Mass Production Start-up
Activities on High Efficiencymicrocrystalline Tandem SolarCells
ATSUSHI MORII*1

HIROMU TAKATSUKA*1YASUHIRO YAMAUCHI*1

KENJI TAGASHIRA*1

2SATOSHI SAKAI*3

YOSHIAKI TAKEUCHI*

The solar cell has attracted attention as a ‘trump card’ in the struggle to reduce burdens on the global environment. Mitsubishi
Heavy Industries, Ltd. (MHI) is already selling an amorphous solar cell for practical use. More recently, MHI has researched anddeveloped a thin film type silicon solar cell for mass production. These new cells can obtain outputs up to 1.5 times higher thanconventional amorphous solar cells, and at lower cost. In October 2007, MHI began producing the cells at a newly constructedfactory with an annual production capacity of 40 MW.
coefficient smaller than that of amorphous Si film, the film
thickness of the i layer of the microcr ystalline tandem
type solar cell must be at least fivefold greater than that of
the amorphous solar cell. To ensure the same production
capacity as that of amorphous cell without increasing thenumber of film deposition chambers, the film depositionrate must be fivefold faster, with simple calculation. To
meet these demands, MHI developed a super-high-rate film
deposition technology for fabricating the microcrystalline
Si film. The functionality of this new technology is superior
to that of the VHF plasma-enhanced CVD film deposition
technology developed earlier for the amorphous solar cell. 2

1. Features of the microcrystalline tandem solar cell
MHI has been producing and selling1 amorphous solar
cells for many years. In October 2007, MHI started toproduce a microcrystalline tandem type solar cell with a far
higher efficiency than the amorphous cell. MHI plans toproduce three kinds of products, with generation outputs of
130 W, 140 W, and 150 W. These cells use a substrate of thesame size as the amorphous cell.
In a microcrystalline tandem type solar cell, films ofmicrocrystalline cells are laminated in series over the filmof a conventional amorphous cell (Fig. 1). Figure 2 shows
an example of spectral sensitivities of the microcrystalline
tandem type solar cells. The microcrystalline cell filmabsorbs long-wavelength light, a type of light unabsorbableby the amorphous cell film, for higher energy efficiency. But
because the microcrystalline Si film has an optical absorption

Sunlight

Top cell(amorphous)

Glass
substrate
Transparent
electrode

5Amorphousi layer2

Bottom cell Microcrystalline(microcrystalline) i layer
Backcontact
electrodeFig. 1 Structure of microcrystalline tandem
type solar cell
T he cros s-se ctional str uc ture of themicrocrystalline tandem type solar cell.

If the film deposition rate is increased, a performancedecrement usually results. To prevent this, the National
Institute of Advanced Industrial Science and Technology,Tandem type cell
Top cell(amorphous Si)

43

Bottom cell
(microcrystalline Si)

10300

500
700900
Wavelength (nm)Current density (mA/cm2)

Sunlight (1kW/m2)

2. High-rate film deposition

14121086
4
20

Voc : 1.39V
Isc : 13.3 mA/cm2
FF : 0.726
Eff : 13.4%
0.5

11.5

Voltage (V)Ultraviolet
ray band

Visiblelight

Infraredray bandFig. 2 Spectral sensitivity characteristics of
microcrystalline tandem type solar cell
The density of photons converted into electricity foreach wavelength of sunlight.

*1 Nagasaki Shipyard & Machinery Works*2 Nagasaki Research & Development Center, Technical Headquarters*3 Advanced Technology Research Center, Technical Headquarters

Fig. 3 Power generation characteristics
of microcrystalline tandem type cell
The characteristics of the current andvoltage of the power generated by a small
microcrystalline tandem type cell (area: 12cm2).

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)59

a res ea rch col labor at or w it h M H I , develop ed a f i l mdeposition method (high pressure depletion method) for
the input of high power. This new method applies a film
deposition pressure (several hundred Pa) higher than thepressure applied by the conventional method, and works
with a narrower gap between the electrode and substrate.
Overall, the new method achieves a microcrystalline Si
single cell efficiency of 9% at a film deposition rate of 2
to 3 nm/s. 3 MHI, meanwhile developed a more efficient
microcrystalline tandem type solar cell using a small plasma

Efficiency of bottom cell (%)

10CVD device designed to respond to a substrate of 5 cm × 5cm. This new cell achieves a 13.4% initial efficiency with thetandem type cell, as shown in Fig. 3. 4 By amalgamatingthese technologies and applying a new type of electrode fordepositing microcrystalline Si film with reduced plasma
localization under the high-pressure conditions describedabove, MHI obtained a bottom cell (microcrystalline Sitype) efficiency of 8.8% at a high film deposition rate of about
2 nm/s. And by applying the electrode cooling structure
based on the results of the technological development of thelarge-area high-rate uniform film deposition, MHI achieved
an efficiency of 8.6% at a film deposition rate of 2.5 nm/s, as
shown in Fig. 4.

Suppression of substrate deformation by electrode cooling:
optimization of film deposition condition

3. Improvement of availability factor of CVD equipment

9The production capacity of the cell film in the plasma CVD
equipment is not determined solely by the film deposition
rate. Instead, it is calculated by multiplying five parameters:
the substrate size, film deposition rate, availability factor,
efficiency, and yield. For plasma CVD equipment, the film
is generally stuck onto the electrodes and heaters in the film
deposition chamber when the film deposition is continued.
It thus becomes necessary to periodically open the film
deposition chamber, remove the components, and exchangethese components with clean replacements. A f ter thereassembly in this operation, the film deposition distribution
must be adjusted and the film deposition chamber must
be baked. These steps reduce the availability factor. To
address this problem, MHI has developed a new way toclean solar cell production equipment using a plasma-Applicationof new type electrode8

Conventional
electrode

7

Thermal deformation(cracking) of the substrate
makes it impossible to optimize
the film deposition condition

60.0

0.5

1.0

1.5

2.0

2.5

3.0

Film deposition rate (nm/s)
Fig. 4 Relation between the film-deposition rate of the microcrystalline
bottom cell and efficiency
As the figure shows, the new electrode is more efficient than theconventional electrode. The efficiency does not decrease at
the film-deposition rate of 2.5 nm/s, the rate at which the glasssubstrate deformation is suppressed by electrode cooling.

Temperatureof the centerof the substrate (°C)

Substrate deformation
amount (mm)300280260240220200180160140120
100181614121086420-2: With electrode cooling: Without electrode cooling

Fluctuationin temperatureof substrate

Without electrode cooling
Substrate deformation

The temperature rise during deposition
can be suppressed with the electrode cooling structure.
Stabilized
condition
0200

Filmdeposition

230

400
600
800
Elapsing time (s): With electrode cooling: Without electrode cooling
StabilizedFilmcondition
deposition

1000

240

250°C1200Variationin deformationof substrate

Cooling type electrode
Without substrate deformation

Deformation can be prevented
with the electrode cooling structure.
16002004006008001000

180250°C

1200

Elapsing time (s)Fig. 5 Results of analysis of the glass substrate temperature and amount of deformation during deposition of the microcrystalline bottom cell
The simulation calculation results of temperature fluctuation and the amount of glass substrate deformation during deposition, according
to existence of substrate cooling.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)

60

1.4m

1.1mFilm deposition rate :2.6 nm/s±14.3%-30-1501530 (%)

we developed an analysis technology by combining theheat transfer simulation and stress analysis. As the stressgenerated in the glass substrate can be calculated using
this technology, the likelihood of substrate cracking can bepresumed from the analysis result of the cracking strengthof the glass. Figure 5 shows the analysis results. It has
also become possible to suppress the deformation andcracking of the substrate by adopting a structure to remove
the heat generated due to the electrical discharge fromthe electrode, with the electrode cooled. In the absence ofelectrode cooling, we could predict a rise in the substratetemperature to 40ºC or higher during the film deposition,
followed by substrate cracking (about 1 substrate per 1,000)
due to the temperature difference between the front andback surfaces and interior of the substrate. By cooling theelectrode, the rise of substrate temperature during the film
deposition can be suppressed to 10ºC or less, and the chanceof substrate cracking can be suppressed to 1/100 or less, incomparison with the condition without cooling.
From these results, we have been able to deposit film
stably with the large electric power inputs required for115-120110-115105-110100-105

95-100

90-95

85-90

80-85

< 80706050403020
100Module Output (Relative value) (%)
Fig. 8 Output distribution of cell module
The output distribution characteristics of a tandem typesolar cell during mass production.

200: MHI (Tandem): Monocrystalline: MHI (Tandem): Monocrystalline

180
160
140

2000
1800
1600
1400
1200
1000

120
100
80

800
600

60
402002200

12 1 2 3 4 5 6 7 8 9 10 1112 1 2 3 4 5 6 7 820052006
Month400
2000

Accumulation of Power Generation( kWh/Wp (DC) )cleaning technology originally applied for the cleaning ofsemiconductor film deposition equipment with f luorinebase gas. With this development, film deposition can becontinued for several months without opening the film
deposition chamber, and the availability factor is increased
by about 20% as a relative value.
For the tandem type solar cell production facility, MHI
has used these technologies to develop plasma C V Dequipment. The company has developed an electric powersource and an electrode capable of supplying the very-highfrequency power required for high-rate film deposition. To
control the uniformity of the film distribution, we divided
the electrodes and added a function to adjust the f ilm
deposition rates of the various divisions of electrodes. We
chose not to use a special robot for transfer of the glass
substrate. Instead, we adopted a system for transfer with amechanical mechanism to reduce the mechanical trouble inthe vacuum. For the temperature control of the substrates
in the plasma CVD equipment, we predict the heat transferfrom the load lock room to the unloading room using a heattransfer simulation. To reduce the substrate deformation,

Amount of Monthly Power Generation( kWh/Wp (DC) )Fig. 7 Appearance of microcrystalline tandem
production factory
Photograph of a microcrystalline tandem production
factory with an annual capacity of 40 MW.

Frequency (%)

Fig. 6 Distribution of film deposition rate of microcrystalline silicon film
The distribution of the microcrystalline silicon film deposition rate ofa large-area substrate (area : 1.1 m × 1.4 m).

Fig. 9 Annual power generation characteristics of microcrystalline tandem type solar cell
The figure compares the power generation characteristics of a microcrystalline
tandem type solar cell with those of a monocrystal silicon solar cell, measured
outdoors in the city of Kassel, Germany.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)61

optimal production, we developed and designed plasma
CVD equipment capable of depositing the microcrystalline
cell film for the microcrystalline tandem type solar cellusing very high frequency, at a film-deposition rate fivetimes faster than the conventional method. In fields test
conducted in Germany to assess the performance of thisnew microcrystalline tandem type solar cell, we confirmedthat the cell has higher annual power outputs than thecrystal type. In the future we intend to improve the output
per cell module and produce products of even higher quality.
The development of the microcrystalline tandem typesolar cell and microcrystalline Si high-rate film deposition
technology has been commissioned by the New Energy andIndustrial Technology Development Organization.

high-rate microcrystalline film deposition. As shown inFig. 6, we confirmed a large-area high-rate uniform film
deposition of microcrystalline Si at a film deposition rate of
2.6 nm/s ± 14.3% using a large-area substrate of 1.1 m × 1.4m. By applying the above-mentioned design technologies,
it has become possible to stably transfer the 1.1 m × 1.4 m
substrate and to deposit the microcrystalline silicon film athigh rate.
The new factory shown in Fig. 7 began the production
from October 2007, and it is scheduled to start the full-scale
production of 40 MW from FY 2008. The photograph at thetop of this paper shows the appearance of a microcrystalline
tandem type solar cell.
The output distribution of the cell module is shown inFig. 8. The outputs of the cell modules fall within the rangeof average value ± 5%, and we can see that the respective
modules have a high repeatability of production. We beganselling 130 W products in F Y 2007 and plan to sell 150
W products within FY 2008, after improving the module
output in parallel with the production. Figure 9 shows theannual fluctuation of power generation per standard outputof microcrystalline tandem type solar cell, as measured
by a German performance evaluation organization. The
power generation characteristics shown in these data
exceed the power generation characteristics of the crystalt ype. Microcr ystalline silicon consists of amorphoussilicon filled with columnar crystal grains. The electricalcharacteristic lies between that of a crystal type and that ofan amorphous type. Thus, the temperature coefficient of
the output also stands between the coefficients of the crystal
and amorphous types. This means that the power producedby the microcrystalline type will exceed that produced by
the crystal type during the higher-temperature months ofsummer.

References
1. Takatsuka, H. et al., Mitsubishi Thin Film Photovoltaic Module,
Mitsubishi Juko Giho Vol.41 No.1 (2004)
2. Takatsuka, H. et al., Development of a-Si/Microcrystalline-Si
Tandem-type
Photovoltaic Solar Cell, Mitsubishi Heavy Industries Technical
Review Vol.42 No.3 (2005)
3. T. Matsui, M. Kondo and A. Matsuda, Proceedings 3rd World
Conference on Photovoltaic Energy Conversion, Osaka (2003)
p.15704. S. Goya, Y. Nakano, N. Yamashita, et al., Proceedings of 3rd World
Conference on
Photovoltaic Energy Conversion, Osaka (2003) p.1849

Atsushi Morii

Hiromu Takatsuka

Yasuhiro Yamauchi

Kenji Tagashira

Yoshiaki Takeuchi

Satoshi Sakai

4. Conclusion
From October 2007, we began producing a microcrystalline
tandem type solar cell with a power output far superior
to that of our conventional amorphous solar cell. For

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 1 (Mar. 2008)

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