U.S. patent application number 13/339117 was filed with the patent office on 2012-04-19 for photovoltaic cell with buffer zone.
This patent application is currently assigned to MH SOLAR CO., LTD.. Invention is credited to Bernard L. Sater.
Application Number | 20120090677 13/339117 |
Document ID | / |
Family ID | 41680419 |
Filed Date | 2012-04-19 |
United States Patent
Application |
20120090677 |
Kind Code |
A1 |
Sater; Bernard L. |
April 19, 2012 |
PHOTOVOLTAIC CELL WITH BUFFER ZONE
Abstract
Systems and methods that provide a barrier for protection of
active layers associated with a vertical multi junction (VMJ)
photovoltaic cell. Buffer zone(s) in form of an inactive layer(s)
arrangement safe guard the active layers against induced stress or
strain resulting from external forces/thermal factors (e.g.,
welding). The buffer zone can be in form of a rim on a surface of
an end layer of a cell unit, to act as a protective boundary for
such active layer, and to further partially frame the VMJ cell for
ease of handling and transportation.
Inventors: |
Sater; Bernard L.;
(Strongsville, OH) |
Assignee: |
MH SOLAR CO., LTD.
Taipei
TW
|
Family ID: |
41680419 |
Appl. No.: |
13/339117 |
Filed: |
December 28, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12536987 |
Aug 6, 2009 |
8106293 |
|
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13339117 |
|
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|
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61088936 |
Aug 14, 2008 |
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Current U.S.
Class: |
136/255 |
Current CPC
Class: |
H01L 31/18 20130101;
H01L 31/04 20130101; H01L 31/0352 20130101; Y02E 10/50
20130101 |
Class at
Publication: |
136/255 |
International
Class: |
H01L 31/06 20120101
H01L031/06 |
Claims
1. A photovoltaic cell comprising: a vertical multi junction (VMJ)
photovoltaic cell that includes a plurality of integrally bonded
cell units, each cell unit with a plurality of layers that form a
PN junction(s); and a buffer zone that safeguards the plurality of
layers from at least one of a stress and strain induced on the VMJ
photovoltaic cell.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 12/536,987 filed on Aug. 6, 2009 entitled
"PHOTOVOLTAIC CELL WITH BUFFER ZONE", which claims the benefit of
U.S. Provisional Application Ser. No. 61/088,936 filed on Aug. 14,
2008 entitled "SOLAR CELL WITH BUFFER ZONE" the entireties of which
are hereby incorporated by reference.
BACKGROUND
[0002] Limited supply of fossil energy resources and their
associated global environmental damage have compelled market forces
to diversify energy resources and related technologies. One such
resource that has received significant attention is solar energy,
which employs photovoltaic systems to convert light into
electricity. Typically, photovoltaic production has been doubling
every two years, increasing by an average of 48 percent each year
since year 2002, making it the world's fastest-growing energy
technology. By midyear 2008, estimates for cumulative global solar
energy production stands to at least 12,400 megawatts.
[0003] Accordingly, solar concentrators represent promising
approaches for mitigating costs associated with photovoltaic (PV)
cells. In general, PV concentrators employ low cost materials such
as large area glass mirrors to intensify sunlight, and reduce
amount of required semiconductor material deemed expensive. In
effect, PV concentrators can reduce a dollar-to-watt cost barrier,
which typically impedes conventional PV Industry. Moreover, PV
concentrators can provide performance advantages, as high cell
efficiencies and sun tracking become prevalent.
[0004] A significant challenge to achieve increased cost
effectiveness is enabling silicon photovoltaic cells to operate
efficiently at high intensities while maintaining relatively low
manufacturing costs. To meet such challenge, high voltage silicon
vertical multi-junction (VMJ) photovoltaic cells have been proposed
as an attractive solution. Nonetheless, active layers positioned at
ends of such cells are susceptible to damage. In addition, such end
layers are more susceptible to mechanical stress caused during
manufacturing, and/or to thermal stress induced during high
intensity operation resulting from a mismatch of thermal expansion
coefficients of contact metals applied thereto (e.g., as electrical
leads.) For example, welding of metal contacts to such end layers
can adversely affect properties of the active layers, and hence
degrade over all performance of the cell. Similarly, intrinsic
mechanical stress induced during such fabrication can negatively
affect behavior for adjacent underlying diffused junctions.
[0005] Likewise, problems can arise due to stress from thermal
cycling in high intensity operation, which is caused by
differential thermal expansion coefficients of the electrical
leads. Such thermal expansion mismatch can further induce thermal
strain, which can also affect the underlying active junctions, and
degrade overall cell performance or threaten the long-term
structural integrity of the VMJ cell.
[0006] Such problems are further compounded in cell arrays, wherein
a plurality of VMJ cells are connected in series (e.g., via lead
contacts), and the poorest performing cell limits operations of the
overall array.
SUMMARY
[0007] The following presents a simplified summary in order to
provide a basic understanding of some aspects of the claimed
subject matter. This summary is not an extensive overview. It is
not intended to identify key/critical elements or to delineate the
scope of the claimed subject matter. Its sole purpose is to present
some concepts in a simplified form as a prelude to the more
detailed description that is presented later.
[0008] The subject innovation supplies a buffer zone(s) at end
layers of a high voltage silicon vertical multi junction (VMJ)
photovoltaic cell, to provide a barrier that protects the active
layers while providing an ohmic contact. Such buffer zone(s) can be
in form of an inactive layer(s) arrangement that is additionally
stacked upon and/or below end layers of the VMJ cell. The VMJ cell
itself can include a plurality of cell units, wherein each cell
unit employs several active layers (e.g., three) to form a PN
junction and a "built-in" electrostatic drift field (which enhances
minority carrier movement toward the PN junction.)
[0009] As such, various active layers such as nn+ and/or p+n
junctions located at either ends of a VMJ cell (and as part of cell
units thereof) can be safeguarded against adverse forms of stress
and/or strain (e.g., thermal/mechanical compression, torsion,
moment, shear and the like--which can be induced in the VMJ during
fabrication and/or operation thereof.) Moreover, the buffer zone
can be formed via materials that have substantially low resistivity
ohmic contact, either metals or semiconductors, such that it will
not contribute any substantial series resistance loss in the
photovoltaic cell at operating conditions. For example, the buffer
zone can be formed by employing low resistivity silicon wafers that
are p-type doped, so that when using other p-type dopants such as
aluminum alloys in manufacturing the VMJ photovoltaic cell, it will
mitigate a risk of auto-doping (in contrast to employing n-type
wafers that can create unwanted pn junctions--when an object is to
create a substantially low resistivity ohmic contact. It is to be
appreciated that the subject innovation can be implemented as part
of any class of photovoltaic cells such as solar cells or
thermophotovoltaic cells. Additionally, aspects of the subject
innovation also can be implemented in other class(es) of
energy-conversion cells such as betavoltaic cells.
[0010] In related aspects, the buffer zone can be in form of a rim
on a surface of an end layer of a cell unit, which acts as a
protective boundary for such active layer and further frames the
VMJ cell for ease of handling and transportation. Likewise, by
enabling a secure grip to the VMJ cell, such rim formation also
eases operation related to the anti reflective coating (e.g.,
coating can be applied uniformly when the cell is securely
maintained during operation, such as by mechanically clamping
thereon.) Moreover, the buffer zones (e.g., the inactive layers
positioned at ends of the VMJ) can be physically positioned
adjacent to other buffer zones during the deposition--and hence any
unwanted dielectric coating material that inadvertently penetrates
down onto the contact surfaces can be readily removed without
damaging active unit cells. The buffer zone can be formed from
substantially low resistivity and highly doped silicon (e.g., a
thickness of approximately 0.008'') Such buffer zone can
subsequently contact conductive leads that partition or separate a
VMJ cell from another VMJ cell in a photovoltaic cell array.
[0011] According to a further aspect, the buffer zone can be
sandwiched between an electrical contact, and the active layers of
the VMJ cells. Moreover, such buffer zones can have thermal
expansion characteristics that substantially match those of the
active layers, hence mitigating performance degradation (e.g.,
mitigation of stress/strain caused when leads are welded or
soldered in manufacturing.) For example, highly doped low
resistivity silicon layers can be employed, which match the thermal
expansion coefficient (3.times.10.sup.-6/.degree. C.) of all active
unit cells. Accordingly, substantially strong ohmic contacts can be
provided to the active unit cells, which additionally mitigate
stress problems caused by welding/soldering and/or from mismatched
thermal expansion coefficients in contact materials. Other examples
include introducing metallic layers, such as tungsten
(4.5.times.10.sup.-6/.degree. C.), or molybdenum
(5.3.times.10.sup.-6/.degree. C.), which are chosen for thermal
expansion coefficients substantially similar to the active silicon
(3.times.10.sup.-6/.degree. C.) p+nn+unit cells. The metallization
applied to the outer layers of the low resistivity silicon layers
of the buffer zone, or to the metallic layers of electrodes that
are alloyed to the active unit cells, can be welded or soldered
without introducing deleterious stress to the high intensity solar
cell or photovoltaic cell--wherein such outer layers serve as ohmic
contacts; rather than segments of unit cells in series with the
other unit cells.
[0012] Various aspect of the subject innovation can be implemented
as part of wafers having miller indices (111) for orientation of
associated crystal planes of the buffer zones, which are considered
mechanically stronger and slower etching than (100) crystal
orientation silicon typically used in making active VMJ unit cells.
Accordingly, low resistivity silicon layers can have a different
crystal orientation than that of the active unit cells, wherein by
employing such alternative orientation, a device with improved
mechanical strength/end contacts is provided. Put differently,
edges of (100) orientated unit cells typically etch faster and
essentially round off corners of the active unit cells with such
crystal orientation--as compared to the inactive (111) orientated
end layers--hence resulting in a more stable device structure with
higher mechanical strength for welding or otherwise connecting end
contacts.
[0013] To the accomplishment of the foregoing and related ends,
certain illustrative aspects (not to scale) of the claimed subject
matter are described herein in connection with the following
description and the annexed drawings. These aspects are indicative
of various ways in which the subject matter may be practiced, all
of which are intended to be within the scope of the claimed subject
matter. Other advantages and novel features may become apparent
from the following detailed description when considered in
conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 illustrates a schematic block diagram of an
arrangement for buffer zones as part of a vertical multi junction
(VMJ) cell in accordance with an aspect of the subject
innovation.
[0015] FIG. 2 illustrates a particular aspect of a unit cell, an
array of which can from a VMJ cell in accordance with a particular
aspect of the subject innovation.
[0016] FIG. 3 illustrates an exemplary cross section for a buffer
zone in form of a rim formation on surfaces of unit cells located
at either end of a VMJ.
[0017] FIG. 4 illustrates a related methodology of employing buffer
zones at end layers of a high voltage silicon vertical multi
junction (VMJ) photovoltaic cell, to provide a barrier that
protects the active layers thereof.
[0018] FIG. 5 illustrates a schematic cross sectional view for a
solar assembly that includes a modular arrangement of photovoltaic
(PV) cells, which can implement VMJs with buffer zones.
DETAILED DESCRIPTION
[0019] The various aspects of the subject innovation are now
described with reference to the annexed drawings, wherein like
numerals refer to like or corresponding elements throughout. It
should be understood, however, that the drawings and detailed
description relating thereto are not intended to limit the claimed
subject matter to the particular form disclosed. Rather, the
intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the claimed
subject matter.
[0020] FIG. 1 illustrates a schematic block diagram of an
arrangement for buffer zones as part of vertical multi junction
(VMJ) cell in accordance with an aspect of the subject innovation.
The VMJ 115 itself is formed from a plurality of integrally bonded
cell units 111, 117 (1 to n, n being an integer), wherein each cell
unit itself is formed from stacked substrates or layers (not
shown). For example, each cell unit 111, 117 can include a
plurality of parallel semiconductor substrates stacked together,
and consisting of impurity doped semiconductor material, which form
a PN junction and a "built-in" electrostatic drift field that
enhance minority carrier movement toward such PN junction.
Accordingly, various active layers such as nn+ and/or p+n
junctions, or pp+ and/or pn+ junctions, located at either ends of a
VMJ cell 115 (and as part of cell units thereof) can be safeguarded
against adverse forms of stress and/or strain (e.g.,
thermal/mechanical compression, torsion, moment, shear and the
like--which can be induced in the VMJ during fabrication and/or
operation thereof.)
[0021] Moreover, each of the buffer zones 110 112 can be formed via
material that have substantially low resistivity ohmic contact
(e.g., any range with upper limit less than approximately 0.5
ohm-cm), while mitigating and/or eliminating unwanted auto doping.
For example, the buffer zones 110, 112 can be formed by employing
low resistivity wafers that are p-type doped, with other p-type
dopants such as aluminum alloys, to mitigate a risk of auto-doping
(in contrast to employing n-type wafers that can create unwanted pn
junctions--when it is desired to create a substantially low
resistivity ohmic contact.)
[0022] FIG. 2 illustrates a particular aspect of a unit cell, an
array of which can form a VMJ cell. The unit cell 200 includes
layers 211, 213, 215 stacked together in a substantially parallel
arrangement. Such layers 211, 213, 215 can further include impurity
doped semiconductor material, wherein layer 213 is of one
conductivity type and layer 211 is of an opposing conductivity
type--to define a PN junction at intersection 212. Likewise, layer
215 can be of the same conductivity type as layer 213-yet with
substantially higher impurity concentration, hence generating a
built-in electrostatic drift field that enhances minority carrier
movements toward the PN junction 212. Such unit cells can be
integrally bonded together to form a VMJ, wherein a buffer zone of
the subject innovation can be positioned to safe guard the VMJ and
associated unit cells and/or layers that form them.
[0023] According to a further aspect, to fabricate the VMJ from a
plurality of cells 200, initially identical PNN+ (or NPP+)
junctions can be formed to a depth of approximately 3 to 10 .mu.m
into flat wafers of high resistivity (e.g., more than 100 ohm-cm)
of N type (or P type) silicon--having a thickness of approximately
0.008 inch. Subsequently, such PNN+ wafers are stacked together
with a thin layer of aluminum interposed between each wafer,
wherein each wafer's PNN+ junction and crystal orientation can be
oriented in the same direction. Moreover, aluminum-silicon eutectic
alloys can be employed, or metals such as molybdenum or tungsten
with thermal coefficient(s) that substantially matches the thermal
coefficient of silicon. Next, the silicon wafers and aluminum
interfaces can be alloyed together, such that the stacked assembly
can be bonded together. Moreover, aluminum-silicon eutectic alloys
can also be employed. It is to be appreciated that various N+-type
and P-type doping layers can be implemented as part of the cell
units and such arrangements are well within the realm of the
subject innovation.
[0024] Buffer zones with substantially low resistivity can also be
supplied in form of an inactive layer(s) arrangement that is
additionally stacked upon and/or below end layers of the VMJ
cell--hence implementing a barrier that protects the active layers
against adverse forms of stress and/or strain (e.g.,
thermal/mechanical compression, torsion, moment, shear and the
like--which can be induced in the VMJ during fabrication and/or
operation thereof.)
[0025] FIG. 3 illustrates an exemplary cross section for a buffer
zone in form of a rim formation 310 (312) on surfaces of an end
layer 331 (341) of unit cells 330 (340), which in part forms the
VMJ cell 300. Such rim formations 310, 312 act as a protective
boundary for active layers of the cell units, and further partially
frame the VMJ cell 300 for ease of handling and transportation
(e.g., a low resistivity buffer zone and edge or end contact of the
VMJ cell.) Likewise, by enabling a secure grip to the VMJ cell 300,
the rim formation also eases operation related to the anti
reflective coating (e.g., coating can be applied uniformly when the
cell is securely maintained during operation, such as by
mechanically clamping thereon.) Moreover, such rim formations can
physically be positioned adjacent to other rim formations during
the deposition process, wherein any unwanted dielectric coating
material that inadvertently penetrates down onto the contact
surfaces can be readily removed without damaging the unit cells
330, 340. The rim formation 310 (312) representing the buffer zone
can be formed from substantially low resistivity and highly doped
silicon (e.g., a thickness of approximately 0.008''), wherein the
rim formation can subsequently contact conductive leads that
partition a VMJ cell from another VMJ cell in a photovoltaic cell
array. Moreover, because of the substantially low resistivity of
the buffer zone, the conductive leads are not required to have full
electrical contact to the buffer zone. As such, they can be partial
contacts, such as a point contact, or a series of point contacts,
while nevertheless providing good electrical contact. It is to be
appreciated that FIG. 3 is exemplary in nature, and other
variations--such as the buffer zone 310 formed in manufacturing
reaching the surfaces of 300 with 310 bonding to active layers
341--are well within the realm of the subject innovation. For
example, the shape of 310 can represent a partial lead contact to
the metalized layer on the buffer zone as discussed earlier.
[0026] The conductive leads can be in form of electrode layers,
which are formed by depositing a first conductive material on a
substrate--and can comprise; tungsten, silver, copper, titanium,
chromium, cobalt, tantalum, germanium, gold, aluminum, magnesium,
manganese, indium, iron, nickel, palladium, platinum, zinc, alloys
thereof, indium-tin oxide, other conductive and semiconducting
metal oxides, nitrides and silicides, polysilicon, doped amorphous
silicon, and various metal composition alloys. In addition, other
doped or undoped conducting or semi-conducting polymers, oligomers
or monomers, such as PEDOT/PSS, polyaniline, polythiothene,
polypyrrole, their derivatives, and the like can be employed for
electrodes. Furthermore, since some metals can have a layer of
oxide formed thereupon that can adversely affect the performance of
the VMJ cell, non-metal material such as amorphous carbon can also
be employed for electrode formation. It is to be appreciated that
the rim formation of FIG. 3 is exemplary in nature and other
configurations for the buffer zone such as, rectangular, circular,
cross sections having a range of surface contact with the active
layers are well within the realm of the subject innovation.
[0027] Moreover, various aspect of the subject innovation can be
implemented as part of wafers having miller indices (111) for
orientation of associated crystal planes of the buffer zones, which
are considered mechanically stronger and slower etching than (100)
crystal orientation silicon typically used in fabricating active
VMJ unit cells. Accordingly, low resistivity silicon layers can
have a different crystal orientation than that of the active unit
cells, wherein by employing such alternative orientation, a device
with improved mechanical strength/end contacts is provided. Put
differently, edges of (100) orientated unit cells typically etch
faster to essentially round off corners of the active unit cells
with such crystal orientation--as compared to the inactive (111)
orientated end layers, hence resulting in a more stable device
structure with higher mechanical strength for welding or otherwise
connecting end contacts.
[0028] FIG. 4 illustrates a related methodology 400 of employing
buffer zones at end layers of a high voltage silicon vertical multi
junction (VMJ) photovoltaic cell, to provide a barrier that
protects the active layers thereof. While the exemplary method is
illustrated and described herein as a series of blocks
representative of various events and/or acts, the subject
innovation is not limited by the illustrated ordering of such
blocks. For instance, some acts or events may occur in different
orders and/or concurrently with other acts or events, apart from
the ordering illustrated herein, in accordance with the innovation.
In addition, not all illustrated blocks, events or acts, may be
required to implement a methodology in accordance with the subject
innovation. Moreover, it will be appreciated that the exemplary
method and other methods according to the innovation may be
implemented in association with the method illustrated and
described herein, as well as in association with other systems and
apparatus not illustrated or described. Initially, and at 410
multiple cell units with PN junctions are formed as described in
detail supra. As explained earlier each cell unit itself can
include a plurality of parallel semiconductor substrates that are
stacked together. Each layer can consist of impurity doped
semiconductor material that form a PN junction, and further include
a "built-in" electrostatic drift field that enhance minority
carrier movement toward such PN junction. Subsequently and at 420,
a plurality of such cell units are integrated to shape a VMJ. Next
and at 430 a buffer zone can be implemented that contacts end
layers of the VMJ, to provide a barrier that protects the active
layers thereof. Such buffer zone(s) can be in form of an inactive
layer(s) arrangement that is additionally stacked upon and/or below
end layers of the VMJ cell. The VMJ can then be implemented as part
of a photovoltaic cell.
[0029] FIG. 5 illustrates a schematic cross sectional view 500 for
a solar assembly that includes a modular arrangement 520 of
photovoltaic (PV) cells 523, 525, 527 (1 through k, where k is an
integer). Each PV cell can employ a plurality of VMJs with buffer
zones according to an aspect of the subject innovation. Typically,
each of the PV cells (also referred to as photovoltaic cells) 523,
525, 527 can convert light (e.g., sunlight) into electrical energy.
The modular arrangement 520 of the PV cells can include
standardized units or segment that facilitate construction and
provide for a flexible arrangement.
[0030] In one exemplary aspect, each of the photovoltaic cells 523,
525, 527 can be a dye-sensitized solar cell (DSC) that includes a
plurality of glass substrates (not shown), wherein deposited
thereon are transparent conducting coating, such as a layer of
fluorine-doped tin oxide, for example. Such DSC can further include
a semiconductor layer such as TiO.sub.2 particles, a sensitizing
dye layer, an electrolyte and a catalyst layer such as Pt-(not
shown)- which can be sandwiched between the glass substrates. A
semiconductor layer can further be deposited on the coating of the
glass substrate, and the dye layer can be sorbed on the
semiconductor layer as a monolayer, for example. Hence, an
electrode and a counter electrode can be formed with a redox
mediator to control of electron flows therebetween.
[0031] Accordingly, the cells 523, 525, 527 experience cycles of
excitation, oxidation, and reduction, which produce a flow of
electrons, e.g., electrical energy. For example, incident light 505
excites dye molecules in the dye layer, wherein the photo excited
dye molecules subsequently inject electrons into the conduction
band of the semiconductor layer. Such can cause oxidation of the
dye molecules, wherein the injected electrons can flow through the
semiconductor layer to form an electrical current. Thereafter, the
electrons reduce electrolyte at catalyst layer, and reverse the
oxidized dye molecules to a neutral state. Such cycle of
excitation, oxidation, and reduction can be continuously repeated
to provide electrical energy.
[0032] What has been described above includes various exemplary
aspects. It is, of course, not possible to describe every
conceivable combination of components or methodologies for purposes
of describing these aspects, but one of ordinary skill in the art
may recognize that many further combinations and permutations are
possible. Accordingly, the aspects described herein are intended to
embrace all such alterations, modifications and variations that
fall within the spirit and scope of the appended claims.
[0033] Furthermore, to the extent that the term "includes" is used
in either the detailed description or the claims, such term is
intended to be inclusive in a manner similar to the term
"comprising" as "comprising" is interpreted when employed as a
transitional word in a claim.
* * * * *