The Study of Improving the Conversion Efficiency and Reducing the Thickness of the HIT Solar Cell
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The Study of Improving the Conversion Efficiency and Reducing the Thickness of the HIT Solar Cell Yasuko Hirayama,Hirotada Inoue,Kenta Matsuyama,Yasufumi Tsunomura,Daisuke Fujishima, Ayumu Yano,Shigeharu Taira,Takeshi Nishiwaki,Toshio Asaumi,Toshihiro Kinoshita,Mikio Taguchi,Hitoshi Sakata,and Eiji Maruyama Sanyo Electric Co., Ltd., 7-3-2, Ibukidai-Higashimachi, Nishi-ku,Kobe City, Hyogo, Japan, 651-2242 ABSTRACT In order to reduce the power-generating cost of silicon solar cells, it is necessary to achieve a high conversion efficiency using a thinner crystalline silicon (c-Si) substrate. The HIT solar cell is an amorphous silicon (a-Si) /crystalline silicon (c-Si) heterojunction solar cell that makes it possible to realize excellent surface passivation and hence high open circuit voltage (Voc). In addition, its symmetrical structure and a low-temperature fabrication process that is under 200C provide advantages in reducing thermal and mechanical stresses within the device so that it can easily be applied to thinner solar cells. We fabricated HIT solar cells using thin wafers from 58-98 μm, and achieved a 22.8% conversion efficiency with a HIT solar cell using a 98-μm-thick wafer, and an excellent Voc value of 0.747 V with a HIT solar cell using a 58-μmthick wafer. INTRODUCTION The “HIT” stands for Heterojunction with Intrinsic Thin-layer. It is an original Sanyo hybrid type solar cell that has an amorphous silicon (a-Si)/crystalline silicon (c-Si) heterojunction. In 1980, Sanyo was the first in the world to commercialize an a-Si solar cell, and has led the world in processing technologies for a-Si thin film and thin-film type solar cells so far. The HIT solar cell was developed based on this high-quality a-Si fabrication technology. Since commercializing it in 1997, Sanyo has steadily extended production of the HIT solar cell. The strong points of the HIT solar cell are its high Voc achieved by the excellent junction properties of the a-Si/c-Si heterojunction and a good temperature coefficient in output power. Thus, the HIT module can generate more annual output power than conventional types. In order to increase the popularity of solar power, a major cost cut is required. As such, the conversion efficiency must be raised, and the materials used for the module must be reduced. A thinner wafer is required for this purpose, because the silicon wafer accounts for a large part of the module cost. Figure 1 shows our progress in the conversion efficiency of the HIT solar cell at the R&D stage. We achieved an excellent conversion efficiency of 23.0% with a practical size of 100.4 cm2 and 22.8% with a thin c-Si wafer of 98 μm in 2009 [1]. In this paper, we describe our approaches for achieving high performance HIT solar cells with a thinner silicon wafer.
Conversion Efficiency(%) )
24
23
R&D
23.0% (Feb.2009) >200 μm
22 22.8% (Jul.2009)
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