16% mc-Si CELL EFFICIENCIES USING INDUSTRIAL IN-LINE PROCESSING
A. W. Weeber, A.R. Burgers, M.J.A.A. Goris, M. Koppes, E.J. Kossen, H.C. Rieffe, W.J. Soppe, C.J.J. Tool, J.H. Bultman
ECN Solar Energy, P.O. Box 1, NL-1755 ZG Petten, The Netherlands
Phone: +31 224 564113; Fax: +31 224 568214; email: weeber@ecn.nl
ABSTRACT: A simple in-line industrial process is developed for multicrystalline silicon (mc-Si) solar cells with an
average efficiency of 16%. The best cell has a confirmed efficiency of 16.5%. The process consists of an acidic etch
for texturing, homogeneous spin-on phosphorous and a belt furnace emitter diffusion, MicroWave PECVD of silicon
nitride layers, and screen-printed metallization. The silicon nitride layer serves as antireflection coating and provides
bulk and surface passivation. We demonstrate that the effective lifetime of Float Zone Si does not degrade during this
in-line process, which means that the process can be applied to obtain high-efficiency solar cells. Detailled
characterization and computer simulation showed that implementation of already proven technologies in the current
cell processing could lead to efficiencies around 18%.
Key words: multicrystalline Si, passivation, texturization
1.
INTRODUCTION
The trend towards cost-effective multicrystalline
silicon (mc-Si) PV technology is to use large and thin
wafers and obtain high efficiencies. It is expected that
200×200 mm2 wafers with a thickness of 200 µm will be
applied in the future. It will be difficult to handle these
fragile wafers in today's high-efficiency processes where
the wafers are placed vertically in batch systems, such as
for POCl3 diffusion and parallel plate plasma enhanced
chemical vapour deposition (PECVD) for passivating
layers.
It has been postulated that high efficiencies could only
be obtained when clean batch processes, such as POCl3
diffusion, are used and that selective emitters would be
required. The purpose of this work is to demonstrate a
simple in-line industrial process for 16% mc-Si solar cells
on large wafers. A belt furnace diffusion for a
homogeneous emitter combined with acidic texturing and
in-line MicroWave Remote PECVD is used as a 16% mcSi solar cell technology. With this process an important
step towards the development of an industrial process for
high efficiencies (>18%) on large (>400 cm2) and thin
(<200 µm) wafers is made.
2.
EXPERIMENTAL
A simple industrial process sequence was used for
solar cell processing and consists of the following process
steps: acidic etching for texturing (different recipes were
used), belt furnace emitter diffusion, MicroWave Remote
PECVD of silicon nitride (SiNx:H), screen-printed
metallization and firing (see Table 1). 156 cm2 mc-Si
wafers from different suppliers (wafer A and wafer B)
were used for solar cell processing. Float Zone (FZ)
material (148 cm2 which corresponds to 5 inch semisquare) was used as reference material. Before and after
emitter diffusion the bulk lifetime of FZ material was
measured to determine changes in material quality during
the diffusion. After the diffusion the emitter was removed
with wet chemical etching. To measure the lifetime both
sides of the FZ material were passivated with SiNx:H. The
lifetime was measured using the Quasi Steady State
Photoconductance (QSSPC) method [1].
Current-voltage (IV) measurements were performed
using the class A solar simulator at ECN with six current
probes per busbar. The measurements were performed
according to the ASTM-E948 norm [2]. The best cells
were sent to NREL to confirm the measurements.
The Internal Quantum Efficiency (IQE) was
determined from the spectral response and the reflectance.
PC-1D5.5 [3] was used for modelling to identify
which improvements are necessary to increase the
efficiency to over 18%.
Table 1: Simple in-line solar cell processing on 156 cm2
mc-Si wafers and 148 cm2 FZ wafers.
Advanced in-line solar cell processing sequence
1. Recipe T1 and T2 acidic etching for saw damage
removal and surface texturing (T1 is being used in
the industry)
2. 65 Ω/sq spin-on phosphorous source and infrared
heated belt furnace emitter diffusion
3. SiNx:H deposition with MicroWave Remote
PECVD system
4. Screen-printing of the Ag front side and full Al rear
side metallization
5. Simultaneous firing of the front and rear side
metallization and Al Back Surface Field (BSF)
formation using an infrared heated belt furnace.
Slightly different firing conditions were used for
texture T1 and T2.
6. Edge isolation
3.
RESULTS AND DISCUSSION
3.1 Cell results
An acidic etch for simultaneous removal of the saw
damage and texturing results in a rough surface that
improves the light coupling and results in higher
efficiencies. Two recipes were used on mc-Si wafers.
Texture T1 results in a somewhat less rough surface,
which gives a higher VOC but a lower JSC, than texture T2.
Etch T1 is tested on a large amount (several thousands) of
wafers and is being applied in the solar cell industry. This
etch was optimized on throughput, lifetime of the
chemical bath, process stability, mechanical and electrical
Figure 1: Surface morphology of an etched mc-Si wafers.
The picture was made using an optical microscope.
Table 2: Cell results (156 cm2, untabbed and single layer
anti-reflection coating). The group of wafers A consists of
4 cells and B of 25 cells. Jsc measurements of ECN and
NREL are within 2% specification.
wafer
A median
A best
B median
B best
B best
NREL
FZ
FZ NREL
JSC
(mA/cm2)
34.2
34.4
34.6
35.0
34.5
FF
η (%)
T1
T1
T2
T2
T2
VOC
(mV)
617
618
611
621
623
0.769
0.774
0.765
0.774
0.768
16.2
16.3
16.2
16.8
16.5
T2
T2
629
631
34.7
34.1
0.789
0.780
17.2
16.8
texture
3.2 Modelling
To assess the effect of further efficiency improvement
the IQE of the best wafer A and B cells were measured
and analyzed using PC-1D (Figure 2 and Figure 3). As a
reference the IQE of the FZ cell is also presented. It can be
seen that the red response of the FZ is somewhat better
than that of the mc-Si cells.
Using PC-1D we try to determine the following fit
parameters: bulk lifetime τbulk, effective rear side surface
recombination velocity Srear, internal rear side reflection
Rrear, effective front side recombination velocity Sfront and
second diode recombination current density J02. Since it is
difficult to distinguish between τbulk and Srear, the FZ cell
is an important reference because τbulk can be fixed at 1000
µs (with this lifetime the diffusion length is about six
times the cell thickness and does not limit the efficiency).
Using this known lifetime, Srear can be fitted with the IQE
between 800 and 1000 nm, and Rrear with that around 1100
nm. Since the wafer B cell is processed the same way as
the FZ cell, these fitted values for Srear and Rrear can be
used to determine the bulk lifetime τbulk of the wafer B
cell. This lifetime was also used for the wafer A cell and
the Srear for that cell was fitted (wafer A cell is processed
slightly different: texture T1). Other combination of τbulk
and Srear for the wafer A cell will result in comparable fit
quality. The fitting of the other parameters is more
straightforward. The surface recombination velocity at the
front side Sfront is determined by the IQE at shorter
wavelengths. All losses at the front side (absorption in
SiNx:H layer, additional recombination due to texturing)
are included in that Sfront. This means that it is
overestimated compared to the actual value. The series
resistance and second diode recombination current J02
were varied for final adjustment of VOC and FF. The
results are presented in Table 3. The following remarks
can be made on these results and will be discussed in more
detail afterwards:
•
Output parameters of PC-1D simulations correspond
to measured parameters, only the modelled JSC seems
to be somewhat larger;
•
A lifetime τbulk of 1000 µs results in a good fit for the
FZ cell, which confirms that it does not limit the red
response of that cell;
•
The lower VOC and FF for the mc-Si cell can be
explained by the larger J02 for that cell;
•
Very good Srear is obtained;
•
Lower than expected internal reflection at the rear
side for both cells;
•
Sfront for the FZ is larger than that for the mc-Si.
100
80
IQE (%)
yield. A relative gain of 6% in short current and efficiency
compared to alkaline saw damage removal was
demonstrated [4]. The reflection at a wavelength of 1000
nm on bare Si is around 20% when etch T1 is applied. For
T2 it is around 18%. For alkaline saw damage removal the
reflection is around 30%. The morphology of an acid
etched wafer is presented in Figure 1.
The emitter diffusion was performed in an infrared
heated belt furnace with a metal belt. This belt contains
potential lifetime killers and it is important to find out if
the material quality degrades during the belt diffusion. We
checked this by measuring the effective lifetime τeff of FZ
material before and after diffusion. In both cases τeff=360
µs and is limited by the surface passivation. That means
that τeff of FZ does not degrade during belt furnace
diffusion.
The cell results are summarized in Table 2. For 156
cm2 untabbed mc-Si solar cells with a single layer
antireflection coating (ARC) a confirmed efficiency of
16.5% is obtained. With an additional MgF2 ARC the
efficiency should be about 17%. These results are
comparable to efficiencies on 156 cm2 mc-Si presented by
Duerinckx et al. [5] who used more complex processing
such as selective emitters. The record efficiency for large
area mc-Si (144 cm2) is 17.6% and has been achieved with
the completely batch processed Buried Contact Solar Cell
concept [6].
Wafer B cells with texture T2 have larger JSC than
wafer A cells due to the lower reflection of T2 compared
to T1. JSC for the best mc-Si cells is comparable to that of
the FZ cell. Voc and FF are better for the FZ cell.
60
FZ
16.3% cell
measured and PC-1D fit
40
20
0
300
500
700
900
wavelength (nm)
1100
Figure 2: Measured and PC-1D fitted IQE of the best cell
from group A compared to measured red response of the
FZ solar cell.
The output parameters for the PC-1D fit correspond to
the measured values. This means that the proposed
procedure using data of an FZ reference cell can be
applied to fit a multicrystalline cell. Furthermore, the good
fit using a lifetime of 1000 µs for the FZ cell confirms that
no degradation of τeff occurs during belt furnace
processing and that high efficiencies can be obtained with
belt furnace processing.
The JSC values modelled with PC-1D seems to be
somewhat larger than the measured ones. So far, it was not
possible to fit both the IV results and the IQE with PC-1D
in a way that both results match perfectly. Since the results
were within the 2% experimental error the fit procedure
seems to be good enough.
The fitted Srear for the Al BSF is 100 cm/s. This is
almost a perfect passivation of the rear side. Taking into
account the uncertainties in the measurements it can be
said that Srear is certainly below 200 cm/s.
The obtained reflection at the rear side of 60% is much
lower than measured earlier (76-78%, [7]). However, it
was impossible to get a good fit of the IQE at wavelengths
above 1000 nm with other than 60% reflection at the rear.
The lower VOC for the mc-Si cell compared to that of
the FZ can only be explained by assuming a higher J02
than the FZ cell. This higher J02 will result from additional
recombination in the depletion region and/or
inhomogeneities in the mc-Si cell. The latter can be
inhomogeneities in material quality [8] or inhomogeneities
in contact resistances [9].
Table 4: Mid-term and long-term improvements of
different material and cell properties and the effect on cell
output.
16.5% cell
with PC-1D fit
IQE (%)
80
60
20
0
300
FZ cell with
PC-1D fit
500
700
900
wavelength (nm)
1100
Figure 3: Measured and PC-1D fits of the IQE of the
16.5% best wafer B cell and the FZ cell.
property/
cell output
τbulk (µs)
Srear (cm/s)
Rrear (%)
Sfront (cm/s)
J02 (nA/cm2)
modelled VOC (V)
modelled JSC
(mA/cm2)
modelled FF
modelled η (%)
parameter
τbulk (µs)
Srear (cm/s)
Rrear (%)
Sfront (cm/s)
J02 (nA/cm2)
modelled VOC (V)
modelled JSC
(mA/cm2)
modelled FF
modelled η (%)
17% FZ 16.5% mc-B
1000
80
100
100
60
60
6⋅105
3⋅105
3
19
627
623
35.1
35.1
16% mc-A
80
300
60
2⋅105
51
619
34.3
Jsc (mA/cm2)
Table 3: PC-1D parameters to model the best
multicrystalline wafer A and B cells and the 17% FZ cell.
0.766
16.8
0.772
16.4
mid-term
long term
150
100
80
105
19
637
36.5
200
100
90
104
6
654
37.3
0.766
16.8
0.771
17.9
0.782
19.1
37.0
645
36.5
640
36.0
635
35.5
630
35.0
625
34.5
620
initial lifetime
0.779
17.1
16.5%
multi
80
100
60
3⋅105
19
623
35.1
Voc (mV)
100
40
3.3 Mid term and long term perspectives
Using the results presented in Table 3 material and
cell properties that could be improved on the mid term can
be identified. We have shown in other studies that τbulk
can be improved to 150 µs [10], Sfront to 105 cm/s [11], and
that Rrear to 80% [7]. J02 is already good. The effect on the
cell output of these improvements and those for the long
term are presented in Table 4. From this table it can be
seen that efficiencies close to 18% can be realized with a
comparable in-line process, and with parameters that have
been obtained previously. The challenge is to integrate the
process steps, which will lead to the improved parameters,
in the presented cell processing sequence. The effect of the
mid term improvements of individual material and cell
properties on VOC and JSC can be seen in Figure 4. It is
clearly visible that Rrear and Sfront are the most important
factors to improve JSC. The most important factor to
improve VOC is Sfront. The same holds for the long-term
improvements. Figure 5 shows the individual effects for
the long term starting with 18% efficiency. The effect on
efficiency can be seen in Figure 6. As expected from the
effects on VOC and JSC, Sfront and Rrear are the most
important factor to improve the cell efficiency. The IQEs
for the modelled cells is presented in Figure 7. The
improvement due to better effective front surface
passivation can be seen at shorter wavelengths. The
improved red response going from the 16.5% to 18% cell
is due to a better τbulk and a better Rrear. The improvement
to the 19% cell is mainly due to the better Sfront.
J02
Rrear
Sfront
Figure 4: Cumulative effect on JSC and VOC of individual
mid-term improvements.
660
37.0
655
36.5
650
36.0
645
35.5
640
35.0
5
Voc (mV)
Jsc (mA/cm2)
37.5
635
initial lifetime
J02
Rrear
Sfront
Figure 5: Cumulative effect on JSC and VOC of individual
long-term improvements. The end result of the mid term
improvements are seen as the initial situation for the long
term improvements.
efficiency (%)
long term improvements
19.0
18.5
18.0
17.5
ACKNOWLEDGEMENTS
17.0
mid term improvements
16.5
16.0
initial
lifetime
J02
Rrear
Sfront
Figure 6: Cumulative effect of individual mid- and longterm improvements on the efficiency. The end result of the
mid term improvements are seen as the initial situation for
the long term improvements.
19% cell
100
80
IQE (%)
It is shown that completely in-line cell processing with
belt furnaces can be used for high-efficiency solar cell
processing. τeff of FZ material does not degrade during
emitter diffusion on a standard metal belt.
With the in-line process a 16.5% confirmed efficient
156 cm2 mc-Si solar cell was made using wet chemical
texturing, a homogeneous emitter and firing through
SiNx:H combined with screen printing. PC-1D modelling
showed that 18% cell efficiency could be achieved when
τbulk, Rrear and Sfront values that were reached in the past,
are implemented in the current cell processing. Further
improvements could result in efficiencies above 19%.
Tom Moriarty and Keith Emery of NREL are
acknowledged for performing IV measurements.
Hans ter Beeke, Gertjan Langedijk and Paul
Trentelman of ECN are obliged for technical assistance.
This work has been carried out with financial support
of NovemSenter and performed within the Dutch DEN
programme
20.0
19.5
CONCLUSIONS
16.5%
cell
60
40
18% cell
20
0
300
500
700
900
wavelength (nm)
1100
Figure 7: IQEs for the modelled cells. The IQE of the
16.5%, that after the mid term improvements resulting in a
18% cell and the same for the 19% cell.
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