Light-Induced Effects in a-Si:H Films and Solar Cells
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LIGHT-INDUCED EFFECTS IN a-Si:H FILMS AND SOLAR CELLS
YOSHIYUKI UCHIDA AND HIROSHI SAKAI Fuji Electric Corporate Research & Development, 2-2-1 Nagasaka, Yokosuka 240-01, Japan
Ltd.
ABSTRACT The present state of a-Si:H solar cells and studies on the light-induced effects in a-Si:H films and a-Si:H solar cells are presented. The conversion efficiencies attained are 11.5%, 10.1% and 7.5% (8.5% in active area) for 2 2 1.0cm singlejunction, multijunction cells and 1200cm submodule, respectively. The density of light-induced defects, ANs, changes with deposition temperature and annealing temperature. This suggests a strong correlation between the ANs and the network structure in a-Si:H films. Field testing results indicate that the degradation of a-Si:H modules can be suppressed within 10% of the original efficiency after 2 years operation. It is also indicated that the multijunction cells are more stable to light exposure than single p-i-n cells.
INTRODUCTION The reversible, light-induced changes in dark conductivity and photoconductivity of a-Si:H films is known as the Staebler-Wronski (SW) effect. In subsequent work ESR experiments have shown that the spin density (dangling bond density) in a-Si:H is increased by light illumination and is recovered by annealing. Extensive research has been carried out in ouder to understand the mechanism of such light-induced, metastable changes and the origin of light-induced defects. Effects of impurities like oxygen, nitrogen, their complexes and also the internal mechanical stress in a-Si:H films have been
proposed as the origin, but their role in the SW effect is still controversial. Recently we have made a series of studies on the effects of deposition conditions, annealing and hydrogen evolution of a-Si:H films on the SW effect. The results will be discussed in detail below. The a-Si:H solar cells also show a metastable change in performance upon prolonged illumination. It is believed that a large part of the performance change is due to the SW effect of the intrinsic a-Si:H layer. We first reported [1] that the light-induced performance change is suppressed by optimizing the design and the fabrication process of the a-Si:H solar cell. The techniques which we found to help p-i-n type solar cells include the following items: (a) minimizing the i-layer thickness. (b) adopting a pin structure in which light comes through the p-type layer instead of the n-type layer. (e)
doping of the i-layer with a trace amount of boron atoms to change the
i-layer from slightly n-type to intrinsic. These techniques have been applied by many institutes and the stability of their a-Si:H solar cells have been markedly improved. In this paper results of illumination tests and field tests for a-Si:H solar cells and modules produced at our laboratory will be presented and discussed. RECENT PROGRESS IN a-Si:H SOLAR CELLS
Single p-i-n cells In p-i-n type a-Si:H cells, it is important to minimize an absorption loss in the window-side doped layer. As an n-type layer with excellent
Mat. Res. Soc. Sy
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