**8. Acknowledgements**

The authors wish to express their gratitude to Prof. Pere Roca i Cabarrocas of LPICM, Ecole Polytechnique, Palaiseau, France for providing all the experimental results on "HIT" cells on P-types wafers, that have been simulated in this article. We are also grateful to him for many in-depth discussions and constant encouragement during the course of this work. The authors also wish to thank Prof. C. Baliff, of IMT, University of Neuchâtel, Switzerland, M. Nath of the Energy Research Unit, IACS, Kolkata, India and J. Damon-Lacoste of TOTAL, S. A. for many helpful discussions.

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**14** 

**Fabrication of the Hydrogenated** 

*1Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology 2Max-Planck-Institut für extraterrestrische Physik* 

*3Graduate School of Engineering Science,* 

*Osaka University* 

*1,3Japan 2Germany* 

**Amorphous Silicon Films Exhibiting High Stability Against Light Soaking** 

Satoshi Shimizu1,2, Michio Kondo1 and Akihisa Matsuda3

A hygrogenated amorphous silicon (a-Si:H) thin film solar cell was first reported in 1976 [Carlson, & Wronski, 1976]. Since then, intensive works have been carried out for the improvement of its performances. Attempt to increase the conversion efficiencies of the thin film solar cells, a multi junction solar cell structure was proposed and has been investigated [Yang et al., 1997; Shah et al., 1999; Green, 2003; Shah et al., 2004]. It consists of the intrinsic layers having different optical bandgaps in order to absorb the sunlight efficiently in a wide

The density of photo-generated carriers is determined by the light absorption coefficient and the defect density of a material. The absorption coefficient of a-Si:H in a visible light region is one order magnitude higher than that of c-Si:H due to the direct transition phenomenon. Therefore, a thin a-Si:H layer absorbs sufficient photons. This is a huge advantage for the thin film based solar cell technology in which mass production should be definitely taken

However, a-Si:H has another aspect known as a Staebler-Wronski effect, i.e., the number of unpaired Si dangling bonds increases with light soaking, which lowers photocarrier density by decreasing carrier lifetime [Staebler & Wronski, 1977]. Indeed, conversion efficiencies of a-Si:H based solar cells deteriorate generally by 15-20 % due to this phenomenon. On the other hand, it is possible to suppress this deterioration to some extent by reducing a film thickness of a-Si:H with efficient light-trapping structures [e.g., Müller et al., 2004]. Indeed, the fabrication of the highly stabilized a-Si:H single junction solar cell by the precise optimizations of the optical properties and the i-layer thickness has been reported [Borrello et al., 2011]. Besides those intensive efforts, establishing the technique for fabricating highly stable a-Si:H films is essentially very important to extract its maximum potential for the

**1. Introduction** 

spectrum range.

into account.

solar cell applications.

interface roughness, *Journal Applied Physics* , Vol 88, No. 11 (December, 2000) pp. 6436-6443, ISSN 0021-8979.

News release by SANYO on 22nd May, 2009, SANYO Develops HIT Solar Cells with World's Highest Energy Conversion Efficiency of 23.0%. < http://panasonic.net/sanyo/news/2009/05/22-1.html>.
