Proceedings of 6th Thin Film Materials & Devices Meeting
November 2-3, 2009, Kyoto, Japan
http://www.tfmd.jp/

High-Pressure H2O Vapor Heat Treatment for Characteristic Improvement
in Crystalline Silicon Solar Cells
M. Hasumi, T. Nagao, S. Yoshidomi and T. Sameshima
Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588,
JapanE-mail : mhasumi@cc.tuat.ac.jp
Keywords: solar cell, defect passivation, conversion efficiency, effective carrier lifetime, H2O
vapor treatment

Abstract. Improvement in characteristics of commercial single crystalline silicon solar cells
was demonstrated by high-pressure H2O vapor heat treatment. The short circuit current, open
circuit voltage and conversion efficiency increased from 10.6 to 11.4 mA/cm2, 0.515 to 0.526 V,
and 11.6 to 12.5%, respectively by the heat treatment at 210oC in 2.0 x 105 Pa H2O vapor for 30
min. This result indicated that high-pressure H2O vapor heat treatment effectively modified thepassivation layer of silicon solar cells and reduced the defects of photo induced carrier
recombination. However, the degradation of conversion efficiency from 13.7 to 12.2% was
observed by the heat treatment at 210oC in 1.0 x 106 Pa H2O vapor for 30 min because of theincrease in serial resistance loss by the oxidation of silver electrode.

Introduction
Solar cells are promising devices for producing electrical powers without increasing thedensity of carbon dioxide in the atmosphere, and the markets of solar cell have been expanded
due to these demands rapidly. Especially, crystalline silicon solar cells have been mostpromising because of their high conversion efficiency, reliability and the use of safe materials.
Technical developments aiming at increasing the conversion efficiency and decreasing
manufacturing cost are important. For example, there have been developments on surface
texture for introducing sunlight into silicon solar cells with no significant reflection loss, multiband gap engineering for carrier generating from sunlight with wide spectra, defect reduction in
crystalline silicon bulk for decreasing carrier recombination probability, and transparent
electrodes with a low resistivity for decreasing joule heating loss [1].
We have developed high pressure H2O vapor heat treatment as a technology for defectpassivation of polycrystalline silicon thin film transistors (poly-Si TFTs) [2]. The improvement
in TFT characteristics was achieved by simple process applied to the samples with 1.3 x 106 Pa
H2O vapor environment at temperatures ranging from 200 to 300oC, with duration for 180 min[3, 4]. This treatment is also attractive for low-cost process. Moreover defect passivation oflayered structures of silicon and silicon oxide is achieved. Metal electrodes do not block
passivation effect of high pressure H2O vapor heat treatment. Therefore, most people have usedhigh pressure H2O vapor heat treatment at a process steps after fabricating TFT structures. We
therefore take an interest in applying high pressure H2O vapor heat treatment to crystalline
silicon solar cells with transparent electrodes, metal wires and passivation insulators for simple
and low cost defect passivation.
In this study, we propose the high-pressure H2O vapor heat treatment as a novel and simple

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method for the improvement in characteristics of commercial crystalline silicon solar cells by
the passivation of defects in the surface and grain boundary of solar cells. The improvement isachieved by the reduction of photo induced carrier recombination loss. Through investigation ofdifferent conditions, we propose effective conditions of high pressure H2O vapor heat treatment
for improvement in solar cells characteristics. In addition, we discuss the subject to apply theH2O vapor annealing to the solar cell fabrication process.

Experimental
Commercial single crystalline silicon solar cells with an official conversion efficiency of
13.5 %, with a structure of SiNx/ITO/n+Si/p-Si/Al were prepared for the investigation. Front
surface electrode wires were made by silver paste printing and that of back surface wassilver/aluminum [5]. The solar cells were divided into small pieces with area ranging from 1 to
2 cm2. Conversion efficiency distributed from 8.4 to 16.3 % for the divided samples withaverage conversion efficiency of 12.4 %. This result has suggested that a distribution ofefficiency exists in one sheet of solar cell. In order to prevent the variation in the treatment
resulting by the variation in the initial solar cell characteristics, 16 samples were treatedsimultaneously for each H2O vapor annealing condition.
High-pressure H2O vapor heat treatment was applied to the samples placed in a pressure
proof chamber with ice made from purified water. By heating the chamber, the water
evaporated and the pressure inside the chamber increased. The treatment temperature wasvaried in the range of 170 to 230oC. The pressure inside the chamber changed from 1.0 x 105 to
1.0 x 106 Pa by controlling the ice volume. The duration was changed from 10 to 180 min.
Electrical properties of initial and the H2O vapor heat treated samples were measured by
Agilent 4156C parameter analyzer. A light emitting diode array with a wavelength of 590 nm at
an intensity of 24 mW/cm2 was used for the light illumination source. The open circuit voltage(Voc), short circuit current (Isc), fill factor and conversion efficiency were experimentallymeasured.

Results and Discussion
12

Current [mA/cm 2]

Improvement
in
solar
cell
characteristics was achieved by thehigh-pressure H2O vapor heat treatment
on specific conditions. For example, Fig.
1 shows the solar cell characteristics
observed in a sample with and without
heat treated at 210oC in 2.0 x 105 Pa H2Ovapor environment for 30 min. Voc, Isc
and conversion efficiency increased from10.5 to 11.5 mA/cm2, 0.525 to 0.544 V,
and 12.4 to 13.7%, respectively by theH2O vapor heat treatment. Those
improvements in solar cell characteristics
are assumed to be caused by thepassivation of defects and the decrease in
the probability of photo induced carrier
recombination in the solar cell.

H2O Vapor
Annealed

108Initial64210oC, 2.0x105 Pa, 30min

2
0
0

0.2

0.4

0.6

Voltage [V]Fig. 1 Solar cell characteristics of initial andH2O vapor annealed single crystalline siliconsolar cell.

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Figure 2 shows Voc of 16 samples heat treated simultaneously, for annealing temperature of(a) 190oC, (b) 210oC and (c) 230oC. Initial Voc is in horizontal axis and those for H2O vapor
heat treatment at 2.0 x 105 Pa for 30 min in vertical axis. Voc increased by H2O vapor heatFig. 2 Voc for initial samples in horizontal
axis and those for H2O vapor treatedsamples with 2.0 x 105 Pa for 30 min at (a)190oC, (b) 210oC and (c) 230oC in vertical
axis.

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Fig. 3 Isc for initial samples in horizontal
axis and those for H2O vapor treatedsamples with 2.0 x 105 Pa for 30 min at (a)190oC, (b) 210oC and (c) 230oC in verticalaxis.

treatment at 210oC and 230oC for most of the samples. The initial average Voc varied slightly inthe ranging from 0.520 to 0.529 V for the samples prepared for each annealing condition.
Increment in average Voc were 0.004, 0.011 and 0.018 V, respectively by the high pressure H2Ovapor heat treatment at 190, 210 and 230oC. Those increment increased as the treatment
temperature increased from 190 to 230oC.
Figure 3 shows Isc of 16 samples heat treated simultaneously for annealing temperature of(a) 190oC, (b) 210oC and (c) 230oC. Initial Isc in horizontal axis and those for H2O vapor heat
treatment at 2.0 x 105 Pa for 30 min in vertical axis. The average Isc increased from 10.6 to
11.4 mA/cm2 by heat treatment at 210oC. On the other hand, it decreased from 12.0 to 11.6
mA/cm2 for the samples treated at 230oC. The decrease in Isc was concerned with thedegradation of fill factor. The average fill factor decreased as the treatment temperature
increased to 230oC. The origin of fill factor degradation is discussed later. The increment in
Isc by the H2O vapor annealing at 210oC was larger for samples with lower initial Isc, as shownin Fig. 3. This result suggests that the H2O vapor heat treatment is capable of making Isc
uniform by reducing carrier annihilation sites.
Simultaneous increases in Voc and Isc for the samples shown in Figs. 2(b) and 3(b) clearly
indicate that the H2O vapor treatment at 210oC increases the density of photo-induced carriers
and the effective minority carrier lifetime. The change in solar cell characteristics as a functionof effective minority carrier lifetime were numerically calculated using Silvaco Co. Ltd.
Luminous device simulator. Increase in Voc and Isc by the increase in effective minority carrier
lifetime were confirmed by the calculation [6].
Conversion efficiency was improved by the H2O vapor treatment at 210oC. Figure 4 shows
the distribution of conversion efficiency for initial and H2O vapor annealed samples with 2.0 x
105 Pa for 30 min at 210oC. The initial average conversion efficiency of 11.7 % improved to12.5 % by the heat treatment at 210oC. The improvement in conversion efficiency by H2Ovapor heat treatment was achieved mainly by the increase of Voc and Isc through decrease in theprobability of photo induced carrier recombination and the increase in the minority carrier
lifetime. However, average conversion efficiency for the samples H2O vapor heat treated at
230oC decreased from 13.3 to 12.9 %. The origin of decrease in conversion efficiency is the
rapid degradation of fill factor by the H 2O vapor heat treatment.
For each treatment pressure, the ratios of averaged solar cell characteristics after H2O vapor
treatment to the initial were obtained(hereinafter average ratio). Figure 5
shows the average ratios of Voc, Isc, fill
factor and conversion efficiency as afunction of H2O vapor pressure forsamples H2O heat treated at 210oC for
30 min. Solar cell characteristics were
not changed for the samples annealed in
1.0 x 105 Pa H2O vapor. The samples
treated in 2.0 x 105 Pa H2O vapor
showed the highest improvement withaverage ratios of Voc, Isc and conversion
efficiency of 1.02, 1.08 and 1.08,
respectively. However, at higher pressureof 1.0 x 106 Pa, the improvements in Voc
and Isc were low and fill factor andconversion efficiency decreased to 0.86 Fig. 4 Distribution of conversion efficiency for
and 0.89, respectively.
initial and H2O vapor annealed samples with 2.0
Dependence of average ratios of x 105 Pa for 30 min at 210oC.

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Ratio of vapor treated toInitial solar cell characteristics

Ratio of vapor treated toinitial solar cell characteristics

solar cell characteristics on duration of H2O vapor heat treatment is shown in Fig. 6. H2Ovapor heat treatment was applied simultaneously for 16 samples at 210oC in 2.0 x 105 Pa for 10,
30 and 180 min. It is insufficient for 10 min duration to improve the solar cell characteristics,
though Isc was slightly increased. The Voc, Isc and conversion efficiency were improved by theH2O vapor heat treatment for 30 min. On the other hand, the average ratios of fill factor andconversion efficiency decreased to 0.90 and 0.93, respectively in the case of 180 min duration.
Degradation of fill factor and conversion efficiency for the H2O vapor heat treated samples
were observed in the cases of treatment temperature at 230oC, pressure in 1.0 x 106 Pa H2Ovapor and duration for 180 min. We discuss possible origin of those degradations. Fill factor
decreased from 0.57 to 0.42 by H2O
1.2
vapor annealing at 210oC in 1.0 x 106 Pa
for 30 min. It is well known that such a
210oC, 30 min
degradation of fill factor can be
1.1
explained by the increase in the serial
resistance loss. Simple model calculationIsc
was made by a conventional equivalent
1.0
circuit model with serial resistance (Rs)Voc
connected to the equivalent circuit ofinitial solar cell. Figure 7 shows the solar
Efficiency
0.9cell characteristics of initial and heat
treated samples at 210oC in 1.0 x 106 Pa
Fill Factor
H2O vapor environment for 30 min (bold0.8lines) and calculated current voltage
characteristics for Rs of 5, 12 and 25
0.0 0.2 0.4 0.6 0.8 1.0 1.2
(thin lines). The calculated solar cell
Treatment Pressure (MPa)characteristics shows good agreementwith the curve of H2O vapor annealed
Fig. 5 Average ratios of Voc, Isc, fill factor andsample for Rs = 12 . The color of silver
conversion efficiency of the samples H2O vapor
electrode lines pasted on the surface
treated to initial as a function of treatmentchanged from white to gray by the H2O
pressure.
ovapor heat treatment at 210 C in 1.0 x
1.2
106 Pa for 30 min. Moreover, the210oC, 2.0 x 105 Pa
resistance of silver electrode lines afterIsc
the treatment increased by 8 times.
1.1
These
observations
suggestthat
degradation of solar cell characteristics
by high pressure H2O vapor heat
treatment was caused by the oxidation of
1.0
Voc
Efficiency
silver electrodes resulting in increase of
Rs. This suggests that high pressure H2Ovapor heat treatment should be carried
Fill Factor
0.9
out before silver electrode pasting inorder to look for much more
improvement in solar cell characteristics.
0.8
The defects of poly-Si TFT are more
0
50
100
150
200
effectively reduce by H2O vapor heat
Treatment Time (min)treatment at temperatures ranging fromo6
Fig. 6 Average ratios of Voc, Isc, fill factor and
200 to 300 C in 1.3 x 10 Pa. The
conversion efficiency of the samples H2O vapor
possibility of further improvement intreated to initial as a function of duration.
characteristics of crystalline silicon solar

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12cells by H2O vapor treatment is still
remained.

Initial12 

2Current (mA/cm )10

Summary8
6

Rs = 5 
25 

Improvement in characteristics of4commercial single crystalline silicon solar
H2O Vapor Annealed
cells was demonstrated by high-pressure2
for 30 min at 210oC in 1.0 x 106 Pa
H2O vapor heat treatment. The
0characteristics of crystalline silicon solar
5
0
0.2
0.4
0.6
cells improved by 2.0 x 10 Pa H2O vaporoheat treatment at 210 C for 30 min.
Voltage (V)Average Isc, Voc and conversion efficiencyof simultaneously H2O vapor heat treated Fig. 7 Solar cell characteristics observedo in a
16 samples were increased from 10.6 to sample as fabricated6and heat treated at 210 C for
11.4 mA/cm2, 0.515 to 0.526 V, and 11.6 30 min in 1.0 x 10 Pa H2O vapor environment
to 12.5%, respectively. By appling the (bold lines) and calculated current-voltage
H2O vapor heat treatment, steam characteristics for serial resistance (Rs) of 5, 12
penetrated into the solar cell and defects and 25 (thin lines).
were
effectively
passivated.
The
improvement of solar cell characteristics is assumed to be caused by the decrease in theprobability of photo induced carrier recombination, and then the increase in the minority
carrier lifetime.
On the other hand, degradation of fill factor and conversion efficiency was observed by the
1.0 x 106 Pa H2O vapor heat treatment at 210oC for 30 min and 2.0 x 105 Pa H2O vapor heat
treatment at 210oC for 180 min. The degradation of solar cell characteristics was caused by theincrease of serial resistance loss according to the oxidation of silver electrodes. It is easily
avoidable by modifying the process of electrode fabrication. These results suggest that thecharacteristics of crystalline silicon solar cells may be easily improved by the low temperatureand low cost process i.e. by the high-pressure H2O vapor heat treatment. Moreover, thistreatment has a potential for further improvements in characteristics of crystalline silicon solar
cells.
Acknowledgements
The authors would like to thank Mr. M. Shimokawa, Mr. Y. Kanda, Mr. K. Ukawa, Mr. T.
Haba and Mr. Y. Mizutani for their support and Ms. M. Kimura for assistance. We also thankDesign System Co. Ltd. for their technical support. This work is partially supported by New
Energy and Industrial Technology Development Organization (NEDO) as part of the Innovative
Solar Cells R&D Program P07026.
References[1] Y. Hamakawa: Taiyodenchi (Solar Photovoltaic Cells) (Coronasha, Tokyo, 2004) [inJapanese].
[2] T. Sameshima and M.Satoh: Jpn. J. Appl. Phys. 36 (1997) L687
[3] H. Watakabe and T. Sameshima: Appl. Phys. A77 (2003) 141.
[4] T. Sameshima, K. Sakamoto and K. Asada: Appl. Phys. A69 (1999) 221.
[5] http://www.e-tonsolar.com/index.asp (Accessed 2009 Aug.)[6] Y. Kanda and M. Shimokawa: private communication.

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