the paper 2

N-TYPE HIGH EFFICIENCY BIFACIAL SILICON SOLAR CELL WITH THE EXTREMELY HIGH
BIFACIALITY OF 96% IN AVERAGE FABRICATED BY USING CONVENTIONAL DIFFUSION METHOD
Shinobu Gonsui1, Shinji Goda1, Koichi Sugibuchi1, Naoki Ishikawa1, Kazuhiro Honda2, Hideaki Zama2
1
PVG Solutions Inc., 455-1 Minato, Saijo, Ehime 793-0046, Japan
2
Institute for Super Materials, ULVAC,-Inc., 2500 Hagisono, Chigasaki, Kanagawa, 253-8543, Japan
ABSTRACT: In this paper, we report our latest progress of n-type bifacial cell. The bifacial cell is made of
156x156mm n-type Cz-Si wafer and fabricated by almost the same process as that of conventional p-type cell. We
have produced the bifacial cell in mass-production level and the cell shows the efficiency of 19.4% and high
bifaciality of 96%, e.g., rear side efficiency of 18.6%, in average. By the improvement of the cell with both selective
phosphorous diffusion and ALD-AlOx passivation, the efficiency of 20.16% and the bifaciality of 95% are achieved.
This improvement is confirmed to be easily added to mass-production. The field test using the bifacial module shows
the output gain of around 20% compared with the monofacial module. This is caused by the incident reflected and
scattered light from the rear side of the system.
Keywords: Bifacial, Boron, c-Si, n-type, Silicon Solar Cell,
1
Introduction
Recently more and more researches have been
reported on n-type cells with different structures such as
SHJ, IBC and MWT in addition to the conventional ptype cell. The bifacial cell is the one of them and has
started to get attention [1,2].
The first characteristic of the bifacial cell is the
ability to get the light from both sides of the cell.
Manufactures can decide which to choose the only front
side efficiency or the high bifaciality at some cost of the
front side efficiency. When the module is installed to be
slanted, higher front side efficiency is needed. On the
other hand, when the module is vertically installed, the
bifaciality of the cell should be higher. As mentioned
above, the bifacial cell can be made selectively to meet
different demands of some module applications. The
second characteristic is that there is no need to implement
new equipment because of the compatibility of not only
cell but also module manufacturing technique with the
conventional p-type cell. Therefore the initial investiment
can be cut. Although many researches on ion
implantation for doping process are reported and very
high efficiency is achieved, it may be for quite a while to
be widely used because of the high equipment cost [3].
PVG Solutions Inc. has developed 156x156mm ntype bifacial cell for three years and successfully started
30MW/year mass-production. The cell efficiency reaches
up to 19.4% in average using the wide range wafer
resistivity. The most distinct characteristic is its high
bifaciality of 96% in average, that is, the rear side
efficiency is 18.6%. Also the front side efficiency of
20.0% has been achieved with 6-inch Cz wafer in
developmental level and confirmed to be easily added
into industrial level.
It is essential for the realization of further high
efficiency bifacial cell to apply an appropriate passivation
to the boron emitter, i.e. a dielectric with the negative
fixed charge such as AlOx not as conventional SiNx and
SiOx [4,5]. ULVAC has already developed a massproductive ALD-AlOx equipment for MEMS application,
and it has applied for PV passivation, resulting to be the
carrier lifetime of over 1 msec. PVG Solutions Inc. and
ULVAC have developed AlOx passivation and improved
cell efficiency in developmental level. In this paper the
result will be reported.
2
EarthON cell
2.1 Cell structure and process
Figure 1: (a) Cross-section of EarthON cell, which is ntype bifacial cell. (b) Manufacturing process of EarthON
cell. This cell can be made by conventional equipment.
Figure 1 shows the cross-section and manufacturing
process of EarthON cell. EarthON cell is made of
156x156mm and 200µm thickness n-type Cz-Si wafers
(standard solar grade with wide resistivity of 2~12
ohm・cm), which is also used for the purpose of
development. At first, the wafer is etched by alkaline
solution to form textured surface on both sides. The
boron emitter and the phosphorous BSF are formed by
diffusion in the tube furnace in turn. After passivation,
SiNx layer is deposited on both sides by PECVD for
antireflection. Then silver metallization is formed on both
sides using screen printing and fired in beam furnace.
Finally edge isolation is treated.
These bifacial cells are produced in Saijo Factory:
this factory is in Saijo-city, Ehime, Japan and has
30MW/year capacity. The average cell efficiency is
19.4% and the average bifaciality is 96%, that is, the rear
side efficiency is 18.6%. Figure 2 shows the result
measured by AIST, and figure 3 shows the cell efficiency
distribution of the latest production of 100,000 cells. The
avarage efficiency of 19.43% and stable production have
been achieved.
Figure 2: I-V characteristics of EarthON cell measured
by AIST. Measured cell is one of the cells of massproduction.
Figure 3: Efficiency distribution of the latest production
of 100,000 cells (September 2013). This indicates narrow
distribution.
2.2 Selective phosphorous diffusion
In order to achieve higher cell efficiency, selective
phosphorous diffusion is adopted at first and tried to
reduce recombination rate of the rear side of the cell. The
cell structure is shown in figure 4. This process is the
same as selective emitter of p-type cell.
Figure 5: Correlation between the sheet resistance of n+
region (A < B < C) and the ratios of the cell
characteristics against the standard cell
Voc is increased significantly in the cell with the
sheet resistance of A, and this contributes to an
improvement of the cell efficiency. Although it is said
that the emitter sheet resistance of 100 ~ 120 ohm/sq
(corresponding to C) is appropriate for p-type selective
emitter, it seems that some degree of phosphorous
concentration is needed for rear n+ region of n-type
bifacial cell to keep enough BSF effect.
Table I shows the ratios of the cell characteristics on
the cell finished the optimization of the n+/n++ region
and the metallization pattern, against the standard cell. In
addition, table II shows an example of the cell results.
According to table I, the cell efficiency is improved
approximately 2% relatively by adopting selective
phosphorous diffusion. Figure 6 shows the ratio of IQE
on the cell adopting selective phosphorous diffusion
against the standard cell. Red response is significantly
increased, and that also demonstrates the recombination
rate of the rear side is reduced.
Table I: Ratios of the cell characteristics on the cell
finished the optimization of the n+/n++ region and the
metallization pattern, against the standard cell.
Max.
Avg.
Min.
Eff.
1.031
1.018
1.007
Voc
1.019
1.014
1.006
Isc
1.009
1.005
1.000
FF
1.004
0.999
0.987
Table II: An example of the cell results adopting selective
phosphorous diffusion
Figure 4: Cross-section of the selectively diffused cell.
Figure 5 shows the correlation between the sheet
resistance of n+ region and the ratios of the cell
characteristics against the standard cell (figure 1(a)).
Plots and bars indicate the average and the variation,
respectively. Three level n+ regions are formed (A, B and
C, C is the highest sheet resistance) and the sheet
resistance of the n++ region is constant.
Eff.[%]
Standard process
Avg.
19.08
Selective P diffusion
Avg.
19.51
Voc[mV]
Isc[A]
FF[%]
639.1
9.01
79.17
650.5
9.04
79.33
Figure 6: Ratio of IQE on the cell adopting the selective
phosphorous diffusion against the standard cell. Red
response is significantly increased.
Table III shows the result of the cell adopting the
selectively phosphorous diffusion and fine Ag
metallization. Although it is possible to obtain further
high efficiency by increasing the number of fingers, the
incident light from rear side is decreased to lower the
output gain from rear side.
Table III: I-V characteristic of the cell adopting the
selective phosphorous diffusion and fine Ag
metallization (in-house measurement).
Avg.
Max.
Eff.[%]
19.85
20.01
Voc[mV]
651.1
653.6
Isc[A]
9.17
9.19
FF[%]
79.45
79.64
2-3. ALD-AlOx passivation
Secondly, current passivation is replaced with ALDAlOx, which is one of the methods to make bifacial cell
more efficient. Conventional passivation materials, e.g.,
SiNx and SiOx, have positive fixed charge and are
adopted for phosphorous emitter of p-type cell. When
they are used for boron emitter of n-type bifacial cell, an
inappropriate inversion layer is formed between the
emitter and the passivation layer [6]. This inversion layer
causes reverse field effect and prevents minority carriers
from moving laterally along the emitter, and that results
in parasitic shunting [7]. To solve this problem, it is
essential to choose a passivation material with negative
fixed charge.
In this work, AlOx layer is deposited by thermal
ALD on an experimental apparatus produced by ULVAC
and that followed by post-annealing in tube furnace. The
cell structure is shown in figure 7. Figure 8 shows the
ratios of the cell characteristics on three improved cells
with AlOx passivation layer and/or lightly diffused boron
emitter (higher sheet resistance) against the standard cell
(figure 1(a)). Plots and bars indicate the average and the
variation, respectively.
Figure 7: Cross-section of the cell with ALD-AlOx.
Figure 8: Ratios of the cell characteristics on three
improved cells with AlOx passivation layer and/or lightly
diffused boron emitter (higher sheet resistance) against
the standard cell
Voc is increased significantly by adopting the AlOx
layer, which shows good passivation characteristic.
Although Isc is increased in the cell with lightly diffused
emitter, it is necessary to adopt AlOx in order to keep FF
high. This is why lateral resistance caused by the positive
fixed charge is compensated by applying the negative
fixed charge in AlOx. Table IV shows an example of the
cell results. Figure 9 shows the ratios of IQE on the cells
with only AlOx and both AlOx and lightly diffused
emitter against the standard cell. Blue response is
significantly increased by AlOx and lighter boron
diffusion, respectively, and that also demonstrates the
improvement of the front side of the cell. Although, as
mentioned above, we succeeded to improve cell
characteristics by applying AlOx passivation, further
improvement is expected by the optimization of postannealing of AlOx and SiNx deposition on AlOx.
Table IV: An example of the cell results
Eff.[%]
Voc[mV]
Isc[A]
Standard process
18.77
638.9
9.01
AlOx passivation
19.16
645.7
9.06
AlOx passivation & Lighter B diffusion
19.48
647.9
9.18
FF[%]
77.95
78.26
78.32
Figure 9: Ratios of IQE on the cell with only ALD-AlOx
and both ALD-AlOx and lighter boron diffusion, against
the standard cell. Blue response is significantly increased.
2-4. High efficiency solar cell
In this part, the cell which both selective phosphorous
diffusion and AlOx passivation are applied is mentioned.
In addition, fine Ag metallization is adopted. Table V
shows the cell characteristics of the latest experiment and
high bifaciality of 95.1% is achieved. In comparison with
the maximum cell characteristics shown in table III, the
improvement of the cell efficiency is lower than expected
regardless of applying AlOx passivation. While the
reasons is under the consideration, the one may be the
influence of the lack of passivation layer on the rear side.
That demonstrates the gain caused by the incident
reflected and scattered light from the rear side of the
system. Moreover, the difference between two bifacial
systems shows the effect of the reflection material.
Although these results indicate the effectiveness of the
bifacial cell/module, more and more research on their
application must be carried out.
Table V: Cell characteristics of the latest experiment
Avg.
Max.
3
Front
Rear
Front
rear
Eff.[%]
20.05
19.07
20.16
19.18
Voc[mV]
654.6
651.3
655.9
653.1
Isc[A]
9.29
8.80
9.30
8.82
FF[%]
78.83
79.49
78.98
79.54
EarthON module
3.1 Module characteristic
Conventional technique, the same as p-type
cell/module, can be used to assemble EarthON modules,
and these bifacial modules have been produced by the
manufactures over the world. Standard modules, with
glass/glass or glass/backsheet (clear), and roof-integrated
modules are commercially installed. Also these modules
are certificated by TÜV and UL, etc. An output power of
258.3W is measured by TÜV Rheinland Taiwan Ltd in
standard test conditions: this bifacial module consists of
60 cells with 19.4% average efficiency and has clear
backsheet at the rear. Although applying the standard (not
clear) backsheet causes scattered light inside of the
module to increase the output power, the bifacial module
with clear backsheet transmit radiation and cannot get
this benefit. Instead, the bifacial modules can get the light
from the rear side.
3.2 Field test
Field test has shown the gain from the rear of bifacial
arrays and the effect of reflection material under the array.
This field test has been conducted using two bifacial
systems, and the one A is on the reflection material,
industrial waste crushed scallop shells, and the other B is
on the grass (figure 10). In winter, both systems are on
the snow. The gain from the rear is evaluated by
comparing performance ratio (PR) between bifacial
arrays and monofacial array. Performance ratio (PR),
calculated in the following equation, is often used to
compare the PV systems under the different conditions
and means the output power per 1 kW system with 1
kW/m2 irradiation.
PR of monofacial system is calculated using
measured irradiation. Table VI shows PR and the output
gain against monofacial system of each system.
According to PR, both bifacial systems shows better PR
than that monofacial system shows, which is generally
around 0.9. In addition, both bifacial systems show the
output gain of 20% and 10% in average, respectively.
Figure 10: Tested bifacial systems, A is on the reflection
material (scallop shell) and B is on the grass. In winter,
both systems are on the snow.
Table VI: PR and the output gain against monofacial
system of each system, PR of monofacial system is
calculated using measured irradiation, and maximum
25% output gain is obtained. (the result of 11 months)
PR
Output
gain[%]
4
Max.
Avg.
Min.
Max.
Avg.
Min.
bifacialA
1.16
1.04
0.83
25.7
18.7
5.6
bifacialB
1.08
0.96
0.89
20.3
10.0
4.5
monofacial
0.93
0.88
0.79
std.
Summary
High efficiency n-type 156x156mm bifacial cell is
reported. Applying both phosphorous selective diffusion
and ALD-AlOx passivation, efficiency of 20.1% and high
bifaciality of 95%, i.e. rear efficiency of 19.1%, are
achieved. Further development is expected to optimize
AlOx passivation and improve metallization process and
material, diffusion profiles of both sides and rear
passivation. Field test using bifacial module demonstrates
an output gain of approximately 20% with the reflection
material. This result shows the effectiveness of bifacial
module.
5
References
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Frankfurt, 2012
[2] O. Schultz Wittmann et al., Proc. 27th EPVSEC,
p.596, Frankfurt, 2012
[3] J. Benick et al., Proc. 27th EPVSEC, p.676, Frankfurt,
2012
[4] J. Benick et al., Appl. Phys. Lett. 92, 253504 (2008).
[5] J. Schmidt et al., Energy Procedia. 15, 30, (2012).
[6] F. Werner et al., Energy Procedia. 27, 319, (2012).
[7] I. Cesar et al., Proc. 24th EPVSEC, p.1552, Hamburg,
2009