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Title

Application of crystalline silicon surface
oxidation to silicon heterojunction solar cells

Author(s)Oikawa, Takafumi; Ohdaira, Keisuke; Higashimine,
Koichi; Matsumura, Hideki

Citation

Current Applied Physics, 15(10): 1168‑1172

Issue Date

2015‑07‑07

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/14706

RightsCopyright (C)2015, Elsevier. Licensed under theCreative Commons Attribution‑NonCommercial‑
NoDerivatives 4.0 International license (CC BY‑
NC‑ND 4.0).
[http://creativecommons.org/licenses/by‑nc‑
nd/4.0/] NOTICE: This is the author's version ofa work accepted for publication by Elsevier.
Takafumi Oikawa, Keisuke Ohdaira, Koichi
Higashimine, Hideki Matsumura, Current Applied
Physics, 15(10), 2015, 1168‑1172,
http://dx.doi.org/10.1016/j.cap.2015.07.004

Description

Japan Advanced Institute of Science and Technology

Application of Crystalline Silicon Surface Oxidation to Silicon
Heterojunction Solar Cells
Takafumi Oikawa1, Keisuke Ohdaira1,2,*, Koichi Higashimine1,2 and Hideki
Matsumura1,2
1Japan Advanced Institute of Science and Technology
1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
2

CREST, Japan Science and Technology Agency

4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
Tel: +81-761-51-1563, Fax: +81-761-51-1149
E-mail: ohdaira@jaist.ac.jp

ABSTRACT
We study the effect of ultra-thin oxide (SiOx) layers inserted at the interface of siliconheterojunction (SHJ) solar cells on their open-circuit voltage (VOC). The SiOx layers canbe easily formed by dipping c-Si into oxidant such as hydrogen peroxide (H2O2) andnitric acid (HNO3). We confirm the prevention of the undesirable epitaxial growth of Si
layers during the deposition of a-Si films by the insertion of the ultra-thin SiOx layers.
The formation of the SiOx layers by H2O2 leads to better effective minority carrier
lifetime (τeff) and VOC than the case of using HNO3. c-Si with the ultra-thin SiOx layers
formed by H2O2 dipping, prior to deposition of a-Si passivation layers, can have highimplied VOC of up to ~0.714 V.

Keywords:

1

Heterojunction solar cell, Ultra-thin SiOx layer, Cat-CVD, Passivation, Epitaxial growth2

1 INTRODUCTION
The demand of solar cells has been growing in the last decade, and for more spread ofphotovoltaics, further improvement in the conversion efficiency is required [1]. Since
crystalline silicon (c-Si) wafer-based solar cells are the most popular structures in thephotovoltaic market at present, the enhancement of their conversion efficiency is ofgreat importance [1]. Of a variety of Si wafer-based solar cells, Si heterojunction (SHJ)
solar cells have recently received much attention [2-6]. The SHJ solar cell is composedof a c-Si wafer and hydrogenated amorphous Si (a-Si) films, and has a plenty ofadvantages such as high conversion efficiency accompanied by high open-circuit
voltage (VOC) and low manufacturing temperatures. Intrinsic a-Si (i-a-Si) films are
utilized for the passivation of crystalline Si (c-Si) surfaces owing to the effect ofabundant hydrogen in the a-Si films. The best reported VOC is 0.750 V with anefficiency of 24.7% for a conventional SHJ structure [2] and 0.740 V with an efficiencyof 25.6% for a heterojunction back-contact (HBC) structure [3]. The SHJ solar cells
have thus been expected to be in the mainstream of next-generation c-Si solar cells.
For high-efficiency SHJ solar cells, i-a-Si passivation layers realizing sufficiently low
surface recombination velocity is essential. The i-a-Si passivation films are generally
formed by plasma-enhanced chemical vapor deposition (PECVD), in which the
suppression of plasma damage onto c-Si surfaces is often problematic. In contrast,
catalytic CVD (Cat-CVD), often also referred to as hot-wire CVD, can form i-a-Si
passivation films without plasma damage since its gas decomposition mechanism is notbased on plasma process but through catalytic cracking on a heated catalyzing wire [7,8].
Cat-CVD is thus expected to be superior for the fabrication of SHJ solar cells. It should
be noted that the passivation property of a-Si/c-Si interfaces is governed not only by the3quality of i-a-Si itself and a degree of damage onto c-Si during deposition but also by
the presence or the absence of the epitaxial growth of Si [9,10]. Silane (SiH4) molecules
are decomposed to one Si and four H atoms on a heated catalyzed wire in Cat-CVD [1116], and due to the existence of bare Si radicals, the epitaxial growth of Si is more likely
to occur in Cat-CVD [17]. The development of a method to prevent the Si epitaxial
growth is thus required particularly for the application of Cat-CVD to the formation of
a-Si passivation films. One of the possible approaches is to vanish the periodicity of cSi lattice on the surface by forming an amorphous thin film on the c-Si. The formationof ultra-thin Si oxide (SiOx) layers is a promising candidate for such a thin film since it
can be easily formed by the oxidation of a c-Si surface and does not prevent carriertransport if it is sufficiently thin [18-21].
In this study, we have attempted to form ultra-thin SiOx layers on c-Si surfaces forpreventing the epitaxial growth during the Cat-CVD of a-Si films by dipping c-Si
wafers into hydrogen peroxide (H2O2) or nitric acid (HNO3). We have also applied thec-Si surface oxidation to the fabrication of SHJ solar cells to confirm the effect ofoxidation process against solar cell performance.

2 EXPERIMENTAL DETAILSWe used double-side-polished floating-zone (FZ) n-type c-Si wafers which have(100) orientation, a thickness of 290 µm, a resistivity of ~2 Ωcm, and a bulk lifetime(τb) of >10 ms. We first cleaved c-Si wafers into 2 cm × 2 cm-sized pieces, and nativeoxide layers on the Si wafers were removed in a 5% HF solution diluted with deionizedwater. The wafers were then immediately dipped in 0.5-10 wt% H2O2 or HNO3 for 30
seconds at room temperature to form the ultra-thin SiOx layers on c-Si surfaces. We

4

used H2O2 and HNO3 solutions with impurity range in parts per million. It should be
noted that longer dipping in the oxidants does not lead to any different results, which isprobably because of low treatment temperature and resulting saturation of oxidation.
The thickness of the ultra-thin SiOx layers evaluated by ellipsometry was 0.8-1.0 nm.
Some of the ultra-thin SiOx layers formed were characterized by attenuated totalreflectance Fourier transform infrared (ATR-FTIR) spectroscopy. We prepared i-a-Si/cSi/i-a-Si and i-a-Si/SiOx/c-Si/SiOx/i-a-Si structures to confirm the effect of epitaxial
growth and passivation quality. In these experiments, a-Si layers were deposited in a
Cat-CVD system with changing substrate temperatures. Deposition pressure and SiH4
flow rate were fixed at 1.0 Pa and 10 sccm, respectively. Deposition duration was also
fixed at 30 second, which resulted in the a-Si thickness of 8-9 nm. We used a tungsten
wire as catalyzer heated at 1800 °C for all the experiments. After the deposition of a-Si
films, the samples were annealed at 200 °C for 30 min in air in order to improve thequality of a-Si/c-Si and a-Si/SiOx/c-Si interfaces. The emergence of epitaxial growth
was evaluated by spectroscopic ellipsometry and cross-sectional transmission electron
microscopy (TEM). The quality of the a-Si/c-Si interfaces was characterized from theeffective minority carrier lifetime (τeff) measured by the microwave photoconductivity
decay (μ-PCD) method.
For SHJ solar cell fabrication, n-a-Si/i-a-Si films were deposited on one side of c-Si
after the formation of ultra-thin SiOx layers and p-a-Si/i-a-Si films were then depositedon the other side. We used PH3 and B2H6 gases diluted with helium to 2.25% to form
the doped a-Si layers. 80-nm-thick indium tin oxide (ITO) films were sputtered on theboth sides of the samples at a substrate temperature of 100 °C. We then formed combshaped Ag electrodes formed by screen printing and successive annealing at 200 °C.

5The deposition condition of the a-Si films is summarized in Table I. Note that the values
in the parentheses represent those used for SHJ solar cell fabrication. We also prepared
solar cells without the oxidation of c-Si surfaces for comparison. We measured τeff andimplied VOC by μ-PCD and quasi-steady-state photoconductivity (QSSPC), respectively,
before the formation of ITO films and Ag electrodes. We characterized the solar cell
performance by current density-voltage (J-V) measurement under 1-sun (AM1.5, 100
mW/cm2) illumination.

3 RESULTS AND DISCUSSION
3.1 Prevention of epitaxial growth
Figure 1 shows the thickness of i-a-Si films on c-Si wafers with and without
oxidation under various oxidant concentrations up to 10 wt% for H2O2 and 30 wt% for
HNO3 as a function of substrate temperature during the deposition of i-a-Si films. The
thickness of i-a-Si films on c-Si without ultra-thin SiOx layers decreases significantlywith increase in substrate temperatures. This is probably because deposited Si layers are
epitaxially grown in some parts, and such epitaxially grown c-Si layers with same
optical properties as c-Si wafers cannot be distinguished by spectroscopic ellipsometry.
Reduction in a-Si film thickness thus indicates the emergence of epitaxial growth. On
the contrary, reduction in i-a-Si film thickness is less significant in the case of thesamples with oxidation. This tendency is more remarkable for the structures treated inhigher oxidant concentration. These results clearly indicate that the ultra-thin SiOx
layers formed on c-Si can effectively prevent the epitaxial growth.
The results above indicate that the epitaxial growth can be suppressed both by H2O2
and HNO3 treatment. One can also see in Fig. 1 that epitaxial growth is adequately

6

prevented at a deposition temperature of around 180 °C even at a H2O2 concentration of
as low as 0.5 wt% in H2O2, while the effective suppression of epitaxial growth isrealized only at much higher HNO3 concentration of about 30 wt%. The reason for thisdifference may originate from different oxidation mechanism depending on oxidant,
which will be discussed in section 3.4.
According to these results, we have clearly confirmed the effectiveness of inserting
thin SiOx layers for the prevention of epitaxial growth. In the case of high oxidant
solution concentration, under which epitaxial growth is sufficiently suppressed, it is thus
possible to deposit a-Si passivation layers at higher temperatures. This would leads tothe utilization of a-Si films with better film quality.
We also confirm the prevention of the epitaxial growth by forming ultra-thin SiOx
layers on c-Si by TEM observation. Figures 2(a) and (b) show the cross-sectional TEM
images of a-Si/c-Si and a-Si/SiOx/c-Si structures. Note that these TEM images wereobserved in the SHJ structures, and deposited Si exists in the region of 8-9 nm from c-Si
interface. In the case of the sample without oxidation, epitaxially-grown c-Si isdefinitely seen at some places. On the other hand, the epitaxial growth is almost
completely prevented in the sample with H2O2 oxidation. This result clearly
demonstrates the benefit of the SiOx layer insertion to obtain high quality a-Si/c-Si
interfaces.

3.2 Oxidant concentration dependence of minority carrier lifetime
Figures 3 shows τeff of i-a-Si/c-Si/i-a-Si structures with c-Si oxidized in H2O2 or
HNO3 with up to 10 wt% oxidant concentrations. i-a-Si films on the structures were
deposited at 160 °C. The samples without SiOx layers show low τeff of less than 50 µs.

7According to the results shown in Figure 1, the epitaxial growth of defective c-Si
occurs, and the a-Si/c-Si interface quality is thus deteriorated. On the contrary, τeff isimproved in the samples with SiOx layers. This result indicates that the formation ofultra-thin SiOx layers on c-Si surfaces is effective in improving the quality of a-Si/c-Si
heterojunction interfaces. The H2O2-oxidized samples show the maximum τeff of 600 µs
at an oxidant concentration of 3-5 wt%. H2O2 with a concentration of more than 5 wt%
may induce surface damage due to excessive oxidation and resulting reduction in τeff.
Compared to the H2O2-oxidized samples, the dependence of τeff on HNO3 concentration
is not clear. Furthermore, the maximum τeff of H2O2-oxidized samples is much higher
than those of HNO3-oxidized samples. These may be not only due to the degree of
epitaxial growth but also due to the difference of the quality of SiOx layers themselves.

3.3 J-V characteristics
The relationship between the oxidant concentration and VOC or fill factor (FF) of the
SHJ solar cells with ultra-thin SiOx layers formed in H2O2 or HNO3 are shown in Figure
4. The SHJ cells with ultra-thin SiOx layers show better VOC compared to the cells
without oxidation. VOC is nearly independent of oxidant concentration at more than 0.5
wt% in both oxidant solutions. This result is unlikely the i-a-Si passivation experiments,
in which clear dependence of τeff on oxidant concentration is seen. This may be due tothe difference of sample structure. In the SHJ cells, n- and p-a-Si films are additionallydeposited, and these doped a-Si films may affect the quality of a-Si/c-Si interfaces.
Focusing on the absolute values of VOC, the SHJ cell with SiOx layers formed by H2O2
has higher VOC than that with HNO3-oxidized layers. This fact is consistent with8

previous results demonstrating more suppressed epitaxial growth and higher τeff shownin Figures 1-3.
On the other hand, FF shows a different tendency against oxidant concentration
depending on oxidant solutions. The SHJ cells with H2O2 treatment has lower FF thanthe cell without oxidation treatment, and FF decreases to ~0.4 with increasing H2O2
concentration. The SHJ cells with HNO3 treatment have a constant FF regardless of theoxidant concentration. FF is generally affected by series or shunt resistances. The SHJ
solar cells fabricated in this study have sufficiently large shunt resistance, which was
confirmed through pseudo FF (pFF) obtained by Suns-VOC measurement. A possible
impact on the FF is thus series resistance. In the SHJ cells with H2O2 treatment, SiOx
layers themselves may act as series resistance since FF decreases with increase inoxidant concentration. In particular, increase in the thickness and density of SiOx layers
by high concentration treatment can be a significant impact on series resistance and FF.
On the contrary, the FF of the SHJ cells with HNO3 oxidation has no dependence onoxidant concentration. This may be because the thickness of SiOx layer formed by
HNO3 treatment does not increase significantly even higher HNO3 concentration [21].
The previous results show that H2O2 treatment leads to better a-Si/c-Si interface
quality and cell performance. Figure 5 and Table II show J-V characteristics of SHJ
solar cells with and without H2O2 treatment. Note that the deposition conditions of a-Si
films were slightly changed to obtain better solar cell performance, which are shown inparentheses in Table I. We can clearly see the impact of SiOx layer insertion on τeff and
VOC. In particular, implied VOC reaches 0.714 V, which is much larger than the implied
VOC of the sample without H2O2 treatment of 0.591 V. It should be noted that VOC
obtained by J-V measurement is smaller than implied VOC for both SHJ cells. This may

9be due to plasma damage onto a-Si/c-Si interfaces during ITO sputtering [22]. Although
the FFs of J-V curves are insufficient at present due to relatively high series resistance,
this can be partly improved by optimizing the sheet resistance of ITO and the thicknessof a-Si layers.

3.4 ATR-FTIR measurement of oxidized Si (100)In order to understand the different tendency of passivation quality and cellcharacteristics depending on the type of oxidant solution, we have attempted to evaluate
the characteristics of ultra-thin SiOx film using ATR-FTIR measurement. Figure 6
shows the FTIR spectra of Si (100) samples treated in (a) H2O2 and (b) HNO3. One cansee two characteristic peaks at 2110 cm-1 and 2141 cm-1, corresponding to SiH2 andSiH3 stretching vibration band, respectively [23,24], for the samples with oxidation in
low concentration oxidant. These peaks derive from hydrogen-terminated Si surfaces
formed by HF dipping. With increasing H2O2 concentration, SiH2 and SiH3 vibration
bands disappear, while the intensity of a peak at 2243 cm-1 increases. This peakoriginates from SiO3H bonds which can be created if all the back bond Si atoms are
replaced to O by oxidation [25]. The emergence of the SiO3H peak is a clear evidenceof the oxidation of Si. On the other hand, In spite of reduction in SiH2 and SiH3 peaks, a
SiO3H peak does not appear after HNO3 oxidation even at high HNO3 concentration.
This suggests that oxygen atoms in HNO3 solution hardly react with the back bond Si
atoms. SiO3H is a dominant structure on a Si (100) surface oxidized in H2O2 [26]. Whenthe back bonds are oxidized, strain due to the interfacial volume expansion occurs [27].
This may behave as an electrical resistance, which could explain the decrease in FF of
SHJ cells with increase in H2O2 concentration. In the case of HNO3-oxidized samples,10

according to Gondek et al., compared with the oxidation of H-terminated Si surfaces by
H2O2, HNO3-oxidized samples show higher SiOH species (3000-3600 cm-1) than H2O2
oxidized samples [28], although it was not clearly observed in this measurement. This
indicates more adsorption of H2O on HNO3-treated c-Si surfaces [29]. Oxidation by
HNO3 is generally performed at higher HNO3 concentrations and at high temperatures[20,21]. Oxidation in HNO3 at room temperature performed in this study may beinsufficient for effective Si surface oxidation. We should also discuss the suppression of
epitaxial growth by HNO3 treatment even without the formation of SiOx layers. As
mentioned above, a number of OH bonds are adhered on a Si surface after HNO3
treatment. This might contribute to the suppression of epitaxial growth to some extent,
which is less effective for the prevention of epitaxial growth than the case of H2O2
oxidation. The difference in the passivation quality may thus be related to the presenceof SiO3H bonds.

4. CONCLUSION
The formation of ultra-thin SiOx layers by dipping in oxidant solution is applied to
SHJ solar cell process. The introduction of ultra-thin SiOx layers between a-Si/c-Si
interfaces results in improvement in τeff and VOC due to the prevention of the epitaxial
growth. τeff significantly improves when we apply H2O2 treatment with a concentrationof 3-5 wt%. A SHJ solar cell with 4 wt% H2O2 treatments shows VOC and implied VOC
up to 0.683 V and 0.714 V, respectively. The results obtained have clearly demonstrated
the effectiveness of ultra-thin SiOx layers formed by H2O2 dipping for improvement in
the performance of SHJ solar cells.

11

ACKNOWLEDGMENT
The authors would like to thank Prof. Usami and Dr. Takahashi of Nagoya University
for ATR FT-IR measurement. This work was supported by JST CREST program.

12

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16

FIGURE CAPTIONS
Figure 1 Thickness of a-Si films as a function of substrate temperature during film
deposition on c-Si treated in (a) H2O2 and (b) HNO3 at various concentrations.

Figure 2

Cross-sectional TEM images of (a)a-Si/c-Si and (b) a-Si/SiOx/c-Si stacked

structures.

Figure 3τeff of c-Si wafers passivated with i-a-Si films as a function of oxidant

concentration.

Figure 4

Oxidant concentration dependence of VOC and FF of SHJ solar cells treatedusing (a) H2O2 or (b) HNO3.

Figure 5

J-V characteristics of the SHJ solar cells without and with ultra-thin SiOx

layers on c-Si formed by dipping in 4.0 wt% H2O2.

Figure 6

ATR-FTIR spectra of Si (100) surfaces oxidized for 30 seconds at room

temperature in (a) H2O2 or (b) HNO3 solutions with various concentrations.

17

Table I

Process conditions for a-Si deposition.
Gas flow rate /sccm
Ts/°C

SiH4

H2

PH3

B2H6

Pgas /Pa

i-a-Si

160

10

-

-

-

1

n-a-Si250

20 (10)50 (-)

4.4-

2

p-a-Si

250

10

50

-

8 (16)

2

Table II

Characteristics of the SHJ solar cells without and with ultra-thin SiOx layers

on c-Si formed by dipping in 4.0 wt% H2O2.
a-Si/c-Si/a-Si

a-Si/SiOx/c-Si/SiOx/a-Siτeff (μs)

72.6

1480

Imp. VOC (V)

0.591

0.714

Jsc (mA/cm2)

31.1

30.5

Cell VOC (V)

0.568

0.683

FF

0.723

0.668

Efficiency (%)

12.8

13.9

i-a-Si thickness (nm)

8
6
4
2
0
80 100 120 140 160 180 200 220 240

i-a-Si thickness (nm)Substrate temperature (°C)
8
6
4
2
0
80 100 120 140 160 180 200 220 240

Substrate temperature (°C)

700
H 2 O2
HNO3

τeff (µs)

600
500
400
300
200
100
0

0

1

2

3

4

5

6

7

8

9 10 11 12Concentration (wt%)

0.70

0.8

VOC
FF

0.7

0.66

0.6

0.64

0.5

0.62

0.4

0.60

0.3

FF

VOC (V)

0.68

0

1

2

3

4

5

6

7

8

H2O2 concentration (wt%)
0.70

VOC
FF

0.7

0.66

0.6

0.64

0.5

0.62

0.4

0.60

0.3

0

5

10

15

20

25 30 35

HNO3 concentration (wt%)

FF

VOC (V)

0.68

0.8

Current density (mA/cm2)

35
30
25
20

a-Si/c-Si

15
a-Si/SiOx/c-Si
10
5
0
0

0.1

0.2

0.3 0.4 0.5

Voltage (V)0.6

0.7 0.8

Absorbance (arb. unit)
2300

2200

2100

2000

Absorbance (arb. unit)

Wavenumber (cm-1)

2300

2200

2100

Wavenumber (cm-1)

2000