U.S. patent application number 14/676572 was filed with the patent office on 2015-07-23 for back surface field formation in silicon microspheres in a photovoltaic panel.
The applicant listed for this patent is Nthdegree Technologies Worldwide Inc.. Invention is credited to Matthew P. Gess, Theodore I. Kamins, Vera N. Lockett, Tricia A. Youngbull.
Application Number | 20150207020 14/676572 |
Document ID | / |
Family ID | 48608880 |
Filed Date | 2015-07-23 |
United States Patent
Application |
20150207020 |
Kind Code |
A1 |
Youngbull; Tricia A. ; et
al. |
July 23, 2015 |
BACK SURFACE FIELD FORMATION IN SILICON MICROSPHERES IN A
PHOTOVOLTAIC PANEL
Abstract
A PV panel is manufactured using a monolayer of small silicon
sphere diodes (10-300 microns in diameter) connected in parallel.
The spheres are embedded in an uncured aluminum-containing layer on
an aluminum foil substrate in a roll-to-roll process, and the
aluminum-containing layer is heated to anneal the
aluminum-containing layer as well as p-dope the bottom surface of
the spheres. The diffusion of the p-type dopants also creates a
back surface field in the spheres to improve efficiency. A
dielectric layer is formed, and a phosphorus-containing layer is
deposited over the spheres to dope the top surface n-type, forming
a pn junction. The phosphorus layer is then removed. A conductor is
deposited to contact the top surface. Conformal, index-graded
lenses are then formed over each of the spheres to form a thin and
flexible PV panel.
Inventors: |
Youngbull; Tricia A.;
(Tempe, AZ) ; Kamins; Theodore I.; (Palo Alto,
CA) ; Lockett; Vera N.; (Phoenix, AZ) ; Gess;
Matthew P.; (San Tan Valley, AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nthdegree Technologies Worldwide Inc. |
Tempe |
AZ |
US |
|
|
Family ID: |
48608880 |
Appl. No.: |
14/676572 |
Filed: |
April 1, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13718978 |
Dec 18, 2012 |
9035174 |
|
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14676572 |
|
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|
61577607 |
Dec 19, 2011 |
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61577612 |
Dec 19, 2011 |
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Current U.S.
Class: |
438/62 ;
438/63 |
Current CPC
Class: |
Y02E 10/548 20130101;
H01L 31/035281 20130101; Y02E 10/547 20130101; Y02P 70/50 20151101;
H01L 31/02168 20130101; H01L 31/055 20130101; H01L 31/1804
20130101; Y02P 70/521 20151101; H01L 31/068 20130101; H01L 31/075
20130101; Y02E 10/52 20130101 |
International
Class: |
H01L 31/18 20060101
H01L031/18; H01L 31/0224 20060101 H01L031/0224; H01L 31/0352
20060101 H01L031/0352 |
Claims
1. A process for forming a solar cell structure comprising:
providing a substantially flat substrate containing aluminum;
depositing an aluminum-containing ink over the substrate, the
aluminum-containing ink forming an uncured aluminum-containing
layer; depositing a plurality of semiconductor particles on the
uncured aluminum-containing layer so that the particles are
partially embedded in the aluminum-containing layer, the particles
having a top surface portion for being exposed to the sun to
generate electricity and having a bottom surface portion; heating
the aluminum-containing layer to diffuse p-type dopants into the
bottom surface portion to create a back surface field, wherein the
aluminum-containing layer is a conductor electrically contacting
the bottom surface portion, wherein the semiconductor particles
form a monolayer over the aluminum-containing layer; depositing a
dielectric layer over exposed portions of the aluminum-containing
layer; and depositing a conductor over the dielectric layer
electrically contacting the top surface portion, the top surface
portion being of an n-type, wherein, at least after the step of
depositing the conductor over the dielectric layer, the
semiconductor particles are a plurality of diodes adapted to
convert sunlight to electricity.
2. The process of claim 1 wherein the semiconductor particles
comprise silicon spheres, the process further comprising: forming a
mechanical bond where the aluminum-containing layer alloys with the
silicon spheres, while creating a p+ region in the silicon spheres
by: 1) diffusion of silicon from the silicon spheres into the
aluminum-containing layer, and 2) aluminum diffusion from the
aluminum-containing layer into the silicon spheres proximate to an
interface of the silicon spheres and the aluminum-containing
layer.
3. The process of claim 1 wherein the semiconductor particles
comprise silicon spheres and the aluminum-containing ink also
contains silicon, the process further comprising: forming a bond
where the aluminum-containing layer alloys with the substrate by:
1) diffusion of silicon from the aluminum-containing ink into the
substrate, and 2) diffusion of aluminum from the substrate into the
aluminum-containing layer.
4. The process of claim 1 where the substrate comprises an
aluminum-containing foil.
5. The structure of claim 1 where the substrate comprises at least
one of aluminum, silicon, steel, copper, or brass.
6. The process of claim 1 wherein the semiconductor particles
comprise silicon spheres, wherein the aluminum-containing ink is
coated on a moving substrate, and wherein the silicon spheres are
subsequently coated on the moving substrate on top of the
aluminum-containing layer.
7. The process of claim 1 wherein the semiconductor particles
comprise silicon spheres, wherein a bottom of the silicon spheres
opposing the substrate is p-type, the method further comprising
doping at least a top surface of the silicon spheres n-type
subsequent to the step of depositing a dielectric layer over
exposed portions of the aluminum-containing layer.
8. The process of claim 1 wherein the semiconductor particles
comprise silicon spheres, wherein the silicon spheres have an
average diameter less than 300 microns.
9. The process of claim 1 wherein all steps are performed under
atmospheric pressures.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. application Ser.
No. 13/718,978, filed Dec. 18, 2012, which is based, in part, on
U.S. provisional application Ser. No. 61/577,607, filed Dec. 19,
2011, and U.S. provisional application Ser. No. 61/577,612, filed
Dec. 19, 2011, all assigned to the present assignee and
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates to forming photovoltaic (PV) panels,
also known as solar panels or solar cells, and, in particular, to a
technique for forming a silicon-based PV panel using an all
atmospheric pressure printing process.
BACKGROUND
[0003] Crystalline and multicrystalline silicon photovoltaic panels
are traditionally fabricated using a semi-automated process that
requires expensive manufacturing equipment, is relatively
labor-intensive, and requires vacuum processing tools such as
vacuum evaporators and plasma enhanced chemical vapor (PECVD)
deposition chambers. The invention described herein describes a
continuous, roll-to-roll crystalline PV manufacturing method that
requires no vacuum tools. Roll-to-roll (R2R) manufacturing of PV
panels have been demonstrated using plasma deposited amorphous
silicon and slot-die coated copper indium gallium diselenide, but
these processes have not been truly continuous, e.g., R2R equipment
is used but the web is wound up and transported to multiple process
stations. Moreover, the cost of manufacturing per Watt of generated
power from these thin film panels has been financially
unsustainable.
[0004] In the present invention, the PV panels are made of high
efficiency crystalline silicon microspheres 10-150 microns in
diameter, which greatly reduces silicon consumption per panel area.
The PV panel makes very efficient use of the silicon since the
light-incident surface area-to-volume ratio is 2-3 orders of
magnitude greater than planar silicon. The small sphere size also
allows the microspheres to be dispersed in an ink system that is
coated on a web into a closed packed monolayer. High throughput,
low-cost coating of the microspheres and other functional layers,
and formation of the PN junction are all carried out in a
continuous, atmospheric pressure roll-to-roll process.
[0005] US patent application publication no. US2010/0167441,
entitled, Method of Manufacturing a Light Emitting, Photovoltaic or
Other Electronic Apparatus and System, is assigned to the present
assignee and incorporated herein by reference. The publication
describes various techniques to form light emitting diode (LED)
sheets and photovoltaic (PV) panels using arrays of semiconductor
microdiodes. In particular, the PV panels are comprised of
microspheres and may be on the order of 20-40 microns in diameter.
Several methods of manufacturing silicon spheres are known and
include forming spheres from molten silicon in a drop tower,
patterning silicon particle agglomerates on a substrate and melting
them to form spheres by surface tension, or dropping powder through
a plasma reactor.
[0006] To-date, spherical PV modules have been limited by a means
to rapidly produce a nearly closed packed monolayer of silicon
spheres. Monolayer formation of micrometer or nanometer range
spheres has been a significant area of research across a number of
different disciplines over the years. Rapid, inline formation of
true monolayers of micrometer spheres from a high solids fluid is
difficult and, within an industrial setting, has remained a
difficult task. Monolayers occur within very narrow control ranges
where a small variance in print conditions favor either sparse
layers or layer doubling.
[0007] Lee et al, US patent application 2011/0117694 A1 describes
an inkjet printing process to make silicon microsphere diodes in a
monolayer but not in a closed packed array, and inkjet printing is
relatively a low-throughput printing process compared to the
coating processes described herein. Moreover, the PV panel process
uses vacuum tools, specifically plasma enhanced chemical vapor
deposition to form the electrodes. What is needed is a high
throughput (e.g., 10-20 ft/min) R2R monolayer coating process of
silicon microspheres.
[0008] Back surface field (BSF) formation in spherical PV diodes at
low temperature (<640.degree. C.) is also needed to increase
panel efficiency and maintain the structural integrity of the web
during a R2R process. A BSF is an aluminum rich region in a silicon
solar cell that is capable of providing a 1-3% total power
conversion efficiency gain in a solar panel. Typically, the rear
contact for mono-crystalline and multi-crystalline silicon solar
cells is formed by screen printing an aluminum paste on the
back-side of a silicon wafer and firing it at 800-900.degree. C. to
form an ohmic contact and a BSF. U.S. patent application Ser. No.
13/587,380 describes an aluminum-based ink. This ink is utilized to
form a BSF in silicon microspheres using rapid annealing at a peak
temperature of 600.degree. C. on a moving web.
[0009] Various methods of doping the silicon spheres to form diodes
are also known. Typically, lightly doped p-type silicon (1-10
Ohm-cm) is highly doped (le-4 Ohm-cm or less) on the outer surface
with phosphorus, to form a pn+ diode. U.S. Pat. No. 7,214,577
describes using standard diffusion of phosphorus dopants into 1-2
mm diameter silicon spheres before forming the PV panels. This is a
batch process that requires a special process chamber to contain
hazardous gas, and later the spherical diodes must be etched to
remove a portion of the n+ region. The method described in the
present disclosure forms the PN junction in-situ during the R2R
process using laser annealing at atmospheric pressure. This is the
first time laser annealing is used to form PN junctions on
spherical silicon in-line, and it removes the need for etching the
diode in later processes.
[0010] The anodes and cathodes of the diodes are ohmically
connected to printed conductors to form an array of
parallel-connected diodes in a PV panel. Panels may be connected in
a combination of series and parallel to achieve the desired
electrical characteristics.
[0011] Further, the panels described in US patent application
publication no. US2010/0167441 are formed using various processes
that are not practical with a roll-to-roll printing process. This
increases the cost of the panels and decreases manufacturing
throughput of the panels. For instance, a substrate with pre-formed
channels in which the spheres ultimately reside is used. A paste
conductor and the spheres that are not deposited in the channels
must be scraped off the substrate, increasing the difficulty and
cost of forming the panels.
[0012] Further, the processes of US patent application publication
no. US2010/0167441 generally deposit pre-formed lenses over the
diode array, where the shapes of the lenses are not optimized for
the spheres and where the lenses are difficult to optimally
position with respect to the spheres. Due to the large variations
in indices of refraction between the silicon, lens, and air, there
is significant reflection of light. U.S. Pat. No. 8,013,238 aligns
lenses to millimeter sized spherical diodes with a vertical,
elastomeric standoff, requiring the spheres be spaced in a square
array, millimeters apart, which significantly decreases the active
area of the photovoltaic panel. In the present application,
Applicants disclose aligned lenses with a graded refractive index
over a closed packed array of silicon microsphere diodes to reduce
reflection of light from the silicon and allow for a more efficient
PV panel. European patent application EP 1 586 121 B1 describes an
antireflection coating for spherical PVs but the material
deposition method is a vacuum process, so a continuous roll-to-roll
process cannot be performed.
[0013] Other improvements over the processes of US patent
application publication no. US2010/0167441 are also desirable,
which improve the performance of the panels and simplify
processing.
[0014] What is needed is an all atmospheric pressure technique to
fabricate a highly efficient PV panel with an antireflective,
graded index lens at a relatively low cost, using a roll-to-roll
printing process.
SUMMARY
[0015] In one embodiment of an all-atmospheric pressure process to
form PV panels using an efficient roll-to-roll fabrication
technique, an aluminum foil substrate is provided on a roll. As the
metal substrate is unwound through the printing press (referred to
herein as the web), a conductive adhesion layer, such as an
aluminum-containing ink, is coated on the substrate, immediately
followed by coating a silicon sphere slurry on the uncured
aluminum-containing ink. The silicon spheres may be 10-300 microns
in diameter and are pre-doped to be n-type or p-type, or have an
intrinsic conductivity type. Lightly doped p-type microspheres are
assumed in the example. The slurry is spread thin, such as by a
roller, blade, or air knife, to form a closed packed monolayer of
silicon spheres, and the spheres become embedded in the
aluminum-containing ink. The aluminum-containing ink is then cured
(e.g., ink's solvent evaporated) and annealed (aluminum particles
sintered) by an in-line furnace or other heat source. The annealing
causes the contact area of the silicon spheres to alloy with the
aluminum ink to form a back surface field, or highly doped p.sup.+
silicon, and form an ohmic contact with the aluminum. This p.sup.+
layer reduces electron-hole recombination at the sphere's back
surface and thus increases efficiency by a total of 1-3 percentage
points.
[0016] The substrate does not use channels for aligning the spheres
and containing the aluminum-containing layer, which greatly
simplifies the formation of the panel and reduces waste.
[0017] A dielectric is spray-coated over the spheres. The
dielectric is designed to wet off (or wick off) the top of the
microspheres by capillary action and surface tension to form a 1-15
micron thick coating between the microspheres, and then cured. A
residual layer of dielectric approximately 150 nm or less remaining
on the tops of the silicon spheres is not detrimental to forming
the PN junction.
[0018] Phosphoric acid, phosphorus-doped glass, phosphorus-doped
silicon nanoparticles, or a phosphorus-containing silicon precursor
is spray-coated over the spheres and heated, such as by a laser, to
diffuse the n-type dopants into the top surface of the spheres,
forming diodes. Residual phosphorus dopant is then washed from the
tops of the spheres.
[0019] A transparent conductor, such as a conductive ink, is then
slot die coated over the surface of the panel including over the
exposed n-type layer of the spheres. If the conductor material is
deposited as a liquid, the conductor viscosity may be sufficiently
low that it pools around the lower portion of the spheres by
gravity, surface tension, and capillary action, and retracts from
over the spheres to form a conducting network. Such pooling reduces
the resistance of the conductor and avoids issues with the
conductor covering the diode and reducing optical transmission.
[0020] Thin metal bus bars are then printed in selected areas over
the pooled conductor layer and the spheres, forming many low
resistance parallel strips running along the length of the PV
panel. A few wider and thicker, orthogonal metal bus bars may then
be formed in contact with the thin bus bars to carry the solar
cell's cathode current to electrical connectors at an edge of each
PV panel. The aluminum substrate carries the anode current to the
connectors.
[0021] Quantum dots of silicon or other types of quantum dots are
then coated over the top surface of the spheres to conform to the
shape of the spheres. The quantum dots absorb UV light and emit the
light in visible wavelengths. The silicon diodes convert the added
visible light into current, so the incident UV light is not wasted,
and solar cell heating is reduced.
[0022] To reduce reflection and to form an environmental barrier, a
layer or sequence of layers of high-index of refraction
nanoparticles (e.g., doped glass beads or other high-index
particles such as titanium dioxide) in an environmentally robust
transparent binder is deposited over the silicon spheres, creating
an omnidirectional, graded (or stepped) index antireflection
coating. A nanoparticle, as used herein, has a diameter of less
than one micron. The particles preferably have an average diameter
of less than 100 nm. The sizes of the nanoparticles are smaller
than the wavelengths of visible light to limit scattering by the
nanoparticles. The particles have an index of refraction of about
1.7-2.4. The transparent binder has an index of refraction that is
lower than the index of refraction of the nanoparticles, but the
composite creates a greater effective index than the binder alone
and is tuned by changing the concentration of the high-index
nanoparticles. Without additives, increasing the refractive index
of an optically transparent polymer above 1.7 is difficult to
achieve. The layer may contain a mixture of one or more
nanoparticle types of different refractive indices, and the
concentrations of the different nanoparticles may vary between the
upper portion and the lower portion of the lens. The different
index nanoparticles may have different sizes and/or masses to
create the different concentrations as the nanoparticles settle
through the liquid at different rates. A single lens coating layer
may be preferred to simplify the number of coating steps during the
roll-to-roll process. In another embodiment, multiple layers with
different bulk refractive indexes are coated and cured on top of
each other to more precisely grade the refractive index of the
lens.
[0023] In one embodiment, a lower index (e.g., n=1.4) PVDF
(polyvinylidene difluoride) layer, or other suitable transparent
polymer, containing matched index (e.g., n=1.4) particles (e.g.,
transparent doped glass beads) is next deposited over the
nanoparticle layer. This lower index layer may have an effective
index of 1.4 or less (e.g., 1.3). These particles may have an
average diameter between 1-10 microns. These particles, being
preferably much harder than the binder, desirably provide abrasion
resistance. Such protection is a very important advantage in PV
panels.
[0024] The quantum dot layer and the lens layers conform to the
rounded top surfaces of the diodes, creating an optimal optical
structure with minimum reflection.
[0025] The roll is then cut to form individual PV panels, each
panel containing millions of silicon diodes connected in parallel,
and an array of the PV panels is bonded to a support structure to
form a module. The PV panels may be electrically connected in any
combination of series and parallel to achieve the desired voltage
and current characteristics.
[0026] In another embodiment, the p-n silicon diodes are formed
prior to being coated on the substrate. p-type doped or undoped
silicon spheres are initially provided. An outer n.sup.+-type layer
is then formed on the spheres such as by subjecting the spheres to
phosphoric acid in a batch barrel process. The spheres are then
applied to an uncured aluminum-containing ink layer on an aluminum
substrate, and the ink is sintered to make electrical contact
between the bottom portion of the n.sup.+-type layer and the
aluminum-containing ink.
[0027] A dielectric layer is then coated and cured, which wets off
the top of the microspheres by capillary action and surface tension
to form 1-15 micron thick coating between the microspheres.
[0028] The top surfaces of the diodes are then etched away,
exposing the inner p-type silicon (assuming the spheres were
initially doped). A further doping of the p-type silicon may be
conducted if needed, such as for ohmic contact and to form a front
surface field, or if the spheres were not initially p-doped. A
transparent conductor is then printed to contact the p-type
silicon. The remaining processes may be those described above.
[0029] In yet another embodiment, the p-n silicon diodes are formed
prior to being deposited on the substrate, as described above, with
a p-type core and an n.sup.+-type outer layer. The spheres are then
printed on a dielectric layer, such as an adhesive tape. The upper
n.sup.+-type layer of the spheres is then etched to expose the
p-type silicon. A layer of aluminum-containing ink is then printed
over the panel. The aluminum-containing ink is heated to flow the
ink between the spheres so that the aluminum makes ohmic contact
with the n.sup.+-type bottom layer of the spheres. The aluminum
also creates a p.sup.+-type top surface of the spheres. Any
aluminum-containing ink remaining in contact with the top
p.sup.+-type silicon is removed by wet etching.
[0030] If there is concern about diffusion between the adjacent
p.sup.+ and n.sup.+ regions, a thin dielectric layer may be formed
around each sphere between the n.sup.+-type region and the exposed
p-type region, prior to depositing the aluminum-containing ink
layer, to act as a separator after the p.sup.+-type top surface of
the spheres is formed.
[0031] A low temperature dielectric is then printed over the panel
that wets off (or wicks off) the tops of the microspheres by
capillary action and surface tension (and pools around the
perimeters of the microspheres) to form a 1-15 micron thick coating
between the microspheres, exposing the p.sup.+-type silicon.
[0032] A transparent conductor layer is then coated over the panel,
such as by slot die coating. The conductor layer is then heated to
sinter conductive particles in the layer and to make ohmic contact
with the p.sup.+-type silicon. There may be desirable pooling of
the transparent conductor material around the sides of the
p.sup.+-type silicon.
[0033] Metal bus bars are then printed to create a low resistance
path to the p.sup.+-type silicon via the transparent conductor.
[0034] A quantum dot layer and graded lens may then be formed, as
previously described.
[0035] Anode and cathode connectors are then formed leading to the
aluminum-based anode layer and the transparent conductor cathode
layer.
[0036] The panels are then separated, mounted on a support
structure, and electrically interconnected.
[0037] Other embodiments are described in the detailed description.
All steps may be performed under atmospheric pressure
conditions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 is a top down view of a starting thin metal foil
substrate. The substrate, forming a panel or a cell, may be any
size for a roll to roll all-atmospheric pressure printing
process.
[0039] FIG. 2 illustrates an aluminum-containing ink or paste layer
being slot die coated immediately prior to a layer of silicon
spheres being coated, such that the spheres overlie the uncured
conductor layer. These layers may be applied by other means, such
as rotary screen printing or knife-over-roll coating.
[0040] FIG. 3 is a cross-sectional view of the substrate along a
row of p-type silicon spheres, illustrating the silicon spheres
being deposited on the uncured conductor layer to form a closed
packed monolayer. The processes will be shown performed on only a
single sphere for simplicity, although each step of the process may
be simultaneously performed over the entire surface of the PV
panel.
[0041] FIG. 4 illustrates an air knife evaporating the solvents
from the silicon sphere slurry to aid in the formation of a thin
layer, such as a monolayer, while forcing the spheres into the
uncured conductor layer. FIG. 4 also illustrates the silicon sphere
layer and conductor layer being annealed to form a back surface
field in the silicon spheres.
[0042] FIG. 5 illustrates the silicon spheres bonding to the
conductor layer after the process of FIG. 4.
[0043] FIG. 6 illustrates a dielectric barrier layer coated over
the surface of the PV panel.
[0044] FIG. 7 illustrates the dielectric that has wetted off (or
wicked off) the tops of the microspheres to expose the silicon
surface.
[0045] FIG. 8 illustrates an n-type dopant layer deposited over the
silicon surface. n-type dopant atoms are diffused from the n-dopant
layer into the silicon to create pn diodes in-situ.
[0046] FIG. 9 illustrates the n-type dopant layer being washed off
after doping the underlying silicon.
[0047] FIG. 10 illustrates a transparent conductor or other
conductor coated at least along the edges of the silicon spheres,
followed by printing a metal bus bar over a portion of the
conductor. The transparent conductor material, if deposited as a
liquid, may automatically retract from the top surfaces of the
spheres by capillary action and desirably pool around the perimeter
of the spheres to form a conducting network.
[0048] FIG. 11 illustrates the deposition of quantum dots on the
silicon surface.
[0049] FIG. 12 illustrates the deposition of high-index
nanoparticles (<300 nm) in a lower-index dielectric material,
forming part of a graded (or stepped)-index lens for reducing
reflection.
[0050] FIG. 13 illustrates the deposition of lower-index and larger
particles forming part of the graded (or stepped)-index lens for
reducing reflection.
[0051] FIG. 14 illustrates another embodiment of the silicon sphere
as a pre-formed diode.
[0052] FIG. 15 illustrates the silicon diode of FIG. 14 embedded in
a conductive layer such that electrical contact is made to the
n.sup.+-type outer layer.
[0053] FIG. 16 illustrates a dielectric layer formed over the
spheres and etched back to expose the tops of the spheres.
[0054] FIG. 17 illustrates that the tops of the spheres have been
etched away to expose the p-type silicon. The exposed p-type
silicon may have the initial p-type dopant concentration, or a
doping step may be performed to make it p.sup.+-type. A transparent
conductor is then formed to make ohmic contact with the p-type or
p.sup.+-type silicon, followed by the formation of metal bus bars.
The processes of FIGS. 11-13 may then be performed.
[0055] FIG. 18 illustrates the silicon sphere as a pre-formed
diode.
[0056] FIG. 19 illustrates the spheres deposited on a substrate and
a dielectric deposited over the spheres.
[0057] FIG. 20 illustrates the dielectric etched back to expose the
tops of the spheres and the exposed n.sup.+-type silicon etched
away to expose the underlying p-type silicon.
[0058] FIG. 21 illustrates the remaining dielectric being
removed.
[0059] FIG. 22 illustrates an aluminum-containing layer being
deposited and heated to form a p.sup.+-type region on top of each
sphere.
[0060] FIG. 23 illustrates the aluminum-containing layer being
etched down to only ohmically contact the n.sup.+-type portion of
the spheres.
[0061] FIG. 24 illustrates a dielectric mask layer formed over the
spheres to expose the p.sup.+-type regions.
[0062] FIG. 25 illustrates a transparent conductor deposited over
the spheres to contact the p.sup.+-type region and thicker metal
bus bars contacting the transparent conductor.
[0063] FIG. 26 is a schematic view of a portion of the
all-atmospheric pressure printing process being performed using a
roll-to-roll technique.
[0064] FIG. 27 is a top down view of four PV panels, each typically
containing millions of silicon diodes connected in parallel for
converting sunlight into electrical power, where robust metal bus
bars are formed to electrically contact the narrower printed
conductors, and where electrodes are formed to allow the panels to
be connected in series by external conductors.
[0065] FIG. 28 shows the viscocity behavior of the silicon
microsphere slurry as a function of shear rate at slurry
temperatures ranging from 20.degree. C.-60.degree. C. The slurry is
highly thioxotropic at room temperature but exhibits much less
shear thinning at 40.degree. C. Some solvent is lost at 60.degree.
C. which results in a slight increase in viscosity at 100
s.sup.-1.
[0066] FIGS. 29A and 29B are optical images at different
magnifications of a closed packed monolayer of 63-75 micron
microspheres coated on an aluminum web. The microspheres form a
closed packed monolayer and can be within 10-150 microns in
diameter with a +/-10 micron variance is diameter.
[0067] FIG. 30 is an optical image of cross-sectioned silicon
microspheres bonded to Al-containing ink coated on an Al foil
substrate. The interface between the ink and the silicon
microspheres shows the formation of a p+ region or back surface
field region.
[0068] FIG. 31A is a scanning electron microscope image of a
cross-section of the monolayer of silicon spheres embedded in the
cured Al ink.
[0069] FIGS. 31B and 31C are energy dispersive x-ray spectroscopy
images taken in a scanning electron microscope showing an alloyed
region (p+) in the silicon microspheres (FIG. 31B) and an alloyed
(silicon-rich phase) region in the aluminum substrate (FIG.
31C).
[0070] Elements that are similar or identical in the various
figures are labeled with the same numeral.
DETAILED DESCRIPTION
[0071] One embodiment of the invention is a process for forming a
thin PV panel (or solar cell) typically containing millions of
small, substantially spherical silicon diodes electrically
interconnected in parallel. Electricity is generated by the panel
due to the photovoltaic effect. A pn junction is fabricated in
lightly doped silicon, typically p-type and referred to as the
base, by diffusing electron-rich or electron-deficient atoms to a
depth of approximately 1 micron, typically forming a doped n-type
layer referred to as the emitter. Electrical contact is made to the
emitter and base on either side of the pn junction. At this
junction, a depletion region forms from the presence of ionized
donors and acceptors. As photons are absorbed from sunlight, free
carriers are generated. These photogenerated carriers diffuse and
drift to the depletion regions of the p-n junction, drift across
the junction under the built-in electric field, and are collected
at the electrodes, resulting in a net photocurrent. Groups of the
diodes may be connected in a combination of series and parallel to
create a desired operating voltage and current. The power may be
used to, for example, feed into the utility grid or charge a
battery.
[0072] Only photons with energies equal to or somewhat greater than
the band gap of silicon (.about.1.1 eV) are converted to
electricity by the silicon. UV light has a much greater energy than
the bandgap, so much of this absorbed energy is wasted as heat.
There is also significant reflection by the silicon due to the
large differences in the indices of refraction between air (n=1)
and silicon (n=about 4 for visible light). The reflected sunlight
is thus wasted. These are only some of the reasons why
silicon-based PV panels have relatively low power conversion
efficiency, typically less than 20%.
[0073] The below-described embodiments illustrate various
atmospheric pressure printing processes for forming an efficient PV
panel. There is no need for a vacuum chamber, such as for metal
depositions, dielectric depositions, etching, etc., resulting in
the PV panel fabrication process being relatively simple and
inexpensive to implement. This process enables a high throughput
roll-to-roll manufacturing technique. Further, the process makes
very efficient use of silicon.
[0074] FIG. 1 is a top down view of a starting substrate 10. In the
example, the substrate 10 is a flexible aluminum foil and used to
conduct current. In another embodiment, the substrate 10 is any
other metal, such as stainless steel, copper, brass, or other
alloy. The substrate 10 may instead be a dielectric, such as a
polymer sheet. The substrate 10 may be any size, such as 9 inches
by 18 inches. Preferably, the substrate 10 is provided on a roll,
and the technique described is performed as a roll-to-roll process
under atmospheric pressure conditions. In the preferred embodiment,
no channels are formed in the substrate 10.
[0075] In FIG. 2, a conductive layer is formed immediately followed
by depositing a silicon sphere slurry. In another embodiment, the
conductive layer can be cured prior to depositing the silicon
spheres. FIG. 2 illustrates a slot die coating process, although
alternative printing or coating methods (e.g., knife-over-roll
coating) are envisioned. A source 12 of aluminum-containing paste
or aluminum-containing ink 13 is provided to a first slot die head
14. The paste can also be a combination of aluminum and silicon or
other materials. The first slot die head 14 may optionally heat the
aluminum-containing ink 13, and a pump causes the ink 13 to be
ejected from a long narrow slot in the first slot die head 14. Slot
die heads are well known. The location and amount of the deposited
aluminum-containing ink 13 is thus carefully controlled and may be
used to deposit rows of conductor layers.
[0076] A source 16 of doped or intrinsic silicon spheres in a
solvent system (silicon ink 18) is provided to a second slot die
head 20, which also may be heated to control viscosity, and
deposited onto the aluminum-containing ink 13 prior to curing of
the aluminum-containing ink 13. The viscosities of the inks 13 and
18 and percentage of the particles in the solvents may be
controlled to optimize spreading of the ink and the packing density
of the deposited particles. The slot die heads 14 and 20 may be
connected together in what is termed a dual-slot die head for
precise alignment of the inks 13 and 18.
[0077] In one embodiment, thousands of the doped spheres are
deposited across the width of the substrate 10. The spheres are
ideally packed hexagonally (i.e., each sphere has 6 spheres
surrounding it in a horizontal plane) to provide the maximum number
of spheres per unit area. Monolayers occur within very narrow
control ranges where a small variance in print conditions favor
either sparse layers or layer doubling. These difficulties are due
to both the rheology of the fluid and the physical limits of either
doctor blade coating or slot die coating of such highly shear
thinning materials.
[0078] FIG. 28 shows the shear thinning properties of the silicon
sphere slurry as a function of temperature. By coating the ink at
40.degree. C., shear thinning is minimized, so the formation of a
monolayer is possible.
[0079] Moreover, by using the wet on wet approach (e.g., wet
spherical laydown on a wet conductive "glue" layer), a closed
packed or near perfect monolayer is made at 213 cm/min. FIGS. 29A
and 29B show the formation of a closed packed monolayer of
microspheres with diameters of 63-75 microns. This process has been
proven with microspheres ranging in size from 10-150 microns in
diameter, but a high packing density requires a total variance in
diameter of at most 20 microns. The combination of both knife over
roll coating and slot die coating allows a significantly enhanced
coating control range and is an innovation in high speed coating of
shear thinning fluids.
[0080] In contrast to this step, the adhesive layer in the US
publication 2010/0167441, deposited over a flat substrate (no
channels or cavities), is not a metal but is, for example, a
conductive polymer. The resistance of an aluminum layer is lower
than that of a conductive polymer, and the aluminum can be used to
dope the silicon with p-type dopants.
[0081] FIG. 3 is a cross-sectional view of the substrate 10,
showing the coating of the aluminum-containing conductor layer 22
followed by the coating of p-type silicon spheres 26. N-type,
un-doped, or silicon spheres with pre-formed pn junctions may be
used instead. The formation of silicon spheres is described in U.S.
Pat. No. 5,556,791. In one embodiment, the spheres 26 have a mean
diameter somewhere between 10-300 microns. The spheres 26 will
typically not be perfect spheres so are assumed to be substantially
spherical.
[0082] If screen printing is used to form the various layers
described herein, the processes may be performed over a large
surface of the substrate 10 while the substrate 10 is stationary.
In screen printing, a fine mesh has formed on it a mask layer, such
as an emulsion, that is patterned using conventional
photolithographic processes. The mesh is then laid over the
substrate 10. Then the liquid or paste comprising the material to
be deposited is squeegeed over the mesh to force the liquid/paste
through the openings in the mask to deposit the liquid/paste on the
substrate 10 surface. The deposited material is cured, such as by
drying by heat.
[0083] FIG. 4 illustrates an optional air knife 30 that spreads the
silicon ink layer 31 to form a thin layer (e.g., a monolayer) of
the spheres 26, while forcing the spheres 26 into the uncured
conductor layer 22. The air knife 30, blowing heated filtered air
or inert gas, also partially evaporates the ink solvent.
Accordingly, there is synergy in using the air knife 30 versus a
nip roller or blade. The air knife 30 may be directed normal to the
substrate 10 or at an angle. In one embodiment, the air knife 30
blows a narrow-angle (a knife edge) of air. In other embodiments,
any type of spreader may be used. In another embodiment, depending
on the deposition technique, no spreader is needed.
[0084] FIG. 4 also illustrates the silicon ink layer 31 and
conductor layer 22 being cured and annealed in a two-step process
during a roll-to-roll process. The curing step evaporates the ink
solvent. A heater 32 is shown. The heater 32 may be any suitable
heater, including a rapid annealing system. The annealing sinters
the aluminum particles in the conductive layer 22 together, forms a
back surface field in the silicon (a p.sup.+ region), and bonds the
silicon to the conductive layer, providing both mechanical and
electrical connectivity to the underlying substrate. FIG. 5
illustrates the silicon spheres 26 being embedded in the cured
conductive layer 22 after the process of FIG. 4 to create a large
contact area. The p.sup.+ area 34 forms during the Al annealing
step wherever the Al is in contact with the silicon spheres 26. The
Al atoms diffuse into the silicon, forming the highly doped p.sup.+
area 34 and creating ohmic contact. Cross-sectioned images of
silicon microspheres bonded to the Al ink and substrate after this
annealing process are shown in FIGS. 30 and 31A-31C. In FIG. 30,
the "bright" region in the bottom of the silicon microspheres are
the BSF regions and are proven to be Al-rich "dark regions" as
shown in the electron dispersive spectroscopy (EDS) micrograph in
FIG. 31B. In FIG. 31C, an EDS micrograph of the silicon content is
shown. A silicon-rich region in the Al substrate shows that
alloying and hence electrical contact between the Al-ink and
Al-substrate occurs to complete the solar cell anode.
[0085] In FIG. 6, a dielectric 36 is deposited over the surface of
the PV panel, such as by spray coating or ink jet printing. In one
embodiment, the dielectric 36 is a spray coated glass precursor,
such as spin-on-glass (SOG), that is then cured. Spin-on-glass is a
term used to describe a low viscosity glass that can be deposited
by spin-coating or spray-coating. The thickness of the dielectric
36 is on the order of a few to a few tens of microns between the
spheres since the spheres 26 only have a diameter between 10-300
microns. In another embodiment, the dielectric is a dispersion of
polymer microbeads that, when cured, form a continuous insulating
layer.
[0086] In FIG. 7, the dielectric 36 is shown to wet off from the
tops of the silicon spheres 26 to expose the upper silicon surface.
Due to the low viscosity of the dielectric 36, the smooth surface
of the silicon spheres, and the lack of chemical interaction
between the silicon and the dielectric 36, the dielectric 36 pools
along the edges of the silicon spheres 26 due to capillary action,
surface tension, and gravity. This removal of the dielectric layer
from the tops of the spheres may also be referred to as wicking.
Even if there is a thin residual dielectric layer on the tops of
the spheres 26, a subsequent laser diffusion step can overcome this
thin dielectric layer to form a PN junction as described below. In
another embodiment, the microbeads completely wet off the tops of
the silicon microspheres exposing a pristine surface to form the PN
junction.
[0087] In FIG. 8, an n-dopant layer 38 is deposited over the
silicon surface to create pn junction diodes in-line. In one
embodiment, the n-dopant layer 38 is a spray coated or printed
phosphorus-doped glass layer. The dopants in layer 38 are diffused
into the silicon using a pulsed laser. The laser heats the sphere
surface 26 to, for instance, above the melting temperature of
silicon to allow for rapid diffusion of the phosphorus into the
silicon. In another embodiment, the dopant is phosphoric acid and
can be coated directly over the dielectric layer, then diffused
through the thin dielectric on top of the silicon microspheres
using laser annealing to form p-n junctions. In a third embodiment,
the dopant is phosphorus-doped nanosilicon or a
phosphorus-containing silicon precursor which, upon irridation with
the laser, forms a continuous silicon film. The film can either be
amorphous, nanocrystalline, or monocrystalline depending on the
laser conditions. When laser conditions are such to diffuse the
phosphorus beyond the interface between the silicon film and the
microsphere, a homojunction is formed. When the laser conditions
are such to keep the phosphorus within the upper silicon layer, a
heterojunction is formed. A 532 nm laser with a peak power of 45 W
or less and fluences of 20-100 micro Joules, with a focal length up
to 6 mm, is desirable.
[0088] In FIG. 9, the remainder of the n-dopant layer 38 is washed
or etched away, which may also further thin the dielectric layer
36. FIG. 9 illustrates the top portion of the silicon spheres 26
being an n-type portion 40, thus a pn diode is formed in-line using
a roll-to-roll process. An additional dielectric layer may be
deposited, if needed, that is designed to wet from most of the
sphere surface so as to pool around the perimeter of the smooth
spheres by gravity, surface tension, and capillary action.
[0089] In FIG. 10, a transparent conductor or other conductor layer
44 is deposited at least along the edges of the silicon spheres 26
to electrically contact the n.sup.+-type portion 40 of the spheres
26, and annealed to lower the contact resistance. In one
embodiment, the conductor layer 44 is deposited by slot die
coating, entailing forcing the liquid conductor material through a
narrow slot onto the surface. In one embodiment, the transparent
conductor material has a sufficiently low viscosity so as to pool
around the perimeter of the smooth spheres by gravity, surface
tension, and capillary action. The pooling of the transparent
conductor lowers the resistance of the conductor and improves
reliability. Since the transparent conductor substantially wicks
off the tops of the diodes, any reflection problems with the
transparent conductor are avoided, and any mismatch of index of
refraction between the transparent conductor and the silicon
becomes irrelevant. If desired, any thin conductor layer may be
etched off the tops of the spheres using a wet etch.
[0090] If a non-transparent conductor layer is used, any conductor
material over the top of the spheres 26 that significantly
attenuates light in the solar radiation spectrum that can be
absorbed by silicon should be etched away. In one embodiment, a
layer comprising nanometer sized silver particles or wires in a
binder is used as the conductor layer 44. The silver particles or
wires contact each other after curing. In one embodiment, the
conductor layer 44 is about 100-200 nm thick after drying.
[0091] A low resistivity metal bus bar 48 is then selectively
printed over the transparent conductor layer 44, such as by inkjet
printing or rotary screen printing of silver or other conductor.
The resulting structure is then annealed to sinter the silver
particles.
[0092] As previously mentioned, UV light from the sun absorbed by
the silicon diodes generates wasted heat. The UV photons are
absorbed in the upper highly doped emitter regions of the silicon
spheres 26 because of their shallow absorption depth, so any
UV-generated free carriers have a high probability of
recombining.
[0093] In FIG. 11, a layer 50 of nanosilicon quantum dots 52,
having an average diameter between 2-20 nm, is deposited directly
on the silicon surface, such as by spray coating or inkjet
printing. Thus, the spheres 26 are conformally coated. Quantum dots
of desired sizes are commercially available and are known to be
used for converting blue or UV light from LEDs into longer
wavelengths, which may create white light. The material and the
size of the quantum dots determine the emitted wavelength. The
quantum dots are dispersed over the spheres 26 (or over any
transparent conductor) in a liquid, which is then evaporated to
leave a thin layer 50 of the quantum dots 52 over the silicon
spheres 26. The quantum dots 52 absorb the sun's UV light and emit
visible light, such as red light at around 700 nm or less, due to
photoluminescence. The visible light is then converted into current
by the photovoltaic effect of the diodes. Thus, efficiency is
increased and heat is reduced. Plots showing the sizes of the
quantum dots vs. their photoluminescence energy are publicly
available, and optimal sizes depend on the wavelengths in the solar
spectrum and the wavelengths that are most efficiently converted to
electricity by the silicon. The quantum dots 56 are not used to
directly generate current.
[0094] Since the quantum dots 52 are preferably silicon, and the
spheres 26 are silicon, the indices of refraction may be close in
value so as not to increase the reflectivity of the silicon sphere
surface. Further, the quantum dots 52 are applied after all
electrical connections are made to the silicon spheres 26 so the
quantum dot layer does not need to be conductive.
[0095] The quantum dot layer 50, possibly being non-conductive, may
overlie the metal bus bar 48 since electrical contact to the metal
bus bar 48 is made along the edges of the PV panel, where the
quantum dots are not deposited. In one embodiment, the quantum dots
52 may be infused in a transparent conductor layer over the spheres
26.
[0096] Polished silicon reflects about 35-50% of visible light and
50-70% of ultraviolet light due to the large difference in the
refractive indices (n) of air and silicon. FIGS. 12 and 13
illustrate the formation of a stepped-index lens, or graded-index
lens, to reduce the reflectivity of the silicon. Only two lens
layers are shown; however, additional layers can be added to grade
the refractive index to further reduce reflective losses. Due to
the rheology of the lens coating formulation, the bottom surface of
the lens will inherently conform to the spheres 26, maximizing
light transmission to the silicon spheres. In FIG. 12, a layer 54
containing high-index nanoparticles 56 in a binder is deposited
over the quantum dot layer 50. In one embodiment, the nanoparticles
56 have an average diameter less than 300 nm, and preferably 10-100
nm. The shapes of the nanoparticles 56 will not necessarily be
spherical, and the diameter is considered to be the widest diameter
of the shape. The nanoparticles 56 are made of a material with a
high index of refraction of about 1.7-2.4. This is generally higher
than any high-index polymer. Since the nanoparticles 56 have a
higher index than the binder they are infused in, it is critical to
keep their size well below the wavelengths of interest (i.e., 350
nm and greater) or there would be significant absorption and
reflection internal to the layer 54. Due to the small sizes of the
nanoparticles 56, there is little or no reflection or absorption in
the desired electromagnetic range. The binder may be polyvinylidene
fluoride (PVDF) or another suitable polymer or other material that
is a liquid when deposited. Nanoparticles may also be referred to
as beads. In one embodiment, the nanoparticles 56 are transparent
doped glass beads. The nanoparticles 56 and binder may be deposited
by spray coating, printing, or using other atmospheric pressure
deposition techniques. Upon curing, the thickness of the layer 54
may be a few microns. In another embodiment, the nanoparticles 56
are less than 10 microns if some absorption is tolerable.
[0097] In FIG. 13, a layer 58 containing lower-index and larger
particles, preferably transparent glass beads 60 (e.g., silica), is
deposited to form the upper part of the graded-index lens for
reducing reflection. The glass beads 60 may have an index of
between 1.4-1.43 to match the index of the liquid binder (e.g.,
PVDF). The beads 60 may have an average diameter between 1-10
microns. Since the beads 60 are formed to have about the same index
as the binder, there is negligible absorption or reflection by the
beads 60 in the binder. The glass beads 60, being much harder than
the binder, desirably increase the abrasion resistance of the layer
58. If the glass beads 60 are densely packed, it will improve the
moisture barrier characteristics of the layer. The layer 58 may be
deposited by spray coating, printing, or other suitable atmospheric
pressure process.
[0098] The thicknesses of the lens layers 54 and 58 in total may be
less than 15 microns. In one embodiment, the layer 58 forms a
generally hemispherical lens to additionally focus sunlight onto
the silicon sphere. The graded or stepped indices of the lens
provide a good transition from the high index silicon to the low
index air. Additional layers of polymers and/or
polymer-nanoparticle composites, having different indices, may be
inserted between the layers 54 and 58 to create a finer graded lens
to further reduce reflection. Polymers with indices less than 1.7
are commercially available.
[0099] The deposited lens 54, 58 are conformal with the silicon
sphere since they are deposited as a viscous liquid. Hence the
bottom surface of the lens 54 will conform to the sphere shape, and
the bottom of the lens 58 will conform to the top of the lens 54
shape. Hence, both lenses 54, 58 may be made substantially
hemispherical by the natural surface tension of the binder for
maximum light acceptance. The term bead, as used herein, does not
necessarily connote a spherical shape, although the glass beads
used in the lens layers 54 and 58 preferably have rounded
edges.
[0100] In another embodiment, a single conformal lens layer may
contain a mixture of one or more nanoparticles of different
refractive indices, and the concentration of nanoparticles may vary
in concentration between the upper portion and the lower portion of
the lens. In one embodiment, a single lens coating layer is
preferred to simplify the number of coating steps during the
roll-to-roll process. The size and/or mass of the nanoparticles for
each refractive index may be different so that different
sizes/masses of the nanoparticles settle to the bottom of the
liquid layer at different rates, resulting in different layers of
different indices nanoparticles being formed for a graded lens. The
liquid may be heated to adjust its viscosity to enable the
nanoparticles to settle. The optimal sizes may be determined by
testing.
[0101] In another embodiment, the lens layers 54 and 58 are
combined into a single graded-index layer containing the
nanoparticles 56 infused in the lower index of refraction binder
that formed part of the layer 58 in FIG. 13. The binder 58 however
does not contain the glass beads 60. The nanoparticles 56 are mixed
in the binder and deposited as described above in a single step.
The nanoparticles 56 naturally migrate/settle to the bottom of the
binder after deposition. The liquid layer may be heated to greatly
lower its viscosity to control the settling of the nanoparticles
56. Thus, the lens area abutting the spheres 26 (having a high
density of the nanoparticles 56) will have an index of refraction
that is higher than the index of refraction further from the
spheres 26 where there is a low density of the nanoparticles
56.
[0102] In another embodiment, multiple layers with different bulk
refractive indexes are coated and cured on top of each other to
more precisely grade the refractive index of the lens.
[0103] FIG. 14 illustrates another embodiment of a silicon sphere
82 as a pre-formed diode created before it is applied to the
substrate. The silicon spheres 82 may be p-doped. In one
embodiment, the spheres are then subjected to POCl.sub.3 in a batch
barrel process to form an n.sup.+-type shell 84 by diffusion of
phosphorus into the silicon sphere surface. Other techniques such
as a wet process with phosphoric acid may also be used.
[0104] In FIG. 15, the silicon spheres 82 are embedded in a
conductive layer 86 formed over a substrate 88, as previously
described. The conductive layer 86 makes ohmic contact to the
n-type shell 84.
[0105] In FIG. 16, a dielectric layer 90 is formed over the
conductive layer 86 and sides of the silicon spheres 82. The
dielectric layer 90 may be inkjet-printed or spray coated spin-on
glass (SOG) or polymer. The dielectric layer 90 is then cured. The
dielectric layer 90 is then etched back to expose the top of the
silicon spheres 82.
[0106] In FIG. 17, the top portion of the spheres 82 is etched
using an atmospheric pressure chemical etching process, such as a
wet or vapor etch, to expose the inner p-type portion of the
spheres. If needed, the top of the spheres 82 may be additionally
p-doped to form a p.sup.+ type layer by depositing a p-doped layer
over the spheres 82, heating the structure to diffuse the dopant
into the tops of the spheres 82, then removing the remaining dopant
layer. Heating may be done with a laser. This is similar to the
process described with respect to FIG. 8 but using a p-doped
layer.
[0107] A transparent conductor or other conductor layer 92 is then
deposited over the structure so as to contact the p-type silicon.
The conductor layer 92 may be an ink that is deposited by any type
of printing, and then cured. The conductor 92 may be of a type that
inherently pools around the perimeter of the spheres 82 by wicking
off the top surface of the spheres 82 by gravity, surface tension,
and capillary action, as previously described. Any transparent
conductor 92 remaining over the spheres 82 may be acceptable,
however. A non-transparent conductor may also be used. A metal bus
bar 94 is then formed, such as by inkjet printing, over the
transparent conductor layer 92 to reduce the resistance along the
rows of spheres 82. Accordingly, electrical contact is made to the
anodes and cathodes of all the spheres 82 deposited on the
substrate 88, and the diodes are connected in parallel. The number
of diodes connected in parallel, defined by the panel area, may be
determined by the desired current to be generated by the panel.
[0108] The processes of FIGS. 11-13 may then be performed to
deposit the quantum dot layer and the graded lens for improving the
solar panel's power conversion efficiency. An entire panel of
millions of diodes connected in parallel is thus completed, using
all atmospheric pressure processes.
[0109] In a variation of FIGS. 14-17, the diodes may be deposited
having: 1) an n.sup.+-type outer shell and an intrinsic core (an
i-conductivity type); 2) a p-type outer shell and an n-type or
intrinsic core; 3) a p.sup.+-type outer shell and an p-type or
intrinsic core; or 4) an n.sup.+-type outer shell and an n-type or
intrinsic core prior. The outer shell or core may be doped after
the diodes have been deposited.
[0110] FIGS. 18-25 illustrate another embodiment of the invention,
using an all-atmospheric pressure printing process to form a PV
panel.
[0111] FIG. 18 illustrates the silicon sphere 82 as a pre-formed
diode, similar to that shown in FIG. 14. p-type doped silicon
spheres are initially provided, and an outer n.sup.+-type layer 84
is then formed on the spheres such as by subjecting the spheres to
POCl.sub.3 in a batch barrel process.
[0112] As shown in FIG. 19, the spheres 82 are then printed on a
dielectric layer 100, such as an adhesive tape, overlying a
substrate 102. A dielectric 104, such as glass, is then deposited
over the spheres 82. The dielectric 104 may be deposited by spray
coating. A suitable glass may be spin-on-glass (SOG).
[0113] FIG. 20 illustrates the dielectric 104 etched back, such as
with a wet etchant, to expose the tops of the spheres. The exposed
n.sup.+-type silicon is then etched away, such as with a wet or
vapor chemical etchant, to expose the underlying p-type silicon.
The dielectric 104, used as a sacrificial masking layer, is
optional if anisotropic etching of the silicon is used. If the
silicon etch is anisotropic, then the spheres 82 themselves block
etching of the underside of the spheres 82.
[0114] The remaining dielectric 104 is then removed, as shown in
FIG. 21.
[0115] FIG. 22 illustrates an aluminum-containing layer 108 screen
printed over the spheres 82 as a paste. Other deposition techniques
may also be used, such as slot die printing. The
aluminum-containing layer 108 is then heated to flow the ink
between the spheres 82 so that the aluminum makes ohmic contact
with the n.sup.+-type bottom layer of the spheres. The aluminum
also dopes the top surface of the spheres 82 to make it a
p.sup.+-type. A rapid annealing system may be used to heat the
surface of the aluminum-containing layer 108 to p-dope the top
surface of the spheres 82.
[0116] If there is concern about diffusion between the adjacent
p.sup.+ and n.sup.+ regions, a thin dielectric layer may be formed
around each sphere 82 between the n.sup.+-type region and the
exposed p-type region, prior to depositing the aluminum-containing
layer, to act as a separator after the p.sup.+-type top surface of
the spheres is formed.
[0117] In FIG. 23, any aluminum-containing layer remaining in
contact with the top p.sup.+-type silicon is removed by etching so
that the aluminum-containing layer 108 only ohmically contacts the
n.sup.+-type portion of the spheres 82.
[0118] In FIG. 24, a low temperature dielectric 112 is then printed
over the sphere 82 and chemically etched to expose the p.sup.+-type
silicon.
[0119] In FIG. 25, a transparent conductor layer 114 is then
deposited over the spheres 82 by slot die coating or other
atmospheric pressure process. The conductor layer 114 is then cured
to make ohmic contact with the p.sup.+-type silicon. There may be
desirable pooling of the transparent conductor material around the
edges of the p.sup.+-type silicon, and the transparent conductor
may retract from the top surface.
[0120] Metal bus bars 116 are then printed to create a low
resistance path to the p.sup.+-type silicon via the transparent
conductor layer 114.
[0121] A quantum dot layer and graded lens may then be formed, as
previously described. If the top portion of the spheres 82 is
exposed after the transparent conductor layer 114 is formed, the
quantum dot and graded lens layers will conform to the sphere 82
surface.
[0122] Anode and cathode connectors are then formed leading to the
aluminum-containing anode layer and the transparent conductor
cathode layer.
[0123] The panels are then sheeted, mounted on a support structure,
and electrically interconnected.
[0124] Additional variations contemplated by the inventors include
the use of intrinsic silicon spheres or lightly n-doped silicon
spheres as the base material. In either case, the p/n, p-i, or n-i
junction or front surface field (n.sup.+/n or p.sup.+/i or
n.sup.+/i) can be introduced by a doped glass or other dopant
source with laser-mediated diffusion of the dopant, as shown in
FIGS. 8 and 9. The back surface field (p.sup.+/i) or p/n or p-i
junction may be introduced by p-type doping at the bottom of the
spheres using an aluminum dopant from a conductive ink, as
illustrated in FIG. 5.
[0125] FIG. 26 is a schematic view of the all-atmospheric pressure
printing process being performed using a roll-to-roll technique. A
substrate 120, which may be on a roll or be a large sheet, is
positioned under any suitable atmospheric pressure process station
for performing any of the steps described above. The substrate 120
may continuously run through different stations for different
processes (in-line roll-to-roll) and/or may be run under a
particular set of tools at a single station for coverage of the
entire substrate 120 before being moved to another station
(roll-to-roll). The three basic equipment tool types used in the
above processes are for depositing 124, heating/curing 128, and
etching 132. The depositing 124 may be by slot die printing, inkjet
printing, spray coating, screen printing, or other suitable
technique. The heating/curing 128 may be performed by lasers, heat
bars, IR, UV, blowers, or other suitable technique. The etching 132
may be performed by chemical vapor etching, wet etching, mechanical
etching, or other suitable technique. Etchants for all the
materials described herein may be conventional (e.g.,
fluorine-based, chlorine-based), used at atmospheric pressure.
[0126] Accordingly, at least the following features distinguish the
inventive processes over the processes in US patent application
publication no. 2010/0167441: [0127] Embodiments of the present
process form a conformal lens (FIGS. 12 and 13). Conformal lenses
are more optimally shaped and are inherently optimally positioned
over the diodes, so efficiency is improved. The index of refraction
of the lenses is also stepped or graded to reduce the reflectivity
of the silicon microspheres. [0128] Embodiments of the present
process form a quantum dot layer (FIG. 11) overlying the diodes,
which conforms to the shape of the diodes, to improve efficiency
and reduce heat. [0129] Embodiments of the present process etch the
diodes (FIGS. 17 and 20) to expose the core silicon region,
followed by an optional p.sup.+ or n.sup.+ doping step. The exposed
core is then contacted by a conductor. [0130] Embodiments of the
present process deposit an aluminum-containing layer 108 (FIG. 22)
to p.sup.+-dope the tops of the diodes, then the
aluminum-containing layer 108 is etched down to only electrically
contact the bottom n.sup.+-type portion of the diodes. [0131] The
various near atmospheric pressure etch processes enable new process
flows to be used to form the panels and, in some embodiments,
improve the performance of the panels. [0132] Embodiments of the
process deposit a dielectric layer that wicks off the tops of the
semiconductor spheres to substantially expose the tops of the
spheres for doping, obviating the need for etching the dielectric.
The dielectric insulates the anode and cathode conductors. [0133]
Embodiments of the present process deposit a transparent conductor
that pools around the edges of the diodes, wicking off the top
surface. This obviates the need for etching and improves optical
efficiency. [0134] Embodiments of the present process use a
substrate without channels and deposit the silicon spheres over an
uncured or partially cured Al layer (FIG. 3). Annealing the Al
layer allows the Al to p.sup.+ dope the silicon. The resulting Al
layer has a very low electrical resistance, and the Al layer is
impervious to the sustained UV exposure from the sun. [0135]
Embodiments of the present process n-dope the top portions of the
silicon with a layer of phosphorus containing material (FIG. 8),
then etch the remaining material residue away (FIG. 9). Etching the
material residue away improves the optical efficiency and lowers
the resistance of the silicon-transparent conductor contact. [0136]
Other improvements over the prior art also exist.
[0137] FIG. 27 is a top down view of four panels 140, each made by
any of the above-described processes and each containing millions
of silicon diodes connected in parallel for converting sunlight
into electrical power. The metal substrates 10/88/102 are shown to
have a first electrical connection 141 made to it (e.g., an anode
electrode). The metal bus bars 48/94/116 are all connected together
with much larger and lower resistance metal bus bars 142, which may
be formed in the x and y directions. The number of the bus bars 142
depends on the size of the panel and the number of diodes connected
in parallel. A second electrical connection 144 (e.g., a cathode
electrode) is made to the metal bus bars 142.
[0138] The various panels 140 are then connected in any combination
of series and parallel by external conductors, such as wires or
part of a frame, to achieve the desired voltage and current.
[0139] Each panel 140 may also be referred to as a solar cell,
since each cell acts as a single unit that is then interconnected
with other cells, as desired by the user. The solar cells may take
any form and not necessarily be rectangular panels.
[0140] In one embodiment, sunlight is converted to electricity by
the panels 140, and a DC-DC converter converts the electricity to a
suitable voltage to charge a battery.
[0141] Although the diodes are described as being spheres, the
diodes may be generally spherical and still be referred to as
spheres. The exact shape depends on tolerances in the processes and
a certain degree of randomness. The term "semiconductor particles"
is used herein to refer to the diodes having any shape, including
spheres, polyhedrons, or random shapes.
[0142] The various transparent layers and the transparent glass
beads forming the lens need not be 100% transparent at all relevant
wavelengths, given the limitations of the materials, but are still
referred to as transparent in accordance with the common usage in
the art.
[0143] All steps described herein are performed on at least a
panel-level in atmospheric pressure conditions, obviating the need
for any vacuum chambers, allowing the panels to be formed quickly
and inexpensively in a roll-to-roll process. The completed panel is
light weight and flexible.
[0144] The techniques described herein may also be used to form
panels of light emitting diodes. Instead of silicon spheres, the
semiconductor particles may be GaN-based particles (e.g., spheres)
that generate blue light. A layer of phosphor may be deposited over
the semiconducting particles by spray coating or printing to create
white light or any other wavelengths of light. All other processes
described herein, suitable for LEDs, may be the same to make
electrical contact to the anodes and cathodes of the LEDs, or to
dope the LEDs, or to form lenses over the LEDs.
[0145] While particular embodiments of the present invention have
been shown and described, it will be obvious to those skilled in
the art that changes and modifications may be made without
departing from this invention in its broader aspects and,
therefore, the appended claims are to encompass within their scope
all such changes and modifications as fall within the true spirit
and scope of this invention.
[0146] In addition to the presently claimed invention, various
other inventions, assigned to the present assignee, are listed
below along with their inventors.
[0147] QUANTUM DOTS BETWEEN DIODES AND LENS IN A PV PANEL. FIG. 11.
Lixin Zheng, Tricia A. Youngbull
[0148] 1. A solar cell structure comprising: [0149] a plurality of
silicon diodes on a substrate adapted to convert sunlight to
electricity, the diodes having a first surface portion for being
exposed to the sun; and [0150] a quantum dot layer deposited at
least over the first surface portion to conform to the first
surface portion, the quantum dot layer converting the sunlight's UV
wavelengths to emitted visible wavelengths, wherein the one or more
diodes convert the emitted visible wavelengths to electricity.
[0151] 2. The structure of Claim 1 wherein the diodes comprises a
plurality of silicon spheres on a substrate, the diodes having a
top surface of a first conductivity type connected to a first
conductor and a bottom surface of a second conductivity type
connected to a second conductor, the quantum dots being deposited
over the top surface of the diodes and on at least a portion of the
first conductor.
[0152] 3. The structure of Claim 1 wherein the quantum dot layer
comprises a layer of nanosilicon particles having an average
diameter between 2-20 nm.
[0153] 4. The structure of Claim 1 wherein the quantum dot layer
comprises a layer of nanoparticles having an average diameter
between 2-20 nm.
[0154] 5. The structure of Claim 1 wherein the quantum dots emit
light having a wavelength of less than 1000 nm.
[0155] 6. The structure of Claim 1 wherein the one or more diodes
comprises a plurality of silicon spheres on a substrate, the diodes
having an average diameter greater than 10 microns, the diodes
having a top surface of a first conductivity type and a bottom
surface of a second conductivity type, the quantum dots being
deposited over the top surface of the diodes, wherein the quantum
dots have an average diameter less than 20 nm and convert UV light
from the sun into light having a wavelength of less than 1000
nm.
[0156] 7. The structure of Claim 6 further comprising a lens formed
over the quantum dot layer.
[0157] 8. The structure of Claim 7 wherein the lens has a graded
index of refraction.
[0158] 9. The structure of Claim 8 wherein the diodes have an outer
surface formed of a first material having a first index of
refraction, the lens comprising: [0159] a first lens layer
overlying the first surface portion, the first lens layer
comprising transparent first particles having an average first
diameter less than 300 nm, the first particles having a second
index of refraction less than the first index of refraction; and
[0160] a second lens layer overlying the first lens layer, the
second lens layer comprising transparent second particles having an
average second diameter greater than the first diameter, the second
particles having a third index of refraction less than the second
index of refraction.
[0161] 10. The structure of Claim 1 wherein the diodes are
electrically contacted by conductors, and wherein the quantum dot
layer is blanket deposited over the diodes and over the
conductors.
[0162] 11. A method of forming a solar cell structure comprising:
[0163] depositing a plurality of silicon diodes on a substrate
adapted to convert sunlight to electricity, the diodes having a
first surface portion for being exposed to the sun; and [0164]
depositing a quantum dot layer at least over the first surface
portion to conform to the first surface portion, the quantum dot
layer converting the sunlight's UV wavelengths to emitted visible
wavelengths, wherein the one or more diodes convert the emitted
visible wavelengths to electricity.
[0165] 12. The method of Claim 11 wherein the diodes have an
average diameter greater than 10 microns, and wherein the quantum
dot layer comprises a layer of nanosilicon particles having an
average diameter less than 20 nm so as to convert UV light from to
sun to visible light having a wavelength less than 1000 nm.
[0166] 13. The method of Claim 11 further comprising forming a lens
over the quantum dot layer.
[0167] 14. The method of Claim 13 wherein the lens has a graded
index of refraction.
[0168] 15. The method of Claim 14 wherein the diodes have an outer
surface formed of a first material having a first index of
refraction, and wherein forming the lens comprises: [0169]
depositing a first lens layer overlying the first surface portion,
the first lens layer comprising transparent first particles having
an average first diameter less than 300 nm, the first particles
having a second index of refraction less than the first index of
refraction; and [0170] depositing a second lens layer overlying the
first lens layer, the second lens layer comprising transparent
second particles having an average second diameter larger than the
first diameter, the second particles having a third index of
refraction less than the second index of refraction.
[0171] CONFORMAL LENS OVER SPHERICAL DIODES IN A PV PANEL. FIGS. 12
and 13. Tricia A. Youngbull, Lixin Zheng, Vera N. Lockett.
[0172] 1. A process for forming a solar cell structure comprising:
[0173] providing a plurality of diodes on a substrate adapted to
convert sunlight to electricity, the diodes having a rounded top
surface portion of a first conductivity type for being exposed to
the sun and having a bottom surface portion of a second
conductivity type; [0174] providing a first conductor electrically
contacting the bottom surface portion; [0175] depositing a second
conductor electrically contacting the top surface portion; and
[0176] depositing a lens material over the top surface portion,
wherein, only after the lens material is deposited, a bottom
surface of the lens material substantially conforms to the rounded
shape of the top surface portion.
[0177] 2. The process of Claim 1 wherein the step of depositing the
lens material comprises: [0178] depositing a liquid lens material
over the diodes, wherein the liquid lens material substantially
conforms to the top surface portion of the diodes by at least
surface tension; and [0179] curing the liquid lens material to form
a first lens.
[0180] 3. The process of Claim 2 wherein the diodes have an average
diameter less than 300 microns, wherein the first conductor
comprises a metal layer, and wherein the second conductor comprises
a transparent conductor layer, the process further comprising:
[0181] depositing a dielectric layer over the metal layer, the
dielectric layer extending between the diodes; and [0182]
depositing the transparent conductor layer over the dielectric
layer to electrically contact the top surface portion of the diodes
and electrically interconnect the diodes, [0183] wherein the step
of depositing the liquid lens material comprises depositing the
liquid lens material over the dielectric layer between the diodes
and over the transparent conductor layer between the diodes as well
as over the top surface portion of the diodes.
[0184] 4. The process of Claim 3 further comprising depositing a
quantum dot layer over the dielectric layer, over the transparent
conductor layer, and over the diodes prior to depositing the liquid
lens material.
[0185] 5. The process of Claim 2 wherein the liquid lens material
is deposited by one of coating or printing.
[0186] 6. The process of Claim 2 wherein the liquid lens material
is deposited over the plurality of diodes and between the diodes
without masking the diodes.
[0187] 7. The process of Claim 2 wherein the first lens has a first
index of refraction, the process further comprising: [0188]
depositing a second liquid lens material over the first lens, a
bottom surface of the second liquid lens material conforming to a
top surface of the first lens; and [0189] curing the second liquid
material to form a second lens, the second lens having a second
index of refraction lower than the first index of refraction.
[0190] 8. The process of Claim 2 wherein the liquid lens material
comprises first particles transparent to visible light and having
an average diameter less than 10 microns, where the first particles
are in a first liquid binder which, when cured, has a first index
of refraction, the first particles having a second index of
refraction higher than the first index of refraction.
[0191] 9. The process of Claim 8 wherein the first particles have
an average diameter less than 300 nm
[0192] 10. The process of Claim 8 further comprising: [0193]
depositing a second liquid lens material over the first lens, a
bottom surface of the second liquid lens material conforming to a
top surface of the first lens; and [0194] curing the second liquid
material to form a second lens, the second lens having a third
index of refraction lower than the second index of refraction.
[0195] 11. The process of Claim 10 wherein the second liquid lens
material comprises second particles transparent to visible light in
a second liquid binder.
[0196] 12. The process of Claim 11 wherein the second particles
have approximately the third index of refraction, and the second
liquid binder, when cured, also has approximately the third index
of refraction.
[0197] 13. The process of Claim 8 wherein the first particles
comprise glass beads.
[0198] 14. A solar cell structure comprising: [0199] a plurality of
diodes on a substrate adapted to convert sunlight to electricity,
the diodes having a rounded top surface portion of a first
conductivity type for being exposed to the sun and having a bottom
surface portion of a second conductivity type; [0200] a first
conductor electrically contacting the bottom surface portion;
[0201] a second conductor electrically contacting the top surface
portion; and [0202] a first lens, formed of a first lens material
deposited over the top surface portion as a liquid, then cured,
such that a bottom surface of the first lens has substantially
conformed around the rounded shape of the top surface portion.
[0203] 15. The structure of Claim 14 wherein the diodes have a
substantially spherical shape, wherein the first lens material
substantially conforms to the top surface portion of the diodes by
at least surface tension.
[0204] 16. The structure of Claim 14 wherein the diodes have an
average diameter less than 300 microns, wherein the first conductor
comprises a metal layer, and wherein the second conductor comprises
a transparent conductor layer, the panel further comprising: [0205]
a dielectric layer over the metal layer, the dielectric layer
extending between the diodes; and [0206] the transparent conductor
layer being over the dielectric layer to electrically contact the
top surface portion of the diodes and electrically interconnect the
diodes, [0207] wherein the first lens material overlies the
dielectric layer between the diodes and over the transparent
conductor layer between the diodes as well as over the top surface
portion of the diodes.
[0208] 17. The structure of Claim 16 further comprising a quantum
dot layer over the dielectric layer, over the transparent conductor
layer, and over the diodes, the first lens being formed over the
quantum dot layer.
[0209] 18. The structure of Claim 14 wherein the first lens has a
first index of refraction, the panel further comprising: [0210] a
second lens formed over the first lens, the second lens formed of a
second lens material deposited over the first lens, then cured, a
bottom surface of the second lens conforming to a top surface of
the first lens, the second lens having a second index of refraction
lower than the first index of refraction.
[0211] 19. The structure of Claim 14 wherein the first lens
comprises first particles transparent to visible light, the first
particles having an average diameter less than 10 microns, where
the first transparent particles are in a first binder having a
first index of refraction, the first transparent particles having a
second index of refraction higher than the first index of
refraction.
[0212] 20. The structure of Claim 19 wherein the first particles
have an average diameter less than 300 nm.
[0213] 21. The structure of Claim 19 further comprising: [0214] a
second lens formed over the first lens, the second lens formed of a
second lens material deposited over the first lens, then cured, a
bottom surface of the second lens conforming to a top surface of
the first lens, the second lens having a third index of refraction
lower than the second index of refraction.
[0215] 22. The structure of Claim 21 wherein the second lens
comprises second particles transparent to visible light in a second
binder, wherein the second particles have approximately the third
index of refraction, and the second binder also has approximately
the third index of refraction.
[0216] DIELECTRIC WETTING OFF TOPS OF SILICON MICROSPHERES IN PV
PANEL TO INSULATE ANODE AND CATHODE CONDUCTORS. FIGS. 6 and 7. Mark
M. Lowenthal, Tricia A. Youngbull, Lixin Zheng.
[0217] 1. A process for forming a solar cell structure comprising:
[0218] depositing a plurality of semiconductor particles on a
substrate at atmospheric pressure, the particles having a top
surface portion for being exposed to the sun to generate
electricity and having a bottom surface portion; [0219] providing a
first conductor electrically contacting the bottom surface portion,
the bottom surface portion having a first conductivity type; [0220]
depositing a dielectric layer over the first conductor and over the
top surface portion of the particles; [0221] wicking substantially
all of the dielectric layer off the top surface portion by
capillary action so as to pool along the edges of the particles;
[0222] depositing a first layer of material over the top surface
portion at atmospheric pressure, the first layer of material
containing dopants of a second conductivity type; [0223] heating
the first layer of material to dope the top surface portion with
the dopants of the second conductivity type; [0224] removing the
first layer of material at atmospheric pressure; and [0225]
depositing a second conductor over the dielectric layer
electrically contacting the top surface portion.
[0226] 2. The process of Claim 1 wherein the step of heating the
first layer of material to dope the top surface portion with the
dopants of the second conductivity type comprises heating the first
layer of material using a laser.
[0227] ETCHING SILICON DIODES IN PV PANEL TO EXPOSE THEIR INNER
CORE FOR CONDUCTOR CONTACT. FIGS. 17-20. Tricia A. Youngbull,
Theodore I. Kamins.
[0228] 1. A process for forming a solar cell structure comprising:
[0229] depositing a plurality of diodes on a substrate adapted to
convert sunlight to electricity, the diodes having a top surface
portion for being exposed to the sun and having a bottom surface
portion, [0230] wherein, prior to depositing the plurality of
diodes, the diodes have a core portion having a first conductivity
type and an outer shell having another conductivity type; [0231]
etching the top surface portion of the diodes to remove a portion
of the outer shell to expose the core portion; [0232] providing a
first conductor electrically contacting the outer shell at the
bottom surface portion; [0233] depositing a dielectric layer over
the first conductor at least around the diodes; and [0234]
depositing a second conductor electrically contacting the exposed
core portion.
[0235] 2. The process of Claim 1 wherein the diodes have an average
diameter less than 300 microns.
[0236] 3. The process of Claim 1 wherein the diodes have an
n.sup.+-type outer shell and a p-type or intrinsic core prior to
being deposited on the substrate.
[0237] 4. The process of Claim 1 wherein the diodes have a p-type
outer shell and an n-type or intrinsic core prior to being
deposited on the substrate.
[0238] 5. The process of Claim 1 wherein the diodes have a
p.sup.+-type outer shell and a p-type or intrinsic core prior to
being deposited on the substrate.
[0239] 6. The process of Claim 1 wherein the diodes have an
n.sup.+-type outer shell and an n-type or intrinsic core prior to
being deposited on the substrate.
[0240] 7. The process of Claim 1 further comprising doping the
outer shell or the core after the diodes have been deposited.
[0241] 8. The process of Claim 1 wherein the first conductor is a
metal layer formed on the substrate prior to depositing the
plurality of diodes, and the bottom surface portion of the diodes
electrically contacts the metal layer.
[0242] 9. The process of Claim 1 wherein the first conductor is a
metal layer formed on the substrate after depositing the plurality
of diodes.
[0243] 10. The process of Claim 1 wherein the second conductor is a
transparent conductor layer deposited over the exposed core
portion.
[0244] 11. The process of Claim 1 wherein the diodes are deposited
by printing.
[0245] 12. The process of Claim 1 wherein the steps of etching the
top surface portion, providing the first conductor, depositing the
dielectric layer, and depositing the second conductor are performed
without masking the diodes and performed at atmospheric
pressure.
[0246] 13. The process of Claim 1 wherein the diodes are
substantially spherical and have an average diameter less than 300
microns.
[0247] 14. The process of Claim 1 wherein the substrate comprises a
metal layer which is the first conductor.
[0248] 15. The process of Claim 1 wherein the diodes have an
n.sup.+-type outer shell and a p-type core prior to being deposited
on the substrate, the process further comprising: [0249] after
etching the top surface portion of the diodes to expose the core
portion, depositing an aluminum-containing layer over the diodes;
[0250] heating the aluminum-containing layer to further
p.sup.+-dope the exposed core portion; and [0251] etching the
aluminum-containing layer to expose the top surface portion of the
diodes to form the first conductor.
[0252] 16. The process of Claim 1 further comprising depositing a
liquid lens material over the top surface portion of the diodes and
curing the lens material to form a lens having a bottom surface
that conforms to the top surface portion of the diodes.
[0253] 17. A solar cell structure comprising: [0254] a plurality of
diodes on a substrate adapted to convert sunlight to electricity,
the diodes having a top surface portion for being exposed to the
sun and having a bottom surface portion, the diodes having a core
portion having a first conductivity type and an outer shell having
another conductivity type; [0255] the top surface portion of the
diodes being etched away to remove a portion of the outer shell to
expose the core portion; [0256] a first conductor layer
electrically contacting the outer shell at the bottom surface
portion; [0257] a dielectric layer over the first conductor at
least around the diodes; and [0258] a second conductor layer over
the dielectric layer electrically contacting the exposed core
portion.
[0259] 18. The structure of Claim 17 wherein the diodes have an
average diameter less than 300 microns.
[0260] 19. The structure of Claim 17 wherein the diodes have an
n.sup.+-type outer shell and a p-type core.
[0261] 20. The structure of Claim 17 wherein the diodes have a
p-type outer shell and an n-type or intrinsic core.
[0262] 21. The structure of Claim 17 wherein the diodes have a
p.sup.+-type outer shell and a p-type or intrinsic core.
[0263] 22. The structure of Claim 17 wherein the diodes have an
n.sup.+-type outer shell and an n-type or intrinsic core. 23. The
structure of Claim 17 wherein the first conductor is a metal layer
formed on the substrate and the diodes are partially embedded in
the metal layer.
[0264] 24. The structure of Claim 17 wherein the second conductor
layer is a transparent conductor layer deposited over the exposed
core portion.
[0265] 25. The structure of Claim 17 further comprising a lens over
the top surface portion of the diodes, the lens being deposited as
a liquid and cured, causing the lens to have a bottom surface that
conforms to the top surface portion of the diodes.
[0266] DEPOSITING SEMICONDUCTOR SPHERES IN AN UNCURED
ALUMINUM-CONTAINING LAYER TO FORM A SUBSTANTIALLY CLOSED PACKED
MONOLAYER OF SPHERES. FIGS. 3-10. Mark M. Lowenthal, Edward W.
Kahrs, Vera N. Lockett, William J. Ray, Howard Nelson, Tricia A.
Youngbull.
[0267] 1. A process for forming a solar cell structure comprising:
[0268] providing a substantially flat substrate; [0269] depositing
an aluminum-containing layer over the substrate, the
aluminum-containing layer being uncured; [0270] depositing a
plurality of semiconductor particles on the uncured
aluminum-containing layer so that the particles are partially
embedded in the aluminum-containing layer, the particles having a
top surface portion for being exposed to the sun to generate
electricity and having a bottom surface portion; [0271] heating the
aluminum-containing layer to at least partially sinter the
aluminum-containing layer, the bottom surface portion being of a
first conductivity type, wherein the aluminum-containing layer is a
conductor electrically contacting the bottom surface portion,
wherein the semiconductor particles form a monolayer over the
aluminum-containing layer; [0272] depositing a dielectric layer
over exposed portions of the aluminum-containing layer; and [0273]
depositing a conductor over the dielectric layer electrically
contacting the top surface portion, the top surface portion being
of second conductivity type, [0274] wherein, at least after the
step of depositing the conductor over the dielectric layer, the
semiconductor particles are a plurality of diodes adapted to
convert sunlight to electricity.
[0275] 2. The process of Claim 1 wherein the plurality of
semiconductor particles are assembled in the monolayer over the
uncured aluminum-containing layer using a coating process.
[0276] 3. The process of Claim 1 further comprising doping the top
surface portion of the semiconductor particles in-situ to form
diodes.
[0277] 4. The process of Claim 3 wherein the top surface portion is
doped with n-type dopants.
[0278] 5. The process of Claim 1 wherein the semiconductor
particles are p-type when initially deposited on the uncured
aluminum-containing layer.
[0279] 6. The process of Claim 5 further comprising depositing a
phosphorus layer over the top surface portion and heating the
phosphorus layer to diffuse n-type dopants into the top surface
portion.
[0280] 7. The process of Claim 6 further comprising removing the
phosphorus layer prior to depositing the conductor over the
dielectric layer.
[0281] 8. The process of Claim 1 wherein heating the
aluminum-containing layer diffuses p-type dopants into the bottom
surface portion of the semiconductor particles.
[0282] 9. The process of Claim 1 wherein the semiconductor
particles are diodes having a core portion being the first
conductivity type and an outer shell being the second conductivity
type.
[0283] 10. The process of Claim 9 further comprising etching away a
top surface of the semiconductor particles to expose the core
portion prior to depositing the conductor, wherein the conductor
contacts the core portion.
[0284] 11. The process of Claim 1 wherein the semiconductor
particles are diodes having a core portion being the second
conductivity type and an outer shell being the first conductivity
type.
[0285] 12. The process of Claim 11 further comprising etching away
a top surface of the semiconductor particles to expose the core
portion prior to depositing the conductor, wherein the conductor
contacts the core portion.
[0286] 13. The process of Claim 1 wherein the conductor is a
transparent conductor.
[0287] 14. The process of Claim 1 wherein the semiconductor
particles are substantially spherical and have an average diameter
less than 300 microns.
[0288] 15. The process of Claim 1 wherein the substrate is a
dielectric.
[0289] 16. The process of Claim 1 wherein the substrate is
electrically conductive.
[0290] 17. A solar cell structure comprising: [0291] a
substantially flat substrate; [0292] an aluminum-containing layer
over the substrate, the aluminum-containing layer being uncured
when deposited; [0293] a plurality of semiconductor particles
partially embedded in the uncured aluminum-containing layer, the
particles having a top surface portion for being exposed to the sun
to generate electricity and having a bottom surface portion, the
aluminum-containing layer being heated to at least partially sinter
the aluminum-containing layer, the bottom surface portion being of
a first conductivity type, wherein the aluminum-containing layer is
a conductor electrically contacting the bottom surface portion;
[0294] a dielectric layer over exposed portions of the
aluminum-containing layer; and [0295] a conductor over the
dielectric layer electrically contacting the top surface portion,
the top surface portion being of second conductivity type, [0296]
wherein, the semiconductor particles are a plurality of diodes
adapted to convert sunlight to electricity.
[0297] 18. The structure of Claim 17 wherein the plurality of
semiconductor particles are assembled in a monolayer over the
aluminum-containing layer.
[0298] 19. The structure of Claim 17 wherein the top surface
portion is doped with n-type dopants and the bottom surface portion
is p-type.
[0299] 20. The structure of Claim 17 wherein the semiconductor
particles are diodes having a core portion being the second
conductivity type and an outer shell being the first conductivity
type.
[0300] 21. The structure of Claim 17 wherein the semiconductor
particles are substantially spherical and have an average diameter
less than 300 microns.
[0301] 22. The structure of Claim 17 wherein the substrate is a
dielectric.
[0302] 23. The structure of Claim 17 wherein the substrate is
electrically conductive.
[0303] INDEX-GRADED LENS OVER SILICON MICROSPHERES IN A PV PANEL.
FIGS. 11-13. Tricia A. Youngbull, William J. Ray, Lixin Zheng, Mark
M. Lowenthal, Vera N. Lockett, Theodore I. Kamins, Neil O.
Shotton.
[0304] 1. A solar cell structure comprising: [0305] one or more
diodes on a substrate adapted to convert sunlight to electricity,
the diodes having a first surface portion for being exposed to the
sun, the diodes having an outer surface being formed of a first
material having a first index of refraction; [0306] a first lens
layer overlying the first surface portion, the first lens layer
comprising transparent first particles having an average first
diameter less than 300 nm, the first particles having a second
index of refraction less than the first index of refraction; and
[0307] a second lens layer overlying the first lens layer, the
second lens layer comprising transparent second particles having an
average second diameter larger than the first diameter, the second
particles having a third index of refraction less than the second
index of refraction.
[0308] 2. The structure of Claim 1 wherein the first particles have
an average diameter between 50-300 nm.
[0309] 3. The structure of Claim 1 wherein the first particles have
an index of refraction greater than or equal to 1.7.
[0310] 4. The structure of Claim 3 wherein the first particles are
doped to have an index of refraction between 1.7-2.4.
[0311] 5. The structure of Claim 3 wherein the first particles are
infused in a first polymer having an index of refraction less than
1.7.
[0312] 6. The structure of Claim 3 wherein the second particles
have an index of refraction of less than or equal to 1.43.
[0313] 7. The structure of Claim 3 wherein the second particles are
infused in a second polymer having an index of refraction
approximately equal to the index of refraction of the second
particles.
[0314] 8. The structure of Claim 1 wherein the second lens layer
forms an approximately hemispherical lens.
[0315] 9. The structure of Claim 1 further comprising one or more
additional lens layers having different indices of refraction to
create a finer graded lens to further reduce reflection.
[0316] 10. The structure of Claim 1 wherein the first particles and
the second particles are doped glass particles.
[0317] 11. The structure of Claim 1 further comprising a quantum
dot layer deposited over the first surface portion, the quantum dot
layer converting the sunlight's UV wavelengths to emitted visible
wavelengths, wherein the one or more diodes convert the emitted
visible wavelengths to electricity, wherein the first lens layer
and the second lens layer are formed over the quantum dot
layer.
[0318] 12. The structure of Claim 1 wherein the one or more diodes
comprises a plurality of silicon spheres on a substrate, the diodes
having a top surface of a first conductivity type and a bottom
surface of a second conductivity type, the first lens layer and the
second lens layer being deposited over the top surface of the
diodes.
[0319] 13. The structure of Claim 1 wherein the first lens layer
conforms to contours of the first surface portion of the diodes,
and wherein the second lens layer conforms to a top surface of the
first lens layer.
[0320] 14. A solar cell structure comprising: [0321] one or more
diodes on a substrate adapted to convert sunlight to electricity,
the diodes having a first surface portion for being exposed to the
sun, the diodes having an outer surface being formed of a first
material having a first index of refraction; [0322] a single lens
layer overlying the first surface portion, the lens layer
comprising transparent first particles having an average first
diameter less than 300 nm, the particles having a second index of
refraction less than the first index of refraction; and [0323] the
lens layer further comprising a binder material having a third
index of refraction less than the second index of refraction, the
binder material containing the first particles, [0324] wherein the
first particles have a higher density proximate to the first
surface portion of the diodes compared to a density of the first
particles more remote from the first surface portion to cause the
lens to have an index of refraction proximate the first surface
portion that is higher than an index of refraction more remote from
the first surface portion, [0325] and wherein the lens layer
conforms to contours of the first surface portion of the
diodes.
[0326] 15. The structure of Claim 14 wherein the particles have an
average diameter between 50-300 nm.
[0327] 16. The structure of Claim 14 wherein the particles have an
index of refraction greater than or equal to 1.7.
[0328] 17. The structure of Claim 14 wherein the particles are
doped to have an index of refraction between 1.7-2.4.
[0329] 18. The structure of Claim 14 wherein the particles are
infused in a first polymer having an index of refraction less than
1.7.
[0330] 19. The structure of Claim 14 wherein the particles are
doped glass particles.
[0331] 20. The structure of Claim 14 further comprising a quantum
dot layer deposited over the first surface portion, the quantum dot
layer converting the sunlight's UV wavelengths to emitted visible
wavelengths, wherein the one or more diodes convert the emitted
visible wavelengths to electricity, wherein the lens layer is
formed over the quantum dot layer.
[0332] 21. The structure of Claim 14 wherein the one or more diodes
comprises a plurality of silicon spheres on a substrate, the diodes
having a top surface of a first conductivity type and a bottom
surface of a second conductivity type, the lens layer being
deposited over the top surface of the diodes.
[0333] 22. The structure of Claim 14 wherein the binder material
also contains second particles having a fourth index of refraction
less than the second index of refraction and greater than the third
index of refraction, a concentration of the first particles
proximate the first surface portion of the diodes being greater
than a concentration of the second particles to form a graded-index
lens layer.
[0334] 23. A method of forming a solar cell structure comprising:
[0335] depositing a plurality of silicon diodes on a substrate
adapted to convert sunlight to electricity, the diodes having a
first surface portion for being exposed to the sun, the diodes
having an outer surface being formed of a first material having a
first index of refraction; [0336] depositing a first lens layer
overlying the first surface portion, the first lens layer
comprising transparent first particles having an average first
diameter less than 300 nm, the first particles having a second
index of refraction less than the first index of refraction; and
[0337] depositing a second lens layer overlying the first lens
layer, the second lens layer comprising transparent second
particles having an average second diameter greater than the first
diameter, the second particles having a third index of refraction
less than the second index of refraction.
[0338] 24. The method of Claim 23 wherein the first particles have
an average diameter between 20-300 nm.
[0339] 25. The method of Claim 23 wherein the first particles have
an index of refraction greater than or equal to 1.7.
[0340] 26. The method of Claim 23 wherein the first particles are
doped to have an index of refraction of about 1.7-1.9.
[0341] 27. The method of Claim 23 wherein the first particles are
infused in a first polymer having an index of refraction less than
1.7.
[0342] 28. The method of Claim 23 wherein the second particles have
an index of refraction of less than or equal to 1.43.
[0343] 29. The method of Claim 23 wherein the second particles are
infused in a second polymer having an index of refraction
approximately equal to the index of refraction of the second
particles.
[0344] 30. The method of Claim 23 wherein the second lens layer
forms an approximately hemispherical lens.
[0345] 31. The method of Claim 23 wherein the first particles and
the second particles are doped glass particles.
[0346] 32. The method of Claim 23 further comprising depositing a
quantum dot layer over the first surface portion, the quantum dot
layer converting the sunlight's UV wavelengths to emitted visible
wavelengths, wherein the one or more diodes convert the emitted
visible wavelengths to electricity, wherein the first lens layer
and the second lens layer are formed over the quantum dot
layer.
[0347] DEPOSIT DOPING LAYER OVER SEMICONDUCTOR SPHERES IN A PV
PANEL AND DIFFUSING DOPANTS USING LASER ANNEALING. FIGS. 8-10.
Tricia A. Youngbull, Richard A. Blanchard, Theodore I. Kamins,
William J. Ray.
[0348] 1. A process for forming a solar cell structure comprising:
[0349] depositing a plurality of semiconductor particles on a
substrate at atmospheric pressure, the particles having a top
surface portion for being exposed to the sun to generate
electricity and having a bottom surface portion; [0350] providing a
first conductor electrically contacting the bottom surface portion,
the bottom surface portion having a first conductivity type; [0351]
depositing a dielectric layer over the first conductor; [0352]
depositing a first layer of material over the top surface portion
at atmospheric pressure, the first layer of material containing
dopants of a second conductivity type; [0353] heating the first
layer of material to dope the top surface portion with the dopants
of the second conductivity type; [0354] removing the first layer of
material at atmospheric pressure; and [0355] depositing a second
conductor over the dielectric layer electrically contacting the top
surface portion.
[0356] 2. The process of Claim 1 wherein the first layer of
material comprises phosphorus, and the dopants of the second
conductivity type are n-type dopants.
[0357] 3. The process of Claim 1 wherein the first conductor is a
metal layer.
[0358] 4. The process of Claim 1 wherein the step of providing the
first conductor comprises depositing an aluminum-containing layer
over the substrate, the aluminum-containing layer being uncured,
and wherein the step of depositing the plurality of semiconductor
particles comprises: [0359] depositing the plurality of
semiconductor particles on the uncured aluminum-containing layer so
that the particles are partially embedded in the
aluminum-containing layer; and [0360] heating the
aluminum-containing layer to at least partially sinter the
aluminum-containing layer and dope the bottom surface portion with
p-type dopants.
[0361] 5. The process of Claim 1 wherein the first conductor is a
metal layer, and the plurality of semiconductor particles are
assembled in a monolayer over the metal layer using a printing
process.
[0362] 6. The process of Claim 1 wherein the semiconductor
particles are p-type when initially deposited on the substrate,
wherein the step of heating the first layer of material dopes the
top surface portion with n-type dopants to convert the
semiconductor particles to diodes.
[0363] 7. The process of Claim 1 wherein the second conductor is a
transparent conductor.
[0364] 8. The process of Claim 1 wherein the semiconductor
particles have an average diameter less than 300 microns.
[0365] 9. The process of Claim 1 wherein the substrate is a
dielectric and the first conductor is a metal layer over the
substrate.
[0366] 10. The process of Claim 1 wherein the substrate is
substantially flat, wherein the first conductor is a metal layer
over the substrate, wherein the semiconductor particles have an
average diameter less than 300 microns, and wherein the step of
depositing the plurality of semiconductor particles on the
substrate comprises printing the semiconductor particles on the
metal layer.
[0367] 11. The process of Claim 10 wherein the semiconductor
particles are randomly located over the metal layer.
[0368] 12. The process of Claim 1 wherein the step of depositing
the plurality of semiconductor particles, the step of providing the
first conductor, the step of depositing the first layer, and the
step of depositing the second conductor are all performed by
printing.
[0369] 13. The process of Claim 1 wherein there are no masking
steps involved in the process.
[0370] 14. The process of Claim 1 wherein the step of depositing
the dielectric layer over the first conductor comprises also
depositing the dielectric layer over the top surface portion of the
particles and wicking substantially all of the dielectric layer off
the top surface portion by capillary action so as to pool along the
edges of the particles.
[0371] 15. The process of Claim 1 wherein the step of heating the
first layer of material to dope the top surface portion with the
dopants of the second conductivity type comprises heating the first
layer of material using a laser.
[0372] 16. A solar cell structure comprising: [0373] a substrate;
[0374] a plurality of semiconductor particles on the substrate, the
particles having a top surface portion for being exposed to the sun
to generate electricity and having a bottom surface portion, the
particles being of a first conductivity type with the top surface
portion of the particles being doped, in-situ, to be a second
conductivity type, the top surface portion having no doping layer
over it; [0375] a first conductor electrically contacting the
bottom surface portion, the bottom surface portion having the first
conductivity type; [0376] a dielectric layer over the first
conductor; and [0377] a second conductor over the dielectric layer
electrically contacting the top surface portion. [0378] wherein,
the semiconductor particles are a plurality of diodes adapted to
convert sunlight to electricity.
[0379] 17. The structure of Claim 16 wherein the first conductor is
a metal layer and the semiconductor particles are assembled in a
monolayer over the metal layer.
[0380] 18. The structure of Claim 16 wherein the top surface
portion is doped with n-type dopants and the bottom surface portion
is p-type.
[0381] 19. The structure of Claim 16 wherein the semiconductor
particles are substantially spherical and have an average diameter
less than 300 microns.
[0382] 20. The structure of Claim 16 wherein the second conductor
is a transparent conductor.
[0383] 21. The structure of Claim 16 wherein the substrate is a
dielectric and the first conductor is a metal layer over the
substrate.
[0384] WICKING DIELECTRIC LAYER OFF TOPS OF SEMICONDUCTOR SPHERES
AND DOPING EXPOSED SPHERES IN PV PANEL. FIGS. 6-10. Tricia A.
Youngbull, Theodore I. Kamins, Richard A. Blanchard.
[0385] 1. A process for forming a solar cell structure comprising:
[0386] depositing a plurality of semiconductor particles on a
substrate at atmospheric pressure, the particles having a top
surface portion for being exposed to the sun to generate
electricity and having a bottom surface portion; [0387] providing a
first conductor electrically contacting the bottom surface portion,
the bottom surface portion having a first conductivity type; [0388]
depositing a dielectric layer over the first conductor and over the
top surface portion of the particles; [0389] wicking substantially
all of the dielectric layer off the top surface portion of the
particles by capillary action so as to pool along the edges of the
particles; [0390] depositing a first layer of material over the top
surface portion at atmospheric pressure, the first layer of
material containing dopants of a second conductivity type; [0391]
heating the first layer of material to dope the top surface portion
with the dopants of the second conductivity type; [0392] removing
the first layer of material at atmospheric pressure; and [0393]
depositing a second conductor over the dielectric layer
electrically contacting the top surface portion.
[0394] 2. The process of Claim 1 wherein the step of heating the
first layer of material to dope the top surface portion with the
dopants of the second conductivity type comprises heating the first
layer of material using a laser.
* * * * *