U.S. patent application number 16/345751 was filed with the patent office on 2020-02-20 for bifacial tube-type perc solar cell, preparation method thereof, and production device therefor.
The applicant listed for this patent is GUANGDONG AIKO SOLAR ENERGY TECHNOLOGY CO., LTD, ZHEJIANG AIKO SOLAR ENERGY TECHNOLOGY CO., LTD.. Invention is credited to Gang Chen, Jiebin Fang, Nailin He, Ta-Neng Ho, Chun-Wen Lai, Kang-Cheng Lin, Wenjie Yin.
Application Number | 20200058817 16/345751 |
Document ID | / |
Family ID | 60027586 |
Filed Date | 2020-02-20 |
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United States Patent
Application |
20200058817 |
Kind Code |
A1 |
Fang; Jiebin ; et
al. |
February 20, 2020 |
BIFACIAL TUBE-TYPE PERC SOLAR CELL, PREPARATION METHOD THEREOF, AND
PRODUCTION DEVICE THEREFOR
Abstract
The present invention discloses a bifacial tube-type PERC solar
cell, which comprises a rear silver major grid line, a rear
aluminum grid line, a rear surface composite film, P-type silicon,
an N-type emitter, a front surface silicon nitride film, and a
front silver electrode. The present invention also discloses a
method and a device for preparing a bifacial tube-type PERC solar
cell. The present invention absorbs sunlight on both surfaces, has
high photoelectric conversion efficiency, high appearance quality,
and high EL yield, and could solve the problems of both scratching
and undesirable deposition.
Inventors: |
Fang; Jiebin; (Foshan,
Guangdong, CN) ; Lin; Kang-Cheng; (Foshan, Guangdong,
CN) ; Lai; Chun-Wen; (Foshan, Guangdong, CN) ;
He; Nailin; (Foshan, Guangdong, CN) ; Yin;
Wenjie; (Foshan, Guangdong, CN) ; Ho; Ta-Neng;
(Foshan, Guangdong, CN) ; Chen; Gang; (Foshan,
Guangdong, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GUANGDONG AIKO SOLAR ENERGY TECHNOLOGY CO., LTD
ZHEJIANG AIKO SOLAR ENERGY TECHNOLOGY CO., LTD. |
Foshan, Guangdong
Yiwu, Zhejiang |
|
CN
CN |
|
|
Family ID: |
60027586 |
Appl. No.: |
16/345751 |
Filed: |
May 25, 2017 |
PCT Filed: |
May 25, 2017 |
PCT NO: |
PCT/CN2017/086030 |
371 Date: |
April 29, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 16/402 20130101;
H01L 31/068 20130101; H01L 31/1876 20130101; Y02E 10/50 20130101;
H01L 31/18 20130101; C23C 16/308 20130101; C23C 16/458 20130101;
C30B 33/00 20130101; C23C 16/56 20130101; H01L 31/0684 20130101;
C23C 16/403 20130101; H01L 31/022425 20130101; H01L 31/02363
20130101; C23C 16/50 20130101; C23C 16/345 20130101; H01L 31/1868
20130101; H01L 31/0236 20130101; Y02P 70/521 20151101 |
International
Class: |
H01L 31/068 20060101
H01L031/068; H01L 31/0224 20060101 H01L031/0224; C23C 16/30
20060101 C23C016/30; C23C 16/34 20060101 C23C016/34; C23C 16/40
20060101 C23C016/40; H01L 31/0236 20060101 H01L031/0236 |
Foreign Application Data
Date |
Code |
Application Number |
May 18, 2017 |
CN |
201710353392.3 |
Claims
1. A bifacial tube-type PERC solar cell, wherein it comprises a
rear silver major grid line, a rear aluminum grid line, a rear
surface composite film, P-type silicon, an N-type emitter, a front
surface silicon nitride film, and a front silver electrode; the
rear surface composite film, the P-type silicon, the N-type
emitter, the front surface silicon nitride film, and the front
silver electrode are stacked and connected sequentially from bottom
to top; the rear surface composite film includes one or more films
selected from a group comprising an aluminum oxide film, a silicon
dioxide film, a silicon oxynitride film, and a silicon nitride
film; it is deposited at a rear surface of a silicon wafer by a
tubular PECVD device; the tubular PECVD device is provided with
four gas lines of silane, ammonia, trimethyl aluminum, and nitrous
oxide; the four gas lines are used alone or in combination to form
the aluminum oxide film, the silicon dioxide film, the silicon
oxynitride film, and the silicon nitride film; a graphite boat is
employed to load and unload the silicon wafer in the tubular PECVD
device; the rear surface composite film forms 30-500
parallel-arranged laser grooving regions by laser grooving; each
laser grooving region includes at least 1 set of laser grooving
unit; the rear aluminum grid line is connected to the P-type
silicon via the laser grooving regions; the rear aluminum grid line
is perpendicularly connected to the rear silver major grid line;
the pin slot of the graphite boat has a depth of 0.6-0.8 mm; a
diameter of a pin base is 6-15 mm, an angle of inclination of an
inclined surface of a pin cap is 35-45 degrees; a thickness of the
pin cap is 1-1.3 mm.
2. (canceled)
3. The bifacial tube-type PERC solar cell according to claim 1,
wherein 3-5 pin marks are formed on the rear surface of the
bifacial tube-type PERC solar cell.
4. The bifacial tube-type PERC solar cell according to claim 1,
wherein a bottom layer of the rear surface composite film is the
aluminum oxide film, and a top layer is composed of one or more
films selected from a group consisting of the silicon dioxide film,
the silicon oxynitride film, and the silicon nitride film.
5. The bifacial tube-type PERC solar cell according to claim 1,
wherein a bottom layer of the rear surface composite film is the
silicon dioxide film; a middle layer of the rear surface composite
film is the aluminum oxide film; a top layer is composed of one or
more films selected from a group consisting of the silicon dioxide
film, the silicon oxynitride film, and the silicon nitride
film.
6. The bifacial tube-type PERC solar cell according to claim 1,
wherein a thickness of the aluminum oxide film is 5-15 nm, a
thickness of the silicon nitride film is 50-150 nm, a thickness of
the silicon oxynitride film is 5-20 nm, and a thickness of the
silicon dioxide film is 1-10 nm.
7. A method of preparing the bifacial tube-type PERC solar cell
according to claim 1, wherein it comprises the following steps:
S101: forming a textured surface on a front surface and a rear
surface of the silicon wafer, the silicon wafer is the P-type
silicon; S102: performing diffusion on the silicon wafer to form
the N-type emitter; S103: removing phosphosilicate glass and
peripheral p-n junctions formed during the diffusion; polishing the
rear surface of the silicon wafer; the depth of rear surface
etching is 3-6 .mu.m; S104: performing annealing on the silicon
wafer; an annealing temperature is 600-820.degree. C., a nitrogen
flow rate is 1-15 L/min, and an oxygen flow rate is 0.1-6 L/min;
S105: depositing the rear surface composite film on the rear
surface of the silicon wafer, including the following: depositing
the aluminum oxide film using TMA and N.sub.2O; a gas flow rate of
TMA is 250-500 sccm, a ratio of TMA to N.sub.2O is 1 to 15-25; a
plasma power is 2000-5000W; depositing the silicon oxynitride film
using silane, ammonia, and nitrous oxide; a gas flow rate of silane
is 50-200 sccm, a ratio of silane to nitrous oxide is 1 to 10-80; a
gas flow rate of ammonia is 0.1-5 slm, the plasma power is
4000-6000W; depositing the silicon nitride film using silane and
ammonia; the gas flow rate of silane is 500-1000 sccm; a ratio of
silane to ammonia is 1 to 6-15; a deposition temperature of the
silicon nitride film is 390-410.degree. C.; a deposition time is
100-400s; the plasma power is 10000-13000W; depositing a silicon
dioxide film using nitrous oxide; a gas flow rate of nitrous oxide
is 0.1-5 slm; the plasma power is 2000-5000W; the tubular PECVD
device is provided with four gas lines of silane, ammonia,
trimethyl aluminum, and nitrous oxide; the graphite boat is
employed to load and unload the silicon wafer in the tubular PECVD
device; the pin slot of the graphite boat has a depth of 0.6-0.8
mm, the diameter of the pin base is 6-15 mm, the angle of
inclination of the inclined surface of the pin cap is 35-45
degrees, the thickness of the pin cap is 1-1.3 mm; S106: depositing
a passivation film on the front surface of the silicon wafer; S107:
performing laser grooving at the rear surface of the silicon wafer,
wherein a laser wavelength is 532 nm, a laser power is above 14W, a
laser scribing speed is above 12 m/s, and a frequency is above 500
kHZ; S108: printing with a paste for the rear silver major grid
line on the rear surface of the silicon wafer; drying; S109:
printing with aluminum paste at the laser grooving regions to
perpendicularly connect with the paste for the rear silver major
grid lines; S110: printing with positive electrode slurry on the
front surface of the silicon wafer; S111: sintering the silicon
wafer at a high temperature to form the rear silver major grid
line, the rear aluminum grid lines, and the front silver electrode;
S112: performing anti-LID annealing on the silicon wafer to obtain
the bifacial tube-type PERC solar cell.
8. The method according to claim 7, wherein depositing the rear
surface composite film on the rear surface of the silicon wafer
includes the following: depositing the aluminum oxide film using
TMA and N.sub.2O; the gas flow rate of TMA is 250-500 sccm, the
ratio of TMA to N.sub.2O is 1 to 15-25; the plasma power is
2000-5000W; depositing the silicon oxynitride film using silane,
ammonia, and nitrous oxide; the gas flow rate of silane is 50-200
sccm, the ratio of silane to nitrous oxide is 1 to 10-80; the gas
flow rate of ammonia is 0.1-5 slm, the plasma power is 4000-6000W;
depositing the silicon nitride film using silane and ammonia; the
gas flow rate of silane is 500-1000 sccm; the ratio of silane to
ammonia is 1 to 6-15; the deposition temperature of the silicon
nitride film is 390-410.degree. C.; the deposition time is
100-400s; the plasma power is 10000-13000W; depositing a silicon
dioxide film using nitrous oxide; the gas flow rate of nitrous
oxide is 0.1-5 slm; the plasma power is 2000-5000W.
9. A production device for the bifacial tube-type PERC solar cell
according to claim 1, the device is the tubular PECVD device, which
includes a silicon wafer loading area, a furnace body, a gas
cabinet, a vacuum system, a control system, and a graphite boat;
the gas cabinet is provided with a first gas line for silane, a
second gas line for ammonia, a third gas line for
trimethylaluminum, and a fourth gas line for nitrous oxide; the
graphite boat is employed for loading and unloading the silicon
wafer; the graphite boat includes a pin, and the pin includes a pin
shaft, a pin cap, and a pin base; the pin shaft is mounted on the
pin base; the pin cap is connected to the pin shaft; the pin slot
is formed between the pin shaft, the pin cap, and the pin base; the
depth of the pin slot is 0.6-0.8 mm; a diameter of the pin base is
6-15 mm; an angle of inclination of an inclined surface of the pin
cap is 35-45 degrees; a thickness of the pin cap is 1-1.3 mm.
10. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the field of solar cells,
and in particular to a bifacial tube-type PERC solar cell, a
preparation method thereof, and production device therefor.
BACKGROUND OF THE INVENTION
[0002] A crystalline silicon solar cell is a device that
effectively absorbs solar radiation energy and converts light
energy into electrical energy through the photovoltaic effect. When
sunlight reaches the p-n junction of a semiconductor, new
electron-hole pairs are formed. Under the action of the electric
field at the p-n junction, the holes flow from the N zone to the P
zone, and the electrons flow from the P zone to the N zone,
generating current in a connected circuit.
[0003] In a conventional crystalline silicon solar cell, surface
passivation is basically only performed at the front surface, which
involves depositing a layer of silicon nitride on the front surface
of the silicon wafer via PECVD to reduce the recombination rate of
the minority carriers at the front surface. As a result, the
open-circuit voltage and short-circuit current of the crystalline
silicon solar cell are greatly increased, which leads to an
increase of the photoelectric conversion efficiency of the
crystalline silicon solar cell.
[0004] In order to meet the ever-rising requirements for the
photoelectric conversion efficiency of crystalline silicon cells,
people began to study the rear surface passivation of solar cells.
At present, the mainstream method is to use a plate PECVD system to
coat the rear side. The plate PECVD system consists of different
chambers; each chamber coats one film. Once the device is fixed,
the number of layers of a composite film is fixed. Therefore, a
disadvantage of the plate PECVD system is that the combination of
layers in the composite film cannot be flexibly adjusted; thus, it
is impossible to optimize the passivation effect of the rear
surface film, which limits the photoelectric conversion efficiency
of the battery. In addition, the plate PECVD system employs an
indirect plasma method, which gives a less than ideal passivation
effect of the film. The plate PECVD system also has other
disadvantages including low uptime and long maintenance time, which
affects its capacity and yield.
[0005] The present invention employs tubular PECVD technology to
deposit a composite film on the rear surface of a silicon wafer in
order to produce a bifacial high-efficiency PERC solar cell. Due to
the fact that tubular PECVD technology adopts a direct plasma
method and could flexibly adjust the composition and combination of
layers in a composite membrane, the passivation effect of the film
is good; the photoelectric conversion efficiency of the PERC solar
cell can be significantly improved. The excellent passivation
ability and process flexibility of tubular PECVD technology allow
the thickness of an aluminum oxide film to be reduced, thus
reducing the consumption of TMA. In addition, tubular PERC
technology can be easily maintained and has a high uptime. In view
of the above, employing tubular PECVD technology to produce
high-efficiency PERC cells has a significant overall cost advantage
over employing plate PECVD technology.
[0006] Despite the above, the cells produced by tubular PECVD
technology have poor appearance quality and low electroluminescence
(EL) yield due to contradictive problems of undesirable deposition
and scratching; these drawbacks prevent the application of this
technology in mass production.
[0007] In a tubular PECVD coating equipment, a silicon wafer is
first inserted into a graphite boat, and then the graphite boat is
fed into a quartz tube for coating deposition. In the graphite
boat, the silicon wafer is fixed to the graphite boat wall via
three pins; one side of the silicon wafer is in contact with the
graphite boat wall, and a film is deposited on the other side of
the silicon wafer. To allow the formation of a uniform film, the
silicon wafer should be in tight contact with the graphite boat
wall. Therefore, the width of the pin slot is set to be small;
about 0.25 mm. There are two advantages in deposition employing
tubular PECVD: 1. during the insertion process, the silicon wafer
slides against the graphite boat wall and scratches the surface of
the silicon wafer which is adjacent to the graphite boat wall; 2.
during the deposition process, due to the inevitable presence of a
gap between the silicon wafer and the graphite boat wall (and in
particular a large gap at the position of the pins), the reacting
gas would diffuse to the other surface of the silicon wafer and
deposits at the other side, leading to undesirable deposition;
hence, undesirable deposition is more serious at the pins.
[0008] When using tubular PECVD to coat the front surface of a
conventional solar cell, scratching and undesirable deposition do
not affect the quality of the cell product; the reasons are as
follows: 1. there are no p-n junctions and coating on the rear
surface of the silicon wafer, hence scratches would not affect the
electrical performance and the EL yield of the cell; 2. there is no
coating on the rear surface of the silicon wafer, hence the
relatively thin undesirable deposition on the rear surface does not
appear obvious and does not affect the appearance quality of the
cell.
[0009] On the other hand, when using tubular PECVD to form the rear
surface film of a PERC cell, scratching and undesirable coating
would seriously affect the pass rate of the cell product. The
problems are as follows: 1. during the deposition of the rear
surface film, undesirable coating would take place at the edges of
the front surface of the cell. As the PERC cell is coated on both
sides, the coating at the edges of the front surface would be
relatively thick; consequentially, graphite boat marks and color
difference appear at the edges of the front surface of the cell,
affecting the appearance quality; 2. when being inserted into the
graphite boat, the front surface of the silicon wafer would be in
contact with the graphite boat wall, scratching the p-n junctions
at the front surface; as a result, scratches would be present
during the EL test, and the electrical performance of the cell
would be affected.
SUMMARY OF THE INVENTION
[0010] The objective of the present invention is to provide a
bifacial tube-type PERC solar cell which can absorb sunlight on
both surfaces, has high photoelectric conversion efficiency, high
appearance quality, and high EL yield, and could solve the problems
of both scratching and undesirable deposition.
[0011] Another objective of the present invention is to provide a
method of preparing the bifacial tube-type PERC solar cell, which
is simple, can be carried out at a large scale, and is compatible
with existing production lines. The solar cells produced have high
appearance quality and high EL yield, and could solve the problems
of both scratching and undesirable deposition.
[0012] Yet another objective of the present invention is to provide
a device for producing the bifacial tube-type PERC solar cell. The
device has a simple structure, is able to lower production cost,
and has large capacity and yield. The solar cells produced have
high appearance quality and high EL yield, and could solve the
problems of both scratching and undesirable deposition.
[0013] To achieve the first objective above, the present invention
provides a bifacial tube-type PERC solar cell, wherein it comprises
a rear silver major grid line, a rear aluminum grid line, a rear
surface composite film, P-type silicon, an N-type emitter, a front
surface silicon nitride film, and a front silver electrode; the
rear surface composite film, the P-type silicon, the N-type
emitter, the front surface silicon nitride film, and the front
silver electrode are stacked and connected sequentially from bottom
to top;
[0014] the rear surface composite film includes one or more films
selected from a group comprising an aluminum oxide film, a silicon
dioxide film, a silicon oxynitride film, and a silicon nitride
film; it is deposited at a rear surface of a silicon wafer by a
tubular PECVD device; the tubular PECVD device is provided with
four gas lines of silane, ammonia, trimethyl aluminum, and nitrous
oxide; the four gas lines are used alone or in combination to form
the aluminum oxide film, the silicon dioxide film, the silicon
oxynitride film, and the silicon nitride film; a graphite boat is
employed to load and unload the silicon wafer in the tubular PECVD
device; a pin slot of the graphite boat has a depth of 0.5-1
mm;
[0015] the rear surface composite film forms 30-500
parallel-arranged laser grooving regions by laser grooving; each
laser grooving region includes at least 1 set of laser grooving
unit; the rear aluminum grid line is connected to the P-type
silicon via the laser grooving regions; the rear aluminum grid line
is perpendicularly connected to the rear silver major grid
line.
[0016] As an improvement of the technical solution above, the pin
slot of the graphite boat has a depth of 0.6-0.8 mm; a diameter of
a pin base is 6-15 mm, an angle of inclination of an inclined
surface of a pin cap is 35-45 degrees; a thickness of the pin cap
is 1-1.3 mm.
[0017] As an improvement of the technical solution above, 3-5 pin
marks are formed on the rear surface of the bifacial tube-type PERC
solar cell.
[0018] As an improvement of the technical solution above, a bottom
layer of the rear surface composite film is the aluminum oxide
film, and a top layer is composed of one or more films selected
from a group consisting of the silicon dioxide film, the silicon
oxynitride film, and the silicon nitride film.
[0019] As an improvement of the technical solution above, a bottom
layer of the rear surface composite film is a silicon dioxide film;
a middle layer of the rear surface composite film is the aluminum
oxide film; a top layer is composed of one or more films selected
from a group consisting of the silicon dioxide film, the silicon
oxynitride film, and the silicon nitride film.
[0020] As an improvement of the technical solution above, a
thickness of the aluminum oxide film is 5-15 nm, a thickness of the
silicon nitride film is 50-150 nm, a thickness of the silicon
oxynitride film is 5-20 nm, and a thickness of the silicon dioxide
film is 1-10 nm.
[0021] Accordingly, the present invention also provides a method of
preparing the bifacial tube-type PERC solar cell, wherein it
comprises the following steps:
[0022] S101: forming a textured surface on a front surface and a
rear surface of the silicon wafer, the silicon wafer is the P-type
silicon;
[0023] S102: performing diffusion on the silicon wafer to form the
N-type emitter;
[0024] S103: removing phosphosilicate glass and peripheral p-n
junctions formed during the diffusion; polishing the rear surface
of the silicon wafer; the depth of rear surface etching is 3-6
gm;
[0025] S104: performing annealing on the silicon wafer; an
annealing temperature is 600-820.degree. C., a nitrogen flow rate
is 1-15 L/min, and an oxygen flow rate is 0.1-6 L/min;
[0026] S105: depositing the rear surface composite film on the rear
surface of the silicon wafer, including the following:
[0027] depositing the aluminum oxide film using TMA and N.sub.2O; a
gas flow rate of TMA is 250-500 sccm, a gas flow ratio of TMA to
N.sub.2O is 1 to 15-25; a plasma power is 2000-5000W;
[0028] depositing the silicon oxynitride film using silane,
ammonia, and nitrous oxide; a gas flow rate of silane is 50-200
sccm, a gas flow ratio of silane to nitrous oxide is 1 to 10-80; a
gas flow rate of ammonia is 0.1-5 slm, the plasma power is
4000-6000W;
[0029] depositing the silicon nitride film using silane and
ammonia; the gas flow rate of silane is 500-1000 sccm; a gas flow
ratio of silane to ammonia is 1 to 6-15; a deposition temperature
of the silicon nitride film is 390-410.degree. C.; a deposition
time is 100-400s; the plasma power is 10000-13000W;
[0030] depositing a silicon dioxide film using nitrous oxide; a gas
flow rate of nitrous oxide is 0.1-5 slm; the plasma power is
2000-5000W;
[0031] the tubular PECVD device is provided with four gas lines of
silane, ammonia, trimethyl aluminum, and nitrous oxide; the
graphite boat is employed to load and unload the silicon wafer in
the tubular PECVD device; the pin slot of the graphite boat has a
depth of 0.5-1 mm;
[0032] S106: depositing a passivation film on the front surface of
the silicon wafer;
[0033] S107: performing laser grooving at the rear surface of the
silicon wafer;
[0034] wherein a laser wavelength is 532 nm, a laser power is above
14W, a laser scribing speed is above 12 m/s, and a frequency is
above 500 kHZ;
[0035] S108: printing with a paste for the rear silver major grid
line on the rear surface of the silicon wafer; drying;
[0036] S109: printing with aluminum paste at the laser grooving
regions to perpendicularly connect with the paste for the rear
silver major grid lines;
[0037] S110: printing with positive electrode slurry on the front
surface of the silicon wafer;
[0038] S111: sintering the silicon wafer at a high temperature to
form the rear silver major grid line, the rear aluminum grid lines,
and the front silver electrode;
[0039] S112: performing anti-LID annealing on the silicon wafer to
obtain the bifacial tube-type PERC solar cell.
[0040] As an improvement of the technical solution above,
depositing the rear surface composite film on the rear surface of
the silicon wafer includes the following:
[0041] depositing the aluminum oxide film using TMA and N.sub.2O;
the gas flow rate of TMA is 250-500 sccm, the ratio of TMA to
N.sub.2O is 1 to 15-25; the plasma power is 2000-5000W;
[0042] depositing the silicon oxynitride film using silane,
ammonia, and nitrous oxide; the gas flow rate of silane is 50-200
sccm, the ratio of silane to nitrous oxide is 1 to 10-80; the gas
flow rate of ammonia is 0.1-5 slm, the plasma power is
4000-6000W;
[0043] depositing the silicon nitride film using silane and
ammonia; the gas flow rate of silane is 500-1000 sccm; the ratio of
silane to ammonia is 1 to 6-15; the deposition temperature of the
silicon nitride film is 390-410.degree. C.; the deposition time is
100-400s; the plasma power is 10000-13000W;
[0044] depositing a silicon dioxide film using nitrous oxide; the
gas flow rate of nitrous oxide is 0.1-5 slm; the plasma power is
2000-5000W.
[0045] Accordingly, the present invention also provides a
production device for the bifacial tube-type PERC solar cell, the
device is the tubular PECVD device, which includes a silicon wafer
loading area, a furnace body, a gas cabinet, a vacuum system, a
control system, and a graphite boat; the gas cabinet is provided
with a first gas line for silane, a second gas line for ammonia, a
third gas line for trimethylaluminum, and a fourth gas line for
nitrous oxide;
[0046] the graphite boat is employed for loading and unloading the
silicon wafer; the graphite boat includes a pin, and the pin
includes a pin shaft, a pin cap, and a pin base; the pin shaft is
mounted on the pin base; the pin cap is connected to the pin shaft;
the pin slot is formed between the pin shaft, the pin cap, and the
pin base; the depth of the pin slot is 0.5-1 mm.
[0047] As an improvement of the technical solution above, the depth
of the pin slot is 0.6-0.8 mm; a diameter of the pin base is 6-15
mm; an angle of inclination of an inclined surface of the pin cap
is 35-45 degrees; a thickness of the pin cap is 1-1.3 mm.
[0048] The present invention has the following beneficial
effects:
[0049] First, the bifacial tube-type PERC solar cell of the present
invention has a plurality of aluminum grid lines arranged in
parallel at the rear surface of the cell. These aluminum grid lines
not only replace the aluminum back surface field in an existing
monofacial solar cell and allows light absorption on the rear
surface, but also act as minor grid lines of a rear silver
electrode for electron conduction. The bifacial tube-type PERC
solar cell of the present invention not only lowers production cost
by reducing the amount of silver paste and aluminum paste used, but
also allows light absorption on both surfaces, which significantly
expands the scope of application and improves the photoelectric
conversion efficiency of a solar cell.
[0050] Second, the present invention employs a tubular PECVD device
to deposit the rear surface composite film on the rear surface of
the silicon wafer in order to produce the aforementioned bifacial
PERC solar cell provided with aluminum grid lines. The rear surface
composite film includes one or more films selected from the group
comprising an aluminum oxide film, a silicon dioxide film, a
silicon oxynitride film, and a silicon nitride film, and is
deposited at the rear surface of the silicon wafer by a tubular
PECVD device. The tubular PERC device adopts a direct plasma method
in which the plasma directly bombards the surface of the silicon
wafer and causes significant passivation of the layer. The tubular
PECVD device is provided with four gas lines of silane, ammonia,
trimethyl aluminum, and nitrous oxide; the four gas lines are used
alone or in combination to form the aluminum oxide film, the
silicon dioxide film, the silicon oxynitride film, and the silicon
nitride film. By adjusting the ratio of the gas flow of the four
gas lines, it is possible to obtain different films. In particular,
it is possible to obtain a silicon oxynitride film or a silicon
nitride film having different composition ratios and refractive
indexes by adjusting the ratio of the gas flow. The combination
order, thickness, and composition of the composite film can be
flexibly adjusted, and therefore the production process of the
present invention is flexible and controllable; furthermore, it is
possible to reduce cost and obtain a large yield with this
production process. In addition, the rear surface composite film is
optimized to match the rear aluminum grid lines, which gives the
best passivation effect and significantly increases the
photoelectric conversion efficiency of the PERC cell.
[0051] Third, the present invention adjusts the diameters of the
pin shaft and the pin base to reduce the depth of the inside of the
pin slot. As a result, the gap between the silicon wafer and the
pin base at the position of the pins is reduced; consequentially,
the amount of gas reaching and coating the rear surface of the
silicon wafer is reduced, boat marks at the front surface edges of
the cell are thus much less likely to occur. In addition, the
present invention adequately increases the angle of inclination of
the inclined surface of the pin cap and the thickness of the pin
cap, and adjusts the automatic wafer inserter, thereby slightly
increasing the distance between the silicon wafer and the graphite
boat wall, reducing scratching. Increasing the angle of inclination
of the inclined surface of the pin cap also reduces the impact
force on the silicon wafer from the graphite boat when the silicon
wafer is sliding down the inclined surface, reducing breakage
rate.
[0052] Furthermore, silicon nitride is the outer layer of the rear
surface composite film; as the deposition time increases, the
thickness of the film increases, which causes silicon wafer to
bend. In the present invention, the silicon nitride deposition
temperature is limited to 390-410.degree. C., the deposition time
is limited to 100-400s. By shortening the time and temperature of
silicon nitride deposition, the bending of the silicon wafer can be
reduced, and thus the amount of the undesirable coating can be
reduced. The temperature window for silicon nitride deposition is
very narrow, between 390-410 degrees; this temperature allows the
maximum reduction of the undesirable coating. When the temperature
is further lowered, however, the amount of the undesirable coating
increases.
[0053] To meet the requirements of large-scale production and
minimize the negative impact caused by shortening the silicon
nitride deposition time, the present invention maintains a laser
power of above 14W, a laser scribing speed of above 12 m/s, and a
frequency of above 500 kHZ. This allows the absorption of a
sufficiently large amount of laser energy in per unit area of the
rear surface composite film to effectively groove the composite
film; as a result, the aluminum paste subsequently printed is in
contact with the silicon substrate through the laser grooving
regions.
[0054] To conclude, the bifacial tube-type PERC solar cell of the
present invention can absorb sunlight on both surfaces. It has high
photoelectric conversion efficiency, high appearance quality, and
high EL yield; it solves the problems of both scratching and
undesirable deposition. In addition, the present invention also
provides a method and a device for the production of the
aforementioned cell. The production method of the present invention
is simple, can be carried out at a large scale, and is compatible
with existing production lines. The device has a simple structure,
is able to lower production cost, and has large capacity and
yield.
BRIEF DESCRIPTION OF THE DRAWINGS
[0055] FIG. 1 is a sectional view of the bifacial tube-type PERC
solar cell of the present invention.
[0056] FIG. 2 is a schematic diagram of the rear surface structure
of the bifacial tube-type PERC solar cell of FIG. 1.
[0057] FIG. 3 is a schematic diagram of the first embodiment of the
rear surface composite film of FIG. 1.
[0058] FIG. 4 is a schematic diagram of the second embodiment of
the rear surface composite film of FIG. 1.
[0059] FIG. 5 is a schematic diagram of the third embodiment of the
rear surface composite film of FIG. 1.
[0060] FIG. 6 is a schematic diagram of the fourth embodiment of
the rear surface composite film of FIG. 1.
[0061] FIG. 7 is a schematic diagram of the fifth embodiment of the
rear surface composite film of FIG. 1.
[0062] FIG. 8 is a schematic diagram of the sixth embodiment of the
rear surface composite film of FIG. 1.
[0063] FIG. 9 is a schematic diagram of a device for producing the
bifacial tube-type PERC solar cell of the present invention.
[0064] FIG. 10 is a schematic diagram of the graphite boat shown in
FIG. 9.
[0065] FIG. 11 is a schematic diagram of a pin of the graphite boat
shown in FIG. 10.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
[0066] To more clearly illustrate the objectives, technical
solutions and beneficial of the present invention, the present
invention will be further described in detail with reference to the
accompanying drawings.
[0067] An existing monofacial solar cell has an aluminum back
surface field on the rear surface of the cell and covering the
entire rear surface of the silicon wafer. The aluminum back surface
field increases the open-circuit voltage Voc and the short-circuit
current Jsc, forces the minority carriers away from the surface,
decreases the recombination rate of the minority carriers, leading
to increased cell efficiency. However, as the aluminum back surface
field is opaque, the rear surface of the solar cell, which is
provided with the aluminum back surface field, cannot absorb light
energy; light energy can only be absorbed in the front surface.
Consequentially, it is difficult to significantly improve the
overall photoelectric conversion efficiency of the cell.
[0068] In view of the technical problems above, referring to FIG.
1, the present invention provides a bifacial tube-type PERC solar
cell, which consecutively includes a rear silver major grid line 1,
rear aluminum grid lines 2, a rear surface composite film 3, P-type
silicon 5, an N-type emitter 6, a front surface silicon nitride
film 7, and a front silver electrode 8. The rear surface composite
film 3, the P-type silicon 5, the N-type emitter 6, the front
surface silicon nitride film 7, and the front silver electrode 8
are stacked and connected sequentially from bottom to top.
[0069] The rear surface passivation layer forms 30-500 groups of
parallel-arranged laser grooving regions by laser grooving. Each
laser grooving region includes at least 1 set of laser grooving
unit 9. The rear aluminum grid lines 2 are connected to the P-type
silicon 5 via the laser grooving regions. The rear aluminum grid
lines 2 are perpendicularly connected to the rear silver major grid
line 1.
[0070] The present invention improves on the basis of existing
monofacial PERC solar cells. A plurality of rear aluminum grid
lines 2 are provided instead of an aluminum back surface field.
Laser grooving regions are formed at the rear surface composite
film 3 by laser grooving, and rear aluminum grid lines 2 are
printed on these parallel-arranged laser grooving regions to be in
local contact with the P-type silicon 5. The closely and parallelly
arranged rear aluminum grid lines 2 could increase the open-circuit
voltage Voc and the short-circuit current Jsc, reduce the
recombination rate of the minority carriers, thus increase the
photoelectric conversion efficiency of the cell; they could replace
the aluminum back surface field of the existing monofacial cell. In
addition, the rear aluminum grid lines 2 do not completely cover
the rear surface of the silicon wafer, and thus sunlight could
reach the inside of the silicon wafer through the gaps between the
rear aluminum grid lines 2. In this way, it is possible to achieve
the absorption of light energy at the rear side of the silicon
wafer, which greatly improves the photoelectric conversion
efficiency of the cell.
[0071] Preferably, the number of the rear aluminum grid lines 2 is
equal to the number of laser grooving regions and are both 30-500.
More preferably, the number of the rear aluminum grid lines 2 is
80-220.
[0072] FIG. 2 shows the rear surface of the silicon wafer. The rear
aluminum grid lines 2 are perpendicularly connected to the rear
silver major grind line 1. The rear silver major grid line 1 is a
continuous straight grid line. As laser grooving regions are
present at the rear surface composite film 3, during the printing
of the rear aluminum grid lines 2, the aluminum paste fills the
laser grooving regions and as a result, the rear aluminum grid
lines 2 are in local contract with the P-type silicon, which allows
the transmission of electrons to the rear aluminum grid lines 2 and
the accumulation of electrons from the rear aluminum grid lines 2
at the rear silver major grid line 1 that intersects with the rear
aluminum grid lines 2. Thus, it is understood that the rear
aluminum grid lines 2 of the present invention could increase the
open-circuit voltage Voc and the short-circuit current Jsc, reduce
the recombination rate of the minority carriers, and transmit
electrons. They are able to replace the aluminum back surface field
of the existing monofacial cell and the minor grid lines of a rear
silver electrode. Consequentially, they not only lower production
cost by reducing the amount of silver paste and aluminum paste
used, but also allows light absorption on both surfaces, which
significantly expands the scope of application and improves the
photoelectric conversion efficiency of a solar cell.
[0073] Apart from a continuous straight grid line as shown in FIG.
2, the rear silver major grid line 1 of the present invention could
also be arranged in spaced segments. The adjacent segments are
connected by a connecting line. There are 2-8 rear silver major
grid lines 1, each with a width of 0.5-5 mm.
[0074] It should be noted that when two or more sets of laser
grooving unit 9 are provided in each laser grooving region, they
are arranged in parallel; the spacing between adjacent sets of
laser grooving units 9 is 5-480 gm.
[0075] Each set of laser grooving unit 9 includes at least one
laser grooving unit 9; the pattern of the laser grooving unit 9 is
circular, elliptical, triangular, quadrangular, pentagonal,
hexagonal, cruciform or star-shaped.
[0076] The width of the laser grooving region of the present
invention is 10-500 gm; the width of the aluminum grid lines 2
located beneath the laser grooving region is larger than the width
of the laser grooving region and is 30-550 gm. When the width of
the aforementioned the aluminum grid lines 2 is relatively large
(for example 500 gm) and the width of the laser grooving region is
relatively small (for example 40 .mu.m), it is possible to arrange
multiple sets of laser grooving regions side by side on the same
aluminum grid line 2 to ensure that the rear aluminum grid lines 2
have sufficient contact area with the P-type silicon 5.
[0077] It should be noted that the laser grooving units in the
laser grooving regions may be parallel with or perpendicular to the
aluminum grid lines.
[0078] The present invention employs a tubular PECVD device to
deposit the rear surface composite film on the rear surface of the
silicon wafer in order to produce the aforementioned bifacial PERC
solar cell provided with aluminum grid lines. The tubular PERC
device adopts a direct plasma method in which the plasma directly
bombards the surface of the silicon wafer and causes significant
passivation of the layer. As shown in FIGS. 3-8, the rear surface
composite film 3 includes one or more films selected from the group
comprising an aluminum oxide film, a silicon dioxide film, a
silicon oxynitride film, and a silicon nitride film; it is
deposited at the rear surface of the silicon wafer by a tubular
PECVD device. The tubular PECVD device is provided with four gas
lines of silane, ammonia, trimethyl aluminum, and nitrous oxide;
the four gas lines are used alone or in combination to form the
aluminum oxide film, the silicon dioxide film, the silicon
oxynitride film, and the silicon nitride film. By adjusting the
ratio of the gas flow, it is possible to obtain a silicon
oxynitride film or a silicon nitride film having different
composition ratios and refractive indexes. The order of formation
and the thickness of the aluminum oxide film, the silicon dioxide
film, the silicon oxynitride film, and the silicon nitride film are
adjustable; the composition and refractive index of the silicon
oxynitride film and the silicon nitride film are adjustable.
[0079] The four gas lines of silane, ammonia, trimethylaluminum,
and nitrous oxide s can form different layers with different gas
combinations, different gas flow ratios, and different deposition
times. By adjusting the ratio of the gas flow, it is possible to
obtain the silicon oxynitride film or the silicon nitride film
having different composition ratios and refractive indexes. The
combination order, thickness, and composition of the composite film
can be flexibly adjusted, and therefore the production process of
the present invention is flexible and controllable; furthermore, it
is possible to reduce cost and obtain a large yield with this
production process. In addition, the rear surface composite film is
optimized to match the rear aluminum grid lines, which gives the
best passivation effect and significantly increases the
photoelectric conversion efficiency of the PERC cell.
[0080] The apparatus for loading and unloading silicon wafers in
the tubular PECVD device is a graphite boat. The pin slot of the
graphite boat has a depth of 0.5-1 mm. Preferably, the depth of the
pin slot of the graphite boat is 0.6-0.8 mm; the diameter of a pin
base is 6-15 mm; the angle of inclination of an inclined surface of
a pin cap is 35-45 degrees; the thickness of the pin cap is 1-1.3
mm. More preferably, the pin slot of the graphite boat has a depth
of 0.7-0.8 mm; the diameter of the pin base is 8-12 mm; the angle
of inclination of an inclined surface of a pin cap is 35-40
degrees; the thickness of the pin cap is 1-1.2 mm.
[0081] When employing tubular PECVD for rear surface film
deposition, it is difficult to prevent both scratching and
undesirable coating. By adjusting an automatic wafer inserter, the
silicon wafer can be inserted into the pin slot without contacting
the graphite boat wall, during which the silicon wafer and the
graphite boat are kept at a distance to avoid friction between the
silicon wafer and the graphite boat wall. If the distance between
the silicon wafer and the graphite boat plate were too large,
scratching would be less likely to take place, but the possibility
of the undesirable coating would increase as the silicon wafer
would not be closely attached to the graphite boat wall. In
addition, the large distance between the silicon wafer and the
graphite boat wall may prevent the silicon wafer from properly
inserting into the pin slot, and the silicon wafer may fall off as
a result. If the distance between the silicon wafer and the
graphite boat plate were too small, the silicon wafer would be
closely attached to the graphite boat plate. As a result,
undesirable coating would be less likely to take place, but the
possibility of scratching would increase.
[0082] The position of the boat marks at the edges of the front
surface of the cell corresponds to the positions of the pins during
the rear surface coating by PECVD. These marks are formed as a
result of gas flowing to the front surface of the cell from the
position of the pins. The thickness of the pin base is slightly
smaller than the thickness of the graphite boat plate, therefore
there is a gap between the silicon wafer and the pin base at the
position of the pins. When depositing the rear surface film, the
gas enters the gap from two sides below the pin shaft, and then
deposit a film at the front surface edges of the silicon wafer,
i.e., forms semi-circular boat marks.
[0083] The present invention adjusts the diameters of the pin shaft
and the pin base to reduce the depth of the inside of the pin slot.
As a result, the gap between the silicon wafer and the pin base at
the position of the pins is reduced; consequentially, the amount of
gas reaching and coating the rear surface of the silicon wafer is
reduced, boat marks at the front surface edges of the cell are thus
much less likely to occur.
[0084] By adjusting the automatic wafer inserter, after inserting
the silicon wafer into a certain position in the graphite boat, the
suction cup releases its vacuum and thus the silicon wafer falls
onto the inclined surface of the pin cap. Under the action of
gravity, the silicon wafer slides down the inclined surface until
it is closely attached to the graphite boat wall. This type of
insertion is contactless can could reduce scratching of the silicon
wafer.
[0085] The present invention adequately increases the angle of
inclination of the inclined surface of the pin cap and the
thickness of the pin cap, and adjusts the automatic wafer inserter,
thereby slightly increasing the distance between the silicon wafer
and the graphite boat wall, reducing scratching. Increasing the
angle of inclination of the inclined surface of the pin cap also
reduces the impact force on the silicon wafer from the graphite
boat when the silicon wafer is sliding down the inclined surface,
reducing breakage rate.
[0086] The tubular PECVD device employs the graphite boat for
loading and unloading silicon wafers. Pin marks are formed on the
rear surface of the cell; 3-5 pin marks are formed on the rear
surface of the cell.
[0087] The rear surface composite film 3 has various embodiments.
Referring to FIGS. 3, 4, and 5, the bottom layer of the rear
surface composite film is an aluminum oxide film, and the top layer
is composed of one or more films selected from the group consisting
of a silicon dioxide film, a silicon oxynitride film, and a silicon
nitride film.
[0088] In the first embodiment of the rear surface composite film
shown in FIG. 3, the bottom layer 31 of the rear surface composite
film 3 is an aluminum oxide film; the top layer 32 of the rear
surface composite film is composed of a silicon oxynitride film and
a silicon nitride film.
[0089] In the second embodiment of the rear surface composite film
shown in FIG. 4, the bottom layer 31 of the rear surface composite
film is an aluminum oxide film; the top layer 32 of the rear
surface composite film is a silicon nitride film.
[0090] In the third embodiment of the rear surface composite film
shown in FIG. 5, the bottom layer 31 of the rear surface composite
film is an aluminum oxide film; the top layer 32 of the rear
surface composite film is composed of a silicon dioxide film, a
silicon oxynitride film A, a silicon oxynitride film B, and a
silicon nitride film.
[0091] Referring to FIGS. 6, 7, and 8, the bottom layer 31 of the
rear surface composite film is a silicon dioxide film; the middle
layer 32 of the rear surface composite film is an aluminum oxide
film; the top layer 33 is composed of one or more films selected
from the group consisting of a silicon dioxide film, a silicon
oxynitride film, and a silicon nitride film.
[0092] In the fourth embodiment of the rear surface composite film
shown in FIG. 6, the bottom layer 31 of the rear surface composite
film is a silicon dioxide film, the middle layer 32 of the rear
surface composite film is an aluminum oxide film, and the top layer
33 of the rear surface composite film is a silicon nitride
film.
[0093] In the fifth embodiment of the rear surface composite film
shown in FIG. 7, the bottom layer 31 of the rear surface composite
film is a silicon dioxide film, the middle layer 32 of the rear
surface composite film is an aluminum oxide film, and the top layer
33 of the rear surface composite film is composed of a silicon
dioxide film, a silicon oxynitride film A, a silicon oxynitride
film B, and a silicon nitride film.
[0094] In the sixth embodiment of the rear surface composite film
shown in FIG. 8, the bottom layer 31 of the rear surface composite
film is a silicon dioxide film, the middle layer 32 of the rear
surface composite film is an aluminum oxide film, and the top layer
33 is composed of a silicon dioxide film, a silicon oxynitride
film, a silicon nitride film A, and a silicon nitride film B.
[0095] Specifically, the thickness of the aluminum oxide film is
5-15 nm, the thickness of the silicon nitride film is 50-150 nm,
the thickness of the silicon oxynitride film is 5-20 nm, and the
thickness of the silicon dioxide film is 1-10 nm. The actual
thickness of the aluminum oxide film, the silicon nitride film, the
silicon oxynitride film, and the silicon dioxide film may be
adjusted according to actual needs; their embodiments are not
limited to the embodiments described in the present invention.
[0096] To conclude, the bifacial tube-type PERC solar cell of the
present invention can absorb sunlight on both surfaces. It has high
photoelectric conversion efficiency, high appearance quality, and
high EL yield; it solves the problems of both scratching and
undesirable deposition.
[0097] It should be noted that EL (electroluminescence) is used for
testing the appearance and the electrical performance of a cell. It
is possible to use EL to check potential defects of crystalline
silicon solar cells and its components. EL could effectively detect
whether a cell has breakage, cracks, broken grids, scratches,
firing defects, dark spots, mixing of different grades of cells,
and inhomogeneous resistance of the cell, among others.
[0098] Accordingly, the present invention also discloses a method
for preparing a bifacial tube-type PERC solar cell, comprising the
following steps:
[0099] (1) Forming a textured surface on the front and rear
surfaces of the silicon wafer, the silicon wafer is P-type
silicon.
[0100] Using wet etching or dry etching techniques to form a
textured surface on the surface of the silicon wafer by a texturing
device.
[0101] (2) Performing diffusion on the silicon wafer to form an
N-type emitter.
[0102] The diffusion process adopted by the preparation method of
the present invention involves placing the silicon wafer in a
thermal diffusion furnace for diffusion to form an N-type emitter
on the P-type silicon. During diffusion, the temperature is kept
within a range of 800.degree. C. to 900.degree. C. The target sheet
resistance is 70-100 ohms/.quadrature..
[0103] At the rear surface of a tube-type PERC cell, the P-type
silicon is only in contact with the aluminum paste only at the
laser grooving regions instead of over the whole rear surface,
which results in higher series resistance. In order to improve the
performance of the tube-type PERC cell, the present invention
adopts a lower diffusion sheet resistance (70-100
ohm/.quadrature.), which reduces series resistance and increases
photoelectric conversion efficiency.
[0104] During the diffusion process, phosphosilicate glass layers
are formed on the front and rear surfaces of the silicon wafer. The
phosphosilicate glass layers form as a result of the reaction
between POCl.sub.3 and O.sub.2 to form a P.sub.2O.sub.5 deposition
on the surface of the silicon wafer during the diffusion process.
P.sub.2O.sub.5 reacts with Si to form SiO.sub.2 and phosphorus
atoms, thus a layer of SiO.sub.2 containing phosphorus is formed on
the surface of the silicon wafer, which is called phosphosilicate
glass. The phosphosilicate glass layer could collect the impurities
in the silicon wafer during diffusion and could further reduce the
number of impurities in the solar cell.
[0105] (3) Removing the phosphosilicate glass on the front surface
and peripheral p-n junctions formed during diffusion; polishing the
rear surface of the silicon wafer; the depth of rear surface
etching is 3-6 .mu.m.
[0106] In the invention, the diffused silicon wafer is immersed in
an acid bath of a mixed solution of HF (mass percentage 40%-50%)
and HNO.sub.3 (mass percentage 60%-70%) in a volume ratio of 1 to
5-8 for 5-30 seconds to remove the phosphosilicate glass and the
peripheral p-n junctions. The phosphosilicate glass layer is likely
to cause a color difference and cause Si.sub.xN.sub.y to peel off
in PECVD; in addition, the phosphosilicate glass layer contains a
large amount of phosphorus and impurities migrated from the silicon
wafer, and thus it is necessary to remove the phosphosilicate glass
layer.
[0107] The etching depth of a conventional cell is about 2 .mu.m.
The present invention adopts a rear surface etching depth of 3 to 6
.mu.m. By increasing the etching depth of the tube-type PERC cell,
the reflectance of the rear surface, the short-circuit current, and
the photoelectric conversion efficiency of the cell can be
improved.
[0108] (4) performing annealing on the silicon wafer; the annealing
temperature is 600-820.degree. C., the nitrogen flow rate is 1-15
L/min, and the oxygen flow rate is 0.1-6 L/min; the annealing step
improves the doping concentration distribution at the front surface
of the silicon wafer, reducing surface defects caused by
doping.
[0109] (5) depositing the rear surface composite film on the rear
surface of the silicon wafer using a tubular PECVD device,
including the following:
[0110] Depositing an aluminum oxide film using TMA and N.sub.2O;
the gas flow rate of TMA is 250-500 sccm, the ratio of TMA to
N.sub.2O is 1 to 15-25; the plasma power is 2000-5000W.
[0111] Depositing a silicon oxynitride film using silane, ammonia,
and nitrous oxide; the gas flow rate of silane is 50-200 sccm, the
ratio of silane to nitrous oxide is 1 to 10-80; the gas flow rate
of ammonia is 0.1-5 slm, the plasma power is 4000-6000W.
[0112] Depositing a silicon nitride film using silane and ammonia.
The gas flow rate of silane is 500-1000 sccm; the ratio of silane
to ammonia is 1 to 6-15; the deposition temperature of the silicon
nitride film is 390-410.degree. C.; the deposition time is
100-400s; the plasma power is 10000-13000W.
[0113] Depositing a silicon dioxide film using nitrous oxide. The
gas flow rate of nitrous oxide is 0.1-5 slm; the plasma power is
2000-5000W.
[0114] The tubular PECVD device is provided with four gas lines of
silane, ammonia, trimethyl aluminum, and nitrous oxide. The
apparatus for loading and unloading silicon wafers in the tubular
PECVD device is a graphite boat. The pin slot of the graphite boat
has a depth of 0.5-1 mm.
[0115] As a preferred embodiment of this step, depositing the rear
surface composite film at the rear surface of the silicon wafer
using a tubular PECVD device, including:
[0116] Depositing an aluminum oxide film using TMA and N.sub.2O;
the gas flow rate of TMA is 250-500 sccm, the ratio of TMA to
N.sub.2O is 1 to 15-25; the deposition temperature of the aluminum
oxide film is 250-300.degree. C.; the deposition time is 50-300s;
the plasma power is 2000-5000 W.
[0117] Depositing a silicon oxynitride film using silane, ammonia,
and nitrous oxide; the gas flow rate of silane is 50-200 sccm, the
ratio of silane to nitrous oxide is 1 to 10-80; the gas flow rate
of ammonia is 0.1-5 slm; the deposition temperature of the silicon
oxynitride film is 350-410.degree. C., the deposition time is
50-200s; the plasma power is 4000-6000 W.
[0118] Depositing a silicon nitride film using silane and ammonia.
The gas flow rate of silane is 500-1000 sccm; the ratio of silane
to ammonia is 1 to 6-15; the deposition temperature of the silicon
nitride film is 390-410.degree. C.; the deposition time is
100-400s; the plasma power is 10000-13000 W.
[0119] Depositing a silicon dioxide film using nitrous oxide. The
flow rate of nitrous oxide is 0.1-5 slm; the plasma power is
2000-5000 W.
[0120] As a more preferred embodiment of this step, depositing the
rear surface composite film at the rear surface of the silicon
wafer using a tubular PECVD device, including:
[0121] Depositing an aluminum oxide film using TMA and N.sub.2O;
the gas flow rate of TMA is 350-450 sccm, the ratio of TMA to
N.sub.2O is 1 to 18-22; the deposition temperature of the aluminum
oxide film is 270-290.degree. C.; the deposition time is 100-200s;
the plasma power is 3000-4000 W.
[0122] Depositing a silicon oxynitride film using silane, ammonia,
and nitrous oxide; the gas flow rate of silane is 80-150 sccm, the
ratio of silane to nitrous oxide is 1 to 20-40; the gas flow rate
of ammonia is 1-4 slm; the deposition temperature for the silicon
oxynitride film is 380-410.degree. C., the deposition time is
100-200s; the plasma power is 4500-5500 W.
[0123] Depositing a silicon nitride film using silane and ammonia.
The gas flow rate of silane is 600-800 sccm; the ratio of silane to
ammonia is 1 to 6-10; the deposition temperature of the silicon
nitride film is 395-405.degree. C.; the deposition time is
350-450s; the plasma power is 11000-12000 W.
[0124] Depositing a silicon dioxide film using nitrous oxide. The
flow rate of nitrous oxide is 1-4 slm; the plasma power is
3000-4000 W.
[0125] As the most preferred embodiment of this step, depositing
the rear surface composite film at the rear surface of the silicon
wafer using a tubular PECVD device, including:
[0126] Depositing an aluminum oxide film using TMA and N.sub.2O;
the gas flow rate of TMA is 400 sccm, the ratio of TMA to N.sub.2O
is 1 to 18; the deposition temperature of the aluminum oxide film
is 280.degree. C. ; the deposition time is 140s; the plasma power
is 3500 W.
[0127] Depositing an silicon oxynitride film using silane, ammonia,
and nitrous oxide; the gas flow rate of silane is 130 sccm; the
ratio of silane to nitrous oxide is 1 to 32; the gas flow rate of
ammonia is 0.5 slm; the deposition temperature for the silicon
oxynitride film is 420.degree. C., the deposition time is 120s; the
plasma power is 5000 W.
[0128] Depositing a silicon nitride film using silane and ammonia.
The gas flow rate of silane is 780 sccm; the ratio of silane to
ammonia is 1 to 8.7; the deposition temperature of the silicon
nitride film is 400.degree. C.; the deposition time is 350s; the
plasma power is 11500 W.
[0129] Depositing a silicon dioxide film using nitrous oxide. The
flow rate of nitrous oxide is 2 slm; the plasma power is 3500
W.
[0130] The applicant discovered that undesirable coating occurs
primarily during the deposition of silicon nitride. This is because
silicon nitride is the outer (top) layer of the rear surface
composite film; as the deposition time increases, the thickness of
the film increases, which causes the silicon wafer to bend. As a
result, silane and ammonia are more likely to deposit to the front
surface edge of the battery. By shortening the time and temperature
of silicon nitride deposition, the bending of the silicon wafer can
be reduced, and thus the amount of the undesirable coating can be
reduced. Further experiments have shown that the temperature window
for silicon nitride deposition is very narrow, between 390-410
degrees; when the temperature is further lowered, however, the
amount of the undesirable coating increases.
[0131] When depositing the aluminum oxide film, the plasma power is
set to 2000-5000 W; when depositing the silicon oxynitride film,
the plasma power is set to 4000-6000 W; when depositing the silicon
nitride film, the plasma power is set to 10000-13000 W; when
depositing the silicon dioxide film, the plasma power is set to
2000-5000 W. This ensures that different layers have good
deposition rates and improves deposition uniformity.
[0132] The tubular PECVD device is provided with four gas lines of
silane, ammonia, trimethyl aluminum, and nitrous oxide. The
apparatus for loading and unloading silicon wafers in the tubular
PECVD device is a graphite boat. The pin slot of the graphite boat
has a depth of 0.5-1 mm. The technical details of the graphite boat
are the same as described above and will not be described in detail
here.
[0133] (6) Depositing a passivation film on the front surface of
the silicon wafer; the passivation film is preferably a silicon
nitride film.
[0134] (7) Performing laser grooving at the rear surface composite
film of the silicon wafer.
[0135] Laser grooving technique is used to groove the rear surface
composite film of the silicon wafer; the depth of the groove
reaches the rear surface of the P-type silicon. The laser
wavelength is 532 nm, the laser power is above 14W, the laser
scribing speed is above 12 m/s, and the frequency is above 500
kHZ.
[0136] Preferably, the laser wavelength is 532 nm, the laser power
is 14-20W, the laser scribing speed is 12-20 m/s, and the frequency
is 500 KHZ or more.
[0137] As the deposition time of silicon nitride is shortened, the
thickness of the silicon nitride film is decreased, which affects
the hydrogen passivation effect of the rear surface composite film
layer and lowers the photoelectric conversion efficiency of the
cell. Therefore, the silicon nitride deposition time cannot be too
short. In addition, the thinner the silicon nitride film, the lower
the absorption rate of the laser; meanwhile, in order to meet the
requirements of large-scale production, the laser scribing speed
must be kept at 12 m/s, and the laser power must be kept at above
14 W. As a result, the power and frequency of the laser must meet
certain criteria in order to allow the absorption of a sufficiently
large amount of laser energy in per unit area of the rear surface
composite film to effectively groove the composite film, so that
the aluminum paste subsequently printed is in contact with the
silicon substrate through the laser grooving regions.
[0138] (8) Printing with a paste for rear silver major grid lines
on the rear surface of the silicon wafer; drying.
[0139] Printing with a paste for rear silver major grid lines
according to a pattern of rear silver major grid lines. The rear
silver major grid lines are continuous straight grid lines;
alternatively, the rear silver major grid lines are arranged in
spaced segments, wherein the adjacent segments are connected by a
connecting line.
[0140] (9) printing with aluminum paste at the laser grooving
regions to form perpendicular connections with the paste for rear
silver major grid lines.
[0141] (10) printing with paste for a positive electrode on the
front surface of the silicon wafer.
[0142] (11) sintering the silicon wafer at a high temperature to
form rear silver major grid lines, rear aluminum grid lines, and a
front silver electrode.
[0143] (12) performing anti-LID annealing on the silicon wafer to
obtain the bifacial tube-type PERC solar cell product.
[0144] The combination order, thickness, and composition of the
composite film can be flexibly adjusted, and therefore the
production process of the present invention is flexible,
controllable, and is compatible with existing production lines. The
bifacial tube-type PERC solar cell of the present invention can
absorb sunlight on both surfaces. It has high photoelectric
conversion efficiency, high appearance quality, and high EL yield;
it solves the problems of both scratching and undesirable
deposition.
[0145] As shown in FIG. 9, the present invention also discloses a
production device for the bifacial tube-type PERC solar cell. The
device is a tubular PECVD device, which includes a silicon wafer
loading area 1, a furnace body 2, a gas cabinet 3, a vacuum system
4, a control system 5, and a graphite boat 6. The gas cabinet 3 is
provided with a first gas line for silane, a second gas line for
ammonia, a third gas line for trimethylaluminum, and a fourth gas
line for nitrous oxide. The first gas line, the second gas line,
the third gas line, and the fourth gas line are provided inside the
gas cabinet 3 and are is not shown in the drawing.
[0146] As shown in FIGS. 10 and 11, the graphite boat 6 is used for
loading and unloading silicon wafers. The graphite boat 6 includes
a pin 60, and the pin 60 includes a pin shaft 61, a pin cap 62, and
a pin base 63. The pin shaft 61 is mounted on the pin base 63. The
pin cap 62 is connected to the pin shaft 61. A pin slot 64 is
formed between the pin shaft 61, the pin cap 62, and the pin base
63. The depth of the pin slot 64 is 0.5-1 mm.
[0147] As shown in FIG. 11, the depth of the pin slot 64, h, is
preferably 0.6-0.8 mm; the diameter of the pin base 63, D, is
preferably 6-15 mm; the angle of inclination of the inclination
surface of the pin cap 62, .alpha., is preferably 35-45 degrees;
the thickness of the pin cap 62, a, is preferably 1-1.3 mm.
[0148] More preferably, the depth of the pin slot 64, h, is 0.7 mm;
the diameter of the pin base 63, D, is 9 mm; the angle of
inclination of the inclination surface of the pin cap 62, .alpha.,
is preferably 40 degrees; the thickness of the pin cap 62, .alpha.,
is 1.2 mm.
[0149] It should be noted that the depth h of the pin slot is the
depth of the inside of the pin slot, and mainly refers to the depth
of the side of the pin shaft 61 that forms an angle with and the
pin base 63. The depth h of the pin slot=(the diameter of the pin
base-the diameter of the pin shaft)/2. The angle of inclination of
the inclination surface of the pin cap 62, .alpha., refers to the
angle between the inclination surface of the pin cap and the
vertical direction.
[0150] In the prior art, the depth h of the pin slot is 1.75 mm,
the diameter D of the pin base is 9 mm, the angle of inclination a
of the pin cap is 30 degrees, and the thickness a of the pin cap is
1 mm. In the prior art, the pin slot is deeper, which leads to too
big a gap between the silicon wafer and the pin base at the
position of the pins; as a result, a lot of gas is present at the
rear surface of the silicon wafer, leading to the formation of a
large number of boat marks at the front surface edges of the cell.
The pin cap has a small angle of inclination and a small thickness,
leading to small adjustment room for the automatic wafer inserter;
consequentially, it is difficult to effectively lower the
occurrence of scratching.
[0151] When employing tubular PECVD for rear surface film
deposition, it is difficult to prevent both scratching and
undesirable coating. By adjusting an automatic wafer inserter, the
silicon wafer can be inserted into the pin slot without contacting
the graphite boat wall, during which the silicon wafer and the
graphite boat are kept at a distance to avoid friction between the
silicon wafer and the graphite boat wall. If the distance between
the silicon wafer and the graphite boat plate were too large,
scratching would be less likely to take place, but the possibility
of the undesirable coating would increase as the silicon wafer
would not be closely attached to the graphite boat wall. In
addition, the large distance between the silicon wafer and the
graphite boat wall may prevent the silicon wafer from properly
inserting into the pin slot, and the silicon wafer may fall off as
a result. If the distance between the silicon wafer and the
graphite boat plate were too small, the silicon wafer would be
closely attached to the graphite boat plate. As a result,
undesirable coating would be less likely to take place, but the
possibility of scratching would increase.
[0152] The position of the boat marks at the edges of the front
surface of the cell corresponds to the positions of the pins during
the rear surface coating by PECVD. These marks are formed as a
result of gas flowing to the front surface of the cell from the
position of the pins. The thickness of the pin base is slightly
smaller than the thickness of the graphite boat plate, therefore
there is a gap between the silicon wafer and the pin base at the
position of the pins. When depositing the rear surface film, the
gas enters the gap from two sides below the pin shaft, and then
deposit a film at the front surface edges of the silicon wafer,
i.e., forms semi-circular boat marks.
[0153] The present invention adjusts the diameter D of the pin base
and the diameter of the pin shaft to reduce the depth h of the
inside of the pin slot. As a result, the gap between the silicon
wafer and the pin base at the position of the pins is reduced;
consequentially, the amount of gas reaching and coating the rear
surface of the silicon wafer is reduced, boat marks at the front
surface edges of the cell are thus much less likely to occur.
[0154] By adjusting the automatic wafer inserter, after inserting
the silicon wafer into a certain position in the graphite boat, the
suction cup releases its vacuum and thus the silicon wafer falls
onto the inclined surface a of the pin cap. Under the action of
gravity, the silicon wafer slides down the inclined surface until
it is closely attached to the graphite boat wall. This type of
insertion is contactless can could reduce scratching of the silicon
wafer.
[0155] The present invention adequately increases the angle of
inclination a of the inclined surface of the pin cap and the
thickness of the pin cap, and adjusts the automatic wafer inserter,
thereby slightly increasing the distance between the silicon wafer
and the graphite boat wall, reducing scratching. Increasing the
angle of inclination of the inclined surface of the pin cap also
reduces the impact force on the silicon wafer from the graphite
boat when the silicon wafer is sliding down the inclined surface,
reducing breakage rate.
[0156] It should be noted that in the prior art, the problem of
undesirable etching is generally only tackled after the production
is completed. For example, in the alkali polishing method during
the production of PERC crystalline silicon solar cells disclosed in
Chinese patent application No. 201510945459.3, after coating a
silicon nitride film on the front surface by PECVD, the undesirable
silicon nitride coating at the rear surface and the edges are
removed by a belt-type transmission etching method, thereby solving
the problems of poor passivation at the rear surface due to
undesirable etching. However, in the tube-type PERC cell of the
present application, the silicon nitride film is coated on the rear
surface and undesirable coating takes place at the front surface;
p-n junctions present on the front surface would be destroyed if
the alkali polishing method of the above patent were used. By
adjusting the coating process and the coating structure, the
invention avoids undesirable coating during the production process
and solves the problem of undesirable coating from its root. No
additional process is required, which simplifies the overall
process and reduces production cost. The invention is of great
importance for the solar photovoltaic industry, which is extremely
cost sensitive. Moreover, the present invention also solves the
problem of scratching.
[0157] In summary, the present invention has the following
beneficial effects:
[0158] First, the bifacial tube-type PERC solar cell of the present
invention has a plurality of aluminum grid lines arranged in
parallel at the rear surface of the cell. These aluminum grid lines
not only replace the aluminum back surface field in an existing
monofacial solar cell to allow light absorption on the rear
surface, but also act as minor grid lines of a rear silver
electrode for electron conduction. The bifacial tube-type PERC
solar cell of the present invention not only lowers production cost
by reducing the amount of silver paste and aluminum paste used, but
also allows light absorption on both surfaces, which significantly
expands the scope of application and improves the photoelectric
conversion efficiency of a solar cell.
[0159] Second, the present invention employs a tubular PECVD device
to deposit the rear surface composite film on the rear surface of
the silicon wafer in order to produce the aforementioned bifacial
PERC solar cell provided with aluminum grid lines. The rear surface
composite film includes one or more films selected from the group
comprising an aluminum oxide film, a silicon dioxide film, a
silicon oxynitride film, and a silicon nitride film, and is
deposited at the rear surface of the silicon wafer by a tubular
PECVD device. The tubular PERC device adopts a direct plasma method
in which the plasma directly bombards the surface of the silicon
wafer and causes significant passivation of the layer. The tubular
PECVD device is provided with four gas lines of silane, ammonia,
trimethyl aluminum, and nitrous oxide; the four gas lines are used
alone or in combination to form the aluminum oxide film, the
silicon dioxide film, the silicon oxynitride film, and the silicon
nitride film. By adjusting the ratio of the gas flow of the four
gas lines, it is possible to obtain different films. In particular,
it is possible to obtain a silicon oxynitride film or a silicon
nitride film having different composition ratios and refractive
indexes by adjusting the ratio of the gas flow. The combination
order, thickness, and composition of the composite film can be
flexibly adjusted, and therefore the production process of the
present invention is flexible and controllable; furthermore, it is
possible to reduce cost and obtain a large yield with this
production process. In addition, the rear surface composite film is
optimized to match the rear aluminum grid lines, which gives the
best passivation effect and significantly increases the
photoelectric conversion efficiency of the PERC cell.
[0160] Third, the present invention adjusts the diameters of the
pin shaft and the pin base to reduce the depth of the inside of the
pin slot. As a result, the gap between the silicon wafer and the
pin base at the position of the pins is reduced; consequentially,
the amount of gas reaching and coating the rear surface of the
silicon wafer is reduced, boat marks at the front surface edges of
the cell are thus much less likely to occur. In addition, the
present invention adequately increases the angle of inclination of
the inclined surface of the pin cap and the thickness of the pin
cap, and adjusts the automatic wafer inserter, thereby slightly
increasing the distance between the silicon wafer and the graphite
boat wall, reducing scratching. Increasing the angle of inclination
of the inclined surface of the pin cap also reduces the impact
force on the silicon wafer from the graphite boat when the silicon
wafer is sliding down the inclined surface, reducing breakage
rate.
[0161] Furthermore, silicon nitride is the outer layer of the rear
surface composite film; as the deposition time increases, the
thickness of the film increases, which causes silicon wafer to
bend. In the present invention, the silicon nitride deposition
temperature is limited to 390-410.degree. C., the deposition time
is limited to 100-400s. By shortening the time and temperature of
silicon nitride deposition, the bending of the silicon wafer can be
reduced, and thus the amount of the undesirable coating can be
reduced. The temperature window for silicon nitride deposition is
very narrow, between 390-410 degrees; this temperature allows the
maximum reduction of the undesirable coating. When the temperature
is further lowered, however, the amount of the undesirable coating
increases.
[0162] To meet the requirements of large-scale production and
minimize the negative impact caused by shortening the silicon
nitride deposition time, the present invention maintains a laser
power of above 14W, a laser scribing speed of above 12 m/s, and a
frequency of above 500 kHZ. This allows the absorption of a
sufficiently large amount of laser energy in per unit area of the
rear surface composite film to effectively groove the composite
film; as a result, the aluminum paste subsequently printed is in
contact with the silicon substrate through the laser grooving
regions.
[0163] To conclude, the bifacial tube-type PERC solar cell of the
present invention can absorb sunlight on both surfaces. It has high
photoelectric conversion efficiency, high appearance quality, and
high EL yield; it solves the problems of both scratching and
undesirable deposition. In addition, the present invention also
provides a method and a device for the production of the
aforementioned cell. The production method of the present invention
is simple, can be carried out at a large scale, and is compatible
with existing production lines. The device has a simple structure,
is able to lower production cost, and has large capacity and
yield.
[0164] It should be noted that the above embodiments are only
intended to illustrate the technical solutions of the present
invention and are not intended to limit the scope of the present
invention. Although the present invention has been described in
detail with reference to the preferred embodiments, the technical
solutions of the present invention may be modified or equivalently
substituted without departing from the spirit and scope of the
technical solutions of the present invention.
* * * * *