U.S. patent application number 13/536857 was filed with the patent office on 2012-12-06 for light conversion efficiency enhanced solar cell fabricated with downshifting nanomaterial.
Invention is credited to Alex R. Guichard, Steven M. Hughes, Juanita N. Kurtin, Georgeta Masson, Alex C. Mayer, Oun Ho Park, Colin C. Reese, Shawn R. Scully, Manav Sheoran, Paul-Emile B. Trudeau.
Application Number | 20120305860 13/536857 |
Document ID | / |
Family ID | 43450176 |
Filed Date | 2012-12-06 |
United States Patent
Application |
20120305860 |
Kind Code |
A1 |
Kurtin; Juanita N. ; et
al. |
December 6, 2012 |
LIGHT CONVERSION EFFICIENCY ENHANCED SOLAR CELL FABRICATED WITH
DOWNSHIFTING NANOMATERIAL
Abstract
The light conversion efficiency of a solar cell (10) is enhanced
by using an optical downshifting layer (30) in cooperation with a
photovoltaic material (22). The optical downshifting layer converts
photons (50) having wavelengths in a supplemental light absorption
spectrum into photons (52) having a wavelength in the primary light
absorption spectrum of the photovoltaic material. The cost
effectiveness and efficiency of solar cells platforms (20) can be
increased by relaxing the range of the primary light absorption
spectrum of the photovoltaic material. The optical downshifting
layer can be applied as a low cost solution processed film composed
of highly absorbing and emissive quantum dot heterostructure
nanomaterial embedded in an inert matrix to improve the short
wavelength response of the photovoltaic material. The enhanced
efficiency provided by the optical downshifting layer permits
advantageous modifications to the solar cell platform that enhances
its efficiency as well.
Inventors: |
Kurtin; Juanita N.;
(Hillsboro, OR) ; Guichard; Alex R.; (Portland,
OR) ; Hughes; Steven M.; (Hillsboro, OR) ;
Mayer; Alex C.; (Portland, OR) ; Park; Oun Ho;
(Portland, OR) ; Scully; Shawn R.; (Portland,
OR) ; Trudeau; Paul-Emile B.; (Portland, OR) ;
Reese; Colin C.; (Portland, OR) ; Sheoran; Manav;
(Portland, OR) ; Masson; Georgeta; (Portland,
OR) |
Family ID: |
43450176 |
Appl. No.: |
13/536857 |
Filed: |
June 28, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12836511 |
Jul 14, 2010 |
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13536857 |
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61225472 |
Jul 14, 2009 |
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Current U.S.
Class: |
252/519.4 ;
252/500; 977/773; 977/774; 977/890 |
Current CPC
Class: |
Y02E 10/52 20130101;
Y02E 10/542 20130101; C09K 11/565 20130101; C09K 11/02 20130101;
H01L 31/055 20130101 |
Class at
Publication: |
252/519.4 ;
252/500; 977/773; 977/774; 977/890 |
International
Class: |
H01B 1/00 20060101
H01B001/00; H01B 1/10 20060101 H01B001/10; H01B 1/12 20060101
H01B001/12; H01B 1/08 20060101 H01B001/08 |
Claims
1.-27. (canceled)
28. An optical downshifting material, comprising: a liquid or
matrix medium; and a plurality of encapsulated quantum dot
heterostructures (QDHs) disposed in the liquid or matrix medium,
each QDH comprising: a nanocrystalline core comprising a first
semiconductor material; a nanocrystalline shell comprising a
second, different, semiconductor material at least partially
surrounding the nanocrystalline core; and an encapsulating coating
surrounding the nanocrystalline core and nanocrystalline shell
pairing.
29. The optical downshifting material of claim 28, wherein the
material has an onset absorption wavelength approximately in the
range of 500-600 nm.
30. The optical downshifting material of claim 29, wherein the
material has a light emission wavelength greater than approximately
600 nm.
31. The optical downshifting material of claim 30, wherein the
material has a maximum absorption wavelength of approximately 480
nm, and has a maximum emission wavelength of approximately 620
nm.
32. The optical downshifting material of claim 28, wherein the
medium is a matrix medium configured for application to a receiving
surface in the form of a film.
33. The optical downshifting material of claim 32, wherein the
matrix is an inert polymer material matrix.
34. The optical downshifting material of claim 28, wherein the
medium is a liquid medium configured for application to a receiving
surface in the form of a solution.
35. The optical downshifting material of claim 28, wherein the
encapsulating coating passivates the nanocrystalline core and
nanocrystalline shell pairing of each QDH.
36. The optical downshifting material of claim 28, wherein the
encapsulating coating protects the nanocrystalline core and
nanocrystalline shell pairing of each QDH from the liquid or matrix
medium.
37. The optical downshifting material of claim 28, wherein the
encapsulating coating of each QDH is a silica coating.
38. The optical downshifting material of claim 28, wherein the
encapsulating coating of each QDH has a radius approximately in the
range of 1-50 nm.
39. The optical downshifting material of claim 28, wherein the
first semiconductor material of each QDH is selected from the group
consisting of zinc sulfide (ZnS) and cadmium sulfide (CdS).
40. The optical downshifting material of claim 28, wherein the
first semiconductor material of each QDH is cadmium selenide (CdSe)
and the second semiconductor material is cadmium sulfide (CdS).
41. The optical downshifting material of claim 40, wherein the CdSe
nanocrystalline core of each QDH has no dimension greater than
approximately 6 nm, and the CdS nanocrystalline shell has at least
one dimension greater than approximately 15 nm and a second
dimension approximately 1-2 nm thicker than a dimension of the CdSe
nanocrystalline core.
42. A method of fabricating an optical downshifting material, the
method comprising: forming a plurality of encapsulated quantum dot
heterostructures (QDHs), each QDH comprising: a nanocrystalline
core comprising a first semiconductor material; a nanocrystalline
shell comprising a second, different, semiconductor material at
least partially surrounding the nanocrystalline core; and an
encapsulating coating surrounding the nanocrystalline core and
nanocrystalline shell pairing; and suspending the plurality of QDHs
in a liquid or matrix medium.
43. The method of claim 42, wherein each QDH is formed by first
forming a micelle around each nanocrystalline core and shell
pairing and then growing the encapsulating coating inside of the
micelle.
44. The method of claim 42, wherein the medium is a matrix medium,
the method further comprising: applying, in the form of a film, the
matrix medium having the plurality of QDHs suspended therein to a
receiving surface of a device.
45. The method of claim 42, wherein the medium is a liquid medium,
the method further comprising: applying, in the form of a solution,
the liquid medium having the plurality of QDHs suspended therein to
a receiving surface of a device.
46. The method of claim 42, wherein suspending the plurality of
QDHs in the liquid or matrix medium comprises protecting, by the
encapsulating coating, the nanocrystalline core and nanocrystalline
shell pairing of each QDH from the liquid or matrix medium.
47. The method of claim 42, wherein forming each of the plurality
of encapsulated QDHs comprises forming a cadmium selenide (CdSe)
nanocrystalline core, forming a cadmium sulfide (CdS)
nanocrystalline shell at least partially surrounding the CdSe
nanocrystalline core, and forming a silica coating surrounding the
CdSe nanocrystalline core and CdS nanocrystalline shell pairing.
Description
RELATED APPLICATION
[0001] This application claims the benefit under 35 U.S.C.
.sctn.119(e) of U.S. Provisional Patent Application No. 61/225,472,
filed Jul. 14, 2009.
COPYRIGHT NOTICE
[0002] .COPYRGT. 2010 Spectrawatt, Inc. A portion of the disclosure
of this patent document contains material that is subject to
copyright protection. The copyright owner has no objection to the
facsimile reproduction by anyone of the patent document or the
patent disclosure, as it appears in the Patent and Trademark Office
patent file or records, but otherwise reserves all copyright rights
whatsoever. 37 CFR .sctn.1.71(d).
TECHNICAL FIELD
[0003] This disclosure relates to solar cell devices and, in
particular, to a solar cell having a photoabsorptive nanomaterial
layer that downshifts certain wavelengths of incident light into a
wavelength region that is better absorbed by the photovoltaic
material of the solar cell.
BACKGROUND INFORMATION
[0004] "First generation" solar cells are based on the use of
crystalline silicon as the photovoltaic material. These silicon
solar cells, which have light conversion efficiency of about
16-21%, currently have the highest light conversion efficiency of
solar cells in high-volume production (excluding high-cost III-V
inorganic cells intended for use in outer space). However, the cost
of electricity from these silicon solar cells is still higher in
dollar per watt than most currently available retail peak
electricity rates. The cost of electricity per watt generated by a
solar cell can generally be changed in either of two ways: the
light conversion efficiency of the solar cell can be increased, or
the cost of producing the solar cell can be decreased.
[0005] FIG. 1A is a graph showing the spectral irradiance versus
wavelength, and FIG. 1B shows the maximum fraction of general
spectral irradiance utilized by silicon. Due to the band gap of
silicon, the maximum fraction of light that can be used is about
44%. Due to thermodynamics and recombination inside the silicon
material, the maximum practical attainable light conversion
efficiency is about 26%.
[0006] The concept of enhancing solar cell performance by employing
spectral converters has been proposed. Downconversion techniques
are disclosed by Trupke, T. and Green, M. A., "Improving solar cell
efficiencies by down-conversion of high energy photons," Journal of
Applied Physics, v. 92, no. 3, Pgs. 1668-1674 (2002) and by
Richards, B. S., "Enhancing the performance of silicon solar cells
via the application of passive luminescence conversion layers,"
Solar Energy Materials & Solar Cells 90, Pgs. 2329-2337 (2006).
Downconversion generally refers to the absorption of one high
energy photon and the subsequent emission of multiple low energy
photons, such that overall photon energy is conserved.
[0007] Also, the concept of absorption and re-emission of one
photon, with the loss to heat of the difference in energy between
photons (hereafter referred to as "downshifting," not to be
confused with downconversion) has been proposed. Batchelder, J. S.
et al., "Luminescent Solar Concentrators: 1) Theory of Operation
and Techniques for Performance Evaluation," Applied Optics 18, Pgs.
3090-3110 (1979) and Currie, M. J., et al., "High Efficiency
Organic Solar Concentrators for Photovoltaics," Science 321, Pgs.
226-228 (2008) describe past wavelength conversion approaches,
which have been based largely on dyes that have very high
luminescence quantum efficiencies but have limited spectral
tunability and degrade rapidly. Such limited spectral tunability
and rapid degradation are discussed by Kinderman, R., et al., "I-V
Performance and Stability of Dyes for Luminescent Plate
Concentrators," Journal of Solar Energy Engineering 129, Pgs.
277-282 (2007). Dyes also typically have a very large overlap
between their absorption and emission spectrum, such that
self-absorption losses are significant.
[0008] Use of a CdSe quantum dots (QD) as a converting material has
been proposed by Van Sark, W. G. J. H. M., "Enhancement of Solar
Cell Performance by Employing Planar Spectral Converters," Applied
Physics Letters 87, 151117 (2005).
[0009] Other materials that have been tested as wavelength
converters include silicon nanocrystals, such as those described by
Svrcek, V., et al., "Silicon Nanocrystals as Light Converter for
Solar Cells," Thin Solid Films 451-452, Pgs. 384-388 (2004). These
materials also have disadvantages.
[0010] More cost-effective solar cell technology is, therefore,
still desirable.
SUMMARY OF THE DISCLOSURE
[0011] The light conversion efficiency of a solar cell can be
increased by employing a downshifting nanomaterial to supplement
the activity of a photovoltaic material.
[0012] In some of embodiments, a photovoltaic material is
wafer-based.
[0013] In some of embodiments, a photovoltaic material is tailored
to emphasize absorption in a wavelength region emitted by the
downshifting material.
[0014] In some of embodiments, the wavelength absorption range of a
photovoltaic material is relaxed in wavelength regions that are not
emitted by the downshifting material or in wavelength regions of
poor absorbance of the photovoltaic material.
[0015] In some embodiments, the solar cell platform is modified to
enhance collection or absorption of wavelength-specific photons
within the downshifting material at the expense of absorption of
the same wavelength-specific photons within the solar cell
platform.
[0016] In some of embodiments, the production cost of the solar
cell platform is favored over its light conversion efficiency.
[0017] In some embodiments, the downshifting material includes a
tunable nanomaterial.
[0018] In some embodiments, the nanomaterial is a quantum dot
heterostructure (QDH).
[0019] In some embodiments, the quantum dot heterostructure is a
multi-component nanocrystal which has been specifically tailored in
size, chemical composition, and shape to be a downshifting
material.
[0020] In some embodiments, the nanomaterial or quantum dot
heterostructure has a non-spherical shell.
[0021] In some embodiments, the nanomaterial or quantum dot
heterostructure is individually encapsulated.
[0022] In some embodiments, the downshifting material is
solution-processible, i.e., the material is created in solution and
can be applied as a liquid.
[0023] In some embodiments, the solution processed material is
stabilized in a matrix and the matrix is deposited as a film.
[0024] In some embodiments, a solar cell employing a downshifting
material is used as a low cost rooftop device.
[0025] In some embodiments, enhanced efficiency provided by the
optical downshifting layer permits advantageous modifications to
the solar cell platform that enhances its efficiency as well.
[0026] Additional aspects and advantages will be apparent from the
following detailed description of preferred embodiments, which
proceeds with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1A is a graph showing the spectral irradiance versus
wavelength.
[0028] FIG. 1B is a graph showing the spectral irradiance generally
available for collection and the fraction of the spectral
irradiance effectively absorbed by silicon.
[0029] FIG. 1C is a graph showing the spectral irradiance generally
available for collection, the fraction of the spectral irradiance
effectively absorbed by silicon, and an exemplary fraction of the
spectral irradiance that could be used for downshifting.
[0030] FIG. 2A is an enlarged simplified cross-sectional view of an
exemplary solar cell including a photovoltaic material and an
optical downshifting layer.
[0031] FIG. 2B is an enlarged portion of the cross-sectional view
of FIG. 2A showing absorption and emission occurring in the optical
downshifting layer.
[0032] FIG. 3 is simplified enlarged drawing of an encapsulated
nanomaterial, such as a quantum dot heterostructure.
[0033] FIG. 3A is a TEM image of an exemplary CdSe/CdS quantum dot
heterostructure.
[0034] FIG. 3B is a TEM image of an exemplary CdSe/CdS quantum dot
heterostructure grown to a different aspect ratio than that of FIG.
3A.
[0035] FIG. 3C is a TEM image of exemplary encapsulated quantum dot
heterostructures: CdSe dot cores having rod-shaped CdS shells
individually encapsulated in a silica encapsulating material.
[0036] FIG. 3D is a TEM image of an exemplary manganese-doped ZnSe
quantum dots.
[0037] FIG. 4 is a graph of absorption and emission versus
wavelength, showing absorption and emission by exemplary quantum
dot heterostructures.
[0038] FIG. 5 is a graph of exemplary external quantum efficiency
(EQE) of a silicon solar cell versus wavelength, showing the EQE
differences between solar cells with and without an optical
downshifting layer.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0039] Within the fixed portion spectral irradiance from the Sun,
silicon has a region of light conversion activity from about 0.3
micron to about 1.2 microns due to the 1.1 eV band gap of silicon
for absorbing incident photons and generating electron and hole
charge carriers, as shown in FIG. 1B. For silicon, this region is
characterized at least in part by a primary light absorption
spectrum from about 0.6 micron to about 1 micron, in which silicon
effectively absorbs photons and generates electron and hole charge
carriers. FIG. 1B shows, however, that silicon is far less
effective at absorbing sunlight in about the 0.2 micron to almost
0.6 micron spectral region. Other photovoltaic materials exhibit
areas of the spectral region that are less effective at absorbing
sunlight than in their respective primary light absorption
spectra.
[0040] For a typical solar cell platform using conventional
photovoltaic material and having a limited primary light absorption
spectrum, the efficiency of the solar cell platform can be
increased by adding a spectral shifting layer. The spectral
shifting layer shifts incident spectral radiation having
wavelengths in a supplemental light absorption spectrum, which is
substantially outside of the primary light absorption spectrum, to
photons having wavelengths in the primary light absorption
spectrum. The spectral shifting layer is also transmissive to
incident photons having wavelengths in the primary light absorption
spectrum and allows them to pass into the photovoltaic material so
that the photovoltaic material can directly absorb these incident
photons.
[0041] For example, in some embodiments, the spectral shifting
layer is an optical downshifting layer that can absorb photons
having higher band gap energies (shorter wavelengths) that are not
readily absorbed by the photovoltaic material and emit photons
having lower band gap energies (longer wavelengths) within primary
light absorption spectrum that are readily absorbed by the
photovoltaic material. Thus, a solar cell can be made to absorb the
spectral irradiance more efficiently by employing such a spectral
shifting layer in conjunction with the photovoltaic material. FIG.
1C is a graph showing the spectral irradiance generally available
for collection, the fraction of the spectral irradiance effectively
absorbed by silicon, and an exemplary fraction of the spectral
irradiance that could be used by an optical downshifting layer.
[0042] FIG. 2A is an enlarged simplified cross-sectional view of an
exemplary solar cell 10 including a solar cell platform 20 having a
photovoltaic material 22 positioned between an emitter electrode 24
and a rear contact electrode 26, which electrodes are spaced-apart
from each other. (Emitter electrode 24 may also be referred to as
top, upper, front, or sun-facing electrode 24, and rear contact
electrode 26 may also be referred to as bottom, lower, back, or
earth-facing electrode 26.) Photovoltaic material 22 includes at
least one charge-separating junction 27 where differently doped
regions of bulk photovoltaic material 22 meet. Although p-n type
charge-separating junctions 27 are most common, n-p type, p-i-n
type, and other type junctions may be employed. Skilled persons
will also appreciate that even though charge-separating junction 27
is shown as a line (with a planar interface texture and no volume),
charge-separating junction 27 may have a nonplanar interface
texture and may include a volume (including a height) of bulk
material due to doping gradients. In some embodiments, photovoltaic
material 22 and emitter and rear contact electrodes 24 and 26 form
respective charge-separating junction interfaces 28 and 29 for
separating the electron and hole charge carriers for collection by
emitter electrode 24 and rear contact electrode 26.
[0043] With reference to FIG. 2A, emitter electrode 24 directly or
indirectly supports an optical downshifting layer 30. In some
embodiments, optical downshifting layer 30 is deposited directly on
a textured external surface 34 of emitter electrode 24. In some
embodiments, one or more supplemental layers 32, such as an
anti-reflective coating (ARC), may be positioned on or applied to
external surface 34 of emitter electrode 24 between emitter
electrode 24 and optical downshifting layer 30. Front grid contacts
38 may extend from emitter electrode 24 through supplemental layers
32 and optical downshifting layer 30 to convey electrical current
from emitter electrode 24 to the surface of solar cell 10.
[0044] FIG. 2B is an enlarged portion of the cross-sectional view
of FIG. 2A, showing absorption and emission occurring in optical
downshifting layer 30. The components of optical downshifting layer
30 are not drawn to scale. In particular, optical downshifting
layer 30 absorbs a photon 50 having a wavelength in the
supplemental light absorption spectrum of photovoltaic material 22
and emits a photon 52 having a wavelength in the primary light
absorption spectrum of photovoltaic material 22.
[0045] Photovoltaic material 22 can be any material that actively
absorbs light and converts it to electricity. For example,
photovoltaic material 22 can be any conventional photovoltaic
material such as a first, second, or third generation photovoltaic
material. Exemplary photovoltaic materials 22 include, but are not
limited to, crystalline silicon (c-Si) or multicrystalline silicon
(mc-Si); nanocrystalline silicon; amorphous silicon (a-Si);
micromorphous silicon; gallium arsenide (GaAs); InP; InAs;
combinations of GaAs, InP, or InAs; other III-V-based photovoltaic
materials; cadmium telluride (CdTe), copper indium selenide (CIS),
copper indium gallium di-selenide (CIGS), active organic
photovoltaic materials, dye-sensitized solar cell (DSC) materials,
and active quantum dot (QD) ensembles. Quantum dot ensembles are
disclosed in detail in U.S. patent application Ser. No. 12/606,908,
entitled Solar Cell Constructed with Inorganic Quantum Dots and
Structured Molecular Contacts, which is herein incorporated by
reference. In some embodiments, photovoltaic material 22 forms a
wafer-based solar cell platform 20.
[0046] In some embodiments, any photovoltaic material 22 having
limited spectral activity in the blue wavelength region could be
employed. In some embodiments, silicon-based photovoltaic materials
22 have a primary light absorption spectrum from about 0.6 micron
to about 1 micron. In some embodiments, CdTe-based photovoltaic
materials 22 have a primary light absorption spectrum from about
500 nm to about 900 nm.
[0047] In terms of efficiency, a photovoltaic material 22
preferably has a light conversion efficient of at least 10%. In
some embodiments, a lower light conversion efficiency may be
adequate if the production costs are low. In some embodiments, a
moderately or highly efficient photovoltaic material 22 is
preferred, especially in area-constrained applications. In
particular, some preferred photovoltaic materials 22 have light
conversion efficiency of greater than 15%.
[0048] In some embodiments, photovoltaic material 22 is selected
based on low production costs. In many embodiments, photovoltaic
material 22 is selected based on the lowest production cost for the
highest efficiency. In many embodiments, other selection factors,
such as longevity and reliability, as well as primary absorption
spectrum, are also or alternatively considered for choosing a
suitable photovoltaic material 22. For example, a typical solar
cell module based on silicon-containing solar cell platforms has a
25-year warranty. Thus, desirable longevity for photovoltaic
material 22 may be even longer.
[0049] Because the effect of downshifting layer 30 is additive, the
light conversion efficiency of photovoltaic material 22 is not
critical. Although practical optimized light-conversion efficiency
is preferred, even a photovoltaic material 22 with non-optimal
light conversion efficiency can be used as a platform. Such
platforms with suboptimal efficiency may be intentionally selected
by design or not. For example, a solar cell platform 20 having a
photovoltaic material 22 generated in a screen-printed process can
be used because the blue part of the spectrum, where a
screen-printed solar cell platform 20 is most inefficient, will be
downshifted by downshifting layer 30.
[0050] In some embodiments, optical downshifting layer 30 has a
sharp, tunable absorption onset at a wavelength where the external
quantum efficiency (EQE) of photovoltaic material 22 begins to
drop. In some embodiments, the supplemental light absorption
spectrum is substantially outside of the primary light absorption
spectrum of the active solar device so as to avoid reduction of
light conversion activity of the photovoltaic material in the
primary light absorption spectrum of the active solar device. In
some embodiments, the majority of the supplemental light absorption
spectrum is outside of the primary light absorption spectrum. In
some embodiments, the supplemental light absorption spectrum is
entirely outside of the primary light absorption spectrum.
[0051] In some embodiments, the onset absorption wavelength falls
between 500 nm and 600 nm. Thus, in some examples, the supplemental
light absorption spectrum of optical downshifting layer 30 may
include wavelengths shorter than or equal to 600 nm. In some
embodiments, the tunable absorption spectrum ranges downward to
include the lower range of significant spectral irradiance. In some
embodiments, the tunable absorption spectra ranges downward to
include wavelengths of 300 nm or shorter. In some examples, the
supplemental light absorption spectrum of optical downshifting
layer 30 may include wavelengths longer than or equal to 200
nm.
[0052] In some embodiments, optical downshifting layer 30 has an
emission spectrum that is separated from the supplemental light
absorption spectrum such that the emitted light is not reabsorbed
in optical downshifting layer 30. In some embodiments, the emission
spectrum is separated from the supplemental light absorption
spectrum by greater than 50 nm or by an energy gap of greater than
0.5 eV. In some embodiments, optical downshifting layer 30 has an
independently tunable emission spectrum. In some embodiments,
optical downshifting layer 30 has an emission spectrum that emits
photons having a wavelength greater than 600 nm. In some
embodiments, optical downshifting layer 30 has an emission
efficiency of greater than 90%. In some embodiments, optical
downshifting layer 30 is simple and inexpensive to lay down on top
of the solar cell platform 20, using a quick, solution-based
process at ambient pressure.
[0053] In some embodiments, optical downshifting layer 30 includes
one or more nanomaterials to efficiently absorb in the supplemental
light absorption spectrum and emit photons in the primary light
absorption spectrum of a given photovoltaic material 22, i.e., the
nanomaterial forming optical downshifting layer 30 absorbs photons
where the spectral absorbance of photovoltaic material 22 is low
and reemits photons at a wavelength where the spectral absorbance
of photovoltaic material 22 is high.
[0054] Nanomaterials are materials with at least one nano-scale
dimension, are often grown colloidally, and have been made in the
form of dots, rods, tetrapods, and even more exotic structures.
(See Scher, E. C.; Manna, L.; Alivisatos, A. P. Philosophical
Transactions of the Royal Society of London. Series A:
Mathematical, Physical and Engineering Sciences 2003, 361, 241 and
Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos,
A. P. Nat Mater 2003, 2, 382-385.) Their sizes generally range from
3 nm to 500 nm. Due to the quantum size effects which arise from a
material having dimensions on the order their electron's bohr
radius, the band gap of the material can also be tuned (See
Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226-13239 and
Bawendi, M. G.; Steigerwald, M. L.; Brus, L. E. Annual Review of
Physical Chemistry 1990, 41, 477-496.) In addition to facilitating
tunability of the band gap for absorption and emission, the
nanomaterials often have near perfect crystallinity, allowing them
to attain extremely high photoluminescence (See Talapin, D. V.;
Nelson, J. H.; Shevchenko, E. V.; Aloni, S.; Sadtler, B.;
Alivisatos, A. P. Nano Lett. 2007, 7, 2951-2959 and Xie, R.;
Battaglia, D.; Peng, X. J. Am. Chem. Soc. 2007, 129,
15432-15433.)
[0055] The nanomaterials can be suspended in an inert polymer
matrix material that could also be used as the encapsulating
material for solar cell platform 20. Nanomaterials are highly
suitable for use as optical downshifting layer 30 and offer serious
advantages over dyes. The nanomaterials are solution processible,
highly controllable semiconductor nanostructures synthesized by
low-cost solution-based methods and can be made to have the exact
optical properties desired for optical downshifting layer 30.
Because of their unique structure and composition, nanomaterials
can be more stable than dyes.
[0056] Semiconductor nanocrystals have unique physical and
electronic properties due to their quantum confinement. Because of
their high surface area to volume ratios, the optical and
electrical properties of semiconductor nanocrystals are highly
governed by their surface defects. In a homogeneous dispersion of
nanocrystals in a matrix, there is a strong nanocrystal-nanocrystal
interaction induced by the hydrophobicity of the organic ligand,
possibly leading to aggregation.
[0057] In some embodiments, optical downshifting layer 30 includes
nanomaterials, particularly quantum dot heterostructures (QDHs),
encapsulated discretely by secondary materials through a micelle
approach. FIG. 3 is simplified enlarged drawing of an encapsulated
quantum dot heterostructure 60. With reference to FIG. 3,
encapsulated quantum dot heterostructure 60 includes a quantum dot
heterostructure having a core 62 surrounded by one or more shells
64. Shell 64 is further encapsulated by an encapsulating material
66, such as a silica sphere.
[0058] By discretely encapsulating each semiconductor nanocrystal,
it is possible to homogeneously disperse the nanocrystals in a
matrix media, as well as protect the nanocrystal surface from the
external environment. Therefore, the introduction of the
encapsulating materials 66 greatly helps to both passivate
nanocrystal surface defects and isolate the individual nanocrystals
for better dispersion. Thus, the encapsulating materials 66
minimize the interaction among the nanocrystals, improving the
stability as well as the homogeneity in a matrix media.
[0059] For such optical downshifting materials 30, outer
encapsulating materials 66 can be grown on individual nanocrystals
non-epitaxially. Micelles are formed using a pair of polar and
non-polar solvents in the presence of a compatible surfactant. The
surface polarity of a nanocrystal can be modified so that only a
single nanocrystal will reside in an individual micelle.
Subsequently, an inorganic or organic encapsulating material 66 may
be selectively grown inside of the micelle, which acts as a
spherical template. By adding additional precursors, an inorganic
or polymeric encapsulating material 66 can be further grown on the
nanocrystal. (See Selvan, S. T.; Tan, T. T.; Ying, J. Y. Adv.
Mater., 2005, 17, 1620-1625; Zhelev, Z.; Ohba, H.; Bakalova, R. J.
Am. Chem. Soc., 2006, 128, 6324-6325; and Qian, L.; Bera, D.;
Tseng, T.-K.; Holloway, P. H. Appl. Phys. Lett., 2009, 94,
073112.)
[0060] Thus, by tuning the synthetic conditions, a single
nanocrystal can be incorporated in a silica sphere as shown in FIG.
3 (and FIG. 3C). Throughout the encapsulation process, the
nanocrystal surfaces are well passivated to avoid aggregation.
Additionally, this passivation endows the nanocrystals with
photoluminescence quantum yields of and near unity. For
encapsulated nanocrystals, the matrix compatibility can be
dependent on the surface of the encapsulating sphere, not the
nanocrystal. Since the surface of the encapsulating material 66 is
spatially removed from the nanocrystal surface, alterations to the
exterior of the encapsulating material 66 do not adversely affect
the electronic or optical properties of the nanocrystal.
[0061] Semiconductor nanocrystals, such as cadmium selenide or
indium phosphide, have widely been studied for control over both
their composition and shape. (See Scher, E. C.; Manna, L.;
Alivisatos, A. P. Philosophical Transactions of the Royal Society
of London. Series A: Mathematical, Physical and Engineering
Sciences 2003, 361, 241 and Talapin, D. V.; Rogach, A. L.;
Shevchenko, E. V.; Kornowski, A.; Haase, M.; Weller, H. J. Am.
Chem. Soc 2002, 124, 5782-5790.)
[0062] Thus, in addition to spherically-shaped nanostructures,
various non-spherical nanostructures have been demonstrated
including, but not limited to, nanorods, nanotetrapods, and
nanosheets. Non-spherical semiconductor nanocrystals have different
physical and electronic properties from those of spherical
semiconductor nanocrystals. These properties can be employed
advantageously in downshifting optical layer 30.
[0063] In some embodiments, the nanomaterials are quantum dot
heterostructures. A quantum dot heterostructure is a nanomaterial
which has been specifically engineered, including but not limited
to, tailoring in size, chemical composition, shape, optical, and/or
electrical properties, to perform a particular function. In
particular, the quantum dot heterostructures can be multi-component
nanocrystals tailored for downshifting applications.
[0064] In some embodiments, optical downshifting material 30 may
include individually encapsulated nanomaterials, particularly
quantum dot heterostructures, employing one type of core material,
one type (composition) of shell material, and one shape of shell
material. In some embodiments, optical downshifting material 30 may
include individually encapsulated quantum dot heterostructures,
particularly quantum dot heterostructures, employing two or more
varieties of individually encapsulated quantum dot
heterostructures, such as a first type of individually encapsulated
quantum dot heterostructure, employing a first type of core
material, a first type of shell material, and a first shape of
shell material and a second type of individually encapsulated
quantum dot heterostructure, employing the first type of core
material, the first type of shell material, and at least one or
more different shapes of shell material, such as rods and
tetrapods.
[0065] In some embodiments, the second type of individually
encapsulated quantum dot heterostructure employs a first type of
core material, at least one or more different types of shell
material, such as ZnS or CdS, and the first or at least one or more
different shapes of shell materials. In such embodiments, each
shell material may be associated with a specific shape, or each
shell material may be formed with a plurality of shapes. In some
embodiments, the second type individually encapsulated quantum dot
heterostructures employs at least one or more different types of
core materials, the first or one or more different types of shell
materials, and the first or one or more different types of shell
shapes. In such embodiments, each core material may be associated
with specific shell materials and/or shapes, or each core material
may be associated with one or more shell materials and/or
shapes.
[0066] As described above, some advantages of employing a quantum
dot heterostructure as an optical downshifting layer 30 include an
increased solar cell efficiency in the blue part of the solar
spectrum, where typical photovoltaic materials 22 have a low
quantum efficiency. The QDH optical downshifting layer 30 also
provides a decreased thermal load in the solar cell platform 20
because the blue photons are absorbed above and never reach
photovoltaic material 22, where most of them would be converted to
heat inside solar cell platform 20.
[0067] In one example, the onset of absorption and separation
between absorption and emission peaks can be tuned by 0.5 eV or
more, enabling long wavelength light to pass through unhindered to
solar cell platform 20 beneath and minimizing re-absorption within
optical downshifting layer 30. Pradhan, N., Peng, X., "Efficient
and Color-Tunable Mn-Doped ZnSe Nanocrystal emitters: Control of
Optical Performance via Greener Synthetic Chemistry," J. Am. Chem.
Soc., 129 (11), Pgs. 3339-3347 (2007); Talapin, D. V., Nelson, J.
H., Shevchenko, E. V., Aloni, S., Sadtler, B., Alivisatos, A. P.,
"Seeded Growth of Highly Luminescent CdSe/CdS Nanoheterostructures
with Rod and Tetrapod Morphologies," Nanoletters v. 7, no. 10 Pgs.
2951-2959 (2007); and Sholin, V., Olson, J. D., Carter, S. A.
"Semiconducting polymers and quantum dots in luminescent solar
concentrators for solar energy harvesting," Journal of Applied
Physics, v. 101, no. 12, Pg. 123114 (2007) describe methods for
tuning nanomaterials.
[0068] In some embodiments, the quantum dot heterostructures can
include following inorganic compounds and/or any combination of
alloys between them: CdSe, CdS, CdTe, ZnSe, ZnS, ZnTe, CuS.sub.2,
CuSe.sub.2, In.sub.2S.sub.3, In.sub.2Se.sub.3, CuGaSe.sub.2,
CuGaS.sub.2, CuInS.sub.2, CuInSe.sub.2, PbSe, PbS, SiO.sub.2,
TiO.sub.2, ZnO, ZrO. These materials can be arranged in cores 62,
core-shells, and core-shell-shells, with or without organic
ligands, such as phosphonic acids, carboxylic acids, amines.
[0069] In some examples, quantum dot heterostructures including
CdSe, CdSe/ZnS, CdSe/CdS, or CdTe have provided very high
luminescence. Quantum dot heterostructures based on the II-VI
chalcogenides are very well understood as high efficiency emitters.
In solution, the quantum dot heterostructures particles have
quantum efficiencies as high as 95%. The quantum dot
heterostructure materials may be distributed in matrices of
polydimethylsiloxane, polyvinylbutyral, or ethylvinylacetate, for
example, and may be incorporated into the encapsulating material
and cover both emitter electrode 24 and rear contact electrode
26.
[0070] FIG. 3A is a transmission electron microscopy (TEM) image of
an exemplary CdSe/CdS quantum dot heterostructure. FIG. 3B is a TEM
image of an exemplary CdSe/CdS quantum dot heterostructure grown at
a temperature lower than that of FIG. 3A. In some embodiments, the
core 62, such as a CdSe core 62 may have no physical dimension
greater than 6 nm, while the extended shell 64, such as a CdS
extended shell 64, has at least one dimension greater than 15 nm
and another dimension that is about 1-2 nm thicker than the CdSe
core 62. Thus, the thickness of the shell 64 may be as thin as 1 or
2 nm along some axes and may be greater than 9 nm along other axes.
The casing of the oxide encapsulating material 66 can range between
1 and 50 nm in radius.
[0071] In some embodiments, quantum dot heterostructure materials
of optical downshifting layer 30 exhibit a tunable onset of
absorption at a wavelength of around 600 nm, with a range of
approximately 50 nm. In all cases, the absorption rapidly increases
moving to the ultraviolet. The gap between absorption onset and
peak emission in the quantum dot heterostructure materials of
optical downshifting layer 30 may also be tuned between 50 nm to
200 nm in separation.
[0072] In some embodiments, quantum dot heterostructure materials
of optical downshifting layer 30 exhibit supplemental light
absorption ranges having supplemental light absorption spectrum
with a maximum absorption wavelength at about 600 nm and a minimum
absorption wavelength in the ultraviolet. In some embodiments, the
supplemental light absorption spectrum of the QDH materials covers
a range of at least 250 nm. In some embodiments, the range of the
supplemental light absorption spectrum may be shorter, and the
maximum absorption wavelength may also be shorter to provide for a
gap between absorption and emission in the QDH materials of optical
downshifting layer 30. In some embodiments, the minimum emission
wavelength is about 50 to 200 nm longer than the maximum absorption
wavelength.
[0073] In one example, optical downshifting layer 30 having quantum
dot heterostructure material absorbs photons at a maximum
absorption wavelength of about 480 nm and emits photons at a
minimum emission wavelength of about a 620 nm wavelength. In some
embodiments, the emission will be slightly shifted more into to the
red spectrum. For example, using a CdTe core material instead of
CdSe provides red shifted emission.
[0074] In some examples, optical downshifting layers 30, including
a quantum dot heterostructure of CdSe quantum dots with a graded
shell of CdS that transitions into ZnS, have maximum absorption
wavelengths between about 500 and 700 nm and have supplemental
light absorption spectrums extending to shorter wavelengths. In
some embodiments, the minimum emission wavelength is about 50 to
100 nm longer than the maximum absorption wavelength. In one
example, optical downshifting layer 30, having a CdSe-based quantum
dot heterostructure material, has a maximum absorption wavelength
of about 450 nm and a minimum emission wavelength of about 550 nm,
with a re-emission efficiency of greater than 80%.
[0075] In some examples, optical downshifting layer 30 includes
quantum dot heterostructures of CdSe quantum dots with a rod-shaped
CdS shell 64, encapsulated in a silica encapsulating material 66.
This quantum dot heterostructure material exhibits maximum
absorption at wavelengths shorter than 500 nm and maximum emission
at wavelengths between 550-700 nm. The silica shell serves the dual
purpose of enhancing the passivation of the nanocrystals for
increased photoluminescence re-emission efficiency, while also
preventing nanocrystal aggregation and increasing the
dispersability of the nanocrystal in the downshifting layer matrix.
FIG. 3C is a TEM image of exemplary quantum dot heterostructures of
CdSe quantum dot cores 62 having rod-shaped CdS shells 64
individually encapsulated in a silica encapsulating material
66.
[0076] In some embodiments, optical downshifting layer 30 includes
CdSe quantum dots with a rod-shaped CdS shell 64, which is further
covered by a second shell 64 of ZnSe, which is encapsulated in a
silica encapsulating material 66.
[0077] Non-Cd based quantum dots can also be employed. In one
example, doped ZnSe has been used in matrices of
polydimethylsiloxane, polyvinylbutyral, or ethylvinylacetate. In
some examples, the ZnSe is doped with manganese. FIG. 3D is a TEM
image of an exemplary is manganese-doped ZnSe quantum dot
heterostructure.
[0078] In some examples, ZnSe quantum dots have a maximum
absorption wavelength between 350 and 450 nm. The minimum emission
wavelengths largely depend on the nature of the particular
structures but range from a shift of about 100 to 200 nm longer
than the maximum absorption wavelength. In some examples, InP
quantum dot heterostructure materials have a maximum absorption
wavelength ranging from 500 to 750 nm, with minimum emission
wavelengths that are longer.
[0079] FIG. 4 is a graph of absorption and emission versus
wavelength, showing exemplary quantum dot heterostructure activity
with absorption in a solid line and emission in a dashed line.
Skilled persons will appreciate that the concept of employing
quantum dot heterostructures for use as downshifting layer 30 is
not dependent on the specific morphology of the solar cells
platforms 20 or its junctions. The labeling on the left of the
graph shows relative absorbance, and the labeling on the right of
the graph shows relative photoluminescence.
[0080] To illustrate the expected efficiency increase due to
downshifting, the losses associated with downshifting have been
calculated to determine that a maximum EQE of about 80% is
achievable in the absorbing window of optical downshifting layer
30. Light emitted within the escape cone of optical downshifting
layer 30 is primarily responsible for the 80% limit.
[0081] FIG. 5 is a graph of exemplary EQE versus wavelength,
showing the EQE differences between a solar cell with and without
optical downshifting layer 30. The thick line shows the typical EQE
response of a multicrystalline solar cell platform 20 (made at ECN
in the Netherlands) in the absence of optical downshifting layer
30, and the thin line shows the EQE in the presence of optical
downshifting layer 30.
[0082] In one embodiment, an optimal design will choose an optical
downshifting layer 30 with an absorption onset (maximum absorption
wavelength) at the wavelength the EQE of photovoltaic material 22
drops below 80%. In one example, the photovoltaic material 22 is
multicrystalline silicon. Consequently, the EQE would be 80% or
higher at the low end of the primary light absorption spectrum of
multicrystalline silicon. The example shown in FIG. 5 presents an
increased conversion efficiency of about 0.5% absolute, due solely
to the downshifting effect.
[0083] While a 0.5% increase in conversion efficiency may appear to
be a minimal improvement, incorporation of optical downshifting
layer 30 allows the design rules for solar cell platform 20 and its
photovoltaic material 22 to be relaxed, such as by decoupling the
short wavelength response from the long wavelength response.
[0084] Skilled persons will appreciate that the use of a
blue-absorbing optical downshifting layer 30 will work
advantageously with a cell technology whose external quantum
efficiency drops sharply in the blue. Traditionally, solar cell
platforms 20 based on silicon photovoltaic materials 22 have been
optimized to have a spectral response over the widest possible
wavelength range at the expense of wavelength specific response.
So, by attempting to capture some blue light with these
conventional photovoltaic materials 22 or conventional solar cell
platforms 20, the resulting conventional solar cell platforms 20
have diminished capacity in red regions of their spectral
response.
[0085] By employing an optical downshifting layer 30 that performs
best at short wavelengths, a solar cell designer can tailor the
solar cell platform 20 or its photovoltaic material 22 to increase
the response in certain visible and near infrared (NIR) wavelengths
at the expense of its performance in the blue.
[0086] In some embodiments, the structure of multicrystalline
silicon in solar cell platform 20 can be modified in order to
optimally pair it with a blue-absorbing optical downshifting layer
30.
[0087] In some embodiments, at least a 1-2% absolute gain in light
conversion efficiency is possible with small modifications, such as
lowering the resistance of the emitter electrode 24 to the front
grid contact 38, and such as tuning the optics of solar cell
platform 20 to respond better to longer wavelength light.
Therefore, an optimally integrated solar cell 10 having a
blue-absorbing optical downshifting layer 30 and a modified
photovoltaic material 22 (or its environment) can have a light
conversion efficiency increase of greater than 1% absolute. This
increase can be accomplished at an additive cost of 8-10 cents for
each wafer, for example. The concurrent increase in efficiency and
power output would, however, lower the overall cost per watt of a
standard multicrystalline silicon solar cell 20, for example, by
roughly 40 cents per watt.
[0088] In some embodiments, external surface 34 of emitter
electrode 24 of solar cell platform 20 is modified to increase the
light conversion efficiency in the primary absorbance spectrum. In
some examples, the doping level (such as total amount of dopant) of
emitter electrode 24 is modified (typically increased) to reduce
series resistance. In some examples, the emitter electrode 24 can
possess a sheet resistance of order 5-30 Ohms/square (such as a
surface doping concentration of greater than 1e.sup.20/cm.sup.3 and
an emitter thickness ranging from 1.0-0.5 .mu.m), representing a
drastic reduction from the level of 65-110 Ohms/square (such as a
surface doping concentration of 1-2e.sup.20/cm.sup.3 to about
5e.sup.19/cm.sup.3 and an emitter thickness ranging from 0.4-0.2
.mu.m) for conventional solar cells that do not have an optical
downshifting layer 30. For example, higher phosphorous surface
concentration results in a lower specific contact resistance for a
tunneling based carrier transport mechanism. A thicker 5-30 ohms/sq
emitter would not only lead to reduced solar cell shunting issues
and hence a higher yield but also provide a wider solar cell
process window. It is also noted that dopant density at the emitter
surface decreases toward the p-n junction. For example, a
phosphorus density of about 2e.sup.20/cm.sup.3 at the emitter
surface decreases to about 1e.sup.17/cm.sup.3 at the p-n
junction.
[0089] In some examples, the increased doping level emitter
electrode 24 may reduce the blue response of solar cell platform 20
because a more highly doped emitter electrode 24 may become less
transmissive to blue photons. However, the blue photons, which are
absorbed by optical downshifting layer 30 at a much greater
efficiency than they can be absorbed by photovoltaic material 22,
are downshifted into the primary absorbance spectrum. The
downshifted photons are not significantly adversely affected by the
increased doping emitter electrode 24 and make their way into
photovoltaic material 22, where they are readily absorbed and
converted in holes and carriers that are captured by emitter
electrode 24 (and/or rear contact electrode 26). In some examples,
a gas or liquid mixture containing phosphorous can be used to dope
p-type Si and convert it to n-type emitter electrode 24.
Conventional thicknesses of the emitter electrode are 200-400 nm,
but in embodiments employing optical downshifting layer 30, the
emitter electrode 24 can be as little as 200-400 nm, but also
thicker than 400 nm and as thick as 1 micron. In particular, higher
doping can easily yield specific contact resistance values that are
less than 3 mohm-cm2 to achieve good fill factors.
[0090] Skilled persons will appreciate that doping and other
parameters of rear contact electrode 26 may also be modified.
[0091] In some embodiments, the doping level of emitter electrode
24 is modified (typically increased) and the gap between front grid
contacts 38 is increased, thereby decreasing the fractional shading
of the underlying layers. The increased doping level of emitter
electrode 24 compensates for increased resistivity incurred by the
increased gap between front grid contacts 38, so the overall
resistance of the solar cell platform 20 is about the same (or not
significantly adversely affected). However, the increased gap
between front grid contacts 38 reduces shading of photovoltaic
material 22 so that more of it is exposed to ambient light, thereby
increasing the capture efficiency of photons incident on solar cell
10. For example, the spacing between grid contacts 38 can be
increased from roughly 2 mm to greater than 3 mm. This increased
gap has the effect of decreasing the number of grid contacts 38
needed and thereby decreasing the grid electrode shading percentage
from greater than 6% to less than 5%. Although the increased doping
of emitter electrode 24 decreases the response of the solar cell
platform 20 to blue wavelengths, for example, optical downshifting
layer 30 more than adequately compensates for the loss, as
previously explained.
[0092] The width 80 of front grid contacts 38 can additionally or
alternatively be reduced to reduce the shading of photovoltaic
material 22 so that more of it is exposed to ambient light, thereby
increasing the capture efficiency of photons incident on solar cell
10. The increased doping level of emitter electrode 24 can
compensate for increased resistivity incurred by the decreased
cross-sectional area of front grid contacts 38, so the overall
resistance of the solar cell platform 20 is about the same (or not
significantly adversely affected).
[0093] In some embodiments, other aspects of front grid contacts 38
are changed, such as configuration, spacing, height, or other
features, to optimize contact resistance. For example, the height
of front grid contacts 38 can be reduced to decrease shading of
photovoltaic material 22 by offsetting the decreased resistivity
with increased doping of emitter electrode 24, such as previously
described.
[0094] With reference again to FIGS. 2A and 2B, a supplemental
layer (not shown) or coating material that is reflective to some or
all of the wavelengths falling outside the primary light absorption
spectrum may be applied to external surface 34 of emitter 24. Thus,
photons 52 with wavelengths in the supplemental light absorption
spectrum that reach external surface 34 or the supplemental layer
may be reflected back into, and thereby have a second opportunity
to be absorbed by, optical downshifting layer 30.
[0095] In some embodiments, rear contact electrode 26 has an
external surface 56 that may be coated with a material that is
reflective to one or both of the primary light absorption and the
supplemental absorption spectrum. Thus, in these spectra the
photons that reach rear contact electrode 26 can be reflected back
into photovoltaic material 22 and optical downshifting layer 30
with an opportunity to be absorbed.
[0096] In some embodiments, the supplemental layer, such as a
passivation layer, can be added above optical downshifting layer 30
to act in place of or in addition to, supplemental layer 32,
allowing optical downshifting layer 30 to act as an anti-reflective
coating (ARC) instead of nitride, for example. The anti-reflective
coating tool is currently the most expensive piece of equipment in
the production line of standard solar cell platforms 20 and
requires the most maintenance and associated infrastructure.
Replacing the traditional anti-reflective coating with a low cost
layer would measurably impact the cost of the producing solar cell
platform 20 and the overall cost of solar cell 10. The passivation
layer associated with optical downshifting layer 30 can replicate
the effect of the anti-reflective coating to avoid the loss of
efficiency otherwise afforded by the anti-reflective coating.
[0097] In some embodiments, a quantum dot heterostructure optical
downshifting layer 30 can, itself, offer a lower reflectance and
enhanced anti-reflective properties to solar cell platform 20 due
to employment of an intermediate index of refraction in the matrix
material containing the quantum dot heterostructures.
[0098] Additional processing and material costs for thin film
quantum dot heterostructures as described above are low due to the
low-temperature, solution-based deposition processes utilized.
Therefore, the price of electricity per watt from solar cells 10
having a QDH optical downshifting layer 30 is substantially lower
than the price of those without one.
[0099] Finally, lifetime concerns associated with the quantum dot
heterostructure materials for optical downshifting layer 30 are
substantially mitigated, since along with photovoltaic material 22,
optical downshifting layer 30 undergoes the standard and
well-understood encapsulation process into modules. As long as the
QDH optical downshifting materials themselves are fundamentally
stable, the solar cell 10 will have an operating lifetime that is
comparable to the operating lifetimes (30+ years) of conventional
silicon solar cell platforms 20.
[0100] A typical solar cell module based on silicon-containing
solar cell platforms 20 has a 25-year warranty, so any additive or
replacement material preferably has a similar longevity or at least
a comparable one. Alternatively, the additive material can be
shorter lived if it provides adequate benefit during its lifespan
and is not detrimental thereafter. For example, if optical
downshifting layer 30 lasts only 10 years, but contributes
meaningfully to the power generation for those 10 years, such
lifetime would be sufficient as long as optical downshifting layer
did not block (such as become nontransmissive to wavelengths in the
primary absorption spectrum) the solar cell platform 20 from
performing its function after the first ten years. However, in the
context of 25 years, a 1-2 year lifetime would not long be enough
for most embodiments.
[0101] It will be obvious to those having skill in the art that
many changes may be made to the details of the above-described
embodiments without departing from the underlying principles of the
invention. For example, skilled persons will appreciate that
subject matter revealed in any sentence, paragraph, or embodiment
can be combined with subject matter from some or all of the other
sentences, paragraphs, or embodiments except where such
combinations are mutually exclusive or inoperable. The scope of the
present invention should, therefore, be determined only by the
following claims.
* * * * *