U.S. patent application number 11/416707 was filed with the patent office on 2007-05-17 for bifacial cell with extruded gridline metallization.
This patent application is currently assigned to PALO ALTO RESEARCH CENTER INCORPORATED. Invention is credited to David K. Fork, Stephen P. Shea.
Application Number | 20070107773 11/416707 |
Document ID | / |
Family ID | 38292736 |
Filed Date | 2007-05-17 |
United States Patent
Application |
20070107773 |
Kind Code |
A1 |
Fork; David K. ; et
al. |
May 17, 2007 |
Bifacial cell with extruded gridline metallization
Abstract
Provided is a bifacial photovoltaic arrangement comprising a
bifacial cell which included a semiconductor layer having a first
surface and a second surface, a first passivation layer formed on
the first surface of the semiconductor layer and a second
passivation layer formed on the second surface of the semiconductor
layer, , and a plurality of metallizations formed on the first and
second passivation layers and selectively connected to the
semiconductor layer. At least some of the metallizations on the
bifacial photovoltaic arrangement comprising an elongated metal
structure having a relatively small width and a relatively large
height extending upward from the first and second passivation
layers.
Inventors: |
Fork; David K.; (Los Altos,
CA) ; Shea; Stephen P.; (Waynesville, NC) |
Correspondence
Address: |
Mark S. Svat;FAY, SHARPE, FAGAN, MINNICH McKEE, LLP
SEVENTH FLOOR
1100 SUPERIOR AVENUE
CLEVELAND
OH
44114-2579
US
|
Assignee: |
PALO ALTO RESEARCH CENTER
INCORPORATED
|
Family ID: |
38292736 |
Appl. No.: |
11/416707 |
Filed: |
May 3, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11336714 |
Jan 20, 2006 |
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11416707 |
May 3, 2006 |
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11282882 |
Nov 17, 2005 |
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11336714 |
Jan 20, 2006 |
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11282829 |
Nov 17, 2005 |
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11282882 |
Nov 17, 2005 |
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Current U.S.
Class: |
136/256 |
Current CPC
Class: |
H01L 31/022425 20130101;
Y02P 70/50 20151101; H01L 31/02363 20130101; H01L 31/056 20141201;
Y02E 10/547 20130101; H01L 31/18 20130101; H01L 31/1876 20130101;
H01L 31/048 20130101; H01L 31/0684 20130101; Y02E 10/52
20130101 |
Class at
Publication: |
136/256 |
International
Class: |
H01L 31/00 20060101
H01L031/00 |
Claims
1. A bifacial photovoltaic arrangement comprising: a bifacial cell
including, a semiconductor layer having a first surface and a
second surface; a first passivation layer formed on the first
surface of the semiconductor layer and a second passivation layer
formed on the second surface of the semiconductor layer; and a
plurality of metallizations formed on the first and second
passivation layers and selectively connected to the first surface
and the second surface of the semiconductor layer, wherein at least
some metallizations comprise an elongated metal structure having a
relatively small width and a relatively large height extending
upward from the first and second passivation layers.
2. The photovoltaic arrangement of claim 1, further including the
metallizations having contact portions extending through openings
in the first passivation layer to the first surface of the
semiconductor .
3. The photovoltaic arrangement of claim 2, further including the
metallizations having contact portions extending through openings
in the second passivation layer to the second surface of the
semiconductor layer
4. The photovoltaic arrangement of claim 1, further including at
least one support portion formed along a side edge of at least one
of the metallizations, wherein the support portion comprises a
transparent material.
5. The photovoltaic arrangement of claim 1, further including a
plurality of the bifacial cells in electrical contact with each
other to form a bifacial photovoltaic module.
6. The photovoltaic arrangement of claim 5, further including a
reflector on one of a layer of glass or plastic, the reflector
being integrated into the arrangement in a manner to be able to
reflect light to backsides of the bifacial solar cells.
7. The photovoltaic arrangement of claim 6, wherein the integrated
reflector is on an outer surface of the layer of plastic or
glass.
8. The photovoltaic arrangement of claim 7, further including one
of a layer of protective paint of plastic laminate provided to
protect the reflector.
9. The photovoltaic arrangement of claim 6, wherein the integrated
reflector is on an inner surface of the layer of plastic or
glass.
10. The photovoltaic arrangement of claim 9, wherein the integrated
reflector is metal.
11. The photovoltaic arrangement of claim 10, further including a
transparent insulator positioned to separate the integrated metal
reflector from the bifacial cells.
12. The photovoltaic arrangement of claim 11, wherein the
transparent insulator has a higher melt temperature than an
adjacent lamination material.
13. The photovoltaic arrangement of claim 6, wherein the integrated
reflector is patterned so that no conductive path exists to short
bifacial solar cells together.
14. The photovoltaic arrangement of claim 6, wherein the integrated
reflector consists of dielectric films.
15. The photovoltaic arrangement of claim 14, wherein the
dielectric films selectively reflect light that is useable to
produce electricity in the bifacial solar cells and pass light that
would heat the bifacial solar cells without generating appreciable
electricity.
16. The photovoltaic arrangement of claim 6, wherein the reflector
is a Lambertian.
17. The photovoltaic arrangement of claim 6, wherein the reflector
is a specular reflector.
18. The photovoltaic arrangement of claim 5, wherein the bifacial
cells are in a bifacial cell module with a plastic laminate front
layer and a glass back layer.
19. The photovoltaic arrangement of claim 1, wherein the
passivation layers of the bifacial cell are configured of amorphous
silicon.
20. The photovoltaic arrangement of claim 1, wherein the
semiconductor layer has a thickness of about 150 microns or less,
and the metallizations on the first and second surfaces have
substantially the same mechanical moments.
21. The photovoltaic arrangement of claim 1, wherein the
metallizations cover less than 10% of the first and second
surfaces.
22. A method for producing a bifacial photovoltaic device, the
photovoltaic device including a semiconductor layer, one or more
doped regions, a first surface, a second surface, and a plurality
of conductive lines disposed over the first surface and the second
surface and contacting one or more doped regions at the first
surface and the second surface, the method comprising: forming a
blanket passivation layer on each of the first surface and the
second surface of the semiconductor; and utilizing a direct-write
metallization apparatus arrangement to deposit the conductive lines
to contact the doped regions of the semiconductor layer.
23. The method of claim 22, further including, utilizing a
non-contact patterning apparatus arrangement to define a plurality
of openings through the first passivation layer and the second
passivation layer, each the openings exposing a corresponding one
of the one or more doped regions of the semiconductor layer; and
utilizing the direct-write metallization apparatus to deposit
contact portions into the openings.
24. The method according to claim 22, wherein the direct-write
metallization apparatus is an inkjet-type printhead.
25. The method according to claim 22, wherein the direct-write
metallization apparatus is a dispensing nozzle.
26. The method according to claim 22, wherein utilizing the
direct-write metallization apparatus to deposit the contact portion
into each of the plurality of openings comprises depositing a
silicide-forming metal into each of the openings.
27. The method according to claim 26, wherein utilizing the
direct-write metallization apparatus further comprises depositing a
second metal onto the silicide forming metal, wherein the second
metal has a greater electrical conductivity than the silicide
forming metal.
28. The method according to claim 27, wherein the silicide forming
metal deposited in openings through the first passivation layer is
different from the suicide forming metal deposited in openings
through the second passivation layer.
29. The method according to claim 27, wherein the second metal
deposited onto the suicide forming metal deposited in openings
through the first passivation layer is different from the second
metal deposited onto the silicide forming metal deposited in
openings through the second passivation layer.
30. The method according to claim 22, wherein the semiconductor
layer comprises at least one of crystalline silicon, amorphous
silicon, CdTe (Cadmium Telluride), or CIGS
(copper-indium-gallium-diselenide).
31. The method according to claim 22, wherein the conductive lines
have an aspect ratio exceeding 2:1.
32. The method according to claim 22, wherein the conductive lines
cover less than 10% of the first surface and the second
surface.
33. A bifacial photovoltaic arrangement comprising: a bifacial cell
including, a semiconductor layer having a first surface and a
second surface, and a thickness of about 150 microns or less; a
first passivation layer formed on the first surface of the
semiconductor layer and a second passivation layer formed on the
second surface of the semiconductor layer; and a plurality of
metallizations formed on the first and second passivation layers
and selectively connected to the semiconductor layer, the
metallizations on the first passivation layer and the
metallizations on the second passivation layer having sufficiently
similar mechanical moments to maintain the semiconductor layer
substantially un-warped.
34. The photovoltaic arrangement according to claim 33, wherein the
matching of the mechanical moments to maintain the semiconductor
layer substantially un-warped, includes using at least some
multilayer metallizations.
Description
INCORPORATION BY REFERENCE
[0001] This application is a Continuation-in-Part Application of
U.S. patent application Ser. No. 11/336,714, filed on Jan. 20,
2006, by David K. Fork et al., and entitled "Solar Cell Production
Using Non-Contact Patterning and Direct-Write Metallization"; U.S.
patent application Ser. No. 11/282,882, filed on Nov. 17, 2005, by
David K. Fork et al., and entitled, "Extrusion/Dispensing Systems
and Methods"; and U.S. patent application Ser. No. 11/282,829,
filed on Nov. 17, 2005, by David K. Fork et al., and entitled,
"Extrusion/Dispensing Systems and Methods," each of which are
hereby incorporated by reference in their entireties.
BACKGROUND
[0002] This application relates to the conversion of light
irradiation to electrical energy, and more particularly, to methods
and tools for producing bifacial photovoltaic devices (e.g.,
bifacial solar cells) and arrangements of bifacial devices (e.g.,
bifacial solar cell modules) that convert solar energy to
electrical energy.
[0003] Solar cells are typically photovoltaic devices that convert
sunlight directly into electricity. Solar cells commonly include a
semiconductor (e.g., silicon) that absorbs light irradiation (e.g.,
sunlight) in a way that creates free electrons, which in turn are
caused to flow in the presence of a built-in field to create direct
current (DC) power. The DC power generated by several PV cells may
be collected on a grid placed on the cell. Current from multiple PV
cells is then combined by series and parallel combinations into
higher currents and voltages. The DC power thus collected may then
be sent over wires, often many dozens or even hundreds of
wires.
[0004] Presently, the majority of solar cells are manufactured
using a screen printed process which screen prints front and back
contacts. The back contact is commonly provided as a layer of
aluminum. The aluminum layer will cover most if not all the back
layer of the silicon wafer, thereby blocking any light which would
reflect onto the back surface of the silicon wafer. These types of
solar cells therefore receive and convert sunlight only from the
front exposed surface.
[0005] However, another type of known solar cell is a bifacial
solar cell, which acquires light from both surfaces of the solar
cell and converts the light into electrical energy. Solar cells
which are capable of receiving light on both surfaces are available
on the market. One example is the HIT solar cell from Sanyo
Corporation of Japan, as well as bifacial solar cells sold by
Hitachi Corporation, also of Japan.
[0006] Drawbacks with existing bifacial solar cells include those
related to the manufacturing processes. Various ones of these
drawbacks are similar to those drawbacks existing in the
manufacture of single-sided solar cells, such as discussed, for
example, in U.S. patent application Ser. No. 11/336,714, previously
incorporated herein by reference. As discussed in that document,
desired but largely unavailable features in a wafer-processing tool
for making solar cells are as follows: (a) never breaks a
wafer--e.g. non contact; (b) one second processing time (i.e., 3600
wafers/hour); (c) large process window; and (d) 24/7 operation
other than scheduled maintenance less than one time per week. The
desired but largely unavailable features in a low-cost metal
semiconductor contact for solar cells are as follows: (a) Minimal
contact area--to avoid surface recombination; (b) Shallow contact
depth--to avoid shunting or otherwise damaging the cell's pn
junction; (c) Low contact resistance to lightly doped silicon; and
(d) High aspect metal features (to avoid grid shading while
providing low resistance to current flow).
[0007] It is particularly desirable to provide feature placement
with high accuracy for feature sizes below 100 microns. By
minimizing the feature sizes, more surface area is available for
the accumulation and conversion of solar light. Features on the
order of 10 microns or smaller can suffice for extracting current.
For a given density of features, such a size reduction may reduce
the total metal-semiconductor interface area and its associated
carrier recombination by a factor of 100.
[0008] Further, a major cost in solar cell production is that of
the silicon layer itself. Therefore, the use of thinner layers is
desirable as one way of reducing costs associated with the
manufacture of solar cells. However, with existing technology, the
manufacture of thin crystalline (silicon) layers (e.g., 150 microns
or less) is not commercially feasible, if not impossible, due to
the previously mentioned unavailable features, and because the
contact layers such as silver, aluminum, etc., cause the
semiconductor layers to warp or bow.
[0009] In addition, such thin devices in general have a problem
that not all light is absorbed by the thin cell. To reach high
efficiency using a thin silicon layer, cells require a design which
permits a higher percentage of the light to be absorbed. Ideally, a
high efficiency thin cell of any material in construction will
accept light incident on it from either side with minimal loss, and
then trap the useful portion of the solar spectrum so that it is
absorbed to create photovoltaic energy.
SUMMARY
[0010] Provided is a bifacial photovoltaic arrangement which
includes a semiconductor layer having a first surface and a second
surface. A first passivation layer is formed on the first surface
of the semiconductor layer, and a second passivation layer is
formed on the second surface of the semiconductor layer. A
plurality of metallizations are formed on the first and second
passivation layers and are selectively connected to the
semiconductor layer. At least some of the metallizations include an
elongated metal structure having a relatively small width and a
relatively large height extending upward from the first and second
passivation layers
[0011] In accordance with another aspect of the present
application, a bifacial photovoltaic arrangement includes a
bifacial cell having a semiconductor layer with a first surface and
a second surface, and a thickness of about 150 microns or less. A
first passivation layer is formed on the first surface of the
semiconductor layer, and a second passivation layer is formed on a
second surface of the semiconductor layer. A plurality of
metallizations are formed on the first and second passivation
layers, and selectively connect to the semiconductor layer. The
metallizations on the first passivation layer and metallizations on
the second passivation layer have sufficiently similar mechanical
moments to maintain the semiconductor layer un-warped.
[0012] In accordance with a further aspect of the present
application, a method is provided for producing a bifacial
photovoltaic device. The method includes forming a blanket
passivation layer on each of a first surface and a second surface
of a semiconductor layer. A direct-write metallization apparatus
arrangement is utilized to deposit the conductive lines to contact
the doped regions of the semiconductor region.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] These and other features, aspects and advantages of the
present application will become better understood with regard to
the following description, appended claims, and accompanying
drawings, where:
[0014] FIG. 1 is a flow diagram showing a simplified method for
producing photovoltaic devices according to an embodiment of the
present application;
[0015] FIG. 2 is a simplified diagram showing an assembly for
producing photovoltaic devices according to an embodiment of the
present application;
[0016] FIG. 3 depicts a slotted conveyor system to carry wafers in
accordance with an embodiment of the present application;
[0017] FIG. 4 depicts an overhead clamp conveyor arrangement for
processing wafers in accordance with the present application.
[0018] FIG. 5 is a perspective view showing a portion of a bifacial
photovoltaic device during a patterning portion of the production
process of FIG. 1;
[0019] FIG. 6 is a top plan view depicting a laser-based patterning
apparatus utilized in the patterning portion;
[0020] FIG. 7 is a perspective view showing a portion of a bifacial
photovoltaic device during a first phase of a metallization portion
of the production process of FIG. 1;
[0021] FIG. 8 is a perspective view showing a portion of a bifacial
photovoltaic device during a second phase of the metallization
portion;
[0022] FIG. 9 is a perspective view showing an inkjet-type printing
apparatus utilized during the metallization portion in accordance
with the present application;
[0023] FIG. 10 is a simplified side-view diagram showing an
extrusion-type dispensing apparatus utilized during the
metallization portion in accordance with the present
application;
[0024] FIG. 11 is a cross-sectional side view showing an extrusion
nozzle utilized during a metallization portion according to an
embodiment of the present application;
[0025] FIGS. 12(A) and 12(B) are cross-sectional side views showing
gridlines formed on a photovoltaic device by the extrusion nozzle
during the metallization separation according to an embodiment of
the present application;
[0026] FIG. 13 is a cross-sectional side view showing a simplified
extrusion nozzle and a multilayer gridline obtained during the
metallization operation in accordance with an embodiment of the
present application;
[0027] FIG. 14 is a perspective view showing a bifacial
photovoltaic device produced in accordance with the present
application;
[0028] FIG. 15 depicts a side view of a single-sided cell with
metallized gridlines;
[0029] FIG. 16 depicts the concept of the solar cell of FIG. 15,
having a bowed effect due to a mismatch of coefficients of thermal
expansion between the wafer and gridlines;
[0030] FIG. 17 depicts metallization on a solar cell, wherein the
coefficients of metal expansion between metallization on a top
surface and a bottom surface are equalized to eliminate the bowing
of FIG. 16.
[0031] FIG. 18 is a simplified cross-sectional view of a bifacial
photovoltaic device module containing multiple bifacial solar
cells;
[0032] FIG. 19 is a simplified cross-sectional view of an
alternative bifacial photovoltaic device module; and
[0033] FIG. 20 is a simplified cross-sectional view of an
alternative bifacial photovoltaic device module.
DETAILED DESCRIPTION
[0034] The present application relates to improvements in bifacial
photovoltaic devices (e.g., bifacial solar cells) and bifacial
photovoltaic arrangements (e.g., bifacial solar cell modules) that
can be used, for example, to convert solar power into electrical
energy. The following description is presented to enable one of
ordinary skill in the art to make and use the application as
provided in the context of a particular application and its
requirements. As used herein, directional terms such as "upper",
"lower", "side", "front", "rear", are intended to provide relative
positions for purposes of description, and are not intended to
designate an absolute frame of reference.
[0035] Further, the semiconductor material described herein is at
times referred to as a semiconductor layer, it is to be understood
this term and its variants are intended to be broadly understood as
the material used in the solar device to absorb the solar radiation
for conversion into electrical energy, while the term solar device
is at times called a solar cell, photovoltaic cell, photoelectric
cell, among other descriptions. Therefore, use of the term
semiconductor layer (and its variants) will, among other
descriptions, be understood to encompass wafers as well as thin
film materials used to make solar devices, including bifacial solar
devices. Various modifications to the preferred embodiment will be
apparent to those with skill in the art, and the general principles
defined herein may be applied to other embodiments. Therefore, the
present application is not intended to be limited to the particular
embodiments shown and described, but is to be accorded the widest
scope consistent with the principles and novel features herein
disclosed.
[0036] FIG. 1 is a flow diagram indicating basic processing steps
utilized to produce bifacial photovoltaic devices in accordance
with an embodiment of the present application. FIG. 2 is a
simplified block diagram illustrating an assembly 230 for
processing photovoltaic devices using a method such as but not
limited to that of FIG. 1.
[0037] The flow diagram of FIG. 1 begins with a step 110 of forming
devices with doped diffusion regions of a semiconductor layer, and
first and second passivation layers over the doped semiconductor
regions. The formation of such devices is well known in the art. In
step 120 a non-contact patterning apparatus is used to define
openings through the first passivation layer on the first surface
of the device. Thereafter, in step 130, a direct-write
metallization apparatus is employed to deposit contact structures
through the defined openings of the first passivation layer and
onto the first semiconductor layer surface over the doped fusion
regions. The process then moves to step 140, where the
semiconductor layer is heated to a temperature sufficient to set
the deposited contact structures on the first surface side. This
operation is undertaken so the deposited contact structure material
will not be negatively impacted when, as will be explained in the
following steps, the device is flipped or rotated such that the
front side of the device becomes the back side, thereby exposing
the back side for processing.
[0038] More particularly, once the semiconductor layer has been
heated so as to set the deposited contact structures, the process
moves to step 150 where the device is flipped or rotated to present
the back side for processing. In step 160 another non-contact
patterning apparatus is used to define openings in the second
passivation layer. Thereafter, in step 170, another direct-write
metallization apparatus is used to deposit contact structures
through the defined openings of the second passivation layer and
onto the second semiconductor layer surface over the doped
diffusion regions. Of course, the above steps may be undertaken up
to. step 130 or alternatively 140. In this case, a single side of
the semi-conductor layer is processed, so the device can be
employed as a single-sided photovoltaic (solar cell) device. At
times when used as a single-sided device, the second passivation
layer may not be needed.
[0039] Referring now to FIG. 2, a system to manufacture the
bifacial photovoltaic devices is discussed in more detail.
Particularly an upper surface 213 and lower surface 213' of a
semiconductor wafer 212 has been treated to include one or more
doped upper surface regions 214, and doped lower surface regions
214' (while doped regions 214' are not fully visible in FIG. 2,
they are, in this embodiment configured the same as doped regions
213). Blanket passivation layer 215 is formed on the upper surface
213 over doped regions 214 and blanket passivation layer 215' is
formed on the lower surface 213' over doped regions 214'. As
referred to herein, the photovoltaic device is generally identified
as "device 211", and at each stage of the processing cycle is
referenced with an appended suffix indicating the device's current
processing stage (e.g., prior to and during loading, the device is
referenced as "device 211T1", with the suffix "T1" indicating a
relatively early point in the process cycle). The operations used
to provide device 211T1 with doped regions 214, 214' and
passivation layers 215, 215' are performed using well-known
processing techniques, and thus the equipment utilized to produce
device 211T1 is depicted generally in FIG. 2 as wafer processing
system 210.
[0040] After initial treatment, device 211T1 is transferred to an
optional loading mechanism 220 of processing system (tool) 230,
which loads device 211T1 onto a conveyor 235. In accordance with
the present concepts, processing system 230 includes at least one
non-contact patterning device 240, and at least one direct-write
metallization device 250, sequentially arranged in the conveying
direction of conveyor 235 (e.g., to the right in FIG. 2). As used
herein, "direct-write metallization device" is defined as a device
in which the metallization material is ejected, extruded, or
otherwise deposited only onto the portions of the semiconductor
layer where the metallization is needed (i.e., without requiring a
subsequent mask and/or etching process to remove some of the
metallization material). In the present application, metallizations
may refer to gridlines contact portions, and/or bus bars alone or
in combination as used, for example, in the configuration of solar
cells. Further, a plurality of metallizations may mean a number of
individual gridlines, irrespective of whether they are physically
and/or electrically joined together, and/or joined to a bus bar or
bus bars. Thus metallizations are not to be thought of as a blanket
layer of metallization, such as for example found as the backsides
of many types of single-sided solar cells.
[0041] A heater apparatus 260 may be provided when it is necessary
to apply a sufficient temperature to the directly written metal
material in order to set the directly written metal material such
that when device 211T4 is flipped or turned, the metal material
will not be smeared or otherwise negatively affected. In one
embodiment, the heater apparatus may be an inductive heater, an
electric heater, a microwave heater, or other appropriate heating
mechanism located near conveyor 235 of processing system 230,
whereby the device 211T4 is able to move past or through heater
apparatus 260 without being removed from conveyor 235. To set the
directly written metal material, the heater supplies a temperature
based on the specific characteristics of the metal material. In
many instances, the temperature necessary to set the metal material
deposited by direct-write metallization apparatus 250 will be in
the range of 120.degree. C. to 140.degree. C. However, the
temperature and amount of heat applied may vary depending on the
particular materials used.
[0042] In an alternative embodiment, heater apparatus 260 may be
located distanced from conveyor 235, and therefore the processing
system will include additional components to off-load device 211T4
to the distanced apparatus, and thereafter return device 211T4 back
to the conveyor.
[0043] In another alternative embodiment, the metal bearing
material may contain compounds that polymerize or otherwise
stabilize mechanically (that is convert from liquid to solid) in
the presence of heat or light (particularly ultraviolet light). A
light source may be provided to cure a portion of the applied metal
material causing it to substantially retain its shape throughout
the remainder of the process flow.
[0044] With attention to still another embodiment, the direct-write
metallization process may employ a hot-melt material. Such a
material may come in the form of a phase charge paste, where for
example, it is in a liquid state as it passes through a heated
printhead, and then solidifies or freezes as it is placed into
contact with a substrate. In this case, the substrate may be the
passivation layers and/or surfaces of the semiconductor layer. One
typical phase change paste will include wax as the hot melt
material, along with the appropriate metal material.
[0045] Following setting of the metal material on device 211T5, the
device is rotated or flipped by flipping apparatus 270 to place the
front surface, which has been processed, face down on the conveyor
235 in order to expose the back surface for processing. Such a
flipping apparatus would be well known in the art. To process the
back surface of device 211T5, processing system 230 includes a
second non-contact patterning apparatus 240' and a second
direct-write metallization apparatus 250'. By use of these
components, the second surface of device 211T6 is processed by
non-contact patterning apparatus 240', and device 211T7 is
processed by direct-write metallization apparatus 250', in a manner
similar to that as described in connection with non-contact
patterning apparatus 240 and direct-write metallization apparatus
250.
[0046] Processing system 230 also includes an optional off-loading
mechanism 280 for removing processed devices 211T8 from conveyor
235 after processing by direct-write metallization apparatus 250'
is completed. The removed devices are then transferred to a
post-metallization processing system 290 for subsequent processing.
Optional loading mechanism 220 and off-loading mechanism 280
operate in a manner well known to those skilled in the art, and
therefore is not described in additional detail herein.
[0047] In an alternative embodiment, a heater apparatus similar to
heater apparatus 260 may also be included in the system, following
the direct-write metallization apparatus 250'. However, this heater
is optional, since to fully process device 211T8, it will be heated
to a temperature higher (e.g., approximately 600.degree.
C.-900.degree. C.) than the setting temperature (e.g., 120.degree.
C. to 140.degree. C.). Therefore, the setting of the metal material
on the back surface and final heating of the entire device may be
accomplished in a single step by a heater arrangement of the
post-metallization processing system 290.
[0048] Of course, other process flows may be used depending on
specific devices being manufactured, and the apparatuses and
arrangement of apparatuses within the processing system. For
example, a processing system employing the concepts of
to-be-discussed FIGS. 3 and 4 would not need to employ flipping
apparatus 270.
[0049] With continuing attention to processing system 230, conveyor
235 is depicted in FIG. 2 as a belt-type conveyor mechanism in
which a planar belt conveys devices 211T1 to non-contact patterning
devices 240, 240' and direct-write metallization devices 250, 250'
with at least one side or surface of the devices in contact with
the conveyor belt. The use of belt-like conveyor 235 in the
depicted generalized system is intended to be exemplary and not
limiting.
[0050] More particularly, and prior to continued discussion of
FIGS. 1 and 2, an alternative to manufacturing the bifacial solar
cells with the semiconductor layer positioned flat on conveyor 235,
is to locate device 211T1 such that both the front surface and back
surface are exposed at the same time to permit simultaneous, or
near simultaneous operations on both surfaces. As illustrated in
the side view of FIG. 3, processing system 230 may be configured
with a conveyor 235a having a slot 236 into which device 211T1 is
loaded. Slot 236 is sized to securely hold device 211T1 such that
device 211T1 does not move back and forth, or slide, during
processing. The width of slot 236 defines the amount of tension
placed on device 211T1 and the depth of slot 236 is selected to
permit sufficient access for complete processing. The specific
width and depth of slot 236 is dependant on the material, size and
thickness of device 211T1. Slot 236 may be a plurality of
individual, separately defined slot areas or one continuous slot,
and may be formed on top of the belt surface or manufactured within
the belt.
[0051] FIG. 4 depicts a further conveyor embodiment wherein the
conveyor is an overhead clamp type conveyor 237 and device 211T1 is
held by an overhead clamp 238, having engaging arms 238a, 238b. The
engaging arms are sufficiently sized and of appropriate tension to
maintain device 211T1 steady during processing.
[0052] By positioning device 211T1 in a substantially vertical
position, it is possible to undertake processing operations without
the requirement of flipping device 211T1. Therefore, operations on
the two sides may be done simultaneously, or sequentially. For
example, non-contact patterning apparatuses 240 and 240' may be
aligned across from each other on opposite sides of device 211T1 (a
similar arrangement may be made with direct-write metallization
apparatuses 250, 250'). In an alternative arrangement, these
apparatuses may be offset from each other, such that only a single
operation is being performed on either side of the surface at one
time. Still further, the overhead conveyor implementation of FIG. 4
may be designed whereby the overhead clamp 238 swivels such that
all of the processing apparatuses are located on a single side, and
device 211T1 is rotated to be processed by the different
apparatuses.
[0053] It is to be appreciated that, while device 211T1 of FIGS. 3
and 4 is depicted in a round or circular configuration, the present
concepts are equally applicable to square or pseudo-square
configurations (e.g., that is round semiconductor layers that are
squared-up by chopping off four edges) as known in the art, and the
use of the round or circular configuration is not intended to be
limiting.
[0054] Returning now to FIG. 2, non-contact patterning apparatus
240 is utilized to define a plurality of openings 217 through
passivation layer 215, whereby each opening 217 exposes a
corresponding one of said one or more regions on surface 213 of the
semiconductor wafer 212. The results of such processing are
illustrated in FIG. 5. Particularly, in accordance with a present
embodiment, non-contact patterning device 240 is a laser-based
ablation device capable of generating laser pulses LP of sufficient
energy to ablate (remove) portions of passivation layer 215 to form
openings 217 that expose surface portions 213A of substrate 212
without the need for cleaning or other processing prior to
metallization. An advantage of using laser ablation, when compared
to methods such as chemical etching, is that wafer 212 need not be
rinsed and dried after the ablation is performed. Avoidance of
rinsing and drying steps enables the rapid and successive
processing of the contact opening followed by the
metallization.
[0055] In an alternative embodiment, a particle-beam generating
apparatus or other appropriate device which can form openings, such
as openings 217, may be used in place of the laser-based
patterning. It is to be appreciated that when non-contact
patterning apparatus 240' processes the back side of device 211T5,
a similar layout of openings 217' through passivation layer 215'
will be formed on the back side of the device. For convenience of
explanation, a separate figure is not provided, and as such a
two-sided processing is depicted in FIG. 2.
[0056] In a further alternative embodiment, the non-contact
patterning device is not a laser- or particle-beam generating
device used to form the contact openings through the passivation
layers. Rather, a solar paste may be used which can include a glass
frit in an organic vehicle. Upon heating, the organic vehicle
decomposes and the glass frit softens and then dissolves the
surfaces of the passivation layers, creating a pathway to the
semiconductor layer.
[0057] It is to be appreciated the embodiments used to make
connections from the semiconductor layer to the metallizations,
such as the contact portions, gridlines, etc. may result in
situations where less than all of the intended connections are
made, due, for example, to imperfect manufacturing, such as
misalignment over a doped region, incomplete formation of openings,
etc. Therefore, the connections may be considered to be selective
connections, where this may mean all the intended connections, or
some amount less than all of the intended connections, are actually
made.
[0058] In accordance with a specific embodiment to form the
above-mentioned openings 217, FIG. 6 illustrates, non-contact
patterning apparatuses 240, 240' include a scanning-type laser
apparatus 240-1 in which laser pulses LP generated by a laser 310
are directed by way of beam conditioning optics 320 onto a rotating
mirror 330 and through a suitable scan lens 340 such that laser
pulses LP are directed in a predetermined scan pattern across
passivation layers 215, 215' (e.g., silicon nitride). Laser
apparatus 240-1 is similar to those used for writing the
electrostatic image on the photoreceptor of a xerographic print
engine. The throughput of such a laser-processing tool can be on
the order of one semiconductor layer per second, which is a
comparable printing speed to a low to medium range laser printer.
The spot size (i.e., the average diameter D of openings 217, 217')
determines the size of each ablated contact opening 217, 217'. This
size is typically in the range of 5 to 50 microns in diameter.
[0059] In an alternative embodiment, laser-based non-contact
patterning apparatus 240-1 includes a femtosecond laser. The
advantage of using a femtosecond laser is that the laser energy can
be focused to sufficient power that the electric field is strong
enough to ionize the atoms in the passivation layer. This enables
energy absorption in spite of the fact that the laser's photon
energy may be less than the band gap energy of the dielectric
passivation. Thus, passivation material can be ablated with less
debris or finer debris. Debris that are generated can be removed by
a stream of gas flow to prevent their redeposition onto the
device.
[0060] Returning again to FIGS. 1 and 2, after patterning of first
or upper passivation layer 215 is completed, device 211T2 is
transported via conveyor 235 to a point located below direct
metallization apparatus 250, where direct-write metallization
apparatus 250 is utilized to deposit at least contact
(metallization) portions 218 into each opening 217. Contact
portions 218 facilitate electrical connection of current-carrying
conductive (metallization) gridlines 219 to the diffusion regions
formed in wafer 212. Upon completion of the metallization process
by direct-write metallization apparatus 250, device 211T4 is heated
by heater apparatus 260 to a setting temperature (e.g., 120.degree.
C. to 140.degree. C.) to set the metal material deposited by the
direct-write metallization apparatus 250. Thereafter, device 211T5
is flipped by flipper apparatus 270 and the second or back surface
of device 211T6 is presented for processing by non-contact
patterning apparatus 240', and device 211T7 is presented to
direct-write metallization apparatus 250' for processing, each in a
manner as previously described in connection with non-contact
patterning apparatus 240 and direct-write metallization apparatus
250.
[0061] Next, device 211T8 is provided to optional wafer-off loading
mechanism 280, which transports device 211T8 to post-metallization
processing system 290. Thus, in this embodiment, openings 217',
contact (metallization) portions 218', and current-carrying
conductive gridlines 219', are formed in a manner similar to that
as described in connection with the processing of the first or
upper side of device 211T1-211T3. It is to be understood, however,
even though the present embodiment employs similar processes on the
front surface and back surface of device 211, this is not required.
Also, the placement of openings 217, 217', contact portions 218,
218' and conductive gridlines 219, 219', do not need to have
corresponding patterns and locations on each side of the device.
Still further, the materials used for metallization on each side do
not need to be the same. Rather, it is common to use different
materials for the different sides.
[0062] Turning attention to FIGS. 7 and 8, the direct-write
processing described above is illustrated. Particularly, FIG. 7
depicts the sequential deposition of contact material CM from
direct-write metallization apparatus 250 into each opening 217
formed in passivation layer 215 such that contact portions 218 are
formed directly on exposed portions 213A of substrate 212. Note
that contact portions 218 do not necessarily fill openings 215. In
accordance with another aspect of the present application, contact
portions 218 include a silicide-forming metal that diffuses slowly
in silicon. Specific examples of metals currently believed to be
suitable for this purpose include nickel (Ni), cobalt (Co) and
titanium (Ti). These metals are not only less expensive than silver
but they are also demonstrated to enable a lower contact resistance
by a factor of 30 or more. For the n-type emitter contact of the
device, the metal may be selected from among the rare earth
elements. These are known to form low barrier height electrical
contacts to lightly doped n-type silicon. The ink or paste bearing
the silicide forming metal may optionally contain a dopant such as
phosphorous or boron to provide additional doping of the contact
region during the thermal processing steps applied to the deposited
metal. The ink or paste bearing the silicide forming metal may
optionally contain nano-particles of metal. The small size of the
metal particles improves both the dispersion of the particles in
the ink and the reactivity with the silicon.
[0063] FIG. 8, illustrates the further processing of direct-write
metallization apparatus 250, which includes a second deposition
head or nozzle. This second deposition head or nozzle deposits a
second (relatively highly conductive) metal MM into openings 215 to
form a conductive plug 219L on contact portions 218, and optionally
deposits the second metal on passivation layer 215 to form metal
lines 219U in order to complete the production of current-carrying
conductive lines 219. In accordance with an aspect of the
application, second metal MM is different from contact metal CM
(discussed above) in that, instead of being selected for its
ability to form a silicide on silicon, second metal MM is selected
for its electrical conductance, and as such typically has a greater
electrical conductivity than contact metal CM. In one specific
embodiment, second metal MM comprises copper, which is inexpensive
and has excellent conductivity, and is also easily soldered. Note,
however, that if copper is used as contact metal CM and allowed to
diffuse into wafer 212, the copper will create recombination
centers within the device, and these will degrade cell performance.
Therefore, it is desired that each current-carrying conductive
lines 219 include both a silicide contact structure 218 (e.g.,
nickel silicide) disposed at the silicon/metal interface, and a low
resistance conductor 219L/219U (such as copper) formed on contact
metal 218. In this case, the nickel silicide contact structure 218
also acts as a diffusion barrier to prevent poisoning of the
silicon by the copper conductive plug 219L. A preferred source of
Ni is ink composed of suspended particles of nanophase Ni. As
mentioned above, the processing shown in FIGS. 7 and 8 in
connection with an upper surface of a device is equally applicable
to processing of the lower or back surface of device 211T5.
[0064] The immediate execution of metallization following the
formation of contact openings 217 provides the additional advantage
of limiting the air-exposure of exposed portions 213A. This
short-duration exposure prevents the formation of an oxidized
silicon layer that can otherwise interfere with the formation of
the subsequently formed silicide (discussed below). Subsequent
heating of the device after the set heating by heating apparatus
260, for example, during the post-metallization processing 290 of
FIG. 2, drives off remaining volatile components of the ink or
paste. Then a temperature cycle of the device, optionally located
in a reducing ambient such as hydrogen or forming gas, completes
the contact.
[0065] In an alternative embodiment of the present application, the
direct-write metallization devices of the application may be
utilized to print a seedlayer metallization material (e.g., Ni, Cu
or Ag) inside each opening and in a predetermined pattern on the
passivation layers to form one or more seedlayers. After removal
from the conveyor, device is subjected to plating processes,
whereby conductive lines are formed on seedlayers using known
techniques. This embodiment provides an inherently self-aligned
process particularly well suited to fabrication of bifacial solar
cells. In a preferred embodiment, seedlayer metallization material
would be jet printed, fired, and then plated with additional metal.
It is to be appreciated that in order to form a bifacial cell, such
processing is performed of both sides of the device.
[0066] In accordance with another aspect of the present
application, the direct-write metallization apparatuses may be an
inkjet-type printhead or an extrusion-type dispensing nozzle, as
described in the following exemplary embodiments. By arranging such
non-contact, direct-write metallization apparatuses immediately
downstream of the non-contact patterning apparatus (described
above), the present application enables the precise placement of
metallization over the just-formed contact openings without an
expensive and time-consuming alignment step.
[0067] FIG. 9 is a perspective view of an inkjet-type printing
apparatus 250-1 for printing contact structures 218 and conductive
lines 219 in the manner described above according to an embodiment
of the present application for both sides of the device (e.g.,
211T2, 211T5). Such inkjet-type printing apparatus is disclosed,
for example, in co-owned U.S. patent application Ser. No.
11/282,882, filed Nov. 17, 2005, titled "Extrusion/Dispensing
Systems and Methods" with inventors David K. Fork and Thomas
Hantschel, previously incorporated herein in its entirety. Printing
apparatus 250-1 is mounted over conveyor 235 (partially shown),
which supports device 211T2, (211T5) and includes a print assembly
450 mounted to a printing support structure 480, and a control
circuit 490 (depicted as a computer/workstation).
[0068] Print assembly 450 includes a print head 430 and an optional
camera 470 (having high magnification capabilities) mounted in a
rigid mount 460. Print head 430 includes one or more ejectors 440
mounted in an ejector base 431. Ejectors 440 are configured to
dispense droplets of the appropriate metallization material in a
fluid or paste form onto device 211T2 in the manner described
above.
[0069] Control circuit 490 is configured in accordance with the
approaches described below to provide appropriate control signals
to printing support structure 480. Data source 491 can comprise any
source of data, including input from an in-line sensor (as
described below), a networked computer, a pattern database
connected via a local area network (LAN) or wide area network
(WAN), or even a CD-ROM or other removable storage media. The
control signals provided by computer/workstation 490 control the
motion and printing action of print head 430 as it is translated
relative to device 211T2.
[0070] Note that the printing action can be provided by printing
support structure 480, by conveyor 235, or by both in combination.
Computer/workstation 490 is optionally coupled to receive and
process imaging data from camera 470. In one embodiment, camera 470
provides both manual and automated calibration capabilities for
printing apparatus 250-1.
[0071] By properly calibrating and registering printing apparatus
250-1 with respect to device 211T2 the metallization pattern (e.g.,
contact portions 218 and metal portions 219L and 219U, described
above with reference to FIG. 8) printed by printing apparatus 250-1
can be precisely aligned with openings 215 formed in passivation
layer 215, thereby ensuring a high-yield manufacturing process.
According to an embodiment of the application, apparatus
calibration can be accomplished with a video camera microscope
(such as camera 470) having an optical axis position that is fixed
relative to the ejector positions of the print head. Of course, a
printing apparatus such as the described printing apparatus 250-1,
may also be used in processing device 211T5, ie., the back
surface.
[0072] FIG. 10 is a simplified side-view showing an extrusion-type
dispensing apparatus 250-2 for printing at least one of contact
structure 218 and conductive lines 219 onto wafer 211T2 in the
manner described above according to another embodiment of the
present application. Extrusion-type dispensing apparatus 250-2 is
mounted over conveyor 235 (partially shown), which supports device
211T2, and includes a dispensing nozzle (applicator) 510, an
optional curing component 520, and an optional quenching component
530. In one embodiment, dispensing nozzle 510 includes one or more
openings 515, and is configured to concurrently apply two or more
metallization materials (e.g., a silicide-forming metal paste and a
high-conductivity metal paste) into openings 217 and over
passivation layer 215 to form contact portions 218 and conductive
lines 219. The materials are applied through pushing and/or drawing
techniques (e.g., hot and cold) in which the materials are pushed
(e.g., squeezed, etc.) and/or drawn (e.g., via a vacuum, etc.)
through dispensing nozzle 510 and out one or more openings 515.
Nozzle 510 can be micro-machined with various channels and
structures that receive and converge individual materials. For
instance, nozzle 510 can include N channels, where N is an integer
equal to or greater than one, for merging materials within the
nozzle 510 into a single flow dispensed through opening 515. Each
of the N channels can be used for introducing a different material
and/or multiple channels can be used for introducing a
substantially similar material. Where nozzle 510 includes a single
channel, the different material can be introduced through similar
and/or different ports into the channel. Each channel can extend
through a length (e.g., the entire length or a subset thereof) of
nozzle 510. For instance, one or more of the N channels can be
designed to be shorter than the length of nozzle 510, but
relatively longer than an entrance length in order to produce
laminar flow, wherein flow velocity is stabilized prior to merging
materials. This can be achieved through known micro-machining
techniques such as deep reactive ion etching, wafer bonding,
etc.
[0073] Creating nozzle 510 for laminar flow mitigates and/or
minimizes mixing of materials as the materials traverse through
nozzle 510 and out of opening 515. The N channels may also be
shaped to counteract the effects of surface tension on the
materials as they progress from nozzle 510 to device 211T2.
[0074] Each channel may be uniquely and/or similarly shaped,
including uniform and/or non-uniform shapes. Similar to the
inkjet-type printing apparatus (discussed above), nozzle 510 may be
moved over device 211T2 during dispensing of the materials in order
to produce the desired metallization structures. Curing component
520 and/or quenching component 530 may be utilized to limit the
tendency for the dispensed materials to intermix after extrusion.
For example, curing component may be used to cure the dispensed
materials by thermal, optical and/or other means upon exit from
nozzle 510. Alternatively, quenching component 530 can be used to
cool wafer 212, thereby cooling and solidifying the dispensed
materials immediately after extrusion.
[0075] Of course, an extrusion type dispensing apparatus, such as
extrusion-type dispensing apparatus 250-2, may also be used in
processing of device 211T5, i.e., the back surface.
[0076] To further describe the exemplary embodiment of the
extrusion concepts, attention is directed to FIG. 11, where the
extrusion dispensing apparatus is shown with a dispensing nozzle
510-1 utilized to simultaneously deposit a contact (lower metal)
layer (218A or 218B, as described below) on the surface of wafer
212 and/or passivation layer 215, and one or more conductive
(upper) metal layers (219A or 219B) on contact layer 218A/B. In
this example, the various layers of the gridlines are co-extruded
high aspect ratio metals (e.g., in a range of 2:1 to 10:1, and
sub-ranges within this range), which permit the coverage area of
the metal material to be in a range of less than 10% to 4% of the
total surface area of the device 211T8.
[0077] FIG. 11 particularly illustrates nozzle 510-1 as having two
or more different materials on wafer 212 and passivation layer 215.
Nozzle 510-1 includes the manifold 620 that includes channels,
which are fabricated to facilitate creating laminar flow in order
to merge materials (i.e., contact material CM and metal material
MM) received in each channel within the manifold 620 into a single
flow of separate materials (with material to material contact)
while mitigating mixing of the materials. The channels are
associated with either ports 636 or ports 638, which are used to
introduce the materials into the manifold 620. The two different
materials are introduced into the manifold 620 in an interleaved
manner such that adjacent channels are used for different
materials. The materials traverse (e.g., via a push, a pull, etc.
technique) through corresponding channels and merge under laminar
flow within the manifold 20 to form a single flow of materials that
are extruded through opening 515-1 onto wafer 212 or passivation
layer 215.
[0078] FIG. 12(A) is a cross-sectional end view showing a high
aspect ratio gridline 219A that is extruded using nozzle 510-1
(FIG. 11) in accordance with an embodiment of the present
application. Gridline 219A includes an elongated central metal
structure 219A-1 having a relatively narrow width and a relatively
large height (i.e., in the direction extending away from the
passivation layer/wafer), and transparent supports 219A-2 formed on
one or both sides of central metal structure 219A-1. In one
embodiment, central metal structure 219A-1 includes a highly
conductive metal such as copper, silver or aluminum, and
transparent supports 219A-2 comprise a low melting glass optimized
for its transparency and adherence to the device surface. Although
not shown, a separate print head may be utilized to print a contact
structure inside each contact opening before the extrusion of
gridline 219A. The benefit of this structure is that it allows the
production of bifacial solar cell devices that produce minimal
interruption of sunlight passing into either side of the device. In
one specific embodiment, contact portion 218A comprising a nickel
bearing paste that is deposited at the gridline-substrate interface
(i.e., in the contact openings and on passivation layer 215), and
upper portion 219A consists of a more conductive metal such as
copper, silver or aluminum. The particular metals being selected
being dependant on the side of the bifacial device being
processed.
[0079] FIG. 12(B) is a cross-sectional end view showing another
high aspect gridline 219B in accordance with another embodiment of
the present application. Similar to high aspect ratio gridline 219A
(described above), gridline 219B includes a high aspect ratio
central metal structure 219B-1 and transparent supports 219B-2
formed on each side of central metal structure 219B-1. However,
gridline 219B also includes one or more elongated contact metal
layers 218B-1 and 218B-2 that are co-extruded simultaneously with
and are located below central metal structure 219B-1 and
transparent supports 219B-2. As described above, contact metal
layers 218B-1 and 218B-2 include, for example a silicide-forming
metal (or, after treatment, the silicide formed from such a
metal).
[0080] FIG. 13 is a cross-section showing a second nozzle 510-2 and
a second gridline including a multi-layer stack formed by a
-contact forming metal portion 218B, a conductive metal portion
219B, and a solder wefting material SW. These materials are
respectively extruded through openings 515-21, 515-22, and 515-23
in the manner depicted in FIG. 13. Any of these layers may serve a
dual function, for example, copper is both highly conductive and
can readily be soldered. As with other co-extruded structures, the
complete extrusion may optionally include a transparent or
sacrificial structure to the side or sides of the gridline to
support its high-aspect ratio metal portion. In an embodiment using
a sacrificial support structure, the support structure is burned
away during firing of the metallization.
[0081] As set forth in the following exemplary embodiments, the
processing methods described above may be modified to optimize the
production of bifacial cell-type photovoltaic devices.
Particularly, using direct-write metallization apparatuses as
described herein, the metallized area (e.g., gridlines, bus bars,
contact portions) of a surface can, as previously mentioned, be
less than 10% to 4%of the total surface area of a device.
[0082] In one embodiment, the metallization applied over the
contact openings by the direct write metallization devices
described above (i.e., inkjet-type printing apparatus 250-1 and/or
extrusion-type dispensing apparatus 250-2) may, after subsequent
thermal processing, serve as the complete cell metallization in
preparation for tabbing and stringing the cells for module
assembly. Alternatives to tabbing may also be applicable, for
example the adhesive bonding of the cells to a flexible
backplane.
[0083] In another alternative embodiment, instead of linearly
arranged contact openings 217, 217', continuous line openings (not
shown) are formed by laser pulses LP that are used to provide
contact between the gridlines and the N-type diffusion region.
[0084] Turning to FIG. 14, illustrated is a perspective view
showing a bifacial type photovoltaic device 211-1 that is produced
in accordance with an embodiment of the present application. Device
211-1 generally includes a P-type single crystalline silicon wafer
(substrate) 212-1 disposed between a continuous N-type diffusion
region 214-1, which is formed in an upper surface of wafer 212-1,
and a continuous P-type diffusion regions 214-2 formed in a lower
surface of wafer 212-1. Passivation layer 215-1 is formed over
diffusion region 214-1, and passivation layer 215-2 is formed over
diffusion regions 214-2. Pyramid-like light trapping structures
215-1A are formed on a surface of upper passivation layer 215-1 and
pyramid-like light trapping structures 215-2A are formed on a
surface of lower passivation layer 215-2, according to known
techniques. In addition, current-carrying conductive gridlines
219-1 and 219-2 are, respectively, formed over passivation layers
215-1 and 215-2. Gridlines 219-1 and 219-2 are formed using any of
the methods described above, e.g., to include a contact portion
218, 218', lower metal conductive plugs 219L, 219L', and metal
gridline portions 219U, 219U'. Note that gridlines 219-1, 219-2 are
typically narrow parallel metal lines that extend substantially
across the surface of passivation layers 215-1, 215-2.
[0085] FIG. 14 is depicted as a p-pn junction device. It is to be
appreciated, however, this construction is simply one example of a
bifacial cell which may be constructed in accordance with the
concepts of the present application. Particularly, the above
description should not be considered to limit the types of bifacial
cells which may be made in accordance with the present teachings.
It is to be understood the present concepts are equally applicable
to other solar cell structures (e.g., homojunction, heterojunction,
p-i-n/n-i-p, multijunction). Additionally, while the present
bifacial cell shows contact points on both sides of the cell, the
concepts disclosed herein may also be used to produce a bifacial
cell wherein the contacts are in a single side.
[0086] Turning to another aspect of the present application, and as
previously mentioned, a large cost in the manufacture of a solar
cell, is the cost of the semiconductor silicon layer. Therefore, it
is desirable to employ as thin a semiconductor silicon layer as
possible. In existing solar cells, the semiconductor layer is
between 250 and 300 microns in thickness. However, due to the
non-contact concepts of the present application, semiconductor
layers of 150 microns to 100 microns, or even less, may now be
considered for use.
[0087] However, when such thin semiconductor layers are attempted
to be used, problems may occur. For example, for nearly all
processing methods, including screen printing, as well as the above
non-contact processing concepts, when a thin semiconductor silicon
layer is employed in construction of a solar cell (such as a
single-sided, square solar cell), an undesirable effect results. As
shown in FIG. 15, single-sided, square solar cell 1000 has metal
gridlines 1002 applied to a single surface. In this example, the
coefficient of thermal expansion of the deposited gridlines 1002 is
not matched to the coefficient of expansion for the semiconductor
layer 1004. This results in a warping of the semiconductor layer
1004 (which may be a bowing or other deformation) such as shown in
the top view of FIG. 16. Particularly, it is common for the edges
of the semiconductor layer 1004 to pull up from a planar surface
when the semiconductor wafer is too thin.
[0088] This effect will also occur for bifacial cells which have
gridlines on each surface, since there will also be a mismatch of
thermal expansion between the materials. For example, in existing
bifacial cells, silver is commonly used on one surface (e.g., the
top metallization), and aluminum is used on the other surface
(e.g., the bottom metallization). In this instance, since aluminum
and silver, as well as the silicon of the semiconductor layer, have
significantly different coefficients of thermal expansion, the
bifacial cell warps (e.g., bows) when a thin semiconductor layer is
used (e.g., silicon of about 150 microns or less). Although,
depending on the semiconductor layer type and metal material,
bowing may occur for semiconductor layers thicker than 150
microns.
[0089] An aspect of the present application which addresses this
issue, is to manufacture a bifacial cell to insure that the
mechanical moments of the gridlines on the upper surface and the
lower surface are equal. By matching the mechanical moments, the
semiconductor layer itself can be made thinner than presently
possible, while avoiding the warping effect. As shown by the
bifacial cell 1008 of FIG. 17, semiconductor layer 1010 has a
thickness of about 150 to 100 microns or less. Then the material(s)
1012 on the upper surface of semiconductor layer 1010 and the
material(s) 1014 on the bottom surface of layer 1010 are selected
to have coefficients of thermal expansion which sufficiently match,
so counteracting forces are generated which sufficiently cancel
each other out to permit the semiconductor layer to remain
non-warped.
[0090] Use of the materials in the present application results in
the difference in the coefficient of thermal expansion between the
two sides to be sufficiently minimized so as to maintain the wafer
non-warped. Alternatively, if metals of significantly different
coefficients of thermal expansion are used on the different sides
of the bifacial cell, for example, where silver might be used as
the metal material on one surface and aluminum used as the other
metal material, the present application provides a manner to even
out the stresses. As depicted by dotted line layer 1018 in FIG. 17,
a second layer of silver (or other appropriate metal) may be placed
on top of the aluminum layer (e.g., 1014). The amount of layer 1016
is selected to ensure that the mechanical moments of the upper
metallization (1012) are sufficiently equal to the bottom
metallization (1014, 1016).
[0091] Silicon has a coefficient of thermal expansion of
2.8.times.10.sup.-6/.degree. C. The expansivity of most metals is
substantially higher. Silver, which is the commonly used material
for front emitter contact gridlines, has a coefficient of thermal
expansion of 18.9.times.10.sup.-6/.degree. C. Aluminum, which is
the commonly used material for the blanket collector metallization,
has a coefficient of thermal expansion of
23.1.times.10.sup.-6/.degree. C. When the metal is fired at a
temperature on the order of 850.degree. C., it is either liquid in
the case of aluminum, or it is softened. Stress accumulates during
cooling from the firing temperature. The metal structures attempt
to contract more than the silicon, and thereby develop a tensile
stress. This occurs in existing cells which use a blanket layer of
aluminum, because the aluminum layer covers nearly the entire back
surface and is typically over 20 microns thick, and it has a much
larger mechanical moment than the front surface metallization. This
causes the silicon layer to bow toward the aluminum side. Given
that the gridlines on the front surface of a typical screen printed
semiconductor layer cover about 10% of the area, the mechanical
moment of the silver metallization on the front is more than
10.times. smaller than mechanical moment of the aluminum
metallization on the back. The bowing problem becomes more severe
as the semiconductor layer becomes thinner.
[0092] On the other hand, since the present embodiments provide an
improved contact structure on the back surface of the semiconductor
layer, less contact area is needed, and therefore blanket
metallization is unnecessary. By breaking the back surface
metallization into gridlines and bus bars rather than a blanket
layer, it will be appreciated that the quantity of metal can be
reduced by over 90%. This has the desired improvement that the
mechanical moments of the front and back layers are comparable. The
present disclosure permits for the mechanical moments can be
matched even closer by one or more of the following procedures: (1)
The respective widths of the metallizations on each side of the
semiconductor layer can be tailored to equalize the mechanical
moments; (2) The respective thicknesses on each side of the
semiconductor layer of the metallizations can be tailored to
equalize the mechanical moments; (3) The respective volumes of the
metallizations on each side of the semiconductor layer can be
tailored to equalize the mechanical moments; (4) For multilayer
metallizations, for example on the backside of the semiconductor
layer, one might use an aluminum--nickel--silver layered
metallization to produce the desired matching mechanical moment to
silver metallization on the front side of the semiconductor layer.
Other ones of these embodiments take advantage of the fact that (a)
if the lines on the front and back of the semiconductor layer are
of primarily the same metal, e.g. silver, if the volumes are
approximately equal, the mechanical moments will also be
approximately equal and (b) silver is a better electrical conductor
than aluminum. One method for producing the multilayer line is to
employ vertical coextrusion described in U.S. patent application
Ser. No. 11/282,882, filed on Nov. 17, 2005, and entitled,
"Extrusion/Dispensing Systems and Methods."
[0093] The individual solar cells constructed in accordance with
the previously described processes are commonly incorporated in a
bifacial photovoltaic arrangement such as a bifacial solar module
shown in FIG. 18. Particularly, module 1100 of FIG. 18 includes a
front surface or cover 1102, which may be a transparent layer such
as glass or plastic. In one embodiment, the layer may be a modified
ETFE (ethylene-tetrafluoroethylene) fluoropolymer such as Tefzel
from EI Du Pont de Nemours and Company of Wilmington, Del. (Tefzel
is a registered trademark of EI Du Pont de Nemours and Company). A
layer of transparent lamination 1104, such as ethylene vinyl
acetate (EVA) is provided on either side of a plurality of bifacial
solar cells 1106 and interconnects 1108 connecting the bifacial
solar cells 1106 in a cell string. Such a cell string may provide
series, parallel or series parallel cell connection. A back surface
includes a plastic or glass layer 1110, which carries a reflector
1112 on its outer surface. In one embodiment, reflector 1112 is a
metallized mirror on glass or plastic layer 1110. In one
embodiment, the metallized material may be a thin layer applied
such as on mylar sheets. If, as in the present embodiment,
reflector 1112 is on the outer surface of the plastic or glass
layer 1110, the metallized mirror may be coated with a protective
barrier 1114 for environmental stability. The protective barrier
may be a variety of materials, including paint and/or a plastic
laminate.
[0094] During the module forming process, the layers are compressed
and heated to permit the plastic laminate 1104 to melt and solidify
the layers into a single module.
[0095] With attention to reflector 1112, insertion of reflector
1112 permits light entering through front surface 1102, which
passes through the module without being originally absorbed by
bifacial solar cells 1106, or which do not actually pass through
solar cells 1106, to be reflected to the back side surface of solar
cells 1106, whereby efficiency in the collection and conversion of
the light to electricity is improved. It is to be understood
reflector 1112 is sized and positioned to reflect light to the
bifacial solar cells throughout the module, where the module may
include multiple solar cells extending in horizontal and vertical
directions in the same plane. Thus, in one embodiment, the
reflector will be capable of reflecting light to all or at least a
majority of the backsides of the bifacial solar cells of the
module.
[0096] Turning to FIG. 19, illustrated is another embodiment of a
bifacial photovoltaic module 1120. In this embodiment, front
surface layer 1102, laminate layers 1104, solar cells 1106 and
connectors 1108 are positioned similar to that as shown in FIG. 18.
However, in this embodiment, a transparent insulator 1120 is
provided between one of laminate layers 1104 and reflector 1122
carried on an inside surface of back side layer 1124, which may be
plastic or glass. By this design, metallized mirror 1122, is
separated from the array of solar cells 1106 by transparent
insulating layer 1120 to prevent shorting of the cell string. The
transparent insulating layer 1120 to selected to have a melting
point higher than the other lamination materials (such as EVA), in
order to prevent the solar cell strings from melting through and
shorting to the mirror. Alternatively, the metallization may be
patterned so that no conductive path exists from one cell to the
next.
[0097] Turning to yet another embodiment, module 1130 of FIG. 20 is
configured so front surface 1132 of module 1130 can be conventional
glass, and the back surface 1134 is designed to consist of a single
layer or multi-layer film, one surface of which is reflective. If
the reflective surface is metallic, the same considerations apply
as in the case of the glass substrate described above. The
reflective surface may be made using multiple dielectric layers
1134a, 1134b, 1134n. In this case, the reflective surface is
designed to transmit light having wavelengths which are not
convertible by the solar cell into electricity. Specifically, in
many instances, long-wavelength portions of the solar spectrum may
not be absorbed by the various layers of the solar cell. Rather, if
reflected to the solar cell, the long-wavelength portions will
simply heat the solar cell to undesirable temperatures. It is known
that the efficiency of cells degrade as they heat, where for
approximately every 1.degree. C. increase, there is a roll off in
the efficiency. Thus, in this embodiment, the multiple dielectric
layers function as a dichroic "cold mirror" to reduce the operating
temperature of the module. In this design, only those wavelengths
of the solar spectrum which have a capability of efficiently being
transformed into electrical energy are reflected back to the back
side of the solar cells 1106. When a dichroic mirror layer is part
of the multi-layered structure, other layers of the system may be
designed as transparent, to increase the long-term robustness of
the module.
[0098] Additionally, the previously described bifacial cells may be
configured to have the passivation layer configured as an amorphous
silicon surface passivation against the crystalline silicon wafer
to reduce recombination of the electrons and holes. Further, the
metallization scheme will include a transparent conducting oxide
(such as but not limited to ITO) on each side. Such a cell could be
designed to be operationally similar to that of an HIT cell from
Sanyo Corporation. In this design, the metal paste used for the
structures on the solar cell will be a curable material with a cure
temperature below 400.degree. C. This enables forming the metal
gridlines without removing the hydrogenation in the amorphous
silicon. Such metal pastes are available from vendors such as
Cermet, Inc. of Atlanta, Ga.
[0099] With further attention to the embodiments of FIGS. 18-20,
the reflectors may be Lambertian reflectors or specular reflectors.
In another embodiment, the bifacial module may be designed with a
plastic laminate front layer and a glass back layer.
[0100] Although the present application has been described with
respect to certain specific embodiments, it will be clear to those
skilled in the art that the inventive features of the present
application are applicable to other embodiments as well, all of
which are intended to fall within the scope of the present
application. For example, although the description above is
primarily limited to silicon-based photovoltaic devices, the
various aspects of the present application may also be utilized in
the production of photovoltaic devices on wafers formed by
amorphous silicon, CdTe (Cadmium Telluride), or CIGS
(copper-indium-gallium-diselenide), among others. In another
example, although co-extrusion has been described as a procedure
for obtaining high aspect ratio metal lines, other procedures such
as mono-extrusion could be used where applicable. Also, the
preceding may of course be used to manufacture a single-sided
photovoltaic device.
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