1. Introduction
Because of increased energy demand and global environ-
mental issues such as global warming due to fossil fuels,
renewable and clean energy such as solar energy has attracted
much attention. Government policy and subsidies have induced
the installation of many solar power plants, of which the
accumulated capacity has reached about 303~306.5 GW in
20161,2). The installation of solar power in 2017 is expected to be
about 95 GW3) and total accumulated capacity is expected to be
about 398~401.5 GW in 2017. Countries have reached grid
parity or have not reached grid parity owing to differences of
local electricity price. To increase the number of countries
reaching grid parity, many researchers in the solar cell industry
and at research institutes have sought lower production cost and
higher cell efficiency. PERC has been studied for quite long
time. Recently, more and more cell makers have changed from
Al-back surface field (Al-BSF) structures to PERC structures,
for higher cell efficiency in both MSSC and SSSC4). As shown
in Fig. 1, the local contact of PERC generates higher series
resistance, which induces the necessity of lower resistivity of a
silicon wafer. However, lower resistivity of a silicon wafer
generates another problem because of LID, which is attributed
to higher boron concentration.
LID is a well-known issue in silicon solar cells. The main
sources of LID are B-O complex activation, Fe-B pair
dissociation, and Cu5-8). Contrary to SSSC, MSSC shows no
LID thanks to the low oxygen concentration in multi-crystalline
silicon ingots. The strategy to mitigate LID in SSSC is well
known from many reports5,9). The first strategy is to decrease
oxygen concentration in a single-crystalline silicon ingot9).
There are many methods to decrease oxygen concentration10-13).
Typically, placing magnets around the equipment used to grow
silicon ingots can result in ingots with very low oxygen
concentration via suppression of silicon melt12,13). However, the
price of magnetic equipment is too high to decrease the
manufacturing cost. As a result, magnetic equipment has not
been adopted by the solar industry; it has been adopted,
however, by the semiconductor industry. The second strategy is
to decrease boron concentration, which induces an increase of
the resistivity5). To compensate for the increase of the
resistivity, a mix of gallium and boron as a p-type dopant
material has been proposed5). However, due to the very low
segregation coefficient of gallium (0.008) in comparison with
segregation coefficient of boron (0.8), it is difficult to control
the resistivity along the whole ingot length when using gallium
dopant14).
Current Photovoltaic Research 6(4) 94-101 (2018) pISSN 2288-3274
DOI:https://doi.org/10.21218/CPR.2018.6.4.094 eISSN 2508-125X
Improved Understanding of LeTID of
Single-crystalline Silicon Solar Cell with PERCKwanghun Kim1) ․ Sungsun Baik1) ․ Jaechang Park1) ․ Wooseok Nam1) ․ Jae Hak Jung2)*
1)R&D center, Woongjin Energy, 37, Techno 2-ro, Yuseong-gu, Daejeon 34012, Republic of Korea2)School of Chemical Engineering, Yeungnam University 280 Daehak-Ro, Gyeongsan, Gyeongbuk 38541, Republic of Korea
Received October 16, 2018; Revised December 19, 2018; Accepted December 19, 2018
ABSTRACT: Light elevated temperature induced degradation (LeTID) was noted as an issue in multi-crystalline silicon solar cells
(MSSC) by Ram speck in 2012. In contrast to light induced degradation (LID), which has been researched in silicon solar cells for a long
time, research about both LeTID and the mechanism of LeTID has been limited. In addition, research about LeTID in single-crystalline
silicon solar cells (SSSC) is even more limited. In order to improve understanding of LeTID in SSSC with a passivated emitter rear contact
(PERC) structure, we fabricated four group samples with boron and oxygen factors and evaluated the solar cell characteristics, such as
the cell efficiency, Voc, Isc, fill factor (FF), LID, and LeTID. The trends of LID of the four group samples were similar to the trend of
LeTID as a function of boron and oxygen.
Key words: LeTID, LID, PERC, Single-crystalline silicon solar cell, p-type boron, Oxygen
*Corresponding author: [email protected]
ⓒ 2018 by Korea Photovoltaic Society
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License
(http://creativecommons.org/licenses/by-nc/3.0)
which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
94
K.H. Kim et al. / Current Photovoltaic Research 6(4) 94-101 (2018) 95
Ram speck noted the issue of LeTID in MSSCs, in contrast to
the case of no LID in MSSC, in 201215). The mechanism of
LeTID in MSSC has been the subject of some research16-27). T.
Luka et al. reported micro structural identification of Cu and
grain boundaries in relation with LID at elevated temperature in
MSSC16,17). F. Kersten et al. reported that Hanwha Q CELLS
Q.ANTUM technology is able to suppress LeTID very
well18-20). Jan Schmidt et al. explained the mechanism of LeTID
in relation with a multi-step defects model in MSSC21). In
addition, there are some papers on LeTID in SSSC22). The
mechanism of LeTID in SSSC is still unclear.
We studied the LeTID of SSSC with PERC. We prepared
samples with different oxygen and boron concentrations and
fabricated PERC SSSCs. The samples were measured for solar
cell characteristics and tested for LID and LeTID. The results of
both LID and LeTID showed similar trends.
2. Experimental
2.1 Growing procedure and evaluation
Commercial polysilicon and quartz crucibles were utilized to
prepare four group samples, as described in Table 1. Four
polycrystalline Si batches were charged with different boron
concentration (p-type dopant) in quartz crucibles and were
moved into a commercial single crystal growing chamber. The
chamber was pumped down to vacuum, and then electrical
power was supplied to a graphite heater to melt the poly Si
contained in the quartz crucible. Following the complete
melting of the poly Si, a single crystal seed with (100)
orientation was dipped into the Si melt. The single crystal seed
was pulled upward and a neck process with a diameter of four to
five millimeters was executed to remove slips from the seed.
This process then moved on to the shoulder process to enlarge
the diameter up to 215 mm, which was the target diameter.
Following the shoulder process, the body growing process was
performed to produce a uniform single crystal ingot. The
crucible was rotated counter-clockwise at a speed of 4-10 rpm,
while the crystal was rotated in the opposite direction at 10-12
rpm during the body growing process. Ar flux was 50-100 lpm,
and the pressure in each chamber was maintained at 30-50 Torr
during the body growing process. Samples with different
oxygen concentrations were prepared by deliberately adjusting
the crucible rotation. After the body growing process, a tail
growing process followed to decrease the diameter slowly to
zero mm to minimize the thermal shock during popping out of
the ingot from the remaining silicon melt. After the tail growing
process, a cooling process of both the ingot and the graphite
parts was initiated inside the growing chamber.
After the removal of the ingots from the growing chamber,
the ingots were cropped into cylindrical bricks for minority
carrier life time (MCLT) measurement, and a few slugs were
examined to measure the oxygen and carbon contents. The
MCLT values were measured at the center of the cropped
surfaces of the cylindrical bricks (BCT300, Sinton). Following
thermal donor annealing, resistivity was measured by the
four-probe method (CMT-SR5000, AIT). The interstitial
oxygen (Oi) and substitutional carbon (Cs) contents were
measured at the centers of the polished slugs by FT-IR
spectroscopy (QS2200, Nano metrics).
2.2 Wafering procedure and evaluation
To fabricate P-type PERC SSSCs using these ingots, wafers
with thicknesses of 180 μm were prepared in a commercial wire
sawing machine. The wafers were heat-treated at 800°C for 35
min with Ar gas and then at 1100°C for 60 min with H2O. The
MCLT map of wafers was obtained by u-pcd (Semi Lab).
(a) (b)
Fig. 1. Schematic diagram of (a) conventional A-BSF cell
structure and (b) PERC
Table 1. Four group conditions
Group Number Name Comment
Group-1 Low ResistivityHigher B atoms
Higher O atoms
Group-2 Low Oi
Lower B atoms
Lower O atoms
Group-3Low Resistivity
+ Low Oi
Higher B atoms
Lower O atoms
Group-4 (Reference)Lower B atoms
Higher O atoms
K.H. Kim et al. / Current Photovoltaic Research 6(4) 94-101 (2018)96
2.3 Solar cell procedure and evaluation
PERC SSSCs were fabricated in a commercial solar cell line.
Each group has 6 solar cells and total number of solar cells is 24.
Cell performance was evaluated using a solar simulator
(LOANA, pave-tools GmbH) at Fraunhofer CSP. All samples
of the four material groups were tested first in B-O LID
treatment at 25±4°C of temperature with 800~1000 W of
illumination for 24 hrs. Then using LID Scope of Lay Tec AG at
Fraunhofer CSP, LeTID treatment was carried out for all
samples at a temperature of 130±0.3°C with Isc-injection at a
current equivalent to 1 sun illumination for at least 2 hrs.
3. Results and Discussion
3.1 Ingot characteristics
Fig. 2 shows oxygen concentration, carbon concentration,
resistivity, and MCLT of ingots. Groups 1 and 4 show higher
oxygen concentrations with similar values. Groups 2 and 3 show
oxygen concentrations that are lower by 4.5 ppma and 7.1 ppma,
respectively, as shown in Fig. 2 (a). Groups 2 and 4 show higher
resistivity with similar values. Groups 1 and 3 show resistivity
values lower by 0.88 ohm/cm and 0.76 ohm/cm, respectively, as
presented in Fig. 2 (b). The carbon concentration values of the
four groups are within the mass production deviation and there
is no meaningful difference, as shown in Fig. 2 (c). The MCLT
of Group 2 is the highest value because this sample has the
lowest oxygen and boron concentrations, which elements may
adsorb minority carriers28). The MCLT of Group 1 is the lowest
value because this sample has the highest oxygen and boron
concentrations28). The similar MCLT values of groups 3 and 4
reflect that the effect of oxygen concentration difference ranged
from 4.5 ppma to 7.1 ppma for MCLT, corresponding to an
effect of resistivity difference that ranged from 0.76 ohm/cm to
0.88 ohm/cm for MCLT, as given in Fig. 2 (d).
3.2 Wafers characteristics
Fig. 3 provides a wafer surface image after the texturing
process. There is no meaningful difference from Fig. 3 Table 2
indicates the reflectiveness of all groups. There is no meaningful
difference from Table 2. Fig. 4 shows MCLT pattern images
before and after heat treatment. Before heat treatment, all
images of all groups are similar, with many horizontal lines, and
the average MCLT values of all groups are similar. Contrary to
the images before heat treatment, after heat treatment circular
pattern appeared in the images of Groups 1 and 4 and the images
of Groups 2 and 3 remained the same as the images before heat
treatment. Groups 1 and 4 are relatively higher oxygen con-
centration groups. Haunschild et al.29) reported that dark circular
pattern images after the emitter diffusion process are harmful to
solar cell efficiency ; the dark circular pattern indicates oxygen
(a) (b)
(c) (d)
Fig. 2. Four ingot qualities including (a) Oi, (b) Resistivity, (c) Cs, and (d) MCLT (Red numbers show the difference value compared
to the value of Group-4 as reference)
K.H. Kim et al. / Current Photovoltaic Research 6(4) 94-101 (2018) 97
precipitation in relatively higher oxygen concentration samples.
Our results for the MCLT map images after heat treatment are in
good agreement with Haunschild et al.’s results. In addition, the
average MCLT values of Groups 2 and 3, with lower oxygen
concentration, are much higher than those of groups 1 and 4,
with higher oxygen concentration, after heat treatment.
3.3 Solar cells characteristics
Fig. 5 shows values of Voc, Isc, FF, and cell efficiency of each
group. The Voc of Groups 2 and 3 with lower oxygen con-
centration is higher than it of Group 4 by 2.16 mV and 2.26 mV,
respectively, as shown in Fig. 5(a). The Isc values of Groups 2
and 3, with lower oxygen concentration, are higher than that of
Group 4 by 60.1 mA and 9.3 mA, respectively, as plotted in Fig.
5(b). However, the FF values of Groups 1 and 3, with lower
resistivity, are higher than that of Group 4, by 0.99 % and 1.1 %,
respectively, as presented in Fig. 5(c). This may be attributed to
the better conductivity due to the higher boron concentration30).
The cell efficiency of Group 3, with lower oxygen concentration
and lower resistivity, shows the highest value, as indicated in
Fig. 5(d).
3.4 LID and LeTID characteristics
Fig. 6(a) shows LID values of all groups after one day of light
soaking. The LID of Group 1 is the highest value due to the
relatively many boron and oxygen atoms5). LIDs of Groups 3
and 4 are similar owing to the decrease of oxygen atoms in
Group 3 and the decrease of boron atoms in Group 4. LID of
Group 2 is not in accordance with our expectation. It was
expected to be the lowest number; however, the LID of group 2
is located between the LID of Group 1 and the LIDs of Groups
3 and 4. The reason for this is unclear. LID results for Groups 1,
3, and 4 are reversely proportional to the MCLT results of
Groups 1, 3, and 4, as shown in Fig. 2(d). Fig. 6(b) shows cell
efficiency values of all Groups after one day of light soaking.
Although cell efficiency degradation happened in all groups, the
trend of all groups before/after one day of light soaking
remained the same. The cell efficiency of Group 3 is the highest
value.
Fig. 7 shows Voc degradation at elevated temperature of 130°C
with Isc-injection in a current equivalent to 1 sun illumination.
Only Group 1 exhibits a distinctive degradation from 0 min to 35
min. The degradation and regeneration curve of Group 1 is
similar to the typical curve in the reported reference31). This
(a) Group-1 (b) Group-2 (c) Group-3 (d) Group-4
Fig. 3. Texturing images of all groups
Table 2. Reflectiveness of all groups
Group-1 Group-2 Group-3 Group-4
Reflectiveness 10.8 11.15 11.25 11.35
Group-1 Group-2 Group-3 Group-4
Before
Heat Treatment
After
Heat Treatment
Fig. 4. MCLT pattern images before and after heat treatment (Red numbers show the difference between value of average MCLTand
value of Group-4 as reference)
K.H. Kim et al. / Current Photovoltaic Research 6(4) 94-101 (2018)98
(a) (b)
(c) (d)
Fig. 5. Cell performance values including (a) Voc, (b) Isc, (c) FF, and (d) cell efficiency before LID (Red numbers show the difference
value from value of Group-4 as reference)
(a) (b)
Fig. 6. The (a) LID and (b) cell efficiency after LID (Red numbers show the difference value from value of Group-4 as reference, and
LID [%] is percent calculation based on efficiency)
Fig. 7. Relative Voc degradation at elevated temperature of 130°C with Isc-injection for current equivalent to 1 sun illumination
K.H. Kim et al. / Current Photovoltaic Research 6(4) 94-101 (2018) 99
might be ascribed to the relatively higher boron and oxygen
concentrations. The other groups do not show severe degrada-
tion of Voc.
Fig. 8(a) shows LeTID values of all groups. The LeTID of
Group 1 is the highest value thanks to the relatively higher boron
and oxygen concentrations. Contrary to the LID results of
Groups 2, 3, and 4, the LeTID of Groups 2 and 4 are similar and
the LeTID of Group 3 is the lowest value. In addition, the
deviation of LeTID of each group is larger than that of the LID
of each group. This shows that there might be noise and other
factors beyond boron and oxygen that affect cell efficiency.
However, the trends of all groups for cell efficiency after LeTID
are similar to the trend before LeTID, as shown in Fig. 8(b). As
a result, the three trends of cell efficiency before LID, after LID,
and after LeTID are identical. However, the difference values
for reference Group 4 decreased as the experimental steps
proceeded.
3.5 Similar trends of LID and LeTID in SSSC
Fig. 9 provides a comparison of LID and LeTID for all
groups. The trends of LID and LeTID are very similar. This
means that the large part of the mechanism of LeTID in SSSC
PERC might be the same as the mechanism of LID induced by
boron and oxygen compound degradation5). And, the LeTID
values of Groups 1, 2, and 4 are higher than the LID values of
Groups 1, 2, and 4. The LID and LeTID values of Group 3 are
very close. The LeTID value of Group 4 is much higher than the
LID value of Group 4.
4. Conclusions
In conclusion, we fabricated four types of wafers and PERC
SSSC and evaluated the cell performance, LID, and LeTID. The
groups with lower oxygen concentrations showed higher
MCLT, and the groups with higher boron concentration showed
lower MCLT. After severe heat treatment, the groups with
lower oxygen concentration did not exhibit ring patterns in the
MCLT map images. The groups with lower oxygen concen-
tration had higher Voc and the groups with higher boron
concentration had higher FF. Group 3, with relatively lower
oxygen and higher boron concentration, had the highest cell
efficiency. The LID and LeTID values of group 1 were the
highest values. The LID and LeTID values of Group 3 were the
lowest values. The trend of LID was similar to the trend of
LeTID. This shows that the mechanism of LeTID in PERC
SSSC might be similar to that of LID. The results in this work
are expected to provide a new way to control LeTID in PERC
SSSC, and to enhance the stability of PERC SSSC.
(a) (b)
Fig. 8. The (a) LeTID and (b) cell efficiency after LeTID (Red numbers show the difference value from the value of Group-4 as
reference)
Fig. 9. Comparison of LID and LeTID (Blue and red numbers
show difference values of LID and LeTID compared to
values of Group-4 as reference)
K.H. Kim et al. / Current Photovoltaic Research 6(4) 94-101 (2018)100
Acknowledgements
This work was supported by the New & Renewable Energy
Core Technology Program of the Korea Institute of Energy
Technology Evaluation and Planning (KETEP), funded by the
Ministry of Trade, Industry & Energy, and Republic of Korea
(No. 20163030013700).
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