U.S. patent application number 13/855569 was filed with the patent office on 2013-08-22 for luminescent materials that emit light in the visible range or the near infrared range and methods of forming thereo.
The applicant listed for this patent is John Kenney, William Pfenninger, Nemanja Vockic, Jian Jim Wang. Invention is credited to John Kenney, William Pfenninger, Nemanja Vockic, Jian Jim Wang.
Application Number | 20130217170 13/855569 |
Document ID | / |
Family ID | 44146114 |
Filed Date | 2013-08-22 |
United States Patent
Application |
20130217170 |
Kind Code |
A1 |
Vockic; Nemanja ; et
al. |
August 22, 2013 |
LUMINESCENT MATERIALS THAT EMIT LIGHT IN THE VISIBLE RANGE OR THE
NEAR INFRARED RANGE AND METHODS OF FORMING THEREO
Abstract
Luminescent materials and methods of forming such materials are
described herein. In one embodiment, a luminescent material has the
formula: [A.sub.aSn.sub.bX.sub.xX'.sub.x'X''.sub.x''][dopant],
wherein A is included in the luminescent material as a monovalent
cation; X, X', and X'' are selected from fluorine, chlorine,
bromine, and iodine; a is in the range of 1 to 5; b is in the range
of 1 to 3; a sum of x, x', and x'' is a+2b; and at least X' is
iodine, such that x'/(a+2b).gtoreq.1/5.
Inventors: |
Vockic; Nemanja; (San Jose,
CA) ; Wang; Jian Jim; (Orefield, PA) ;
Pfenninger; William; (Fremont, CA) ; Kenney;
John; (Palo Alto, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Vockic; Nemanja
Wang; Jian Jim
Pfenninger; William
Kenney; John |
San Jose
Orefield
Fremont
Palo Alto |
CA
PA
CA
CA |
US
US
US
US |
|
|
Family ID: |
44146114 |
Appl. No.: |
13/855569 |
Filed: |
April 2, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12958825 |
Dec 2, 2010 |
|
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|
13855569 |
|
|
|
|
61267756 |
Dec 8, 2009 |
|
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Current U.S.
Class: |
438/69 |
Current CPC
Class: |
C23C 14/0026 20130101;
C09K 11/665 20130101; C23C 12/02 20130101; H01L 31/02322 20130101;
C23C 14/024 20130101; C23C 14/5806 20130101; C23C 14/025 20130101;
H01L 31/055 20130101; Y02E 10/52 20130101; H01L 51/001 20130101;
C23C 14/30 20130101; C23C 14/24 20130101; C23C 14/0694
20130101 |
Class at
Publication: |
438/69 |
International
Class: |
H01L 31/0232 20060101
H01L031/0232 |
Claims
1. A method of forming a luminescent material, comprising:
providing a source of A and X, wherein A is selected from at least
one of elements of Group 1, and X is selected from at least one of
elements of Group 17; providing a source of B, wherein B is
selected from at least one of elements of Group 14; subjecting the
source of A and X and the source of B to vacuum deposition to form
a set of films adjacent to a substrate; and heating the set of
films to a temperature T.sub.heat to form a luminescent material
adjacent to the substrate, wherein the luminescent material
includes A, B, and X, one of the source of A and X and the source
of B has a lower melting point T.sub.m1, another of the source of A
and X and the source of B has a higher melting point T.sub.m2, and
T.sub.m1<T.sub.heat<T.sub.m2.
2. The method of claim 1, wherein
T.sub.m1<T.sub.heat<(T.sub.m1+3T.sub.m2)/4.
3. The method of claim 1, wherein
T.sub.m1<T.sub.heat<(T.sub.m1+T.sub.m2)/2.
4. The method of claim 1, wherein the source of A and X includes a
compound having the formula AX, and the source of B includes a
compound having the formula BY.sub.2, where Y is selected from at
least one of elements of Group 17.
5. The method of claim 1, wherein subjecting the source of A and X
and the source of B to vacuum deposition includes: subjecting the
source of A and X to at least one of electron-beam deposition and
thermal evaporation; and subjecting the source of B to thermal
evaporation.
6. The method of claim 1, wherein subjecting the source of A and X
and the source of B to vacuum deposition includes: mixing the
source of A and X and the source of B to form a mixture; and
subjecting the mixture to vacuum deposition to form the set of
films adjacent to the substrate.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. application Ser.
No. 12/958,825, filed on Dec. 2, 2010, which claims the benefit of
U.S. Provisional Application No. 61/267,756, filed on Dec. 8, 2009,
the disclosures of which are incorporated herein by reference in
their entirety.
FIELD OF THE INVENTION
[0002] The invention relates generally to luminescent materials.
More particularly, the invention relates to luminescent materials
that emit light in the visible range or the near infrared range and
methods of forming such materials.
BACKGROUND OF THE INVENTION
[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 13-15
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 luminescent materials described herein.
SUMMARY OF THE INVENTION
[0008] Luminescent materials according to various embodiments of
the invention can exhibit a number of desirable characteristics. In
some embodiments, the luminescent materials can exhibit
photoluminescence with a high quantum efficiency, with a narrow
spectral width, and with a peak emission wavelength located within
a desirable range of wavelengths, such as the visible range or the
near infrared range. Also, these photoluminescent characteristics
can be relatively insensitive over a wide range of excitation
wavelengths. The luminescent materials can have other desirable
characteristics, such as relating to their bandgap energies and
electrical conductivities. Advantageously, the luminescent
materials can be inexpensively and readily formed for use in solar
modules and other applications.
[0009] In one embodiment, a luminescent material has the
formula:
[A.sub.aSn.sub.bX.sub.xX'.sub.x'X''.sub.x''][dopant], [0010]
wherein [0011] A is included in the luminescent material as a
monovalent cation; [0012] X, X', and X'' are selected from
fluorine, chlorine, bromine, and iodine; [0013] a is in the range
of 1 to 5; [0014] b is in the range of 1 to 3; [0015] a sum of x,
x', and x'' is a+2b; and [0016] at least X' is iodine, such that
x'/(a+2b).gtoreq.1/5.
[0017] In another embodiment, a method of forming a luminescent
material includes: (1) providing a source of A and X, wherein A is
selected from at least one of elements of Group 1, and X is
selected from at least one of elements of Group 17; (2) providing a
source of B, wherein B is selected from at least one of elements of
Group 14; (3) subjecting the source of A and X and the source of B
to vacuum deposition to form a set of films adjacent to a
substrate; and (4) heating the set of films to a temperature
T.sub.heat to form a luminescent material adjacent to the
substrate, wherein the luminescent material includes A, B, and X,
one of the source of A and X and the source of B has a lower
melting point T.sub.m1, another of the source of A and X and the
source of B has a higher melting point T.sub.m2, and
T.sub.m1<T.sub.heat<T.sub.m2.
[0018] 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 various
embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] 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.
[0020] FIG. 1 illustrates normalized emission spectra of a set of
luminescent materials, according to an embodiment of the
invention.
[0021] FIG. 2 illustrates a perovskite-based microstructure of
certain luminescent materials, according to an embodiment of the
invention.
[0022] FIG. 3 illustrates X-ray diffraction data for UD930,
according to an embodiment of the invention.
[0023] FIG. 4 illustrates a combined representation of an incident
solar spectrum and measured absorption and emission spectra of
UD930 in accordance with an embodiment of the invention.
[0024] FIG. 5 through FIG. 8 illustrate manufacturing methods to
form luminescent materials, according to some embodiments of the
invention.
[0025] FIG. 9 illustrates a solar module implemented in accordance
with an embodiment of the invention.
[0026] FIG. 10 illustrates measured photoluminescence intensity
plotted as a function of annealing temperature for UD930, according
to an embodiment of the invention.
[0027] FIG. 11(a) illustrates excitation spectra for UD930 at
temperatures in the range of 12K to 300K, according to an
embodiment of the invention.
[0028] FIG. 11(b) illustrates emission spectra for UD930 at
temperatures in the range of 12K to 300K, according to an
embodiment of the invention.
DETAILED DESCRIPTION
[0029] Definitions
[0030] 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.
[0031] 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.
[0032] As used herein, the term "adjacent" refers to being near or
adjoining Adjacent elements can be spaced apart from one another or
can be in actual or direct contact with one another. In some
instances, adjacent elements can be connected to one another or can
be formed integrally with one another.
[0033] As used herein, the terms "substantially" and "substantial"
refer to a considerable degree or extent. When used in conjunction
with an event or circumstance, the terms can refer to instances in
which the event or circumstance occurs precisely as well as
instances in which the event or circumstance occurs to a close
approximation, such as accounting for typical tolerance levels of
the embodiments described herein.
[0034] 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.
[0035] As used herein, the term "size" refers to a characteristic
dimension of an object. Thus, for example, a size of an object that
is spherical can refer to a diameter of the object. In the case of
an object that is non-spherical, a size of the non-spherical object
can refer to a diameter of a corresponding spherical object, where
the corresponding spherical object exhibits or has a particular set
of derivable or measurable characteristics that are substantially
the same as those of the non-spherical object. Alternatively, or in
conjunction, a size of a non-spherical object can refer to an
average of various orthogonal dimensions of the object. Thus, for
example, a size of an object that is a spheroidal can refer to an
average of a major axis and a minor axis of the object. When
referring to a set of objects as having a particular size, it is
contemplated that the objects can have a distribution of sizes
around the particular size. Thus, as used herein, a size of a set
of objects can refer to a typical size of a distribution of sizes,
such as an average size, a median size, or a peak size.
[0036] As used herein, the term "sub-micron range" refers to a
general range of dimensions less than about 1 .mu.m or less than
about 1,000 nm, such as less than about 999 nm, less than about 900
nm, less than about 800 nm, less than about 700 nm, less than about
600 nm, less than about 500 nm, less than about 400 nm, less than
about 300 nm, or less than about 200 nm, and down to about 1 nm or
less. In some instances, the term can refer to a particular
sub-range within the general range, such as from about 1 nm to
about 100 nm, from about 100 nm to about 200 nm, from about 200 nm
to about 300 nm, from about 300 nm to about 400 nm, from about 400
nm to about 500 nm, from about 500 nm to about 600 nm, from about
600 nm to about 700 nm, from about 700 nm to about 800 nm, from
about 800 nm to about 900 nm, or from about 900 nm to about 999
nm.
[0037] As used herein, the term "ultraviolet range" refers to a
range of wavelengths from about 5 nm to about 400 nm.
[0038] As used herein, the term "visible range" refers to a range
of wavelengths from about 400 nm to about 700 nm.
[0039] As used herein, the term "infrared range" refers to a range
of wavelengths from about 700 nm to about 2 mm. The infrared range
includes the "near infrared range," which refers to a range of
wavelengths from about 700 nm to about 5 .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.
[0040] 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 element 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.
[0041] 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. Luminescence can also occur based on
relaxation from excited states of quasi-particles, such as
excitons, bi-excitons, and exciton-polaritons. For example, in the
case of photoluminescence, which can include fluorescence and
phosphorescence, an excited 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.
[0042] As used herein, the term "optical quantum efficiency" or
"OQE" refers to a ratio of the number of photons emitted by a
photoluminescent material to the number of photons absorbed by the
photoluminescent material. In some instances, an optical quantum
efficiency can be represented as: OQE=.eta..sub.1.eta..sub.2, where
.eta..sub.1 corresponds to a fraction of absorbed photons leading
to the formation of excited states, such as excited states of
excitons, and .eta..sub.2 corresponds to an "internal quantum
efficiency," namely a fraction of excited states undergoing
radiative decay that yields emitted photons.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] As used herein, the term "Full Width at Half Maximum" or
"FWHM" refers to a measure of spectral width. In some instances, a
FWHM can refer to a width of a spectrum at half of a peak intensity
value.
[0047] As used herein with respect to a photoluminescent
characteristic, the term "substantially flat" refers to being
substantially invariant with respect to a change in wavelength. In
some instances, a photoluminescent characteristic can be referred
to as being substantially flat over a range of wavelengths if
values of that characteristic within that range of wavelengths
exhibit a standard deviation of less than 20 percent with respect
to an average value, such as less than 10 percent or less than 5
percent.
[0048] 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.
[0049] 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.
[0050] As used herein, the term "electron acceptor" refers to a
chemical entity that has a tendency to attract an electron from
another chemical entity, while the term "electron donor" refers to
a chemical entity that has a tendency to provide an electron to
another chemical entity. In some instances, an electron acceptor
can have a tendency to attract an electron from an electron donor.
It should be recognized that electron attracting and electron
providing characteristics of a chemical entity are relative. In
particular, a chemical entity that serves as an electron acceptor
in one instance can serve as an electron donor in another instance.
Examples of electron acceptors include positively charged chemical
entities and chemical entities including atoms with relatively high
electronegativities. Examples of electron donors include negatively
charged chemical entities and chemical entities including atoms
with relatively low electronegativities.
[0051] A set of characteristics of a material can sometimes vary
with temperature. Unless otherwise specified herein, a
characteristic of a material can be specified at room temperature,
such as 300K or 27.degree. C.
[0052] Luminescent Materials
[0053] Embodiments of the invention relate to luminescent materials
having a number of desirable characteristics. In particular,
luminescent materials according to some embodiments of the
invention can exhibit photoluminescence with a high quantum
efficiency, with a narrow spectral width, and with a peak emission
wavelength located within a desirable range of wavelengths. Also,
these photoluminescent characteristics can be relatively
insensitive over a wide range of excitation wavelengths. Without
being bound by a particular theory, these unusual and desirable
characteristics can at least partly derive from a particular
microstructure of the luminescent materials. Advantageously, the
luminescent materials can be inexpensively and readily processed to
form a variety of products, which, in turn, can be used in solar
modules and other applications.
[0054] Desirable luminescent materials include a class of
semiconductor materials that can be represented with reference to
the formula:
[A.sub.aB.sub.bX.sub.x][dopants] (1)
[0055] In formula (1), A is selected from elements of Group 1, 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
5, such as vanadium (e.g., as V(III) or V.sup.+3), elements of
Group 11, 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 12, 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 13, 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 14, 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 Pb.sup.+2 or as
Pb(IV) or Pb.sup.+4), and elements of Group 15, such as bismuth
(e.g., as Bi(III) or Bi.sup.+3); and X is selected from elements of
Group 17, such as fluorine (e.g., as F.sup.-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 (1), a is an integer that can
be in the range of 1 to 12, such as from 1 to 9 or from 1 to 5; b
is an integer that can be in the range of 1 to 8, such as from 1 to
5 or from 1 to 3; and x is an integer that can be in the range of 1
to 12, such as from 1 to 9 or from 1 to 5. In some instances, x can
be equal to a+2b, such as for purposes of charge balance when
oxidation states of A, B, and X are +1, +2, and -1, respectively.
For example, 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 also
contemplated that X.sub.x in formula (1) can be more generally
represented as X.sub.xX'.sub.x'(or X.sub.xX'.sub.x'X''.sub.x''),
where X and X' (or X, X', and X'') can be independently selected
from elements of Group 17, and the sum of x and x' (or the sum of
x, x', and x'') can be in the range of 1 to 12, such as from 1 to 9
or from 1 to 5. With reference to the generalized version of
formula (1), the sum of x and x' (or the sum of x, x', and x'') can
be equal to a+2b. For example, a can be equal to 1, and the sum of
x and x' (or the sum of x, x', and x'') can be equal to 1+2b. It is
further contemplated that a blend or a mixture of different
luminescent materials represented by formula (1) can be used.
Dopants can be optionally included in a luminescent material
represented by formula (1), and can be present in amounts that are
less than about 5 percent, such as less than about 1 percent or
from about 0.1 percent to about 1 percent, in terms of atomic
percent or elemental composition. The dopants can derive from
reactants that are used to form the luminescent material, or can
derive from moisture, atmospheric gases, or other chemical entities
present during the formation of the luminescent material. In
particular, the dopants can include cations, anions, or both, which
can form electron acceptor/electron donor pairs that are dispersed
within a microstructure of the luminescent material.
[0056] Examples of luminescent materials represented by formula (1)
include those represented with reference to the formula:
[A.sub.aSn.sub.bX.sub.x][dopants] (2)
[0057] In formula (2), A is selected from potassium, rubidium, and
cesium; and X is selected from chlorine, bromine, and iodine. Still
referring to formula (2), x can be equal to a+2b. In some
instances, a can be equal to 1, and x can be equal to 1+2b. Several
luminescent materials with desirable characteristics can be
represented as CsSnX.sub.3[dopants] and include materials
designated as UD700 and UD930. In the case of UD700, Xis bromine,
and, in the case of UD930, X is iodine. UD700 exhibits a peak
emission wavelength at about 695 nm, while UD930 exhibits a peak
emission wavelength at about 950 nm. The spectral width of UD700
and UD930 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 UD700 and
UD930 is stimulated by a wide range of wavelengths of solar
radiation up to the absorption edge of these materials at about 695
nm for UD700 and about 950 nm for UD930. The chloride analog,
namely CsSnCl.sub.3[dopants], exhibits a peak emission wavelength
at about 450 nm, and can be desirable for certain implementations.
Normalized emission spectra of UD700, UD930, and the chloride
analog, as measured using a xenon lamp source at about 300K, are
illustrated in FIG. 1 in accordance with an embodiment of the
invention. Other luminescent materials with desirable
characteristics include CsSn.sub.2X.sub.5[dopants],
Cs.sub.2SnX.sub.4[dopants], and CsSn.sub.3X.sub.7[dopants],
mixtures thereof with, or without, CsSnX.sub.3[dopants], such as a
mixture of CsSnX.sub.3[dopants], CsSn.sub.2X.sub.5[dopants], and
Cs.sub.2SnX.sub.4[dopants], and luminescent materials in which at
least a fraction of cesium is substituted with another monovalent
ion of comparable size, such as CH.sub.3NH.sub.3.sup.+ or other
poly-elemental, monovalent ions. Additional luminescent materials
with desirable characteristics include RbSnX.sub.3[dopants], such
as RbSnI.sub.3[dopants] that exhibits a peak emission wavelength at
about 705 nm, and RbSnBr.sub.3[dopants] that exhibits a peak
emission wavelength at about 540 nm. Further luminescent materials
with desirable characteristics include KSnX.sub.3[dopants], such as
KSnBr.sub.3[dopants] that exhibits a peak emission wavelength at
about 465 nm. Each of these luminescent materials can be deposited
as a film in a single layer or in multiple layers interspersed with
other layers formed from the same luminescent material or different
luminescent materials.
[0058] Additional examples of luminescent materials represented by
formula (1) include those represented with reference to the
formula:
[A.sub.aGe.sub.bX.sub.x][dopants] (3)
[0059] In formula (3), A is selected from potassium, rubidium, and
cesium; and X is selected from chlorine, bromine, and iodine. Still
referring to formula (3), x can be equal to a+2b. In some
instances, a can be equal to 1, and x can be equal to 1+2b. In the
case that A is cesium, and X is iodine, for example, a luminescent
material can sometimes be represented with reference to the
formula:
[CsGeI.sub.3][dopants] (4)
[0060] Additional examples of luminescent materials represented by
formula (1) include those represented with reference to the
formula:
[A.sub.aPb.sub.bX.sub.x][dopants] (5)
[0061] In formula (5), A is selected from potassium, rubidium, and
cesium; and X is selected from chlorine, bromine, and iodine. Still
referring to formula (5), x can be equal to a+2b. In some
instances, a can be equal to 1, and x can be equal to 1+2b. In the
case that A is cesium, and X is iodine, for example, a luminescent
material can sometimes be represented with reference to the
formula:
[CsPbI.sub.3][dopants] (6)
[0062] Additional examples of luminescent materials represented by
formula (1) include those represented with reference to the
formula:
[A.sub.aSn.sub.bX.sub.xX'.sub.x'][dopants] (7)
[0063] In formula (7), A is selected from potassium, rubidium, and
cesium; and X and X' are different and are selected from fluorine,
chlorine, bromine, and iodine. Still referring to formula (7), the
sum of x and x' can be equal to a+2b. In order to achieve desirable
photoluminescent characteristics, at least one of X and X' can be
iodine, which can constitute at least 1/5, at least 1/4, at least
1/3, at least 1/2, or at least 2/3 of a total number of halide
ions. For example, in the case that X' is iodine,
x'/(a+2b).gtoreq.1/5, .gtoreq.1/4, .gtoreq.1/3, .gtoreq.1/2, or
.gtoreq.2/3. In some instances, a can be equal to 1, and the sum of
x and x' can be equal to 1+2b. In the case that A is cesium, X is
chlorine, and X' is iodine, for example, a luminescent material can
sometimes be represented with reference to one of the formulas:
[CsSnClI.sub.2][dopants] (8)
[CsSnCl.sub.2I][dopants] (9)
[CsSn.sub.2Cl.sub.2I.sub.3][dopants] (10)
[CsSn.sub.2Cl.sub.3I.sub.2][dopants] (11)
[CsSn.sub.2ClI.sub.4][dopants] (12)
[CsSn.sub.2Cl.sub.4I][dopants] (13)
[Cs.sub.2SnClI.sub.3][dopants] (14)
[Cs.sub.2SnCl.sub.2I.sub.2][dopants] (15)
[Cs.sub.2SnCl.sub.3I][dopants] (16)
And, in the case that A is cesium, X is bromine, and X' is iodine,
for example, a luminescent material can sometimes be represented
with reference to one of the formulas:
[CsSnBrI.sub.2][dopants] (17)
[CsSnBr.sub.2I][dopants] (18)
[CsSn.sub.2Br.sub.2I.sub.3][dopants] (19)
[CsSn.sub.2Br.sub.3I.sub.2][dopants] (20)
[CsSn.sub.2BrI.sub.4][dopants] (21)
[CsSn.sub.2Br.sub.4I][dopants] (22)
[Cs.sub.2SnBrI.sub.3][dopants] (23)
[Cs.sub.2SnBr.sub.2I.sub.2][dopants] (24)
[Cs.sub.2SnBr.sub.3I][dopants] (25)
And, in the case that A is cesium, X is fluorine, and X' is iodine,
for example, a luminescent material can sometimes be represented
with reference to one of the formulas:
[CsSnFI.sub.2][dopants] (26)
[CsSnF.sub.2I][dopants] (27)
[CsSn.sub.2F.sub.2I.sub.3][dopants] (28)
[CsSn.sub.2F.sub.3I.sub.2][dopants] (29)
[CsSn.sub.2FI.sub.4][dopants] (30)
[CsSn.sub.2F.sub.4I][dopants] (31)
[Cs.sub.2SnFI.sub.3][dopants] (32)
[Cs.sub.2SnF.sub.2I.sub.2][dopants] (33)
[Cs.sub.2SnF.sub.3I][dopants] (34)
[0064] Further examples of luminescent materials represented by
formula (1) include those represented with reference to the
formula:
[A.sub.aSn.sub.bX.sub.xX'.sub.x'X''.sub.x''][dopants] (35)
[0065] In formula (35), A is selected from potassium, rubidium, and
cesium; and X, X', and X'' are different and are selected from
fluorine, chlorine, bromine, and iodine. Still referring to formula
(35), the sum of x, x', and x'' can be equal to a+2b. In order to
achieve desirable photoluminescent characteristics, at least one of
X, X', and X'' can be iodine, which can constitute at least 1/5, at
least 1/4, at least 1/3, at least 1/2, or at least 2/3 of a total
number of halide ions. For example, in the case that X' is iodine,
x'/(a+2b).gtoreq.1/5, .gtoreq.1/4, .gtoreq.1/3, .gtoreq.1/2, or
.gtoreq.2/3. In some instances, a can be equal to 1, and the sum of
x, x', and x'' can be equal to 1+2b. In the case that A is cesium,
X is chlorine, X' is iodine, and X'' is bromine, for example, a
luminescent material can sometimes be represented with reference to
one of the formulas:
[CsSnClIBr][dopants] (36)
[CsSn.sub.2ClI.sub.x'Br.sub.4-x'][dopants], x'=1, 2, or 3 (37)
[CsSn.sub.2Cl.sub.2I.sub.x'Br.sub.3-x'][dopants], x'=1 or 2
(38)
[CsSn.sub.2Cl.sub.3IBr][dopants] (39)
[Cs.sub.2SnClI.sub.x'Br.sub.3-x'][dopants], x'=1 or 2 (40)
[Cs.sub.2SnCl.sub.2IBr][dopants] (41)
And, in the case that A is cesium, X is chlorine, X' is iodine, and
X'' is fluorine, for example, a luminescent material can sometimes
be represented with reference to one of the formulas:
[CsSnClIF][dopants] (42)
[CsSn.sub.2ClI.sub.x'F.sub.4-x'][dopants], x'=1, 2, or 3 (43)
[CsSn.sub.2Cl.sub.2I.sub.x'F.sub.3-x'][dopants], x'=1 or 2 (44)
[CsSn.sub.2Cl.sub.3IF][dopants] (45)
[Cs.sub.2SnClI.sub.x'F.sub.3-x'][dopants], x'=1 or 2 (46)
[Cs.sub.2SnCl.sub.2IF][dopants] (47)
[0066] Certain luminescent materials represented by formula (1) can
have a perovskite-based microstructure. This perovskite-based
microstructure can be layered with relatively stronger chemical
bonding within a particular layer and relatively weaker chemical
bonding between different layers. In particular, certain
luminescent materials represented by formula (1) can have a
perovskite-based crystal structure. This structure can be arranged
in the form of a network of BX.sub.6 octahedral units along
different planes, with B at the center of each octahedral unit and
surrounded by X and with A interstitial between the planes, as
illustrated in FIG. 2 in accordance with an embodiment of the
invention, where B is a cation, X is a monovalent anion, and A is a
cation that serves to balance the total charge and to stabilize the
crystal structure. Certain aspects of a perovskite-based
microstructure can be observed in X-ray diffraction ("XRD") data,
as illustrated in FIG. 3 for UD930 in accordance with an embodiment
of the invention.
[0067] Referring back to FIG. 2, dopants can be incorporated in a
perovskite-based crystal structure, as manifested by, for example,
substitution of a set of atoms included in the structure with a set
of dopants. In the case of UD930, for example, either, or both,
Cs.sup.+1 and Sn.sup.+2 can be substituted with a cation such as
Sn(IV) or Sn.sup.+4 , and I.sup.-1 can be substituted with an anion
such as F.sup.-1, Cl.sup.-1, Br.sup.-1 , O.sup.-2, OH.sup.-1, or
other anions with smaller radii relative to I.sup.-1. The
incorporation of dopants can alter a perovskite-based crystal
structure relative to the absence of the dopants, as manifested by,
for example, shorter bond lengths along a particular plane and
between different planes, such as shorter B--X--B bond lengths
along a particular plane and shorter B--X--B bond lengths between
different planes. In some instances, substitution of I.sup.-1 with
either, or both, of F.sup.-1 and Cl.sup.-1 can lead to shorter and
stronger bonds with respect to Sn.sup.+2 along a particular plane
and between different planes. Without being bound by a particular
theory, the incorporation of dopants can lend greater stability to
a perovskite-based crystal structure, and desirable
photoluminescent characteristics can at least partly derive from
the presence of these dopants. In some instances, substitution of
I.sup.-1 with other halides can be at levels greater than typical
doping levels, such as up to about 50 percent of I.sup.-1 to form
an alloy of mixed halides.
[0068] Certain luminescent materials represented by formula (1) can
be polycrystalline with constituent crystallites or grains having
sizes in the sub-micron range. The configuration of grains can vary
from one that is quasi-isotropic, namely in which the grains are
relatively uniform in shape and size and exhibit a relatively
uniform grain boundary orientation, to one that is anisotropic,
namely in which the grains exhibit relatively large deviations in
terms of shape, size, grain boundary orientation, texture, or a
combination thereof. In the case of UD930, for example, grains can
be formed in an anisotropic fashion and with an average size in the
range of about 200 nm to about 400 nm, such as from about 250 nm to
about 350 nm.
[0069] Several luminescent materials represented by formula (1)
have characteristics that are desirable for solar modules. In
particular, the luminescent materials can exhibit photoluminescence
with a high optical quantum efficiency that is greater than about 6
percent, such as at least about 10 percent, at least about 20
percent, at least about 25 percent, at least about 30 percent, or
at least about 35 percent, and can be up to about 40 percent, about
50 percent, or more, and with a high internal quantum efficiency
that is greater than about 50 percent, such as at least about 60
percent, at least about 70 percent, at least about 75 percent, at
least about 80 percent, or at least about 85 percent, and can be up
to about 95 percent, about 99 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. Incorporation of the luminescent materials
within a resonant cavity waveguide can further narrow the spectral
width.
[0070] In addition, the luminescent materials can have bandgap
energies and resistivities that are tunable to desirable levels by
adjusting reactants 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.
[0071] Moreover, the photoluminescence 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. Also, optical quantum efficiencies of the luminescent materials
can be substantially flat over a range of excitation wavelengths,
such as from about 200 nm to about 1 .mu.m, from about 200 nm to
about 980 nm or from about 450 nm to about 900 nm.
[0072] For example, UD930 has a direct bandgap with a value of
about 1.32 eV at 300K. This bandgap can decrease as temperature
decreases, at least partly resulting from an anharmonicity in a
lattice potential. Without being bound by a particular theory,
photoluminescence for UD930 (and certain other luminescent
materials represented by formula (1)) can occur via exciton
emission. An exciton corresponds to an electron-hole pair, which
can be formed as a result of light absorption. Most semiconductor
materials have relatively small exciton binding energies, so
excitons are typically not present at room temperature. Certain
luminescent materials represented by formula (1) can have
relatively large exciton binding energies, and can be incorporated
in a resonant cavity waveguide to yield suppression of emission in
a vertical direction and stimulated emission along a plane of the
cavity waveguide. The larger a Stokes shift, or exciton binding
energy, the more tolerant the cavity waveguide can be with respect
to imperfections. Thus, the cavity waveguide can be readily formed
in an inexpensive manner, without resorting to techniques such as
Molecular Beam Epitaxy ("MBE").
[0073] Desirable characteristics of UD930 can be further
appreciated with reference to FIG. 4, which illustrates a combined
representation of a solar spectrum and measured absorption and
emission spectra of UD930 in accordance with an embodiment of the
invention. In particular, FIG. 4 illustrates the AM1.5 G 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 UD930 render this material particularly effective for
spectral concentration when incorporated within an emission layer.
In particular, photoluminescence of UD930 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 reflector layers (e.g., above and below the
emission layer) that are tuned to reflect emitted radiation back
towards the emission layer, without significant reduction of
incident solar radiation that can pass through the reflector layers
and reach UD930. Also, the absorption spectrum of UD930 is
substantially flat and extends from the absorption edge at about
950 nm through a large fraction of the AM1.5G solar spectrum into
the ultraviolet. In addition, the peak emission wavelength of about
950 nm (or about 1.32 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, UD930 can broadly absorb a
wide range of wavelengths from incident solar radiation, while
emitting a narrow range of wavelengths that are matched to silicon
to allow a high conversion efficiency of incident solar radiation
into electricity. Furthermore, the absorption spectrum and the
emission spectrum of UD930 overlap to a low degree, thereby
reducing instances of self-absorption that would otherwise lead to
reduced conversion efficiency.
[0074] Methods of Forming Luminescent Materials
[0075] Luminescent materials represented by formula (1) can be
formed via reaction of a set of reactants or precursors at high
yields and at moderate temperatures and pressures. The reaction can
be represented with reference to the formula:
Source(B)+Source(A, X).fwdarw.Luminescent Material (48)
[0076] In formula (48), source(B) serves as a source of B, and, in
some instances, source(B) can also serve as a source of dopants or
halide ions. In the case that B is germanium, tin, or lead, for
example, source(B) can include one or more types of B-containing
compounds selected from B(II) compounds of the form BY, B Y.sub.2,
B.sub.3Y.sub.2, and B.sub.2Y and B(IV) compounds of the form
By.sub.4, where Y can be selected from elements of Group 16, such
as oxygen (e.g., as O.sup.-2); elements of Group 17, such as
fluorine (e.g., as F.sup.-1), chlorine (e.g., as Cl.sup.-1),
bromine (e.g., as Br.sup.-1), and iodine (e.g., as I.sup.-1); and
poly-elemental chemical entities, such as nitrate (i.e.,
NO.sub.3.sup.-1), thiocyanate (i.e., SCN.sup.-1), hypochlorite
(i.e., OCl.sup.-1), sulfate (i.e., SO.sub.4.sup.-2), orthophosphate
(i.e., PO.sub.4.sup.-3), metaphosphate (i.e., PO.sub.3.sup.-1),
oxalate (i.e., C.sub.2O.sub.4.sup.-2), methanesulfonate (i.e.,
CH.sub.3SO.sub.3.sup.-1), trifluoromethanesulfonate (i.e.,
CF.sub.3SO.sub.3.sup.-1), and pyrophosphate (i.e.,
P.sub.2O.sub.7.sup.-4). Examples of tin(II) compounds include
tin(II) fluoride (i.e., SnF.sub.2), tin(II) chloride (i.e.,
SnCl.sub.2), tin(II) chloride dihydrate (i.e.,
SnCl.sub.2.2H.sub.2O), tin(II) bromide (i.e., SnBr.sub.2), tin(II)
iodide (i.e., SnI.sub.2), tin(II) oxide (i.e., SnO), tin(II)
sulfate (i.e., SnSO.sub.4), tin(II) orthophosphate (i.e.,
Sn.sub.3(PO.sub.4).sub.2), tin(II) metaphosphate (i.e.,
Sn(PO.sub.3).sub.2), tin(II) oxalate (i.e., Sn(C.sub.2O.sub.4)),
tin(II) methanesulfonate (i.e., Sn(CH.sub.3SO.sub.3).sub.2),
tin(II) pyrophosphate (i.e., Sn.sub.2P.sub.2O.sub.7), and tin(II)
trifluoromethanesulfonate (i.e., Sn(CF.sub.3SO.sub.3).sub.2).
Examples of tin (IV) compounds include tin(IV) chloride (i.e.,
SnCl.sub.4) and tin(IV) chloride pentahydrate (i.e.,
SnCl.sub.4.5H.sub.2O). It is contemplated that different types of
source(B) can be used, such as source(B) and source(B'), with B and
B' independently selected from elements of Group 14, or as
source(B), source(B'), and source(B''), with B, B', and B''
independently selected from elements of Group 14.
[0077] Still referring to formula (48), source(A, X) serves as a
source of A and X, and, in some instances, source(A, X) can also
serve as a source of dopants. Examples of source(A, X) include
alkali halides of the form AX. In the case that A is cesium,
potassium, or rubidium, for example, source(A, X) can include one
or more types of A(I) halides, such as cesium(I) fluoride (i.e.,
CsF), cesium(I) chloride (i.e., CsCl), cesium(I) bromide (i.e.,
CsBr), cesium(I) iodide (i.e., CsI), potassium(I) fluoride (i.e.,
KF), potassium(I) chloride (i.e., KCl), potassium(I) bromide (i.e.,
KBr), potassium(I) iodide (i.e., KI), rubidium(I) fluoride (i.e.,
RbF), rubidium(I) chloride (i.e., RbCl), rubidium(I) bromide (i.e.,
RbBr), and rubidium(I) iodide (i.e., RbI). It is contemplated that
different types of source(A, X) can be used, such as source(A, X)
and source(A', X'), with A and A' independently selected from
elements of Group 1, and X and X' independently selected from
elements of Group 17, or as source(A, X), source(A', X'), and
source(A'', X''), with A, A', and A'' independently selected from
elements of Group 1, and X, X', and X'' independently selected from
elements of Group 17.
[0078] The reaction represented by formula (48) can be carried out
by combining, mixing, or otherwise contacting source(B) with
source(A, X), and then applying a form of energy. For some
embodiments, source(B) and source(A, X) can be deposited on a
substrate to form a set of films or layers. For example, source(B)
and source(A, X) can be co-deposited on a substrate to form a film,
or can be sequentially deposited to form adjacent films. Examples
of suitable deposition techniques include vacuum deposition (e.g.,
thermal evaporation or electron-beam evaporation), Physical Vapor
Deposition ("PVD"), Chemical Vapor Deposition ("CVD"), Atomic Layer
Deposition ("ALD"), sputtering, spray coating, dip coating, web
coating, wet coating, and spin coating. For other embodiments,
source(B) and source(A, X) can be mixed in a dry form, in solution,
or in accordance with any other suitable mixing technique. For
example, source(B) and source(A, X) can be provided in a powdered
form, and can be mixed using any suitable dry mixing technique. As
another example, source(B) and source(A, X) can be dispersed in a
reaction medium to form a reaction mixture, and the reaction medium
can include a solvent or a mixture of solvents. Once source(B) and
source(A, X) are suitably combined, a form of energy is applied to
promote formation of a luminescent material, such as in the form of
acoustic or vibrational energy, electrical energy, magnetic energy,
mechanical energy, optical energy, or thermal energy. For example,
source(B) and source(A, X) can be deposited on a substrate, and a
resulting set of films can be heated to a suitable temperature to
form the luminescent material. Heating can be performed in air, in
an inert atmosphere (e.g., a nitrogen atmosphere), or in a reducing
atmosphere for a suitable time period. It is also contemplated that
multiple forms of energy can be applied simultaneously or
sequentially.
[0079] The resulting luminescent material can include A, B, and X
as major elemental components as well as elemental components
derived from or corresponding to Y. Also, the luminescent material
can include additional elemental components, such as carbon,
chlorine, hydrogen, and oxygen, that can be present in amounts that
are less than about 5 percent or less than about 1 percent in terms
of elemental composition, and further elemental components, such as
sodium, sulfur, phosphorus, and potassium, that can be present in
trace amounts that are less than about 0.1 percent in terms of
elemental composition.
[0080] Examples of the reaction represented by formula (48) include
those represented with reference to the formula:
BY.sub.2+AX.fwdarw.Luminescent Material (49)
[0081] In formula (49), B is selected from germanium, tin, and
lead; Y is selected from fluorine, chlorine, bromine, and iodine; A
is selected from potassium, rubidium, and cesium; and Xis selected
from fluorine, chlorine, bromine, and iodine. Still referring to
formula (26), it is contemplated that BY.sub.2 can be more
generally represented as BY.sub.2 and B'Y'.sub.2 (or BY.sub.2,
B'Y'.sub.2, and B''Y''.sub.2), where B and B' (or B, B', and B'')
are independently selected from germanium, tin, and lead, and Y and
Y' (or Y, Y', and Y'') are independently selected from fluorine,
chlorine, bromine, and iodine. In the case that B is tin, for
example, BY.sub.2 can be represented as SnY.sub.2, or can be more
generally represented as SnY.sub.2 and SnY'.sub.2 (or SnY.sub.2,
SnY'.sub.2, and SnY''.sub.2), where Y and Y' (or Y, Y', and Y'')
are independently selected from fluorine, chlorine, bromine, and
iodine.
[0082] For example, SnI.sub.2 (or SnCl.sub.2) can be reacted with
CsI to form a luminescent material having a peak emission
wavelength at about 950 nm, such as UD930. As another example,
SnBr.sub.2 can be reacted with CsBr to form a luminescent material
having a peak emission wavelength at about 695 nm, such as UD700.
As another example, SnBr.sub.2 can be reacted with KBr to form a
luminescent material having a peak emission wavelength at about 465
nm. As another example, SnI.sub.2 can be reacted with RbI to form a
luminescent material having a peak emission wavelength at about 705
nm. As a further example, SnBr.sub.2 can be reacted with RbBr to
form a luminescent material having a peak emission wavelength at
about 540 nm.
[0083] Attention next turns to FIG. 5 through FIG. 8, which
illustrate manufacturing methods to form luminescent materials,
according to some embodiments of the invention.
[0084] Referring first to FIG. 5, a substrate 500 is provided. The
substrate 500 serves as a supporting structure during manufacturing
operations, and serves to protect a resulting luminescent material
from environmental conditions. The substrate 500 can be rigid or
flexible, can be porous or non-porous, can be optically
transparent, translucent, or opaque, and can be formed from a
glass, a metal, a ceramic, a polymer, or another suitable material.
For some implementations, the substrate 500 can include a base
substrate and a set of coatings or films that are formed adjacent
to the base substrate to provide a deposition surface for
subsequent manufacturing operations. Examples of suitable coating
materials include oxides, such as silica (i.e., SiO.sub.2 or
.alpha.-SiO.sub.2), alumina (i.e., Al.sub.2O.sub.3), TiO.sub.2,
Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, ZrO.sub.2, HfO.sub.2, SnO.sub.2,
ZnO.sub.2, La.sub.2O.sub.3, Y.sub.2O.sub.3, CeO.sub.2,
Sc.sub.2O.sub.3, Er.sub.2O.sub.3, V.sub.2O.sub.5, and
In.sub.2O.sub.3; and other suitable thin-film dielectric
materials.
[0085] Substrate effects can sometimes occur with respect to
resulting photoluminescence characteristics. For example,
enhancements of about three times or more in photoluminescence
intensity, such as from about 5 times to about 10 times, can occur
for alumina-based ceramic substrates, relative to substrates formed
from certain other materials. Without being bound by a particular
theory, such enhancements can at least partly derive from one or a
combination of the following: a R1 R2 emission process; surface
roughness of the alumina-based ceramic substrate; and high
reflectivity of the alumina-based ceramic substrate, which can
promote reflection of incident radiation back towards a luminescent
material.
[0086] Next, as illustrated in FIG. 5, a set of reactants (that are
precursors of a luminescent material) are deposited adjacent to the
substrate 500. In the illustrated embodiment, source(B) and
source(A, X) are subjected to vacuum deposition, thereby forming a
precursor layer 502 adjacent to the substrate 500. Deposition can
be carried out using a vacuum deposition system that is evacuated
to a pressure no greater than about 1.times.10.sup.-4 Torr, such as
no greater than about 1.times.10.sup.-5 Torr, and down to about
1.times.10.sup.-6 Torr or less. It is contemplated that another
suitable deposition technique can be used in place of, or in
conjunction with, vacuum deposition.
[0087] Deposition of source(B) and source(A, X) can be carried out
sequentially in accordance with the same vacuum deposition
technique or different vacuum deposition techniques. For example,
BY.sub.2 and AX can be evaporated in sequential layers, from two
layers to 30 or more layers total, such as from two layers to 16
layers total, or from two layers to six layers total, and with a
weight or molar ratio of BY.sub.2 to AX from about 99:1 to about
1:99, such as from about 5:1 to about 1:5 or from about 2:1 to
about 1:2. A particular one of BY.sub.2 and AX having a lower
melting point T.sub.m1 can be placed in an evaporator boat and
deposited by thermal evaporation, while another one of BY.sub.2 and
AX having a higher melting point T.sub.m2 can be placed in another
evaporator boat and deposited by thermal evaporation or
electron-beam evaporation. In the case of SnI.sub.2 with a melting
point of about 318.degree. C. (or SnCl.sub.2 with a melting point
of about 246.degree. C.) and CsI with a melting point of about
620.degree. C., SnI.sub.2 (or SnCl.sub.2) can be deposited by
thermal evaporation, while CsI can be deposited by thermal
evaporation or electron-beam evaporation. A thickness of each
individual BY.sub.2-containing layer or each individual
AX-containing layer can be in the range of about 10 nm to about 1.5
.mu.m, such as from about 10 nm to about 1 .mu.m or from about 10
nm to about 300 nm, with a total thickness for all layers in the
range of about 20 nm to about 45 .mu.m, such as from about 40 nm to
about 20 .mu.m or from about 50 nm to about 5 .mu.m.
[0088] Source(B) and source(A, X) can also be co-deposited in
accordance with a particular vacuum deposition technique. For
example, BY.sub.2 and AX can be co-evaporated to form a single
layer or multiple layers, with a weight or molar ratio of BY.sub.2
to AX from about 10:1 to about 1:10, such as from about 5:1 to
about 1:5 or from about 2:1 to about 1:2, and with a total
thickness in the range of about 10 nm to about 1.5 .mu.m, such as
from about 10 nm to about 1 .mu.m or from about 10 nm to about 300
nm. In particular, BY.sub.2 and AX can be mixed in an evaporator
boat and then deposited by thermal evaporation. Mixing of BY.sub.2
and AX can be carried out in a powdered form, or by forming a
pre-melt of BY.sub.2 and AX. In the case of SnI.sub.2 (or
SnCl.sub.2) and CsI, SnI.sub.2 (or SnCl.sub.2) can evaporate at
lower temperatures than CsI, and, therefore, a temperature of the
evaporator boat can be gradually raised as a relative amount of CsI
in a mixture increases.
[0089] Different types of source(B) can be used, and can be
co-deposited with source(A, X) or deposited sequentially with
source(A, X). For example, BY.sub.2 and B'Y'.sub.2 can be mixed in
an evaporator boat and deposited by thermal evaporation, followed
by deposition of AX, and so forth. Mixing of BY.sub.2 and
B'Y'.sub.2 can be carried out in a powdered form, or by forming a
pre-melt of BY.sub.2 and B'Y'.sub.2, with a weight or molar ratio
of BY.sub.2 to B'Y'.sub.2 from about 99:1 to about 1:99, such as
from about 5:1 to about 1:5 or from about 2:1 to about 1:2. As
another example, BY.sub.2, B'Y'.sub.2, and B''Y''.sub.2 can be
mixed in an evaporator boat and deposited by thermal evaporation,
followed by deposition of AX, and so forth. Likewise, different
types of source(A, X) can be used, and can be co-deposited with
source(B) or deposited sequentially with source(B).
[0090] Still referring to FIG. 5, an encapsulation material is next
deposited, thereby forming an encapsulation layer 504 adjacent to
the precursor layer 502. The encapsulation layer 504 serves to
provide protection and sealing of a resulting luminescent material
and to reduce its exposure to oxygen, humidity, and other
contaminants, thereby enhancing stability of resulting
photoluminescence characteristics. Examples of suitable
encapsulation materials include oxides, such as silica, alumina,
TiO.sub.2, Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, ZrO.sub.2, HfO.sub.2,
SnO.sub.2, ZnO.sub.2, La.sub.2O.sub.3, Y.sub.2O.sub.3, CeO.sub.2,
Sc.sub.2O.sub.3, Er.sub.2O.sub.3, V.sub.2O.sub.5, and
In.sub.2O.sub.3; nitrides, such as SiO.sub.xN.sub.2-x; fluorides,
such as CaF.sub.2, SrF.sub.2, ZnF.sub.2, MgF.sub.2, LaF.sub.3, and
GdF.sub.2; nanolaminates, such as HfO.sub.2/Ta.sub.2O.sub.5,
TiO.sub.2/Ta.sub.2O.sub.5, TiO.sub.2/Al.sub.2O.sub.3,
ZnS/Al.sub.2O.sub.3, and AlTiO; and other suitable thin-film
dielectric materials. Additional examples of suitable encapsulation
materials include glasses, such as borosilicate glasses (e.g.,
borofloat.RTM. glass and D 263.TM. glass), phosphate glasses, and
other low melting glasses. A thickness of the encapsulation layer
504 can be in the range of about 10 nm to about 1.5 .mu.m, such as
from about 50 nm to about 500 nm or from about 50 nm to about 300
nm. In the case of a deposited layer of glass, a suitable metal can
be deposited adjacent to the layer of glass to provide a hermetic
seal, such as silver, aluminum, gold, copper, iron, cobalt, nickel,
palladium, platinum, ruthenium, or iridium. Vacuum deposition, such
as electron-beam evaporation, can be used to form the encapsulation
layer 504, along with the other layers of the assembly of layers.
Alternatively, another suitable deposition technique can be used,
such as sputtering.
[0091] The assembly of layers is next subjected to annealing to
promote bonding between the layers as well as to promote reaction
according to formula (48), thereby converting the precursor layer
502 to an emission layer including a luminescent material according
to formula (1). Annealing can be carried out using any suitable
heating technique to apply thermal energy via conduction,
convection, or radiation heating, such as by heating the assembly
of layers using a hot plate, an oven, resist heating, or lamp
heating. It is also contemplated that thermal energy can be applied
in accordance with fast heating cycles to yield rapid thermal
annealing.
[0092] Resulting photoluminescence characteristics can be dependent
upon an annealing temperature and an annealing time period. As
such, an annealing temperature and an annealing time period can be
optimized to yield higher photoluminescence intensities. For
example, a particular one of BY.sub.2 and AX can have a lower
melting point T.sub.m1, another one of BY.sub.2 and AX can have a
higher melting point T.sub.m2, and an optimal annealing temperature
T.sub.heat can be greater than T.sub.m1 and less than T.sub.m2,
such as greater than T.sub.m1 and up to a three-quarters point
(i.e., (T.sub.m1+3T.sub.m2)/4) or a halfway point (i.e.,
(T.sub.m1+T.sub.m2)/2) between the lower melting point and the
higher melting point, although annealing can also be carried out at
higher or lower temperatures. In the case of SnI.sub.2 with a
melting point of about 318.degree. C. and CsI with a melting point
of about 620.degree. C., an optimal annealing temperature
T.sub.heat can be greater than about 318.degree. C. and less than
about 620.degree. C., such as from about 340.degree. C. to about
420.degree. C. or from about 350.degree. C. to about 410.degree. C.
In the case of SnCl.sub.2 with a melting point of about 246.degree.
C. and CsI with a melting point of about 620.degree. C., an optimal
annealing temperature T.sub.heat can be greater than about
246.degree. C. and less than about 620.degree. C. In some
instances, an initial melting can arise from formation of an
eutectic between SnCl.sub.2 and a reaction product of SnCl.sub.2
and CsI. An optimal annealing time period can be in the range of
about 1 sec to about 1 hr, such as from about 5 sec to about 10 min
or from about 5 sec to about 1 min, although annealing can also be
carried out for longer or shorter time periods. Optimal values of
an annealing temperature and an annealing time period can also be
suitably adjusted depending upon, for example, particular reagents
used, a thickness of individual layers within the precursor layer
502, or a total thickness of the precursor layer 502. In some
instances, a reaction between layers of reactants can occur at
temperatures significantly below melting temperatures of the
reactants by way of solid state reactions. In particular, the
layers can be sufficiently thin so that diffusion can occur within,
for example, a few hundred nanometers or less and a time period of
a few seconds to a few minutes, thereby allowing the reactants to
react and to form a luminescent material.
[0093] Referring next to FIG. 6, a substrate 600 is provided, and a
precursor layer 604 is formed adjacent to the substrate 600.
Certain aspects of the manufacturing method of FIG. 6 can be
implemented in a similar manner as described above for FIG. 5, and,
therefore, are not further described herein.
[0094] As illustrated in FIG. 6, a reflector layer 602 is initially
formed adjacent to the substrate 600, followed by formation of the
precursor layer 604 adjacent to the reflector layer 602, and
followed by formation of another reflector layer 606 adjacent to
the precursor layer 604. The pair of reflector layers 602 and 606
serve to provide protection and sealing of a resulting luminescent
material and to reduce its exposure to oxygen, humidity, and other
contaminants. In addition, the pair of reflector layers 602 and 606
serve to reduce loss of emitted radiation and to promote guiding of
the emitted radiation along a lateral direction.
[0095] In the illustrated embodiment, the formation of the
reflector layers 602 and 606 is carried out by depositing a set of
dielectric materials using ALD or another suitable deposition
technique. In particular, each of the reflector layers 602 and 606
is implemented as a dielectric stack, including multiple dielectric
layers and with the number of dielectric layers in the range of 2
to 1,000, such as in the range of 2 to 100, in the range of 30 to
90, or in the range of 30 to 80. Each individual dielectric layer
can have a thickness in the range of about 1 nm to about 200 nm,
such as from about 10 nm to about 150 nm or from about 10 nm to
about 100 .mu.m. Depending upon the number of dielectric layers
forming the reflector layers 602 and 606, a total thickness of each
of the reflector layers 602 and 606 can be in the range of about
100 nm to about 20 .mu.m, such as from about 1 .mu.m to about 15
.mu.m or from about 1 .mu.m to about 10 .mu.m. For certain
implementations, a dielectric stack can include multiple layers
formed from different dielectric materials. Layers formed from
different materials can be arranged in a periodic fashion, such as
in an alternating fashion, or in a non-periodic fashion. Examples
of dielectric materials that can be used to form the reflector
layers 602 and 606 include oxides, such as silica, alumina,
TiO.sub.2, Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, ZrO.sub.2, HfO.sub.2,
SnO.sub.2, ZnO.sub.2, La.sub.2O.sub.3, Y.sub.2O.sub.3, CeO.sub.2,
Sc.sub.2O.sub.3, Er.sub.2O.sub.3, V.sub.2O.sub.5, and
In.sub.2O.sub.3; nitrides, such as SiO.sub.xN.sub.2-x; fluorides,
such as CaF.sub.2, SrF.sub.2, ZnF.sub.2, MgF.sub.2, LaF.sub.3, and
GdF.sub.2; nanolaminates, such as HfO.sub.2/Ta.sub.2O.sub.5,
TiO.sub.2/Ta.sub.2O.sub.5, TiO.sub.2/Al.sub.2O.sub.3,
ZnS/Al.sub.2O.sub.3, and AlTiO; and other suitable thin-film
dielectric materials. Desirably, different materials forming a
dielectric stack have different refractive indices so as to form a
set of high index layers and a set of low index layers that are
interspersed within the dielectric stack. For certain
implementations, an index contrast in the range of about 0.3 to
about 1 or in the range of about 0.3 to about 2 can be desirable.
For example, TiO.sub.2 and SiO.sub.2 can be included in alternating
layers of a dielectric stack to provide a relatively large index
contrast between the layers. A larger index contrast can yield a
larger stop band with respect to emitted radiation, thereby
approaching the performance of an ideal omnireflector. In addition,
a larger index contrast can yield a greater angular tolerance for
reflectivity with respect to incident solar radiation, and can
reduce a leakage of emitted radiation at larger angles from a
normal direction.
[0096] The assembly of layers illustrated in FIG. 6 is then
subjected to annealing to promote bonding between the layers as
well as to promote reaction according to formula (48), thereby
converting the precursor layer 604 to an emission layer that is
sandwiched between the reflector layers 602 and 606.
[0097] Referring next to FIG. 7, a reflector layer 702 is formed
adjacent to a substrate 700, and a precursor layer 704 is formed
adjacent to the reflector layer 702. Certain aspects of the
manufacturing method of FIG. 7 can be implemented in a similar
manner as described above for FIG. 5 and FIG. 6, and, therefore,
are not further described herein.
[0098] As illustrated in FIG. 7, a spacer layer 706 is next formed
adjacent to the precursor layer 704, followed by formation of
another reflector layer 708 adjacent to the spacer layer 706. The
spacer layer 706 and the reflector layer 708 serve to provide
protection and sealing of a resulting luminescent material and to
reduce its exposure to oxygen, humidity, and other contaminants. In
addition, the reflector layer 708 serves to reduce loss of emitted
radiation and to promote guiding of the emitted radiation along a
lateral direction, while the spacer layer 706 provides index
matching for low loss guiding of the emitted radiation. It is
contemplated that the relative positions of the reflector layer 702
and the reflector layer 708, with respect to the precursor layer
704, can be switched for other implementations, and that the spacer
layer 706 (or another similar spacer layer) and the reflector layer
708 (or another similar reflector layer) can be formed adjacent to
the substrate 700, with the spacer layer 706 (or another similar
spacer layer) providing a deposition surface for the formation of
the precursor layer 704.
[0099] In the illustrated embodiment, the formation of the spacer
layer 706 and the reflector layer 708 is carried out by depositing
a set of materials using vacuum deposition or another suitable
deposition technique. In particular, the reflector layer 708 is
implemented so as to be omnireflective over a relatively wide range
of wavelengths, and can be formed by depositing a metal, such as
silver, aluminum, gold, copper, iron, cobalt, nickel, palladium,
platinum, ruthenium, or iridium; a metal alloy; or another suitable
material having broadband reflectivity. A thickness of the
reflector layer 708 can be in the range of about 1 nm to about 200
nm, such as from about 10 nm to about 150 nm or from about 10 nm to
about 100 .mu.m. As illustrated in FIG. 7, the spacer layer 706 can
be formed by depositing a suitable low index material, such one
having a refractive index that is no greater than about 2, no
greater than about 1.5, or no greater than about 1.4. Examples of
suitable low index materials include MgF.sub.2 (refractive index of
about 1.37), CaF.sub.2, silica, alumina, and other suitable
thin-film, low index, dielectric materials. A thickness of the
spacer layer 706 can be in the range of about 1 nm to about 500 nm,
such as from about 50 nm to about 300 nm or from about 10 nm to
about 100 nm.
[0100] The assembly of layers illustrated in FIG. 7 is then
subjected to annealing to promote bonding between the layers as
well as to promote reaction according to formula (48), thereby
converting the precursor layer 704 to an emission layer that is
sandwiched between the reflector layers 702 and 708.
[0101] Referring next to FIG. 8, a bottom substrate 800 is
provided, and a precursor layer 802 is formed adjacent to the
bottom substrate 800. While not illustrated in FIG. 8, it is
contemplated that either, or both, a spacer layer and a reflector
layer can be formed adjacent to the bottom substrate 800 and can
provide a deposition surface for the formation of the precursor
layer 802. Certain aspects of the manufacturing method of FIG. 8
can be implemented in a similar manner as described above for FIG.
5 through FIG. 7, and, therefore, are not further described
herein.
[0102] As illustrated in FIG. 8, a bonding layer 804 is next formed
adjacent to the precursor layer 802, followed by positioning of a
top substrate 806 adjacent to the bonding layer 804, and followed
by bonding and annealing of the assembly of layers. The bonding
layer 804 and the top substrate 806 serve to provide protection and
sealing of a resulting luminescent material and to reduce its
exposure to oxygen, humidity, and other contaminants. It is
contemplated that the bonding layer 804 (or another similar bonding
layer) can be formed adjacent to the top substrate 806, and then
positioned adjacent to the precursor layer 802 to achieve
bonding.
[0103] In the illustrated embodiment, the formation of the bonding
layer 804 is carried out by depositing an adhesive or bonding
material using any suitable deposition technique. In particular,
the bonding layer 804 can be formed by depositing a thermal-curable
adhesive or bonding material, such as a glass (e.g., a spin-on
glass or a sealing glass) or a polymer (e.g., a perfluoropolymer or
an epoxy-based polymer). A thickness of the bonding layer 804 can
be in the range of about 1 nm to about 200 nm, such as from about
10 nm to about 150 nm or from about 10 nm to about 100 .mu.m.
[0104] Bonding can be achieved using fluid pressure, a mechanical
press, or another suitable bonding technique, along with the
application of thermal energy to promote bonding as well as to
promote reaction according to formula (48). It is also contemplated
that bonding can be achieved using an ultraviolet light-curable
adhesive or bonding material, rather than a thermal-curable
adhesive or bonding material. For example, a thin coating of a
pre-polymer (or a set of monomers) can be applied to a set of
surfaces, and, after the surfaces are pressed together, ultraviolet
light exposure can be applied through either, or both, of the
surfaces to cure the pre-polymer (or the monomers).
[0105] Solar Modules
[0106] FIG. 9 illustrates a solar module 900 implemented in
accordance with an embodiment of the invention. The solar module
900 includes a PV cell 902, which is a p-n junction device formed
from crystalline silicon. However, the PV cell 902 can also be
formed from another suitable photoactive material. As illustrated
in FIG. 9, the PV cell 902 is implemented as 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 902. As illustrated in FIG. 9, the PV
cell 902 is configured to accept and absorb radiation incident upon
a side surface 904 of the PV cell 902, although other surfaces of
the PV cell 902 can also be involved.
[0107] In the illustrated embodiment, the solar module 900 also
includes a spectral concentrator 906, which is formed as a slab
having a side surface 908 that is adjacent to the side surface 904
of the PV cell 902. The spectral concentrator 906 includes a set of
luminescent materials as described herein, which can be included
within an emission layer to 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 902. During operation of the solar module 900,
incident solar radiation strikes a top surface 910 of the spectral
concentrator 906, and a certain fraction of this incident solar
radiation penetrates below the top surface 910 and is absorbed and
converted into substantially monochromatic, emitted radiation. This
emitted radiation is guided laterally within the spectral
concentrator 906, and a certain fraction of this emitted radiation
reaches the side surface 904 of the PV cell 902, which absorbs and
converts this emitted radiation into electricity.
[0108] In effect, the spectral concentrator 906 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 902; and (3) conveying the emitted radiation to the PV cell
902, where the emitted radiation can be converted to useful
electrical energy. The spectral concentrator 906 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 906 are further
described below.
[0109] Collection refers to capturing or intercepting incident
solar radiation in preparation for conversion to emitted radiation.
Collection efficiency of the spectral concentrator 906 can depend
upon the amount and distribution of a luminescent material within
the spectral concentrator 906. 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 906, or can occur
within one or more regions of the spectral concentrator 906. The
collection efficiency can also depend upon other aspects of the
spectral concentrator 906, 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.
[0110] 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 906 can depend upon photoluminescence
characteristics of a luminescent material, including its optical
quantum efficiency or its internal quantum efficiency, but can also
depend upon interaction of luminescent centers with their local
optical environment, including via resonant cavity effects.
Depending upon the distribution of the luminescent centers,
conversion of incident solar radiation can occur in a distributed
fashion throughout the spectral concentrator 906, or can occur
within one or more regions of the spectral concentrator 906. 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.
[0111] Conveyance refers to guiding or propagation of emitted
radiation towards the PV cell 902, and the efficiency of such
conveyance refers to the probability that an emitted photon reaches
the PV cell 902. Conveyance efficiency of the spectral concentrator
906 can depend upon photoluminescence 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.
[0112] By performing these operations, the spectral concentrator
906 provides a number of benefits. In particular, by performing the
collection operation in place of the PV cell 902, the spectral
concentrator 906 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 1,000.
Also, the spectral concentrator 906 enhances solar energy
conversion efficiency based on at least two effects: (1)
concentration effect; and (2) monochromatic effect.
[0113] In terms of the concentration effect, the spectral
concentrator 906 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 902. Incident solar radiation is collected via the top surface
910 of the spectral concentrator 906, and emitted radiation is
guided towards the side surface 904 of the PV cell 902. A solar
radiation collection area, as represented by, for example, an area
of the top surface 910 of the spectral concentrator 906, can be
significantly greater than an area of the PV cell 902, as
represented by, for example, an area of the side surface 904 of the
PV cell 902. A resulting concentration factor onto the PV cell 902
can be in the range of about 10 to about 100 and up to about 1,000
or more. For example, the concentration factor can exceed about
10,000 and can be up to about 60,000 or more. In turn, the
concentration factor can increase the open circuit voltage or
V.sub.oc of the solar module 900, 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 902. For example, V.sub.oc
can be increased from a typical value of about 0.55 V, which is
about half the bandgap energy of silicon, to about 1.6 V, which is
about 1.5 times the bandgap energy of silicon. A typical solar
radiation energy flux or intensity 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 906
with respect to the collection, conversion, and conveyance
operations.
[0114] In terms of the monochromatic effect, the narrow band
radiation emitted from the spectral concentrator 906 can be
efficiently absorbed by the PV cell 902, 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 902,
thermalization can mostly occur within the spectral concentrator
906, rather than within the PV cell 902.
[0115] Aspects of Cavity Quantum Electrodynamics can be used to
implement the spectral concentrator 906 as a micro-cavity or a
resonant cavity waveguide. The resulting resonant cavity effects
can provide a number of benefits. For example, resonant cavity
effects can be exploited to control a direction of emitted
radiation towards the PV cell 902 and, therefore, enhance the
fraction of emitted radiation reaching the PV cell 902. This
directional control can involve suppressing emission for optical
modes in non-guided directions, while allowing or enhancing
emission for optical modes in guided directions towards the PV cell
902. In such manner, there can be a significant reduction in loss
of emitted radiation via a loss cone. Also, resonant cavity effects
can be exploited to modify emission characteristics, such as by
enhancing emission of a set of wavelengths that are associated with
certain optical modes and suppressing emission of another set of
wavelengths that are associated with other optical modes. This
modification of emission characteristics can reduce an overlap
between an emission spectrum and an absorption spectrum via
spectral pulling, and can reduce losses arising from
self-absorption. This modification of emission characteristics can
also yield a larger exciton binding energy, and can promote
luminescence via exciton emission. In addition, resonant cavity
effects can enhance absorption and emission characteristics of a
set of luminescent materials, and can allow the use of
semiconductor materials having indirect optical transitions or
forbidden optical transitions. This enhancement of absorption and
emission characteristics can involve optical gain as well as
amplified spontaneous emission, such as via the Purcell effect. In
some instances, the high intensity of emitted radiation within the
spectral concentrator 906 can lead to stimulated emission and
lasing, which can further reduce losses as emitted radiation is
guided towards the PV cell 902.
[0116] In the illustrated embodiment, a local density of optical
states within the spectral concentrator 906 can include both guided
optical modes and radiative optical modes. Guided optical modes can
involve propagation of emitted radiation along the spectral
concentrator 906, while radiative optical modes can involve
propagation of emitted radiation out of the spectral concentrator
906. For a relatively low degree of vertical confinement, the local
density of optical states and emission characteristics are modified
to a relatively low degree. Increasing vertical confinement, such
as by increasing an index contrast between dielectric layers of a
dielectric stack, can introduce greater distortions in the local
density of optical states, yielding modification of emission
characteristics including directional control. Also, by adjusting a
thickness of an emission layer within the spectral concentrator 906
with respect to vertical resonance, radiative optical modes can be
suppressed. This suppression can reduce emission losses out of the
spectral concentrator 906, while enhancing probability of lateral
emission along the spectral concentrator 906 in a direction towards
the PV cell 902. For certain implementations, the emission layer
can be disposed between a pair of reflector layers so as to be
substantially centered at an anti-node position of a resonant
electromagnetic wave, and the pair of reflector layers can be
spaced to yield a cavity length in the range of a fraction of a
wavelength to about ten wavelengths or more. Lateral confinement
can also be achieved by, for example, forming reflector layers
adjacent to side edges and surfaces of the spectral concentrator
906, which are not involved in conveyance of radiation.
[0117] When implemented as a resonant cavity waveguide, a
performance of the spectral concentrator 906 can be characterized
with reference to its quality or Q value, which can vary from low
to high. A relatively low Q value can be sufficient to yield
improvements in efficiency, with a greater Q value yielding
additional improvements in efficiency. For certain implementations,
the spectral concentrator 906 can have a Q value that is at least
about 5, such as at least about 10 or at least about 100, and up to
about 10.sup.5 or more, such as up to about 10,000 or up to about
1,000. In the case of a high-Q resonant cavity waveguide, the
spectral concentrator 906 can exhibit an exciton emission in which
excitons interact with cavity photons to form coupled
exciton-photon quasi-particles referred as exciton-polaritons. The
spectral concentrator 906 can operate in a weak coupling regime or
a strong coupling regime, depending upon an extent of coupling
between excitons and cavity photons or among excitons in the case
of bi-excitons.
[0118] In the strong coupling regime, the spectral concentrator 906
can be implemented as a polariton laser, which can lead to highly
efficient and intense emissions and extremely low lasing
thresholds. A polariton laser can have substantially zero losses
and an efficiency up to about 100 percent. A polariton laser is
also sometimes referred as a zero threshold laser, in which there
is little or no lasing threshold, and lasing derives at least
partly from excitons or related quasi-particles, such as
bi-excitons or exciton-polaritons. The formation of quasi-particles
and a resulting modification of energy levels or states can reduce
losses arising from self-absorption. Contrary to conventional
lasers, a polariton laser can emit coherent and substantially
monochromatic radiation without population inversion. Without being
bound by a particular theory, emission characteristics of a
polariton laser can occur when exciton-polaritons undergo
Bose-condensation within a resonant cavity waveguide. Lasing can
also occur in the weak coupling regime, although a lasing threshold
can be higher than for the strong coupling regime. In the weak
coupling regime, lasing can derive primarily from excitons, rather
than from exciton-polaritons.
[0119] By implementing as a high-Q resonant cavity waveguide in the
form of a polariton laser, the spectral concentrator 906 can
exhibit a number of desirable characteristics. In particular,
lasing can be achieved with a very low threshold, such as with an
excitation intensity that is no greater than about 200 mW
cm.sup.-2, no greater than about 100 mW cm.sup.-2, no greater than
about 50 mW cm.sup.-2, or no greater than about 10 mW cm.sup.-2,
and down to about 1 mW cm.sup.-2 or less, which is several orders
of magnitude smaller than for a conventional laser. Because a
typical solar radiation intensity is about 100 cm .sup.-2, lasing
can be achieved with normal sunlight with little or no
concentration. Also, lasing can occur with a short radiative
lifetime, such as no greater than about 500 psec, no greater than
about 200 psec, no greater than about 100 psec, or no greater than
about 50 psec, and down to about 1 psec or less, which can avoid or
reduce relaxation through non-radiative mechanisms. Furthermore,
lasing can involve narrowing of a spectral width of an emission
spectrum to form a narrow emission line, such as by a factor of at
least about 1.5, at least about 2, or at least about 5, and up to
about 10 or more, relative to the case where there is a substantial
absence of resonant cavity effects. For example, in the case of
UD930, a spectral width can be narrowed to a value in the range of
about 2 nm to about 10 nm, such as from about 3 nm to about 10 nm,
when UD930 is incorporated in a high-Q resonant cavity waveguide. A
narrow emission line from lasing can enhance solar conversion
efficiencies, as a result of the monochromatic effect. In such
manner, lasing and low loss with distance can allow higher
intensities of emissions reaching the PV cell 902 and higher solar
conversion efficiencies. There can be little or no measurable loss
of emissions that are guided towards the PV cell 902. With lasing,
a solar energy conversion efficiency can be up to about 30 percent
or more, such as in the range of about 20 percent to about 30
percent or in the range of about 28 percent to about 30
percent.
EXAMPLES
[0120] The following examples describe specific aspects of some
embodiments of the invention to illustrate and provide a
description for those of ordinary skill in the art. The examples
should not be construed as limiting the invention, as the examples
merely provide specific methodology useful in understanding and
practicing some embodiments of the invention.
Example 1
Formation of Luminescent Material--UD930
[0121] Samples of UD930 were formed in a reproducible manner by
vacuum deposition in accordance with two approaches. In accordance
with one approach, tin(II) iodide and cesium(I) iodide were
evaporated in sequential layers, typically up to six layers total.
In accordance with another approach, tin(II) chloride and cesium(I)
iodide were evaporated in sequential layers, typically up to six
layers total. For certain samples, a thickness of the tin(II)
chloride-containing layer was about 90 nm, and a thickness of the
cesium(I) iodide-containing layer was about 170 nm. Tin(II) iodide
(or tin(II) chloride) was deposited by thermal evaporation, while
cesium(I) iodide was deposited by electron-beam evaporation.
Deposition was carried out at a pressure of about 10.sup.-5 Torr
(or less) on a variety of substrates, including those formed from
glass, ceramic, and silicon.
[0122] Following deposition, resulting samples were annealed in a
glove box in a dry, nitrogen atmosphere to allow a self-limiting
chemical reaction between tin(II) iodide (or tin(II) chloride) and
cesium(I) iodide. In the case of samples formed using tin(II)
chloride, annealing was also sometimes carried out in air. An
optimum annealing temperature was observed to vary somewhat
depending upon a particular sample, but typically was greater than
a melting point of tin(II) iodide at about 318.degree. C. (or a
melting point of tin(II) chloride at about 246.degree. C.). FIG. 10
illustrates measured photoluminescence intensity at about 943 nm
plotted as a function of annealing temperature for tin(II)
iodide/cesium(I) iodide deposited on a silicon substrate (with
native SiO.sub.2), according to an embodiment of the invention.
Annealing at temperatures at or below the melting point of tin(II)
iodide was observed to yield weak photoluminescence intensity,
while a strong band of photoluminescence intensities was observed
after annealing at temperatures of about 50.degree. C. above the
melting point of tin(II) iodide. An optimum annealing time period
was observed to vary somewhat depending upon a particular sample
and a particular substrate used. In the case of samples formed on
glass substrates, an optimal annealing time period typically was
about 15 sec. Stability of photoluminescence was enhanced if the
samples were stored in a dry, nitrogen atmosphere, or were
encapsulated, such as by bonding to a glass substrate using a layer
of epoxy.
Example 2
Characterization of Luminescent Material--UD930
[0123] FIG. 11(a) illustrates excitation spectra obtained for UD930
at temperatures in the range of 12K to 300K, and FIG. 11(b)
illustrates emission spectra obtained for UD930 in the same
temperature range, according to an embodiment of the invention. As
can be appreciated, a variation of the peak positions with
temperature is similar for the excitation and emission spectra.
[0124] A practitioner of ordinary skill in the art requires no
additional explanation in developing the luminescent materials
described herein but may nevertheless find some helpful guidance by
examining the co-pending and co-owned U.S. patent application Ser.
No. 11/689,381 (now U.S. Pat. No. 7,641,815), filed on Mar. 21,
2007, the disclosure of which is incorporated herein by reference
in its entirety.
[0125] 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.
* * * * *