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: jhjung@ynu.ac.kr

ⓒ 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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