A thin-film silicon solar cell and module

PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONS

Prog. Photovolt: Res. Appl. 2005; 13:489–494

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pip.645

A Thin-film Silicon Solar Celland ModuleKenji Yamamoto*,y, Akihiko Nakajima, Masashi Yoshimi, Toru Sawada, Susumu Fukuda,

Takashi Suezaki, Mitsuru Ichikawa, Yohei Koi, Masahiro Goto, Tomomi Meguro,

Takahiro Matsuda, Masataka Kondo, Toshiaki Sasaki and Yuko TawadaKaneka Corporation, 2-1-1, Hieitsuji, Otsu, Shiga, 520-0104, Japan

We have developed a new light-trapping scheme for a thin-film Si stacked module

(Si HYBRID PULS module), where a (a-Si:H/transparent interlayer/microcrystalline

Si) thin-film was integrated into a large-area solar cell module. An initial aperture

efficiency of 13�1% has been achieved for a 910� 455mm Si HYBRID PLUS

module, which was independently confirmed by AIST. This is the first report of the

independently confirmed efficiency of a large-area thin-film Si module with an inter-

layer. The 19% increase of short-circuit current of this module was obtained by the

introduction of a transparent interlayer that caused internal light-trapping. A mini-

module was shown to exhibit a stabilized efficiency of 12%. Outdoor performance of a

Si HYBRID (a-Si:H / micro-crystalline Si stacked) solar cell module has been inves-

tigated for over 4 years with two different kinds of module (top and bottom cell

limited, respectively). The HYBRID modules limited by the top cell have exhibited

a more efficient performance than the modules limited by the bottom cell, in natural

sunlight at noon. Copyright # 2005 John Wiley & Sons, Ltd.

keywords: thin-film; solar cell; plasma chemical vapor deposotion; light trapping; light soaking

INTRODUCTION

In the search for a higher-efficiency thin-film Si solar cell, we have been engaged in the development of a

thin-film crystalline Si (mc-Si) and a-Si/crystalline (mc-Si) stacked solar cell.1–3 We started the mass

production of a-Si/mc-Si stacked (HYBRID) modules in April 2001.

The last few years have witnessed significant developments in this area. One of them is the work that Dr J

Meier of University of Neuchatel reported4 on the 7% efficiency of a mc-Si cell and the 13% initial efficiency of

a-Si/mc-Si stacked cell in 1996. In 1997, further significant progress was made in thin-film mc-Si solar cell

development on glass a substrate, which was fabricated by plasma chemical vapor deposition (CVD) at low

temperature. The cell with a thickness of 2�0 mm, developed by Kaneka Corporation,1,2 reached an efficiency

of 10%. As a next generation of further high-efficiency solar cells, we have developed the new stacked thin-film

Si solar cell module, where a transparent interlayer was inserted between a-Si and mc-Si layer to enhance the

Received 2 December 2004

Copyright # 2005 John Wiley & Sons, Ltd. Revised 15 March 2005

* Correspondence to: Kenji Yamamoto, Kaneka Corporation, 2-1-1, Hieitsuji, Otsu, Shiga, 520-0104, Japan.yE-mail: [email protected]

Contract/grant sponsor: Japanese Ministry of Economy, Trade and Industry.

Special Issue

partial reflection of light back into the a-Si top cell.5 We describe this structure as employing an internal light-

trapping process.

In this paper, it is shown that the Si HYBRID PLUS (a-Si/inter-layer/mc-Si stacked) solar cell modules can be

fabricated in commercially available sizes by using the available production technology.

AMORPHOUS Si/MICROCRYSTALLINE Si STACKED SOLAR CELL

The advantage of a high Isc for our mc-Si solar cell was applied to the stacked cell with the combination of the a-

Si cell. In applying the a-Si cell to the stacked cell, we had to pay attention to the stabilized efficiency, since the

a-Si experiences photodegradation, while the mc-Si cell is stable. We have prepared a three-stacked cell of a-

Si:H/mc-Si/mc-Si, which was less sensitive to degradation as a result of using the thinner a-Si. We have inves-

tigated the stability of a-Si:H /mc-Si /mc-Si (triple) cell. These devices were then subjected to indoor light soak-

ing under the conditions of AM1�5, irradiation at 100 mW/cm2, 48�C, and an open-circuit. The triple cell

showed a stabilized efficiency of 12% as reported elsewhere,3 where a highly stabilized fill factor of 76�2%

was presented.

Towards the achievement of further high-efficiency solar cells, we have engaged in a study of the new stacked

thin-film Si solar cells. Figure 1 shows the concept of internal light trapping of a a-Si/mc-Si stacked pin cell with

interlayer, where the transparent interlayer was inserted between a-Si and mc-Si layer to enhance a partial reflec-

tion of light back into the a-Si top cell. It is important to select the appropriate interlayer to minimize the optical

losses of red light, which affect the sensitivity of the bottom cell. This structure enables the increase of current

Figure 1. Schematic of the interlayer

490 K. YAMAMOTO ET AL.

Copyright # 2005 John Wiley & Sons, Ltd. Prog. Photovolt: Res. Appl. 2005; 13:489–494

of the top cell without increasing the thickness of top cell, which leads to less photodegradation of the stacked

cell. We have reported an initial efficiency of 14�7% for a 1 cm2 (a-Si/interlayer/mc-Si) stacked cell.5

LARGE-AREA THIN-FILM SILICON HYBRID AND HYBRID PLUS MODULES

The internal light trapping technology transfer from a small size device to a large area module has been per-

formed by us. We call this new HYBRID module (a-Si/mc-Si stacked) with transparent interlayer the HYBRID

PLUS. The structure of the HYBRID PLUS is indicated in Figure 1.

The design of the multijunction device structure as well as the optimization of the fabrication process have

been promoted under the concept of realizing high annual output power outdoors, and is labeled the ‘high-per-

formance module’. We applied a large-area dual light source solar simulator to obtain an approximate AM 1�5spectrum and also to demonstrate quasi-summer or winter spectra by changing the ratio of halogen/xenon light

intensity.6 The measurements with the dual light source simulator under various spectral irradiation conditions

have been performed to optimize the type of transparent interlayer. Figure 2 shows the results for three modules

with different kinds of interlayer A, B and C while keeping the thickness of the top a-Si and bottom mc-Si layers

the same. The spectrum was controlled by changing the light intensity ratio of halogen/xenon dual light source

simulator.6 Total irradiation intensity was approximately equivalent in the entire spectral region. A dashed ver-

tical line in Figure 2 indicates a nearly AM 1�5 spectrum condition for this measurement. By increasing the

transmittance of interlayer (from A to B to C), Isc is increased in the blue-rich region of these modules. In other

words, module C with the most transparent interlayer showed the highest Isc when blue-rich conditions. The

transmittance at 1000 nm of interlayer C is 1�5% higher than that of interlayer A. This implies that the bottom

cell current is very much affected by the transmittance of the interlayer.

By applying this measurement technology to optimize the thickness of the top a-Si, interlayer and bottom mc-

Si layer, we have succeeded in fabricating the large area series integrated monolithic Si HYBRID PLUS mod-

ule. It was confirmed that the Isc value as well as the efficiency of the module increased by inserting a trans-

parent interlayer, as shown in Table I. Here, the same top a-Si layer thickness was used for the module without

(module D) and the module with interlayer (E). The remarkable 19% increase in Isc of this module was obtained

by the introduction of a transparent interlayer, which caused internal light trapping.

We have succeeded in obtaining the highest module initial aperture efficiency of 13�1%, which was con-

firmed by an independent laboratory (AIST), as shown in Figure 3.

Figure 2. Short circuit current comparison for three HYBRID modules (three modules with different kinds of interlayer A,

B and C, keeping the thickness of each top a-Si and bottom mc-Si layer the same) measured under various spectral irradiation

conditions. A specific spectrum was obtained by changing the light intensity ratio of halogen/xenon dual light source

simulator. Total irradiation intensity was approximately equivalent over the entire spectral region. A dashed vertical line

indicates a nearly AM 1�5 spectrum condition for this measurement. The transmittance at 1000 nm of interlayer C is 1�5%

higher than that of interlayer A

SILICON SOLAR CELL 491


Table I. I–V characteristics of two types of HYBRID modules without and with ‘improved’ transparent interlayer. The

parameters were normalized by reference to the values of module D (without interlayer)

Voc Jsc Fill factor Efficiency

Module D (without interlayer) 1�00 1�00 1�00 1�00

Module D (with interlayer) 1�00 1�19 0�963 1.15

Figure 3. The performance of HYBRID module confirmed by AIST (National Institute of Advanced Industrial Science and

Technology). The scribed area’s efficiency is shown in this figure

Figure 4. Light-soaking test for HYBRID modules with interlayer



The stabilized efficiency of this Si HYBRID PLUS module has been investigated by using the mini-module,

which was cut from the large area to a size of 14�18 cm2, and the light-soaking acceleration tests of the mini-

module. Figure 4 shows the efficiency changes of this mini-module during light soaking. After 20 h exposure at

5 kW/m2 with a metal halide lamp, the efficiency reached 11�7%. A 12% stabilized efficiency was observed

after 200 h additional light-soaking at 1 kW/m2, 50�C.

It is seen that our acceleration test is too strong to degrade the mini-module and to lead to another additional

degradation mode.

Figure 5. The variation of the device I–V parameters of two kinds of HYBRID modules in the field. Most of the data

obtained at noon on days with relatively clear skies for a total irradiance range of 850–1150 W/m2 were utilized for the

analysis in this work. Pmax and Isc were scaled to the irradiance of 1 kW/m2 linearly and Pmax, Voc and Isc were corrected to

25�C by use of their temperature coefficients. Then, all I–V parameters were normalized by parameters obtained by the solar

simulator measurement under standard test conditions (STC), which was carried out on 20 June 2003.

Pout ¼Pmax

GPOM

1 þ � 25 � Tmodð Þf g

Vocout¼ Vocf1 þ � 25 � Tmodð Þg

where Pmax is the maximum power of the module, GPOM is the irradiance measured with the pyranometer on the plane of the

module, Tmod is the temperature of the module, Voc is the open-circuit voltage, � is the temperature coefficient of perfor-

mance and � is the temperature coefficient of the voltage

SILICON SOLAR CELL 493


OUTDOOR PERFORMANCE OF THIN-FILM Si HYBRID MODULE

Outdoor performance of the Si HYBRID module (a-Si:H / micro-crystalline Si stacked) solar cell module (with-

out interlayer) have been investigated for over 4 years with two different kinds of modules (top and bottom

limited cell, respectively). Figure 5 shows daily device I–V parameters of the top-limited and the bottom-limited

HYBRID modules taken at noon in Otsu, Japan. Isc was normalized at 1 kW/m2. Voc was calibrated to the tem-

perature of 25�C. Each parameter of the module was normalized by each value of STC measurements on 20

May 2003. HYBRID modules limited by the top cell exhibited the more efficient performance rather than the

bottom-limited cell in natural sunlight at noon. This is due to both the spectral and annealing effects of the cell.

Namely, in summer, the top-limited cell shows the higher current due to the relatively high blue-rich spectrum.

CONCLUSION

A stabilized efficiency of 12% was exhibited by the HYBRID PLUS mini module. By applying our developed

measurement technology and selecting the appropriate interlayer, we have shown the high potential of this new

type of HYBRID PLUS module towards the achievement of higher efficiency.

Acknowledgements

This work was supported by the Incorporated Administrative Agency New Energy and Industrial Technology

Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).

REFERENCES

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2. Yamamoto K, Yoshimi M, Suzuki T, Tawada Y, Okamoto Y, Nakajima A. Very thin film crystalline silicon solar cells on

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A thin-film silicon solar cell and module

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