight power over the course of a day.
[0079] FIG. 6 and FIG. 7 illustrate integration of diodes to
address issues related to shading and defects. In particular, FIG.
6 illustrates a row 600 of series connected PV cells, including a
PV cell 602. A total output current through the row 600 can be
affected by a drop in current produced by any one of the series
connected PV cells. For example, if the PV cell 602 is affected by
shading or defects, the total output current can experience a
significant drop, with current produced by remaining PV cells being
substantially dissipated within the affected PV cell 602. In the
illustrated embodiment, a diode 604 is integrated in parallel with
the affected PV cell 602, and can serve as a bypass for the current
produced by the remaining PV cells.
[0080] FIG. 7 illustrates an array 700 of parallel connected rows
702A, 702B, 702C, and 702D of PV cells, each of the rows 702A,
702B, 702C, and 702D including multiple PV cells that are connected
in series. An output voltage at the top of each of the rows 702A,
702B, 702C, and 702D can be substantially the same, and can be
affected by a drop in voltage produced by any one of the series
connected PV cells. For example, if a PV cell 704 included within
the row 702B is affected by shading or defects, the output voltage
of the row 702B can experience a significant drop, with power
produced by remaining rows 702A, 702C, and 702D being substantially
dissipated within the affected row 702B. In the illustrated
embodiment, blocking diodes 706A, 706B, 706C, and 706D are
integrated with respective ones of the rows 702A, 702B, 702C, and
702D, and can serve to reduce power dissipation within the affected
row 702B.
Luminescent Materials
[0081] A variety of luminescent materials can be used to form the
solar modules described herein. Examples include organic
fluorophors, inorganic fluorophors and phosphors, nanoparticles,
and semiconductor materials.
[0082] Inorganic fluorophors having optical transitions in the
range of about 900 nm to about 980 nm can be suitable for use with
PV cells based on silicon. An inorganic fluorophor having a
suitable emission wavelength can be selected based on an atomic
moiety involved. For example, inorganic fluorophors with
luminescence derived from transition or rare earth atoms can be
used. Other examples of inorganic fluorophors include oxides (or
other chalcoginides) with luminescence derived from a defect state
in a crystal.
[0083] Nanoparticles, such as nanoparticles formed from silicon or
germanium, can be useful for spectral concentration. The
nanoparticles can be formed as self-assembled nanoparticles, such
as by vacuum deposition, or as discrete nanoparticles, such as in a
colloidal solution. The nanoparticles can be formed with a high
internal quantum efficiency for photoluminescence by reducing
defect density, typically to less than one defect per nanoparticle.
In addition, surfaces of the nanoparticles can be properly
terminated to enhance the photoluminescence. Emission wavelength of
the nanoparticles can be dependent upon, or controlled by, their
sizes. A narrow distribution of sizes can be desirable, so that a
resulting spectral width is narrow, and there is reduced
self-absorption of emitted light from smaller-sized nanoparticles
by larger-sized nanoparticles.
[0084] Semiconductor materials, such as indium phosphide or InP,
with a bandgap energy that is near and slightly above the bandgap
energy of PV cells can also be used. In particular, semiconductor
materials with a bandgap energy in the range of about 1.1 eV to
about 1.5 eV, such as from about 1.2 eV to about 1.4 eV, at 300K
can be suitable as spectral concentrators for PV cells based on
silicon.
[0085] For example, indium phosphide has a direct, allowed bandgap
energy of about 1.35 eV and an absorption coefficient of about
10.sup.5 cm.sup.-1. Indium phosphide can be deposited by a variety
of processes, such as sputter deposition, Metalorganic Chemical
Vapor Deposition ("MOCVD"), Organometallic Chemical Vapor
Deposition ("OMCVD"), atmospheric chemical vapor deposition, ALD,
Molecular Beam Epitaxy ("MBE") deposition, and so forth. Indium
phosphide, or another semiconductor material, can be deposited as a
film in a single layer or in multiple layers interspersed with
other layers. The other layers can be included for optical and
efficiency purposes and for chemical and environmental protection,
such as silica and alumina as hermetic sealants. The absorption
coefficient of indium phosphide, or another semiconductor material,
in the optical wavelengths of the solar spectrum can be in the
range of about 10.sup.4 cm.sup.-1 or greater at energies larger
than the bandgap edge. A film thickness in the micrometer range,
such as a few micrometers or less, can have an optical density of 2
or more to allow at least about 99 percent of incident solar
radiation to be absorbed. Indium phosphide, or another
semiconductor material, can also be deposited into porous matrices
or deposited as nanoparticles. For example, indium phosphide can be
formed as nanoparticles and dispersed in a matrix such as an
optically stable polymer or an inorganic glass. The total amount of
absorbing semiconductor material can be equivalent to an optical
density of 2 or more to allow at least about 99 percent of incident
solar radiation to be absorbed. Use of a resonant cavity structure
allows the efficient use of semiconductor materials in the form of
thin films. Furthermore, the resonant cavity structure, by
modification of a radiation matrix, allows the use of semiconductor
materials with forbidden optical transitions and indirect optical
transitions in the desired wavelength range for spectral
concentration. Lower bandgap energy materials can also be made to
luminesce by quantum confinement, either in thin films or by
formation of nanoparticles.
[0086] A new class of luminescent materials is disclosed in
co-pending and co-owned U.S. patent application Ser. No.
11/689,381, entitled "Luminescent Materials that Emit Light in the
Visible Range or the Near Infrared Range" and filed on Mar. 21,
2007, the disclosure of which is incorporated herein by reference
in its entirety. This class of luminescent materials includes
semiconductor materials that can be represented with reference to
the formula:
[A.sub.aB.sub.bX.sub.x][dopants] (I)
[0087] In formula (I), A is selected from elements of Group IA,
such as sodium (e.g., as Na(I) or Na.sup.1+), potassium (e.g., as
K(I) or K.sup.1+), rubidium (e.g., as Rb(I) or Rb.sup.1+), and
cesium (e.g., as Cs(I) or Cs.sup.1+); B is selected from elements
of Group VA, such as vanadium (e.g., as V(III) or V.sup.+3),
elements of Group IB, such as copper (e.g., as Cu(I) or Cu.sup.+1),
silver (e.g., as Ag(I) or Ag.sup.+1), and gold (e.g., as Au(I) or
Au.sup.+1), elements of Group IIB, such as zinc (e.g., as Zn(II) or
Zn.sup.+2), cadmium (e.g., as Cd(II) or Cd.sup.+2), and mercury
(e.g., as Hg(II) or Hg+.sup.2), elements of Group IIIB, such as
gallium (e.g., as Ga(I) or Ga.sup.+1), indium (e.g., as In(I) or
In.sup.+1), and thallium (e.g., as Tl(I) or Tl.sup.+1), elements of
Group IVB, such as germanium (e.g., as Ge(II) or Ge.sup.+2 or as
Ge(IV) or Ge.sup.+4), tin (e.g., as Sn(II) or Sn.sup.+2 or as
Sn(IV) or Sn.sup.+4), and lead (e.g., as Pb(II) or Ph.sup.+2 or as
Pb(IV) or Pb.sup.+4), and elements of Group VB, such as bismuth
(e.g., as Bi(III) or Bi+.sup.3); and X is selected from elements of
Group VIIB, such as fluorine (e.g., as F-1), chlorine (e.g., as
Cl.sup.-1), bromine (e.g., as Br.sup.-1), and iodine (e.g., as
I.sup.-1). Still referring to formula (I), a is an integer that can
be in the range of 1 to 9, such as from 1 to 5; b is an integer
that can be in the range of 1 to 5, such as from 1 to 3; and x is
an integer that can be in the range of 1 to 9, such as from 1 to 5.
In some instances, a can be equal to 1, and x can be equal to 1+2b.
It is also contemplated that one or more of a, b, and x can have
fractional values within their respective ranges. It is further
contemplated that X.sub.x in formula (I) can be more generally
represented as X.sub.xX'.sub.xX''.sub.x'', where X, X', and X'' can
be independently selected from elements of Group VIIB, and the sum
of x, x', and x'' can be in the range of 1 to 9, such as from 1 to
5. With reference to the generalized version of formula (I), a can
be equal to 1, and the sum of x, x', and x'' can be equal to 1+2b.
Dopants included in a luminescent material represented by formula
(I) can be present in amounts that are less than about 5 percent,
such as less than about 1 percent, in terms of elemental
composition, and can derive from ingredients that are used to form
the luminescent material. In particular, the dopants can include
cations and anions, which form electron acceptor/electron donor
pairs that are dispersed within a microstructure of the luminescent
material.
[0088] Several luminescent materials represented by formula (I)
have characteristics that are desirable for spectral concentration.
In particular, the luminescent materials can exhibit
photoluminescence with a high internal quantum efficiency that is
greater than about 6 percent, such as at least about 10 percent, at
least about 20 percent, at least about 30 percent, at least about
40 percent, or at least about 50 percent, and can be up to about 90
percent or more. Also, the luminescent materials can exhibit
photoluminescence with a narrow spectral width that is no greater
than about 120 nm at FWHM, such as no greater than about 100 nm or
no greater than about 80 nm at FWHM. Thus, for example, the
spectral width can be in the range of about 20 nm to about 120 nm
at FWHM, such as from about 50 nm to about 120 nm, from about 50 nm
to about 100 nm, or from about 50 nm to about 80 nm at FWHM.
[0089] In addition, the luminescent materials can have bandgap
energies that are tunable to desirable levels by adjusting
ingredients and processing conditions that are used. For example, a
bandgap energy can correlate with A, with the order of increasing
bandgap energy corresponding to, for example, cesium, rubidium,
potassium, and sodium. As another example, the bandgap energy can
correlate with X, with the order of increasing bandgap energy
corresponding to, for example, iodine, bromine, chlorine, and
fluorine. This order of increasing bandgap energy can translate
into an order of decreasing peak emission wavelength. Thus, for
example, a luminescent material including iodine can sometimes
exhibit a peak emission wavelength in the range of about 900 nm to
about 1 .mu.m, while a luminescent material including bromine or
chlorine can sometimes exhibit a peak emission wavelength in the
range of about 700 nm to about 800 nm. By tuning bandgap energies,
the resulting photoluminescence can have a peak emission wavelength
located within a desirable range of wavelengths, such as the
visible range or the infrared range. In some instances, the peak
emission wavelength can be located in the near infrared range, such
as from about 900 nm to about 1 .mu.m, from about 910 nm to about 1
.mu.m, from about 910 nm to about 980 nm, or from about 930 nm to
about 980 nm.
[0090] Moreover, the photoluminescent characteristics described
above can be relatively insensitive over a wide range of excitation
wavelengths. Indeed, this unusual characteristic can be appreciated
with reference to excitation spectra of the luminescent materials,
which excitation spectra can be substantially flat over a range of
excitation wavelengths encompassing portions of the ultraviolet
range, the visible range, and the infrared range. In some
instances, the excitation spectra can be substantially flat over a
range of excitation wavelengths from about 200 nm to about 1 .mu.m,
such as from about 200 nm to about 980 nm or from about 200 nm to
about 950 nm. Similarly, absorption spectra of the luminescent
materials can be substantially flat over a range of excitation
wavelengths encompassing portions of the ultraviolet range, the
visible range, and the infrared range. In some instances, the
absorption spectra can be substantially flat over a range of
excitation wavelengths from about 200 nm to about 1 .mu.m, such as
from about 200 nm to about 980 nm or from about 200 nm to about 950
nm.
[0091] Two semiconductor materials with desirable characteristics
are designated as UD-700 and UD-930. The composition of these
materials is represented as CsSn.sub.bX.sub.1+2b. In the case of
UD-700, X is bromine, and, in the case of UD-930, X is iodine. The
spectral width of UD-700 and UD-930 is narrow (e.g., about 50 meV
or less at FWHM), and the absorption spectrum is substantially flat
from the absorption edge into the far ultraviolet. Photoluminescent
emission of UD-700 and UD-930 is stimulated by a wide range of
wavelengths of solar radiation up to the absorption edge of these
materials at about 700 nm for UD-700 and about 950 nm for
UD-930.
[0092] Desirable characteristics of UD-930 can be further
appreciated with reference to FIG. 8, which illustrates a combined
representation of a solar spectrum and measured absorption and
emission spectra of UD-930 in accordance with an embodiment of the
invention. In particular, FIG. 8 illustrates the AM1.5G solar
spectrum (referenced as (A)), which is a standard solar spectrum
representing incident solar radiation on the surface of the earth.
The AM1.5G solar spectrum has a gap in the region of 930 nm due to
atmospheric absorption. In view of the AM1.5G solar spectrum and
characteristics of PV cells based on silicon, the absorption
spectrum (referenced as (B)) and emission spectrum (referenced as
(C)) of UD-930 render this material particularly effective for
spectral concentration when incorporated within a luminescent
layer. In particular, photoluminescence of UD-930 is substantially
located in the gap of the AM1.5G solar spectrum, with the peak
emission wavelength of about 950 nm falling within the gap. This,
in turn, allows the use of reflectors (e.g., above and below the
luminescent layer) that are tuned to reflect emitted radiation back
towards the luminescent layer, without significant reduction of
incident solar radiation that can pass through the reflectors and
reach the luminescent layer. Also, the absorption spectrum of
UD-930 is substantially flat and extends from the absorption edge
at about 950 nm through substantially the full AM1.5G solar
spectrum into the ultraviolet. In addition, the peak emission
wavelength of about 950 nm (or about 1.3 eV) is matched to the
absorption edge of PV cells based on silicon, and the spectral
width is about 50 meV at FWHM (or about 37 nm at FWHM). The
absorption coefficient of silicon is about 10.sup.2 cm.sup.-1 in
this range of emission wavelengths, and junctions within the PV
cells can be designed to efficiently absorb the emitted radiation
and convert the radiation into electron-hole pairs. As a result,
UD-930 can broadly absorb a wide range of wavelengths from incident
solar radiation, while emitting a narrow range of wavelengths that
is matched to silicon to allow a high conversion efficiency of
incident solar radiation into electricity. Furthermore, the
absorption spectrum and the emission spectrum of UD-930 overlap to
a low degree, thereby reducing instances of self-absorption that
would otherwise lead to reduced conversion efficiency.
[0093] Other luminescent materials that are suitable as spectral
concentrators for silicon include Zn.sub.3P.sub.2, Cu.sub.2O, CuO,
CuInGaS, CuInGaSe, and so forth. Table I below lists a variety of
semiconductor materials that can be used for the applications
described herein.
TABLE-US-00001 TABLE I Examples of Spectral Concentrator Materials
material E.sub.g (eV, 300K) type Ge QD 0.8 to 1.5 Si QD 1.2 to 1.5
InP 1.34 direct Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y 1.2 to 1.4 CdTe
1.475 direct Ga.sub.2Te.sub.3 1.2 direct In.sub.2Se.sub.3 1.3
direct InSe 1.2 indirect In.sub.2Te.sub.3 1.1 direct InTe 1.16
direct CuGaTe.sub.2 1.2 CuInS.sub.2 1.5 Cu.sub.3In.sub.5Se.sub.9
1.1 CuInS.sub.2-xSe.sub.x 1.1 to 1.4 direct
Ag.sub.3In.sub.5Se.sub.9 1.22 AgGaTe.sub.2 1.3 direct AgInSe.sub.2
1.2 direct CuTIS.sub.2 1.4 Cr.sub.2S.sub.3 1.1 FeP.sub.2 0.4
FeSi.sub.2 0.8 Mg.sub.2Si 0.8 MoS.sub.2inte. <1.4
MoSe.sub.2inte. <1.2 WS.sub.2inte. 1.1 Sr.sub.2CuO.sub.2Cl 1.3
direct ZnGeP.sub.2 1.3 direct Zn.sub.3P.sub.2 1.35 indirect
Zn.sub.3P.sub.2 1.4 direct .beta. ZnP.sub.2 1.3 direct KTaO.sub.3
1.5 BaSnO.sub.3 1.4 CrCa.sub.2GeO.sub.4 1.1 LaMnO.sub.3 1.3
Ba.sub.1-xSr.sub.xSi.sub.2 1.2 BaSi.sub.2 1.3 direct ZnGeAs.sub.2
1.12 direct CdSnP.sub.2 1.17 direct Cu.sub.3AsS.sub.4 1.24
CdIn.sub.2Te.sub.4 1.25 direct Na.sub.3Sb 1.1 K.sub.3Sb 1.1 CuO 1.4
indirect Cu.sub.2O 1.4 forbidden, direct Cu.sub.2S 1.3 direct
Cu.sub.2Se 1.2 direct Cd.sub.4Sb.sub.3 1.4 TIS 1.36 direct
BiS.sub.3 1.3 BiI.sub.3 1.35 NiP.sub.2 0.7 SnS 1.1 SnSe 0.9
Ti.sub.1+xS.sub.2 0.7 TiS.sub.3-x 0.9 Zn.sub.3N.sub.2 1.2
Ag.sub.8GeS.sub.6 1.39 Ag.sub.8SnS.sub.6 1.28 CdInSe.sub.2 1.4
HgTlS.sub.2 1.25 BiSeI 1.3 MgGa.sub.2S.sub.4 1.2
[0094] Absorption and emission characteristics are typically
several orders of magnitude lower for semiconductor materials
having indirect optical transitions or forbidden optical
transitions compared to those materials having direct optical
transitions. However, by modification of a radiation matrix,
resonant cavity effects can enhance absorption and emission
characteristics and allow the use of semiconductor materials having
indirect or forbidden optical transitions. Referring to Table I,
CuO is an indirect bandgap semiconductor material having a bandgap
energy of about 1.4 eV, and Cu.sub.2O has a direct but spin
forbidden bandgap energy of about 1.4 eV. By incorporating within a
resonant cavity structure, either, or both, CuO and Cu.sub.2O can
be used for spectral concentration. Still referring to Table I,
Zn.sub.3P.sub.2 has an indirect optical transition of about 50 meV
below a direct optical transition of about 1.4 eV. Cavity effects
can allow coupling of the indirect optical transition to the higher
energy direct optical transition, thereby providing enhanced
absorption and emission for use as spectral concentrators.
[0095] In addition to the characteristics noted above, the
semiconductor materials listed in Table I typically have an index
of refraction greater than about 3. For example, InP has an index
of refraction of about 3.2. Because of internal reflection, less
than about 18% of light within a luminescent stack can exit to air.
In some instances, light normal to a surface of the luminescent
stack can have a Fresnel reflection loss of about 25% to air.
Anti-reflection coatings can be used to enhance optical coupling of
the light from the luminescent stack to a PV cell.
[0096] To reduce self-absorption of emitted light within a
luminescent stack, luminescence can occur via exciton emission. An
exciton corresponds to an electron-hole pair, which can be formed
as a result of light absorption. A bound or free exciton can have a
Stokes shift equal to an exciton binding energy. Most semiconductor
materials have exciton binding energies of less than about 20 meV.
Room temperature is about 25 meV, so excitons are typically not
present at room temperature for these materials. Some semiconductor
materials, such as CdTe and HgTe, have excitons with high binding
energies and are present at room temperature. However, some of
these semiconductor materials may be toxic or relatively expensive.
Other semiconductor materials have intrinsic excitons at room
temperature, such as bismuth triiodide or Bil.sub.3, and can be
desirable for the applications described herein.
[0097] Certain layered semiconductor materials, such as tin and
lead halides, can have bandgap and exciton energies tuned by
separation of inorganic layers with organic components. Examples
include organic-inorganic quantum well materials, conducting
layered organic-inorganic halides containing 110-oriented
perovskite sheets, hybrid tin iodide perovskite semiconductor
materials, and lead halide-based perovskite-type crystals. Certain
aspects of these semiconductor materials are described in Ema et
al., "Huge Exchange Energy and Fine Structure of Excitons in an
Organic-Inorganic Quantum Well," Physical Review B, Vol. 73, pp.
241310-1 to 241310-4 (2006); Mitzi et al., "Conducting Layered
Organic-inorganic Halides Containing 110-Oriented Perovskite
Sheets," Science, Vol. 267, pp. 1473-1476 (1995); Kagan et al.,
"Organic-Inorganic Hybrid Materials as Semiconducting Channels in
Thin-Film Field-Effect Transistors," Science, Vol. 286, pp. 945-947
(1999); Mitzi, "Solution-processed Inorganic Semiconductors," J.
Mater. Chem., Vol. 14, pp. 2355-2365 (2004); Symonds et al.,
"Emission of Hybrid Organic-inorganic Exciton Plasmon Mixed
States," Applied Physics Letters, Vol. 90, 091107 (2007); Zoubi et
al., "Polarization Mixing in Hybrid Organic-Inorganic
Microcavities," Organic Electronics, Vol. 8, pp. 127-135 (2007);
Knutson et al., "Tuning the Bandgap in Hybrid Tin Iodide Perovskite
Semiconductors Using Structural Templating," Inorg. Chem., Vol. 44,
pp. 4699-4705 (2005); and Tanaka et al., "Comparative Study on the
Excitons in Lead-halide-based Perovskite-type crystals
CH.sub.3NH.sub.3PbBr.sub.3 CH.sub.3NH.sub.3PbI.sub.3," Solid State
Communications, Vol. 127, pp. 619-623 (2003), the disclosures of
which are incorporated herein by reference in their entireties.
[0098] Also, other layered materials, such as tin sulfide, tin
selenide, titanium sulfide, and others listed in Table I, can be
tuned by intercalating other materials between the layered
materials. ALD can be used to make layered materials with tuned
bandgap energies and tuned exciton binding energies. Tuning an
exciton to higher energy can reduce self-absorption and enhance the
probability of lasing. Such material-process combination can be
used to develop a low self-absorption luminescent material by tuned
exciton luminescent emission. This can be further combined with a
resonant cavity structure, in either a weak or strong coupling
regime, to produce a low loss, high quantum efficiency,
down-conversion structure. Thermal quenching, namely the reduction
of luminescence intensity with an increase in temperature, can be
reduced or eliminated by generating an exciton with a binding
energy greater than the Boltzman temperature, which is about 25 meV
at room temperature. For solar applications, a binding energy in
the range of about 35 meV to about 50 meV can be desirable. A
larger binding energy can lead to a Stokes shift in the
photoluminescence from the absorption edge that results in an
absorption gap, thereby leading to lower solar energy conversion
efficiencies.
[0099] Another way to reduce self-absorption is via the use of
orientated birefringence. In particular, one way to reduce
self-absorption in a specific direction within a single crystal or
film is to orient a birefringent material. Birefringence refers to
a different refractive index along two or more different directions
of a material. A birefringent material, such as a semiconductor
material, has two or more different bandgap energies along
different crystal axis. If a crystal anisotropy has a bandgap in
the visible region of an optical spectrum, the material can be
referred as being dichoric rather than birefringent. Various
birefringent semiconductor materials can be used as spectral
concentrators, such as CuInSe.sub.2-xS.sub.x, Zn.sub.3N.sub.2, and
perovskites such as CsSn.sub.1+xI.sub.3+2x. Since there are two or
more absorption edges or bandgap energies for a birefringent
material, a resulting film can be deposited in an oriented state
with the higher bandgap energy (i.e., shorter wavelength absorption
edge) along a direction facing towards PV cells. In this case,
emitted light in the direction facing towards the PV cells can have
a lower absorbance because the emission wavelength is longer than
the higher energy bandgap. The use of resonant cavity effects and
Bragg reflectors can suppress emission in other, more highly
self-absorbed directions.
[0100] Thermal quenching and self-absorption can also be reduced by
modifying material characteristics. For semiconductor materials, an
absorption edge can become tilted with increasing temperature and
certain types of doping. This absorption edge tilt can sometimes
lead to increased self-absorption, and can be described by the
Elliott equation. Proper doping and interface or surface
modification can be used to control this absorption edge tilt to
reduce instances of thermal quenching and self-absorption. In the
case of nanoparticles formed of a semiconductor material, coatings
formed on the nanoparticles can alter emission characteristics of
the semiconductor material by the "Bragg Onion" technique.
[0101] The solar spectrum on the surface of the earth ranges from
the ultraviolet into the infrared. Photons absorbed from the
ultraviolet to about 1.3 eV are about 49.7 percent of the total
number of photons and about 46.04 percent of the total energy. Of
the absorbed photons at 100 percent internal quantum efficiency, a
luminescent material with emission at about 1.3 eV can yield a
solar energy conversion efficiency of about 46 percent (for one
photon to one photon mechanism). Multiple photon generation can
yield higher solar energy conversion efficiencies. Silicon
nanoparticles, such as silicon quantum dots, that emit multiple
photons can be used as spectral concentrators described herein to
provide higher conversion efficiencies. Certain aspects of silicon
nanoparticles are described in Beard et al., "Multiple Exciton
Generation in Colloidal Silicon Nanocrystals," Nano Letters,
published on the web on Jul. 24, 2007, the disclosure of which is
incorporated herein by reference in its entirety.
[0102] Since about one half of incident solar radiation is at lower
energy, or longer wavelength, than 1.3 eV (or 950 nm), conversion
efficiency can be increased by up-conversion. Up-conversion can
involve a process where two photons are absorbed and one photon is
emitted at a higher energy. Rare earth atoms can be relatively
efficient at undergoing up-conversion, and other processes, such as
Second Harmonic Generation ("SHG") at relatively high intensities,
can be used to enhance solar energy conversion efficiencies. The
use of resonant cavity effects in a spectral concentrator can
enhance up-conversion and non-linear processes such as SHG. Certain
aspects of up-conversion are described in Sark et al., "Enhancing
Solar Cell Efficiency by Using Spectral Converters," Solar Energy
Materials & Solar Cells, Vol. 87, pp. 395-409 (2005); and
Shalav et al., "Luminescent Layers for Enhanced Silicon Solar Cell
Performance: Up-conversion," Solar Energy Materials & Solar
Cells, Vol. 91, pp. 829-842 (2007), the disclosures of which are
incorporated herein by reference in their entireties.
[0103] Resonant Cavity Effects and Structures
[0104] Described herein is Cavity Quantum Electrodynamics ("CQED")
and its manifestation in the form of resonant cavity effects that
can be advantageously exploited for spectral concentration. For
example, cavity effects can be exploited to control the direction
of emitted light towards a PV cell and, therefore, enhance the
fraction of emitted light reaching the PV cell. In addition, cavity
effects can be exploited to modify emission characteristics, such
as by enhancing emission of a particular set of wavelengths that
are associated with resonant optical modes and suppressing emission
of other wavelengths that are associated with suppressed optical
modes. This modification of emission characteristics can reduce
overlap between an emission spectrum and an absorption spectrum,
and can yield reduced losses arising from self-absorption.
Furthermore, cavity effects can allow the use of semiconductor
materials having indirect optical transitions or forbidden optical
transitions by enhancing their absorption and emission
characteristics. Certain aspects of CQED are described in
Yablonovitch, Physical Review Letters, Vol. 58, pp. 2059-2062
(1987), the disclosure of which is incorporated herein by reference
in its entirety.
[0105] According to CQED, a luminescent center can interact with
its local optical environment. A luminescent center can be any
localized event or moiety that involves emission of light. An
optical environment surrounding a luminescent center can affect the
propagation of light emitted from the luminescent center as well as
the internal spontaneous emission characteristics of the
luminescent center. In particular, positioning a luminescent center
within an optical environment can modify both the directionality of
emitted light (e.g., intensity at any particular range of
wavelengths in a particular range of directions) and the total
spectral emission (intensity at any particular range of wavelengths
averaged over all directions) of the luminescent center.
Accordingly, proper positioning of a luminescent center within a
suitable optical environment can be used to select a range of
directions in which emission occurs for a particular range of
wavelengths. Also, with proper selection of the optical
environment, emission can be substantially suppressed in all
directions for a range of wavelengths. In some instances, the
luminescent center can re-absorb photons emitted in suppressed
directions, or the luminescent center interacts with the optical
environment so as to avoid emitting photons in suppressed
directions.
[0106] Depending on characteristics of a luminescent center and its
optical environment, suppression of emission at some wavelengths
can lead to enhancement of emission at other wavelengths. In
effect, cavity effects can "pull" an inherent peak emission
wavelength to a different peak emission wavelength as modified by
the optical environment. The shift in wavelengths can be as much as
several hundred nanometers (e.g., between about 50 nm to about 100
nm), depending on an inherent emission profile of the luminescent
center.
[0107] For certain applications, emission of light by a luminescent
center within an optical environment can be understood with
reference to Fermi's Golden Rule, which can be represented by the
formula:
.GAMMA. ( .omega. , r -> ) = 2 .pi. 2 f f .mu. ^ ( .omega. , r
-> ) E ^ ( .omega. , r -> ) i 2 .delta. ( .omega. - ( .omega.
f - .omega. i ) ) ( II ) ##EQU00001##
Fermi's Golden Rule represents a relationship between the rate of
spontaneous emission by a luminescent center and a local density of
optical states. In formula (II), .GAMMA.(.omega., {right arrow over
(r)}) represents the rate of spontaneous emission at frequency
.omega. by a luminescent center at position {right arrow over (r)},
f| and |i are possible final and initial (electronic) quantum
states of the luminescent center, {circumflex over (.mu.)}E
represents an interaction between an optical transition element
(e.g., dipole or quadrupole moment) and allowed electric fields,
and .delta. represents the Dirac delta function.
[0108] Formula (II) can be represented in a modified form, in which
the allowed electric fields are separated from the interaction term
by integrating over frequency. This modified form is represented by
the formula:
.GAMMA. ( .omega. ab , r -> ) = .pi. 3 0 a .mu. ^ b 2 N rad ( r
-> , d -> , .omega. ab ) ( III ) ##EQU00002##
As can be appreciated with reference to formula (III), emission of
light by the luminescent center can be viewed as including an
"atom" part, which is a transition matrix between quantum states
and involves the optical transition element, and a "field" part,
which accounts for the local density of optical states and is
represented as N.sub.rad. N.sub.rad can encompass various possible
emissions within the optical environment, and can be viewed in
terms of possible optical modes along orthogonal directions. Some
of these optical modes can be resonant optical modes that have
enhanced contribution to N.sub.rad, while others of these optical
modes can be suppressed optical modes that have reduced
contribution to N.sub.rad. Referring to formula (III), the "atom"
part and the "field" part, in combination, specify the rate of
spontaneous emission by the luminescent center.
[0109] In the absence of cavity effects, a structure can be
implemented with a luminescent stack that is sufficiently thicker
than a coherence wavelength of emitted light. In this case,
emission of light can be substantially isotropic. Emitted light in
directions above the total internal reflection angle remains in the
structure, but emitted light in directions below the total internal
reflection angle leaves the structure and defines a loss cone of
emitted light. Emission losses in the case of omni-directional
emission can be up to 20 percent via the loss cone. Emitted light
that remains in the structure can be subject to further losses. In
particular, emitted light can be at longer wavelengths than
absorbed light, namely it is Stokes shifted. If the extent of the
Stokes shift is relatively small, there can be an overlap of
emission spectrum and absorption spectrum, which can lead to
self-absorption and reduce the fraction of emitted light than can
reach a PV cell. Also, emitted light can be subject to
re-absorption (via self-absorption) and then re-emission, namely it
is subject to photon recycling. Each re-emission event can be
isotropic, with an associated loss cone. Accordingly, emitted light
reaching the PV cell can be reduced by loss of emittcd light below
the total internal reflection angle and a series of re-absorption
and re-emission events. If the internal quantum efficiency of the
luminescent stack is less than 100 percent, then each re-absorption
and re-emission events can have a further associated loss.
[0110] Aspects of CQED can be used to form microcavities or
resonant cavity structures that exhibit resonant cavity effects.
These resonant cavity structures can provide a number of benefits,
including: (1) directional control of emission towards a PV cell
and, therefore, reduction in emission loss via a loss cone; and (2)
spectral pulling, which can reduce overlap between an emission
spectrum and an absorption spectrum and, therefore, reduce
self-absorption.
[0111] Resonant cavity structures can be implemented as, for
example, resonant cavity waveguides, including single-mode and
multi-mode waveguides; photonic crystals; polariton lasers; and
plasmonic structures. For example, a resonant cavity waveguide can
be implemented as a total internal reflection waveguide, including
a luminescent layer sandwiched by a pair of reflectors (e.g., Bragg
or omni-reflector) on top and bottom surfaces of the luminescent
layer. The pair of reflectors serve to reduce a loss cone and,
therefore, reduce loss of emitted light as it is guided towards a
PV cell. Also, a top or solar-side reflector can be implemented for
enhanced overlap between incident solar radiation and the
luminescent layer. As a result of cavity effects, the waveguide can
suppress emission in non-guided directions, while allowing or
enhancing emission in the guided direction towards the PV cell. In
such manner, there can be a significant reduction in loss of
emitted light via the top and bottom surfaces of the luminescent
layer.
[0112] As another example, a resonant cavity waveguide can be
implemented as a Antiresonant Reflecting Optical Waveguide
("ARROW"). An ARROW is typically based on the Fabry-Perot effect
for guiding, rather than total internal reflection, and, in some
instances, can be a more efficient structure. In particular, the
ARROW can provide enhanced photoluminescence and low loss guiding
to a PV cell. The ARROW can allow certain optical modes to be
substantially centered on a low index region (e.g., a non-absorbing
material) or, depending on the implementation, on a high index
region. In such manner, substantial propagation of light can occur
in the non-absorbing material, and self-absorption can be reduced.
Certain aspects of ARROW structures are described in Huang et al.,
"The Modal Characteristics of ARROW structures," Journal of
Lightwave Technology, Vol. 10, No. 8, pp. 1015-1022 (1992);
Litchinitser et al., "Application of an ARROW Model for Designing
Tunable Photonic Devices," Optics Express, Vol. 12, No. 8, pp.
1540-1550 (2004); and Liu et al., "Characteristic Equations for
Different ARROW Structures," Optical and Quantum Electronics, Vol.
31, pp. 1267-1276 (1999); the disclosures of which are incorporated
herein by reference in their entirety.
[0113] Photonic crystals can be implemented to control propagation
of light, both direction and radiative frequency and lifetime. A
photonic crystal is typically implemented as a mesoscopic array of
two or more materials with differing indices of refraction that are
arranged in a substantially periodic manner. For light in the
visible and near infrared ranges, spacing within the array can be
in the range of a few hundred nanometers to a micron or so. The
array can extend in one dimension, two dimensions, or three
dimensions. Examples of photonic crystals include those based on a
Bragg reflector, a planar cavity formed by two opposed dielectric
interference reflectors, and an omni-directional mirror. An example
of a three dimensional photonic crystal is one based on a three
dimensional Bragg grating.
[0114] Resonant cavity structures can be implemented as
one-dimensional, two-dimensional, or three-dimensional structures.
The number of characteristic dimensions associated with a resonant
cavity structure can correspond to the number of quantum-confined
dimensions of the structure. Thus, for example, a resonant cavity
structure can extend in three dimensions, but a subset of those
dimensions can be quantum-confined.
[0115] For example, a resonant cavity waveguide can be implemented
as a slab waveguide to provide one-dimensional confinement.
Examples include total internal reflection slab waveguides,
extensions of total internal reflection slab waveguides that
include multi-layered mirrors or other layers, and slab ARROW
structures. In the event of re-absorption and re-emission,
propagation of light can be modeled as a two-dimensional diffusion
of photonic density. In some instances, one-dimensional confinement
can yield reduced spectral pulling, relative to, for example,
higher dimensional confinement.
[0116] As another example, a resonant cavity waveguide can be
implemented to provide two-dimensional confinement. Depending on a
degree of confinement, propagation of light under such confinement
can be modeled as two-dimensional photon diffusion or
one-dimensional photon diffusion. Various types of lateral
confinement can be implemented, including channel waveguides, ridge
waveguides, and strip-loaded waveguides. For example, a ridge
waveguide can be formed based on slab waveguide by etching,
scratching, or pressing parallel ridges either at a top or
solar-side reflector or at a bottom reflector. Alternatively, or in
conjunction, the ridges can be impressed into a luminescent layer
before applying the bottom reflector. A ridge waveguide can also be
formed in a substrate via conformal coating. Ridge spacing can be
in the range of about 2 to about 10 wavelengths of emitted light.
Spectral pulling can occur, but can sometimes be reduced depending
on a degree of confinement and a coupling among individual guiding
substructures. As another example, strip-loaded waveguides can be
formed from a generally planar substrate by forming depressions or
shallow grooves in the substrate of about 0.5 .mu.m to about 100 nm
or less in depth. Various techniques can be used to form the
depressions, such as by embossing a flexible, plastic substrate. A
luminescent material can be coated over the resulting surface, with
planarization forming the strip-loaded waveguides. The resulting
waveguides can be close enough to produce a coupled mode with a
coupling length of a fraction of a millimeter to several
millimeters.
[0117] Resonant cavity structures can also provide
three-dimensional confinement. For example, a photonic crystal can
be used that includes some level of distortion or defects.
Distortions can provide directional control of emission towards a
PV cell, while suppressing emission in other directions. This
directional control can allow the photonic crystal to guide light
in one or two dimensions. Distortions can also yield spectral
pulling of emission along a propagation direction, while
suppressing emission over substantially an entire spectral range in
other directions. In some instances, spectral pulling can be
achieved by residual interaction with a neighboring photonic
crystal.
[0118] The following provides additional implementation details on
slab waveguides, although other resonant cavity structures
including diffuse or dispersed luminescent centers can be similarly
implemented.
[0119] A luminescent material included in a slab waveguide can be
represented as a set of luminescent centers. Because of potential
overlap between absorption and emission wavelengths, propagation of
light within the waveguide can be modeled as a photon diffusion
process. Lesser overlap can lead to a larger photon diffusion
coefficient, while greater overlap can lead to a smaller photon
diffusion coefficient. Different luminescent materials and
associated luminescent centers can be included in the waveguide,
and these different luminescent materials can have different photon
diffusion characteristics. By positioning a luminescent material
within the waveguide having cavity effects, a spectral direction of
emission can be controlled, thus affecting the direction of a
photon diffusion coefficient.
[0120] One implementation for the slab waveguide can include a
single, substantially uniform layer of a luminescent material.
Edges or surfaces of the luminescent layer that are not coupled to
PV cells can have reflectors thereon, so the slab waveguide can
operate as a total internal reflection slab waveguide. A reflector
can be a coating, a layer, or a film of a dielectric material or a
metal. The luminescent layer can have an index of refraction
greater than a surrounding medium, and, therefore, can guide
emitted light in directions above the total internal reflection
angle towards the PV cells.
[0121] In the slab waveguide, a local density of optical states can
include both guided optical modes and radiative optical modes.
Guided optical modes can involve propagation of light along the
luminescent layer, while radiative optical modes can involve
propagation of light out of the luminescent layer. For a low index
contrast between the luminescent layer and the surrounding medium,
the local density of optical states can differ slightly from that
of free space, and emission characteristics are modified to a low
degree. Increasing confinement, such as by increasing the index
contrast, can introduce greater distortions in the local density of
optical states, yielding enhanced modification of emission
characteristics including directional control. Also, by adjusting a
thickness of the luminescent layer away from vertical resonance,
radiative optical modes can be suppressed. This suppression can
reduce emission losses out of the luminescent layer, while
enhancing probability of lateral emission along the luminescent
layer in a direction towards a PV cell.
[0122] Other implementations of the slab waveguide can have
opposing surfaces of the luminescent layer sandwiched by reflector
stacks. A reflector stack can include multiple coatings, layers, or
films of a dielectric material or a metal, and can distort a local
density of optical states in a similar fashion as increasing an
index contrast. In the case of "perfect" omni-reflecting thin-film
coatings, vertical confinement can be substantially absolute, and
radiative optical modes can be substantially suppressed. Thus,
substantially all emission can be confined to a lateral direction,
providing enhanced probability for conveyance of light to a PV
cell.
[0123] Still other implementations include multi-slab or ARROW
structures that operate with significant propagation of light
outside a luminescent material, while still maintaining a high
degree of vertical confinement. It can be desirable to maintain a
relatively high degree of transmission of solar wavelengths, while
maintaining omni-reflection over emission wavelengths.
[0124] Vertical confinement can convert a three-dimensional photon
diffusion of a bulk luminescent material into a two-dimensional
photon diffusion. A photon diffusion coefficient under such
confinement can be determined based on complex components of
optical mode propagation coefficients, rather than inherent
material characteristics such as an absorption coefficient of the
luminescent material. Decreasing the dimensionality of diffusion
can increase the photon diffusion coefficient, which can lead to
longer propagation distances between randomization events. Further
increases in the photon diffusion coefficient, such as in the
direction of a PV cell, can be achieved by including additional
propagation structure. For example, lateral confinement, such as by
adjusting a width of a luminescent layer, can reduce a local
density of optical states in a lateral direction. As with vertical
confinement described above, increasing lateral confinement can
introduce greater distortions in the local density of optical
states, yielding enhanced modification of emission characteristics
including directional control. In addition, incorporating lateral
confinement can also provide some measure of spectral pulling.
[0125] The performance of a resonant cavity structure can be
characterized with reference to its quality or finesse, which can
vary from low to high. Certain optical devices, such as lasers, can
have a relatively high finesse to operate properly. For spectral
concentrators, a relatively low finesse can be sufficient to yield
improvements in efficiency, with a greater finesse yielding
additional improvements in efficiency. In some instances, the high
intensity of emitted light in a resonant cavity structure can lead
to stimulated emission and spectral hole burning, which can
increase the amount of emitted light reaching a PV cell and yield
further improvements in efficiency.
[0126] A resonant cavity structure can operate in a weak coupling
regime or a strong coupling regime. In the weak coupling regime, a
resonant cavity structure can be implemented as a slab waveguide,
including a luminescent layer sandwiched by a pair of reflectors on
top and bottom surfaces of the luminescent layer. The pair of
reflectors serve to reduce a loss cone and, therefore, reduce loss
of emitted light as it is guided towards a PV cell. As a result of
cavity effects, the waveguide can modify emission wavelength and
intensity and reduce the lifetime of an excited state in the
waveguide. In the strong coupling regime, a resonant cavity
structure can be implemented as a polariton laser that is formed on
a substrate. A polariton laser can have substantially zero losses
and an efficiency up to about 100 percent. A polariton laser is
also sometimes referred to as a zero threshold laser, in which
there is no threshold, and lasing occurs by a coupled
exciton-photon quasiparticle called a polariton. Certain aspects of
polariton lasers are described in Christopoulos et al.,
"Room-Temperature Polariton Lasing in Semiconductor Microcavities,"
Physical Review Letters, Vol. 98, pp. 126405-1 to 126405-4 (2007);
Houdre et al., "Strong Coupling Regime in Semiconductor
Microcavities," C. R. Physique, Vol. 3, pp. 15-27 (2002); and
Kavokin, "Exciton-Polaritons in Microcavities: Present and Future,"
Appl. Phys. A, Vol. 89, pp. 241-246 (2007); the disclosures of
which are incorporated herein by reference in their entireties.
[0127] Characteristics of a luminescent material can determine the
type, desired effect, and resulting efficiency of a resonant cavity
structure. Table II below sets forth a classification scheme
according to an embodiment of the invention.
TABLE-US-00002 TABLE II Spectral Concentrator Resonant Cavity
Structures Internal Quantum Efficiency (@.lamda..sub.emiss) 100%
<100% Inherent 50 Case 1: Case 2: Stokes Shift (meV) Cavity for
direction Cavity for direction Efficiency = 100% Efficiency <
100% <50 Case 3: Case 4: Cavity for direction Cavity for
direction Photon diffusion Spectral pull Stimulated emission
Efficiency < 100% Efficiency .ltoreq. 100%
[0128] For case 1, the internal quantum efficiency of a luminescent
material is 100 percent at a particular set of emission wavelengths
.lamda..sub.emiss. In the case of a silicon PV cell,
.lamda..sub.emiss can be in the range of about 900 nm to about 1000
nm. The inherent Stokes shift of the luminescent material is about
50 meV. With this Stoke shift, remiss is sufficiently spaced apart
from an absorption edge of the luminescent material to reduce
self-absorption, with the relevant absorption coefficient being
less than about 10.sup.-2 cm.sup.-1. The resulting efficiency of a
resonant cavity structure can correspond to the fraction of emitted
light reaching the PV cell. For case 1, the resonant cavity
structure can control direction of emission towards the PV cell to
enhance the resulting efficiency to up to 100 percent.
[0129] For case 2, the internal quantum efficiency of a luminescent
material is less than 100 percent at a particular set of emission
wavelengths .lamda..sub.emiss. The inherent Stokes shift of the
luminescent material is about 50 meV. With this Stoke shift,
.lamda..sub.emiss is sufficiently spaced apart from an absorption
edge of the luminescent material to reduce self-absorption, with
the relevant absorption coefficient being less than about 10.sup.-2
cm.sup.-1. The resulting efficiency of a resonant cavity structure
can be less than 100 percent, and can be bounded at the upper end
by the internal quantum efficiency of the luminescent material. For
case 2, the resonant cavity structure can control direction of
emission towards a PV cell to enhance the resulting efficiency.
[0130] For case 3, the internal quantum efficiency of a luminescent
material is 100 percent at a particular set of emission wavelengths
.lamda..sub.emiss. The inherent Stokes shift of the luminescent
material is less than about 50 meV, and, as a result, there is a
certain level of self-absorption of emitted radiation, with the
relevant absorption coefficient being about 10.sup.-1 cm.sup.-1 or
more and up to about 10.sup.3 cm.sup.-1. Self-absorption can lead
to photon recycling, namely an emitted photon is absorbed and
re-emitted, albeit with 100 percent efficiency. As a result of
self-absorption, the resulting efficiency of a resonant cavity
structure can be less than 100 percent. For case 3, the resonant
cavity structure can control direction of emission towards a PV
cell to enhance the resulting efficiency. Emitted photons can
undergo diffusion with a diffusion length related to the absorption
coefficient. The diffusion can be modeled as a Brownian diffusion,
and, in steady state, there can be a substantially uniform
intensity of emission in the resonant cavity structure, except
within a diffusion length of the absorbing PV cell. In some
instances, the intensity of emission can be sufficient to lead to
stimulated emission and lasing.
[0131] For case 4, the internal quantum efficiency of a luminescent
material is less than 100 percent at a particular set of emission
wavelengths .lamda..sub.emiss. The inherent Stokes shift of the
luminescent material is less than about 50 meV, and, as a result,
there is a certain level of self-absorption of emitted radiation,
with the relevant absorption coefficient being about 10.sup.-1
cm.sup.-1 or more and up to about 10.sup.3 cm.sup.-1.
Self-absorption can lead to photon recycling, and, because the
internal quantum efficiency is less than 100 percent, each
absorption and re-emission cycle can lead to a reduction in the
fraction of emitted light that can reach a PV cell. As a result of
self-absorption, the resulting efficiency of a resonant cavity
structure can be less than 100 percent, and can be bounded at the
upper end by the internal quantum efficiency of the luminescent
material. For case 4, the resonant cavity structure can control
direction of emission towards the PV cell to enhance the resulting
efficiency. In addition, the resonant cavity structure can shift
emission wavelengths into resonance, which can lead to spectral
pulling and reduced self-absorption. An ARROW is one type of
resonant cavity structure that can be used for case 4. In
particular, the ARROW can allow optical modes to propagate with
intensity concentrated in a low index region, thereby reducing the
level of self-absorption.
[0132] A practitioner of ordinary skill in the art requires no
additional explanation in developing the solar modules described
herein but may nevertheless find some helpful guidance regarding
the formation and processing of PV cells by examining the following
references: U.S. Pat. No. 7,169,669, entitled "Method of Making
Thin Silicon Sheets for Solar Cells" and issued on Jan. 30, 2007;
and U.S. Patent Application Publication No. 2005/0272225, entitled
"Semiconductor Processing" and published on Dec. 8, 2005, the
disclosures of which are incorporated herein by reference in their
entireties. A practitioner of ordinary skill in the art may also
find some helpful guidance regarding spectral concentration by
examining the following references: U.S. Pat. No. 4,227,939,
entitled "Luminescent Solar Energy Concentrator Devices" and issued
on Oct. 14, 1980; and A. H. Zewali, "Photon Trapping and Energy
Transfer in Multiple-Dye Plastic Matrices: an Efficient
Solar-Energy Concentrator;" Optics Letters, Vol. 1, p. 73 (1977),
the disclosures of which are incorporated herein by reference in
their entireties. Also, a practitioner of ordinary skill in the art
may find some helpful guidance regarding multi-junction solar
modules by examining Barnham et al., "Quantum-dot Concentrator and
Thermodynamic Model for the Global Redshift," Applied Physics
Letters, Vol. 76, No. 9, pp. 1197-1199 (2000), the disclosure of
which is incorporated herein by reference in its entirety.
[0133] While the invention has been described with reference to the
specific embodiments thereof, it should be understood by those
skilled in the art that various changes may be made and equivalents
may be substituted without departing from the true spirit and scope
of the invention as defined by the appended claims. In addition,
many modifications may be made to adapt a particular situation,
material, composition of matter, method, or process to the
objective, spirit and scope of the invention. All such
modifications are intended to be within the scope of the claims
appended hereto. In particular, while the methods disclosed herein
have been described with reference to particular operations
performed in a particular order, it will be understood that these
operations may be combined, sub-divided, or re-ordered to form an
equivalent method without departing from the teachings of the
invention. Accordingly, unless specifically indicated herein, the
order and grouping of the operations are not limitations of the
invention.
* * * * *
References