U.S. patent application number 12/480699 was filed with the patent office on 2009-12-10 for solar cell, solar module and system and fabrication method thereof.
Invention is credited to Wei-Chih Chang, Yi-Chin Chou, Chorng-Jye Huang, Shih-Cheng Lin, Pin-Sheng Wang.
Application Number | 20090301555 12/480699 |
Document ID | / |
Family ID | 41264167 |
Filed Date | 2009-12-10 |
United States Patent
Application |
20090301555 |
Kind Code |
A1 |
Lin; Shih-Cheng ; et
al. |
December 10, 2009 |
SOLAR CELL, SOLAR MODULE AND SYSTEM AND FABRICATION METHOD
THEREOF
Abstract
A solar cell having an improved structure of rear surface
includes a p-type doped region, a dense metal layer, a loose metal
layer, at least one bus bar opening, and solderable material on or
within the bus bar opening. The solderable material contacts with
the dense aluminum layer. The improved structure in rear surface
increases the light converting efficiency, and provides a good
adhesion between copper ribbon and solar cell layer thereby
providing cost advantages and reducing the complexity in
manufacturing. A solar module and solar system composed of such
solar cell are also disclosed.
Inventors: |
Lin; Shih-Cheng; (Hsinchu
County, TW) ; Chang; Wei-Chih; (Tainan County,
TW) ; Chou; Yi-Chin; (Changhua County, TW) ;
Huang; Chorng-Jye; (Hsinchu City, TW) ; Wang;
Pin-Sheng; (Hsinchu County, TW) |
Correspondence
Address: |
NORTH AMERICA INTELLECTUAL PROPERTY CORPORATION
P.O. BOX 506
MERRIFIELD
VA
22116
US
|
Family ID: |
41264167 |
Appl. No.: |
12/480699 |
Filed: |
June 9, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61060108 |
Jun 9, 2008 |
|
|
|
Current U.S.
Class: |
136/255 |
Current CPC
Class: |
B32B 2327/12 20130101;
H01L 31/1804 20130101; H01L 31/068 20130101; B32B 17/10788
20130101; H01L 31/022425 20130101; Y02E 10/547 20130101; B32B
17/10018 20130101; Y02P 70/521 20151101; Y02P 70/50 20151101; H01L
31/048 20130101 |
Class at
Publication: |
136/255 |
International
Class: |
H01L 31/00 20060101
H01L031/00 |
Claims
1. A solar cell having a front and a rear surface, comprising: a
loose metal layer; a dense metal layer between said loose metal
layer and said rear surface; a p-type doped region between said
dense metal layer and said rear surface; and a bus bar opening
recessed in said loose metal layer to expose a portion of the dense
metal layer.
2. The solar cell of claim 1 wherein a solderable material is
deposited on or within said bus bar opening.
3. The solar cell of claim 1 wherein said dense metal layer is made
of III A group.
4. The solar cell of claim 1 wherein said loose metal layer is made
of III A group.
5. The solar cell of claim 1 wherein said bus bar opening is formed
by removing a partial region of said loose metal layer, and wherein
said bus bar opening is formed by removing said loose metal layer
by a method or a combination of methods selected from the list
including: (1) laser ablation; (2) wet etching; (3) dry etching;
(4) physical removing; (5) ultrasonic cleaning; (6) rubbing.
6. The solar cell of claim 2 wherein said solderable material is
selected from the group consisting of: silver, copper, tin, zinc,
palladium, nickel and alloys or mixtures thereof.
7. The solar cell of claim 1 wherein said dense metal layer
comprises substantial composition of aluminum.
8. The solar cell of claim 1 wherein said loose metal layer
comprises substantial composition of aluminum.
9. The solar cell of claim 1 wherein said bus bar opening is formed
by removing a partial region of said loose metal layer, and wherein
said bus bar opening is formed by removing said loose metal layer
by laser ablation.
10. The solar cell of claim 6 wherein said solderable material is
tin-zinc alloy.
11. The solar cell of claim 2 wherein said solderable material is
deposited on or within said bus bar opening by a process selected
from the group consisting of: (1) sputtering; (2) vapor deposition;
(3) strip or patch coating (4) ink-jet printing, screen printing,
gravure printing, letter printing, thermal printing, dispensing or
transfer printing; (5) stamping; (6) electroplating; (7)
electroless plating, and (8) combinations thereof.
12. The solar cell of claim 1 wherein said p-type doped region is
formed on entire or substantial area of said rear surface of said
solar cell.
13. A solar module comprising: a semiconductor substrate with a
rear surface; a loose metal layer; a dense metal layer between said
loose metal layer and said rear surface; a p-type doped region
between said dense metal layer and said rear surface; at least one
bus bar opening in said loose metal layer; a solderable material on
or within said bus bar opening; and a conductor; wherein said
conductor electrically connects to the said dense metal layer
through said solderable material.
14. The solar module of claim 13 wherein said dense metal layer is
made using the material of III A group.
15. The solar module of claim 13 wherein said loose metal layer is
made using the material of III A group.
16. The solar module of claim 13 wherein said bus bar opening is
formed by removing said loose metal layer by a method or a
combination of methods selected from the list including: (1) laser
ablation; (2) wet etching; (3) dry etching; (4) ultrasonic; (5)
rubbing; or (6) other physical removing.
17. The solar module of claim 13 wherein said solderable material
is selected from a group including: silver, copper, tin, zinc,
palladium, nickel and alloys or mixtures thereof.
18. The solar module of claim 13 wherein said dense metal layer
comprises substantial composition of aluminum.
19. The solar module of claim 13 wherein said loose metal layer
comprises substantial composition of aluminum.
20. The solar module of claim 13 wherein said bus bar opening is
formed by removing said loose metal layer by laser ablation.
21. The solar module of claim 13 wherein said solderable material
is tin-zinc alloy.
22. The solar module of claim 13 wherein said conductor is a metal
alloy comprising tin.
23. The solar module of claim 13 wherein said solderable material
is deposited on or within said bus bar opening by methods
including: (1) sputtering; (2) vapor deposition; (3) strip or patch
coating (4) ink-jet printing, screen printing, gravure printing,
letter printing, thermal printing, dispensing or transfer printing;
(5) stamping; (6) electroplating; (7) electroless plating, or (8)
combinations thereof.
24. The solar module of claim 13 wherein said conductor is coated
by tin alloy.
25. The solar module of claim 13 wherein said p-type doped region
is formed on entire or substantial area of said rear surface of
said solar cell.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to and the benefit of U.S.
provisional application Ser. No. 61/060,108 filed Jun. 9, 2008.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a solar cell and a method
of making the same. More particularly, this invention relates to a
solar cell with an improved rear surface structure, a solar module
and a solar system composed of such solar cell.
[0004] 2. Description of the Prior Art
[0005] For decades, the problems of the petroleum shortage and the
greenhouse effect are getting worse and worse; therefore many
countries invent more and more efforts in the development of energy
saving products, such as solar cell. Worldwide solar system
installations increased by 5,948 MW in 2008, up from 2,826 MW
installed during the previous year. In 1985, annual solar
installation demand was only 21 Megawatts. The market size of solar
cell industry has increased several times from 1985 and has become
a hot business coming after the semiconductor. The solar cells can
be manufactured using a variety of semiconductor material or
organic compound material, however, from the industrial viewpoint;
silicon which is semiconductor is mainly used for the solar
cells.
[0006] The solar cells using silicon can be roughly classified into
a bulk type solar cell using a wafer of mono-crystalline silicon,
multi-crystalline silicon or the like and a thin film solar cell
having a semiconductor film formed on a substrate. The bulk type
solar cell, which is the most commonly known solar cell having the
largest scale of commercial market than other types of solar cells.
It has been well known that in the solar cells, a good ohmic
contact between the electrode and the n/p doped region will make it
easier to collect the electron and hole and further it will also
produce a reduction of ohmic loss and increasing of the
photovoltaic conversion efficiency.
[0007] In the front surface of p type solar cell, the front surface
field is made by doping an n type impurity in the whole front side.
So the front side electrode can be in contact with front n doped
region with good ohmic contact. And further, in the rear surface of
such solar cell, aluminum (Al), which is a cost effective material
with good conductivity is most commonly used for making the p-type
doped region. It has been well known that the p-type doped region
can form the Back Surface Field (BSF) in the rear surface of solar
cell. However, an aluminum oxide layer will be formed upon the
process of forming such p-type doped region. Such aluminum oxide
layer will not only increase contact resistance but also bring the
difficulty for solar cell soldering.
[0008] Several technologies have been disclosed for resolving
foregoing problem, for instance, by depositing a discontinued Al
layer on the rear surface of solar cell to form a discontinued
p-type doped region which is the most commonly used in the
conventional solar cell manufacturing.
[0009] The semiconductor substrate will be exposed in such
discontinued region and a silver paste, or other suitable metal
pastes, can be applied to such discontinued region to form a
solderable interconnection pad.
[0010] However, in such structure, the electron-hole recombination
will be increased, the full-factor (F.F.) and the short-circuit
current (Isc) will be decreased. As a result, photovoltaic
conversion efficiency decreases.
SUMMARY OF THE INVENTION
[0011] Accordingly, it is one object of the invention to provide an
improved rear surface for a solar cell such that the recombination
of electron-hole pairs at the metal/semiconductor interface of the
solar cell is reduced.
[0012] It is another object of the invention to provide a solar
cell with a solderable rear surface which is cost effective in
manufacturing process and material chosen.
[0013] Further, it is a method of the invention to provide an
improved solar cell bettering the efficiency also reducing the
manufacturing cost of solar cells at present
[0014] To achieve the advantages described above, one embodiment of
this invention provides an improved rear surface structure of a
solar cell therefore to be described as: (1) a loose metal layer;
(2) a p-type doped region between the loose metal layer and the
rear surface; (3) a dense metal layer between the loose metal layer
and p-type doped region; (4) at least one bus bar opening made by
removing the loose metal layer; and (5) a solderable material
deposited on or within the bus bar opening. The loose metal layer,
dense metal layer and the p-type doped region thereof are formed by
firing the metal paste. The loose metal layer is a loose structure
composed by metal particles wherein the structure contains many
voids. Relatively, the dense metal layer is a dense structure
composed of metal-silicon alloy that is formed after firing the
metal paste, wherein the composition of the metal-silicon alloy is
metal substantially.
[0015] The bus bar opening is made by removing the loose metal
layer of the pre-selected area on rear surface by the methods
including but not limited to: (1) laser ablation; (2) wet etching
achieved by photo-resist coating and then the loose metal layer is
etched by immersing in chemical solution such as NaCl and H2SO4
contained liquid under a suitable temperature; (3) dry etching
carried out by mixed gas comprising Boron Trichloride (BCl3),
Chloride (Cl2), Chloroform (CHCl3), Nitrogen (N2), and ammonia
(NH3); (4) physical removing by high pressure gas, liquid or solid
particles, and can be realized by spray gun; (5) ultrasonic
cleaning in the water and (6) rubbing. When the bus bar opening is
made by those alternative methods, however, care must be taken to
assure that the dense metal layer should be exposed therefore
providing a good electrical contact with the solderable
material.
[0016] The solderable material is chosen to have a good
solderability with the dense metal layer and ribbon. Thus, for
instance, silver, copper, tin, zinc, palladium, nickel and alloy
thereof could act as a good solderable material in this
invention.
[0017] The solderable material is applied on or within the bus bar
opening by methods including but not limited to: (1) sputtering;
(2) vapor deposition; (3) strip or patch coating (4) ink-jet
printing, screen printing, gravure printing, letter printing,
thermal printing, dispensing or transfer printing; (5) stamping;
(6) electroplating; (7) electroless plating, or (8) combinations
thereof. It is understood that the method of applying solderable
material on or within the bus bar opening will be highly relied on
the composition of the solderable material.
[0018] The metal paste is chosen to act as a p-type doping source,
therefore such metal paste shall be composed of at least one of the
elements of the III A group. Hence, either the loose metal layer or
dense metal layer is made using the metal paste composed of at
least one of the elements of the III A group.
[0019] A method of fabricating foregoing solar cell is also
provided in this invention, The method includes the following
steps: (1) providing a metal paste coated on the entire or
substantial area of the rear surface of solar cell; (2) forming a
loose metal layer, a dense metal layer and a p-type doped region by
firing the metal paste; (3) forming at least one bus bar opening by
selectively removing said loose metal layer; (4) applying a
solderable material on or within the bus bar opening. The p-type
doped region is under said dense metal layer and loose metal layer
and extending on rear surface inwardly and the dense metal is
between p-type doped region and the loose metal layer.
[0020] In addition, a solar module and a solar system composed of
one or more foregoing solar cells are also disclosed in this
invention.
[0021] The basic structure of a solar module of the present
invention is composed of a solar cell assembled with a front film
and a back film which is sandwiched in between the back face member
and a transparent front face member. And such solar cell assembly
is composed of more than one solar cells fabricated by the method
disclosed in the present invention and each solar cell of such
solar cell assembly is interconnected to next one electrically and
mechanically by a conductor.
[0022] Further, the front film and back film are composed of
organic resin compositions such as EVA, EMA, acrylic resins,
silicone elastomers, PVB, epoxy resins or aliphatic
polyesterurethanes and the like.
[0023] Furthermore, in such solar module, generally, glass or a
transparent polymer material such as fluororesin film, an acrylic
resin film and the like is used to for the front face member and
glass materials, metal sheets, hard plastics and the like are used
for the back face member. Still further, the back face member has
an insulating material like a polyethylene terephthalate (PET),
nylon and TEDLAR.RTM. is used between the back face member and the
solar cell assembly.
[0024] There is also a solar system provided having above-mentioned
structure of one or more solar modules with one or more solar cells
encapsulated by front film and back film and then covered by front
face member and back face member, wherein the solar cells are
fabricated by the method disclosed in the present invention. The
structure of such solar system is further composed an inverter. The
applications for the solar system disclosed in the present
invention generally separated by two types: (1) connected with the
utility power grid and (2) disconnected with the utility power
grid.
[0025] These and other objectives of the present invention will no
doubt become obvious to those of ordinary skill in the art after
reading the following detailed description of the preferred
embodiment that is illustrated in the various figures and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a cross-sectional side view of the solar cell
fabricated in the present invention.
[0027] FIG. 2 is a SEM picture in the cross-sectional side view of
the rear surface of the solar cell fabricated in the present
invention.
[0028] FIG. 3 shows the process of the method of making the solar
cell in the present invention.
[0029] FIG. 4 shows one example of a structural view of the solar
module with the solar cells made by the method of the present
invention.
[0030] FIG. 5 is an example of a solar system with a plurality of
solar modules and an inventor described in the present
invention.
DETAILED DESCRIPTION
[0031] In accordance with one embodiment of this invention, an
improved rear surface structure of a solar cell includes: (1) a
loose metal layer; (2) a p-type doped region between the loose
metal layer and the rear surface; (3) a dense metal layer between
the loose metal layer and p-type doped region; (4) at least one bus
bar opening made by removing the loose metal layer; and (5) a
solderable material deposited on or within the bus bar opening. The
loose metal layer, dense metal layer and the p-type doped region
thereof are formed by firing the metal paste. The loose metal layer
is a loose structure composed by metal particles wherein the
structure contains a plurality of voids. Relatively, the dense
metal layer is a dense structure composed of metal-silicon alloy
that is formed after firing the metal paste, wherein the
composition of the metal-silicon alloy is metal substantially.
[0032] The bus bar opening may be made by removing the loose metal
layer of the pre-selected area on rear surface by the methods
including but not limited to: (1) laser ablation; (2) wet etching
achieved by photo-resist coating and then the loose metal layer is
etched by immersing in chemical solution such as NaCl and H2SO4
contained liquid under a suitable temperature; (3) dry etching
carried out by mixed gas comprising Boron Trichloride (BCl3),
Chloride (Cl2), Chloroform (CHCl3), Nitrogen (N2), and ammonia
(NH3); (4) physical removing by high pressure gas, liquid or solid
particles, and can be realized by spray gun; (5) ultrasonic
cleaning in the water and (6) rubbing. When the bus bar opening is
made by those alternative methods, however, care must be taken to
assure that the dense metal layer should be exposed therefore
providing a good electrical contact with the solderable
material.
[0033] The solderable material is chosen to have a good
solderability with the dense metal layer and ribbon. Thus, for
instance, silver, copper, tin, zinc, palladium, nickel and alloy
thereof could act as a good solderable material in this
invention.
[0034] The solderable material is applied on or within the bus bar
opening by methods including but not limited to: (1) sputtering;
(2) vapor deposition; (3) strip or patch coating (4) ink-jet
printing, screen printing, gravure printing, letter printing,
thermal printing, dispensing or transfer printing; (5) stamping;
(6) electroplating; (7) electroless plating, or (8) combinations
thereof. It is understood that the method of applying solderable
material on or within the bus bar opening will be highly relied on
the composition of the solderable material.
[0035] The metal paste is chosen to act as a p-type doping source,
therefore such metal paste shall be composed of at least one of the
elements of the III A group. Hence, either the loose metal layer or
dense metal layer is made using the metal paste composed of at
least one of the elements of the III A group.
[0036] A method of fabricating foregoing solar cell is also
provided in this invention, The method includes the following
steps: (1) providing a metal paste coated on the entire or
substantial area of the rear surface of solar cell; (2) forming a
loose metal layer, a dense metal layer and a p-type doped region by
firing the metal paste; (3) forming at least one bus bar opening by
selectively removing said loose metal layer; (4) applying a
solderable material on or within the bus bar opening. The p-type
doped region is under said dense metal layer and loose metal layer
and extending on rear surface inwardly and the dense metal is
between p-type doped region and the loose metal layer.
[0037] In addition, a solar module and a solar system composed of
one or more foregoing solar cells are also disclosed in this
invention.
[0038] The basic structure of a solar module of the present
invention is composed of a solar cell assembled with a front film
and a back film which is sandwiched in between the back face member
and a transparent front face member. And such solar cell assembly
is composed of one or more solar cells fabricated by the method
disclosed in the present invention and each solar cell of such
solar cell assembly is interconnected to other solar cells
electrically and mechanically by a conductor.
[0039] Further, the front film and back film are composed of
organic resin compositions such as EVA, EMA, acrylic resins,
silicone elastomers, PVB, epoxy resins or aliphatic
polyesterurethanes and the like.
[0040] Furthermore, in such solar module, generally, glass or a
transparent polymer material such as fluororesin film, an acrylic
resin film and the like is used to for the front face member and
glass materials, metal sheets, hard plastics and the like are used
for the back face member. Still further, the back face member has
an insulating material like polyethylene terephthalate (PET), nylon
and TEDLAR.RTM. is used between the back face member and the solar
cell assembly.
[0041] There is also a solar system provided having above-mentioned
structure of one or more solar modules with one or more solar cells
encapsulated by front film and back film and then covered by front
face member and back face member, wherein the solar cells are
fabricated by the method disclosed in the present invention. The
structure of such solar system is further composed an inverter. The
applications for the solar system disclosed in the present
invention generally separated by two types: (1) connected with the
utility power grid and (2) disconnected with the utility power
grid.
[0042] An improved structure of solar cell according to one
embodiment of the invention is shown in FIG. 1 and FIG. 2. The
solar cell includes a semiconductor substrate 101, 201 having a p-n
junction 106 located adjacent to its front surface 107. An
anti-reflective (AR) coating 110 is coated on the front surface 107
of the semiconductor substrate 101, 201. A p-type doped region 102,
202 located adjacent to the rear surface 108 of semiconductor
substrate 101, 201.
[0043] In the preferred embodiment, the metal paste is composed of
aluminum and all area of the rear surface 108 is covered with
aluminum paste and fired at high temperature under 1200.degree. C.
for forming aluminum and silicon alloy. During the cooling down
process, the p-type doped region 102,202, dense aluminum layer
103,203, and loose aluminum layer 104,204 are formed. The dense
aluminum layer 103,203 is in contact with p-type doped region
102,202 so as to form ohmic contacts in between the dense aluminum
layer 103,203, p-type doped region 102,202 and the semiconductor
substrate 101,201. The loose aluminum layer 104,204 is composed of
aluminum particles formed by high temperature reaction of aluminum
paste, and its structure is loose. The loose aluminum layer is a
loose structure composed by metal particles wherein the structure
contains many voids. Relatively, the dense aluminum layer is a
dense structure composed of metal-silicon alloy forming after
firing aluminum paste, wherein the composition of the
aluminum-silicon alloy is aluminum substantially.
[0044] The bus bar opening(s) 109 are made by removing the loose
aluminum layer with laser ablation to thereby expose the underlying
dense aluminum layer 103. The wavelength of laser used in the
preferred embodiment is 300 to 1100 nm. However, care must be taken
to assure that the dense aluminum layer 103,203 should be exposed
therefore providing a good contact electrically with the solderable
material 105.
[0045] A preferred method of manufacturing a completed improved
rear surface structure of solar cell in accordance with one
embodiment of this invention is to be described with reference to
FIG. 3. This method includes the following steps: (1) providing an
aluminum paste coated on the entire or substantial area of the rear
surface 108 of solar cell by printing aluminum paste directly; (2)
forming a loose aluminum layer 104,204, a dense aluminum layer
103,203 and a p-type doped region 102,202 by firing the aluminum
paste; (3) forming at least one bus bar opening 109 by selectively
removing said loose aluminum layer 104,204, wherein the loose
aluminum layer 104,204 is removed by laser ablation with a
wavelength of 300 to 1100 nm; and (4) applying a solderable
material 105 on or within the bus bar opening 109.
[0046] The solderable material 105 is chosen so as to connect the
dense aluminum layer 103,203 and ribbon 403 for the connection of
the solar cell mechanically and electrically to other solar cells.
In the preferred case, the solderable material is composited of
tin-zinc alloy containing 5 to 50% zinc. The solderable material
105 facilitates the formation of solderable contact(s) with ribbon
while at the same time taking the advantage of reducing the
complexity in manufacturing in comparison to the present available
solar cells. In preferred case, the ribbon is coated with tin or
tin-alloy.
[0047] The completed structure of the solar cell is illustratively
shown in FIG. 1. Thereafter the solar cell may be further
processed, e.g., to prepare it for connection to another solar
cell.
[0048] In the preferred structure of solar module shown in the FIG.
4 is composed of a plurality of solar cells 406 interconnected by
ribbon 403 assembled with a front film 402 and a back film 404
which is sandwiched in between the back face member 405 and a
transparent front face member 401. Further, the solar cells 406 are
fabricated by the method disclosed in the present invention. In
preferred solar module, glass is used for the front face member 401
and TEDLAR.RTM. is used for the back face member 405. Further, the
front film 402 and back film 404 are organic resin preferably
composed of EVA.
[0049] A preferred solar system shown in the FIG. 5 is also
provided in present invention, the solar system thereof having the
above-described solar module 501. In particular, such solar system
comprising a plurality of solar modules with solar cells, which are
fabricated by the method disclosed in the present invention. The
structure of such solar system is further integrated with an
inverter 502.
Example
[0050] Hereinafter shows the results of experiments conducted on
solar cell elements fabricated according to the foregoing
embodiment.
[0051] As to substrate, two kinds of flat plate of p-type
multi-crystalline silicon substrate and p-type monocrystallne
silicon substrate of 156 mm.times.156 mm in size both are
fabricated by the foregoing method.
TABLE-US-00001 TABLE 1 Voc (V) Isc (A) FF (%) Eff. (%) Polysilicon
Conventional 0.61 7.76 77.14 14.97 This invention 0.61 7.78 77.62
15.11 Monosilicon Conventional 0.61 8.32 78.17 16.66 This invention
0.61 8.33 78.51 16.81
[0052] Table 1 shows the result of cell efficiency fabricated by
the conventional method and the foregoing method pursuant to the
preferred embodiment. By increasing Isc and FF, the efficiency is
improved 0.14% in multicrystalline and 0.15% in monocrystalline in
this invention.
[0053] In view of the foregoing description of a preferred
embodiment of this invention, various additional alternations,
modifications, variations, and obvious substitutions will occur to
those skilled in the art. It, therefore, should be understood that
the above description of the invention is intended to be
illustrative only, and in no way limiting the scope of the
invention. The invention rather should be understood as being
limited only by the terms of the appended claims.
[0054] Those skilled in the art will readily observe that numerous
modifications and alterations of the device and method may be made
while retaining the teachings of the invention. Accordingly, the
above disclosure should be construed as limited only by the metes
and bounds of the appended claims.
* * * * *