Development of a-Si/Microcrystalline-Si Tandem-type
Photovoltaic Solar CellYASUHIRO YAMAUCHI*1
KEISUKE KAWAMURA* 2
MASAYUKI FUKAGAWA* 2

HIROMU TAKATSUKA*1NOBUKI YAMASHITA*3YOSHIAKI TAKEUCHI*2

Mitsubishi Heavy Industries, Ltd. (MHI) has been carrying out work to develop thin film type photovoltaic solar cells
as a part of efforts to develop and realize clean, "green" sources of energy. MHI has been actively working on developing
very high frequency (VHF) large area, high-speed deposition plasma enhanced chemical vapor deposition (CVD) technology with the aim of realizing reduced fabrication costs. As a result of these efforts, we have already put into practical use
the world's largest sized amorphous silicon photovoltaic cell (1.4 m x 1.1 m) with a power generating efficiency of 8%,
which is currently being sold mainly in markets outside Japan. MHI is currently in the process of developing a highefficiency amorphous-silicon /microcrystalline-silicon tandem thin film photovoltaic module (a-Si/microcrystalline-Si tandem thin film PV module) with a power conversion efficiency of a 12% targeted for the Japanese domestic market basedon this technology. In basic tests, an initial efficiency of 13.1% (stabilized efficiency of 12%) could be attained with small
area cells (32 mm2), and a conversion efficiency of 11% could be attained with a medium sized module (50 cm x 40 cm). We
are currently developing cells with a large area substrate that 1m2 or more is wide in actual size or greater with a target
power efficiency of 12% by combining the above technology with VHF plasma deposition technology. (The image shows asection transmission electron microscope photograph of the cross section of a-Si/microcrystalline-Si tandem PV modules.)application as early as possible(2). VHF plasma enhanced
CVD is a method by which the gas decomposition rate isincreased through the use of a high frequency electriccharge that is tens of MHz to one hundred and several tensof MHz instead of the conventional 13 MHz used to generate plasma. This method makes high-speed, high quality
film forming with low ion damage possible(3). However, as
the wavelength of the electric charge supplied becomesshorter, it becomes more difficult to apply it to a large area
PV module. As a countermeasure to this problem, MHI has
invented a unique ladder shaped electrode as well as a newtechnique called a phase modulation method which controls the phase of the VHF electric charge supplied to theFig. 1electrode (Fig.
1), thereby making it possible for MHI tosucceed in realizing large-area VHF plasma enhanced
CVD(4)(5). MHI now utilizes the above results to produce
1.4 m x 1.1 m a-Si PV modules, which are the largest size
solar cells of their type in the world.

1. Introduction
With the coming into effect of the Kyoto Protocol in February of 2005, more attention than ever is being given to
the development and application of clean, "green" sourcesof energy such as solar power systems and wind power generation systems due to serious concerns regarding theprevention of global warming. Much of this interest has
grown rapidly under government guidance in cooperation
with industry. One such energy source of interest is PV
solar cell. The New Energy and Industrial Technology Development Organization (NEDO) in Japan has been
vigorously pursuing the development of cost reducing technologies crucial to spurring the spread and rapid adoptionof PV systems in society(1). Thin-film silicon PV modules
are considered to be particularly promising for reducing
production costs, because there are essentially no constraints on the availability of the raw materials used intheir production, and they offer great benefits in terms ofscalability for increased production. As a result, NEDO has
also come to support the development of technologies forthin-film Si PV modules as the next generation of solar cells
beyond the current mainstream of crystalline Si solar cells.
In order to realize lower production costs, conversion
efficiency and productivity must be increased. With regards
to raising productivity in particular, it is important to develop a large-area, high-speed, film deposition technology
for plasma enhanced CVD, which is the core technology inthe thin film Si PV module fabrication process. MHI believes that VHF plasma enhanced CVD is quiteadvantageous as a high-speed, film deposition technology,
and has been concerting its efforts to realize its practical*1 Nagasaki Shipyard & Machinery Works*2 Nagasaki Research & Development Center, Technical Headquarters*3 Advanced Technology Research Center, Technical Headquarters

Matching Amplifier
box

Oscillator(60 MHz)

SplitterPhasemodulator
Modulation
signalMatching box

AmplifierFig. 1 Conceptual principle of ladder shaped electrode andphase modulation method

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 3 (Oct. 2005)

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At the same time, MHI also has been developing highefficiency technologies for the production of tandem-type
PV solar cells utilizing the characteristics of VHF plasma
enhanced CVD, and has succeeded in achieving an initial efficiency of 13.1% as a result of basic experiments
incorporating MHI's 5 cm2 glass substrate. The company
is now proceeding to research and develop a technology
for fabricating larger area tandem-type PV modules
based on the basic technologies and techniques acquired
from these experiments under a commission by NEDO.
So far, a 40 cm x 50 cm module has been successfully
produced with a conversion efficiency of 11% (6)(7). A report is presented here of the current state of progress
being made in the development of large area, high-speed
film forming technology with a film deposition rate
faster than 2.0 nm/s for practical application in the fabrication of PV cells having an area of 1 m 2 or larger
that are expected to achieve generation efficiencies of
12% or more.

This is particularly important in being able to introduce such systems in the Japanese domestic market
where a-Si PV systems have been at a disadvantage until now vis-a-vis crystalline silicon systems due to theconstraints on installation space in Japan. In addition,a-Si PV systems can be supplied at a cost less than thatof crystalline silicon systems due to the adoption of MHI's
original large-area, high-speed film deposition technology. Furthermore, since the actual amount of power
produced annually by a-Si PV systems outdoors is about
5-10% more than crystalline systems, they have become
an increasingly competitive product for manufacturersof solar cell systems.

3. Development of high efficiency microcrystalline ilayer deposition technology
Although a microcrystalline-Si film can absorb a
broader range of wavelengths than an a-Si film, it has asmaller optical absorption coefficient than a-Si. Consequently, the i-layer of a microcrystalline PV cell that
forms the bottom cell of a tandem PV module needs to
be at least five times thicker than the a-Si i-layer, as
shown in Fig. 2. As a result, a film deposition rate thatis five times faster is necessary in order to realize thelevel of productivity as that for amorphous silicon.
Normally, an increase in the deposition rate results
in a decline in performance. As a countermeasure to this,the National Institute of Advanced Industrial Scienceand Technology (AIST), which conducts joint researchwith MHI, developed a method of producing plasma (usedin the plasma enhanced CVD method) in which the deposition pressure was increased to a level higher than that
previously used (to several hundred Pa) and in which
the distance between the discharge electrode and the substrate was reduced to less than 10 mm.

2. Objective of developing a-Si/microcrystalline-Si tandem solar cell modules
Figures 2 and 3 show the cross sectional structure ofa tandem PV module and the usable spectrum of solar
light, respectively. A tandem type solar cell module consists of an a-Si cell and microcrystalline-Si cell arranged
in a two-level stacked structure. Since short wavelength
light is absorbed in the a-Si top cell, and long wavelength
light is absorbed in microcrystalline-Si bottom cell, respectively, this type of tandem structure can generate
electricity across a broader spectrum of light than asingle junction a-Si cell. MHI is aiming to develop a PV
module capable of producing electric power with an efficiency of 12% using this structure. Through these efforts,
it was possible to develop amorphous type cells that could
be installed in an area equivalent to that used for crystalline silicon systems, thereby overcoming a notable
shortcoming of amorphous cells in which they require alarge installation area.

Sunlight (1 kW/m2 )5

Photon density(1014/cm2/s/nm)

Sunlight
Glass
TCO
A-Si p-layer
A-Si i-layer
A-Si n-layer

a-Si top cell

Top cell (a-Si)

4
3

Bottom cell
(microcrystalline-Si)

2
1
0
300

Microcrystalline-Si p-layer
Microcrystalline-Si i-layer

Tandem cell

Microcrystalline-Sibottom cell

500

700

900

Wavelength (nm)Microcrystalline-Si n-layer

Ultraviolet
radiation

ZnOAg

Visible light

Infrared region

Fig. 3 Wavelength regions utilized in a tandem PV module
Conversion efficiency is enhanced by stacking the a-Si top cell andmicrocrystalline-Si bottom cell in order to expand the utilized wavelength
region from 300 nm to 1 m.

Fig. 2 Structure of tandem PV module
The deposition of the microcrystalline-Si i-layer needs to be five times
thicker than that of the a-Si i-layer.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 3 (Oct. 2005)

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This newly developed plasma production method madeit possible to increase the deposition rate to 2.0 - 3.0 nm/s,
a five-fold increase over former rates, while achieving anefficiency in excess of 9% in microcrystalline single junction cells produced with this method(8).
On the other hand, since plasma formation becomesincreasingly localized as the deposition pressure rises,
it becomes all the more difficult to achieve uniform film
deposition over a large area. In order to extend the application of the above results into technology fordeveloping units of larger size for practical use, MHI
developed a new type of electrode that can better accommodate higher pressures and closer electrode gaps. This
electrode shortens the distance between the electrodesin the conventional ladder type electrode and improves
the cross sectional configuration of the electrode rods.
A new method of supplying power and new electrodeconfiguration were developed that could accommodate
an increase in VHF power levels in the microcrystalline-Si deposition process. Traditionally, electric powerhas been branched off from two power sources to supply power to multiple electrical feeds surrounding anelectrode using one large area ladder electrode, as
shown in Fig. 1. In the case of microcrystalline-Si deposition, the electrodes are divided and arranged into aneight parallel segmented electrode configuration, in
which power is supplied to each respective electrodeFigfrom multiple, independent VHF power sources (Fig
Fig.. 4 ).
This arrangement has made it possible to control eachindividual electrode independently, thereby improving
the ability to control the amount of power input andthe phase shift of each electrode to a much finer andgreater degree.

In order to achieve a film deposition rate for amorphoussilicon that is several times faster, it is necessary to apply
a comparably greater amount of electric power. As a result, it is no longer possible to ignore the power loss that
occurs in the power feed to the electrodes from the VHF
power source that previously had not been a problem in
the a-Si deposition process. Consequently, a low-loss, coaxial straight cable power supply structure was newly
designed, which made it possible to reduce power loss fromthe power feed from the previous 10% to just 3%.
Heat input from the plasma to the substrate becomes aproblem as more power is introduced into the process. In
other words, since the VHF power added in to enhance thedeposition process eventually converts into heat, it becomesincreasingly difficult to control the temperature of the substrate, unless the something is done to eliminate theadditional heat generated in the process. The desired levelof device performance would then no longer be achievable.
In the worst case, the substrate may undergo warping deformation and heat cracks will appear. Furthermore, thedistance between the electrodes can no longer be maintained evenly due to structural heat deformation of theelectrodes and other parts of the CVD apparatus.
In order to solve these heat related problems, calculations were carried out to predict substrate deformation
during the deposition process based on thermal strength
coupled analysis. The results were then used in the thermal design of the CVD apparatus. As a result, a structure
was adopted by which heat generated during the deposition process is removed from the electrodes by circulating
coolant through the electrodes, thereby making it possible
to better limit and prevent deformation and cracking of
the substrate.
Figure 5 shows the results of this analysis. It was predicted that when there is no electrode cooling, thetemperature of the substrate would rise during film deposition to more than 40oC and that about 1 substrate sheet
in 1000 would crack due to differences in temperature between the front and back surfaces and the interior of thesubstrate. The application of electrode cooling would significantly reduce the rate of such defects by limiting thetemperature to less than 10oC and hence, reduce the probability of substrate cracking to approximately one sheet in
100 000 during film deposition.
Based on these results, stable film deposition has become possible under conditions which allow for the
introduction of the large power input needed for the highspeed deposition of microcrystalline-Si. Hence, ahigh-speed deposition rate of 1.6 nm/s with a uniform film
Fig. 66) or even a rate of 2.0
forming distribution of +/-15% (Fig.
nm/s with a uniform film forming distribution of +/- 20%
on a 1.4 m x 1.1 m substrate could be demonstrated andverified. Development efforts are currently continuing torealize even further improvements in high speed deposition rates and film uniformity.

Split VHF power source

Matching box
Low loss power supply

Segmented electrodes

Low loss power supply
Matching box

Split VHF power source

Fig. 4 Schematic of the electrodes in the large area
plasma enhanced CVD apparatus used fordeposition of microcrystalline-Si
Control is enhanced through the adoption of segmented
electrodes and multiple power source method.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 3 (Oct. 2005)3

oTemperature of center of substrate ( C)

Fluctuations in temperature of substrate

300
280
260
240
220
200
180
160
140
120
100

Without electrode cooling
Deformation of substrate: With electrode cooling: Without electrode cooling

Increases in temperature during
deposition can be controlled byelectrode cooling structure

Condition
stability

230

250oC

240

Film deposition
0

200

400

600

800

1 000

1 200

Extent of substrate deformation (mm)Elapsed time (s)Variations in deformation of substrate
18: With electrode cooling: Without electrode cooling16
14 Condition
stability
12
Film deposition
10
8
Deformation of substrate can6
be prevented with electrode4
cooling structure
20Cooling electrodes
No deformation of substrate

160

180

o

250 C

-2
0

200

400

600

800

1 000

1 200

Elapsed time (s)Current density (mA/cm2)

Fig. 5 Results of analysis of fluctuations in the temperature of the substrate and deformation of the substrateduring deposition based on thermal strength coupled analysis
Temperature distribution and deformation within the substrate are controlled and contained through electrode cooling.

1.1m

2 000
1 000
0

Bottom side

14
12
10
8
6
4

Voc : 1.39 V
Isc : 13.3 mA/cm2
FF : 0.726
Eff : 13.4 %

2

1.4 m

-30 % -15 % 0 % 15 % 30 %0

Deposition rate:
1.6 nm/s 15%

0.5

1

1.5

Voltage (V)Fig. 6 Results of deposition of microcrystalline-Si film on
1.4 m x 1.1 m large area substrate
Deposition rate distribution depicted on a two-dimensional graph.
The microcrystalline-Si film can be deposited on the large area
substrate to a distribution of 15%.

Fig. 7 Characteristic initial power generation curve for a small
area tandem thin film solar cell fabricated with a small
area plasma enhanced CVD apparatus
A world top level initial conversion efficiency of 13.1% could be attained.

nologies and techniques established with this small-scale
apparatus in the VHF plasma enhanced CVD apparatus
used for the fabrication of 40 cm x 50 cm medium sized
substrates. We have also established tandem modularization
technology based on the development of stable film forming
technology that applies a new type of cooled electrode structure and laser scribing technology for use in the fabricationof tandem PV modules, described above, in which a conversion efficiency exceeding 11% could be obtained in a tandem
module of the above size(10).

4. Development of a-Si/microcrystalline-Si tandem-type
PV solar cell
MHI has been developing technologies to increase theefficiency of a-Si/microcrystalline-Si tandem PV modules
using small-scale VHF plasma enhanced CVD apparatus
provided for a 5 cm2 area substrate. Thus far, an initial
tandem-cell conversion efficiency of 13.1% has beenFig. 7
achieved (Fig.
7)(9). MHI has established tandem module
fabrication technology that reflects the high efficiency tech-

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 3 (Oct. 2005)

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In order to integrate these technologies, productivity
and performance verification tests were carried out on atandem a-Si/microcrystalline-Si PV module using a VHF
plasma enhanced CVD apparatus capable of forming a film
on a 1.4 m x 1.1 m substrate for practical use. The performance of the microcrystalline single cell using thisequipment was confirmed to be already equivalent to thatof the small-scale apparatus used for 5 cm2 substrates,
namely a deposition rate of 2.0 nm/s and an efficiency ofFig. 8approximately 8.5% (Fig.
8). The power capacity of the new
system will be increased to achieve the same level of performance as that described above even at film forming speedsof 2.5 nm/s or faster, and an efficiency of 12 % is planned to
be demonstrated for tandem type modules of this size.

Current density (mA/cm2 )25

Voc : 0.524 V
Isc : 22.5 mA/cm2
FF : 0.719: 8.5 %

10
5

0.1

0.2

0.3

0.4

0.5

0.6

Voltage (V)Fig. 8 Initial power generation characteristics of
microcrystalline-Si single junction solar cellon a large area substrate
It has become possible to achieve the same level of
performance for a large area system as the results offactor tests achieved with a small scale plasma enhanced
CVD apparatus.

MHI has been developing a-Si/microcrystalline-Si tandem PV modules as one of the technologies for attaining
the high efficiency required for low cost thin film Si PV
modules. With the success in achieving an initial conversion rate of 13.1% for tandem cells obtained fromexperiments conducted on small area cells used in smallscale apparatus having a substrate with an area of 5 cm2,
electrodes for microcrystalline-Si film deposition and VHF
power supply technologies were newly developed and applied in a new plasma enhanced CVD apparatus. These
achievements have made the practical application of technology for large area, high-speed film deposition of areas
1 m2 in size at a rate of 2.5 nm/s or faster feasible.
In addition, it was confirmed that the microcrystallineSi single cells produced with this apparatus had a level of
performance comparable to that of cells produced with thesmall-scale apparatus. Further improvements in the quality and uniformity of the film are expected to achieve atarget conversion efficiency of 12%, and techniques andtechnologies for realizing lower costs are scheduled.
Lastly, the authors would like to add that developmentof the technologies for large area, high-speed film deposition for microcrystalline-Si tandem PV modules describedin this report was carried out under a commission by
NEDO, as well as with the advice and guidance of various
cooperating universities and research institutes. We would
like to extend our sincere appreciation to NEDO and each
university and research institute that has contributed tothis research work for their kind assistance and guidance.

Hiromu Takatsuka

150

5. Conclusion

Yasuhiro Yamauchi

20

References(1) http://www.nedo.go.jp/taiyoshitsu/ (2003-7)(2) Takeuchi et al., Development of Amorphous Silicon Photovoltaic Solar Cell, Mitsubishi Juko Giho Vol. 37 No. 1 (2000) p.26
(3) H. Curtins, N. Wyrsch, M. Favre and A. V. Shah: Plasma Chem.
Plasma Process. 7 (1987) 267.
(4) MHI's Photovoltaic Power Generation System, Mitsubishi Juko
Giho Vol. 39 No. 3 (2002) p.172(5) Y. Takeuchi, A. Takano, H. Mashima, K. Kawamura, Y.
Yamauchi and H. Takatsuka, "Development of Large Area VHF
Plasma CVD and its Application to Si Thin Film Solar Modules", Proceedings of the 19th European Photovoltaic Solar
Energy Conference and Exhibition (2004) p1378(6) Yonekura et al., High Technology Achieving Photovoltaic Solar
Cell with High Cost-Performance, Mitsubishi Juko Giho Vol.
40 No. 6 (2003) p.320(7) Noda et al., Photovoltaic Module as Source of Infinite Energy
for the Earth, Mitsubishi Heavy Industries Technical Review
Vol. 42 No. 1 (2005)(8) T. Matsui, M. Kondo and A. Matsuda, "Microcrystalline siliconsolar cells grown at 20-30•ð/s by high pressure silane-depletion plasma", Proceedings of 3rd World Conference on Photovoltaic Energy Conversion (Osaka,2003) p.1570(9) S. Goya, Y. Nakano, N. Yamashita, et al., "Development ofamorphous silicon/ microcrystalline silicon tandem solar cells",
Proceedings of 3rd World Conference on Photovoltaic Energy
Conversion (Osaka,2003) p.1849(10) H. Takatsuka, M. Noda, Y. Yonekura, Y. Takeuchi and Y.
Yamauchi, Solar Energy, Vol.77 (2004) p.951

Keisuke Kawamura

Nobuki Yamashita

Masayuki Fukagawa

Yoshiaki Takeuchi

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Technical Review Vol. 42 No. 3 (Oct. 2005)

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