Fabrication of Polycrystalline Si Thin Film for Solar Cells
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tandem solar cell, and 8.9% for a single junction a-Si solar cell[61. In the field of poly-Si thin2
film, a high field effect mobility of more than 400cm /Vs was obtained by using the laser recrystallization method[7]. Based on the above technologies, we have continued to conduct R&D for solar cells with high conversion efficiency and low cost. Although the a-Si solar cell is one of the candidates, its efficiency is not sufficiently high because of its limited photosensitivity. To achieve solar cells with a much higher conversion efficiency, it is necessary to develop materials with high photosensitivity in the long-wavelength region where a-Si films have no photosensitivity. Polycrystalline silicon (poly-Si) thin-film is an effective material for this purpose and a-Si/polySi tandem solar cells are the most feasible[8], considering the cost of solar cells as shown in Fig. 1. 0.2 E
Solar spectrum
S" 0.1 -
-
pLow-cost poly-Si
C
substrate Light a-S,"" poly-Si , , thin-film Vcell cell
Sa-Si
00 500 700 900 1100
HIT
Wavelength (nm)
SPC
Fig. 1 Solar spectrum and target structure of high-efficiency, low-cost solar cells 965 Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
Based on our policy for this development, we investigated a device structure for a heterojunction between a-Si and crystalline Si and the material properties of thin-film poly-Si. For the device structure, we have developed a new-structure solar cell, called HIT[9] based on a p-type a-Si/n-type crystalline silicon heterojunction solar cell. The HIT solar cell offers high performance with low cost. For materials, we have been investigating poly-Si thin films fabricated by the solid phase crystallization (SPC) method[10] as a material for solar cells with photosensitivity in the long-wavelength region. This SPC method has many features suited to the fabrication process of solar cell materials, such as a low process temperature. In this paper, we review our R&D on poly-Si thin-film solar cells using the HIT structure and the SPC method. We also discuss topics for further improvement. HIT STRUCTURE SOLAR CELL Concept of the New HIT Structure Solar Cell Figure 2 shows the structures and doping concentrations of the conventional p/n homojunction (a), p-type a-Si/n-type crystalline Si heterojunction (b), and new HIT (c). As the conventional homojunction needs a high temperature around 8001C, this structure cannot be used in our low-cost solar cell process. In addition, it is difficult to fabricate an abrupt junction because of cross-contamination of the dopants in the homojunction. The p-type a-Si/n-type c-Si heterojunction can solve these problems because this structure has the following attractive characteristics: (1) neither a high temperature nor much energy is necessary for processing. (2) a top cell (a-Si cell) can be fabricated in the same process for tandem use. (3) a shallow, sharply-profiled junction can be easily obtained through a deposition process. But it was found that a slight cross-contamination of dopants rema
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