Recent Progress in Amorphous Silicon Solar Cells and Their Technologies
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io- J
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a-SiGe
a-Si
a-SiC
Conduct vity (S /cm)
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1 n - 12 -
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Optical bandgap (eV) Figure 1. Dark and photoconductivity of a-SiCe:H, (f, V), a-Si:H (9,0), and a-SiC:H (M • ) versus optical bandgap.
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Recent Advances in a-Si Alloy Technologies
on
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talline technologies applicable to welldeveloped single-crystalline silicon solar cell fabrication processes. The second remarkable innovation is a-Si:H (hydrogenated amorphous silicon) technology, which we will discuss. We open our discussion with a brief overview of the present status of a-Si solar cell R&D efforts, with some new insights in device physics. Next, we discuss some new approaches and key technologies for improving solar cell efficiency with stabilized performance using new materials such as a-SiC:H (amorphous silicon carbide), /xcSiC:H (microcrystalline silicon carbide), and a-SiGe:H (amorphous silicon germanium). Also, the progress of conversion efficiency in various types of amorphous silicon solar cells is surveyed and summarized. Finally, aspects of PV system developments and application fields are introduced and used to show that PV is a promising candidate among the renewable energy technologies for the future.
2.1
Since the recent success of valency electron control in glow discharge-produced amorphous silicon carbon alloys (a-SiCH),1 a new amorphous silicon alloys age has opened up and, during the past four years, a group of new materials —including a-SiGe:H, amorphous silicon-nitride (a-SiN:H), and amorphous silicon-tin (a-SiSn:H)— has been developed. The significance of this material innovation is that a remarkable increase of material functions is obtained by controlling electrical, optical, and also photoelectronic properties by adjusting atomic compositions in the mixed alloys. Therefore, a
wide variety of applications is being developed with these new electronic materials. In fact, a-SiC:H/a-Si:H heterojunction solar cells in 1982,2 a-Si:H/a-SiGe:H stacked solar cells by Ovshinsky et al. in 1985,3 superlattice devices,4 a-Si:H/a-SiN:H in thin-film transistors,5 photoreceptors,6 x-ray sensors,7 color sensors,8 etc. have been developed, with some already commercialized. Figure 1 shows the dark- and photoconductivities versus the optical energy gap in nondoped a-Si alloy films deposited under high hydrogen dilution conditions by plasma CVD.9 As can be seen from the figure, a-Si].xCx alloy has a rather high photoconductivity, with CT-^/O-J ratios over 104. Recently, Hamakawa and Okamoto have reported the relation between dark conductivity and the optical energy gap for p- and n-type a-SiC prepared by conventional RF plasma CVD, and p- and n-type /xc-SiC prepared by electron cyclotron resonance (ECR) plasma CVD.10 According to the data, as the optical energy gap increases, the dark conductivity of the films prepared by RF plasma CVD rapidly decreases, while that of the films prepared by ECR plasma CVD remains higher than 10~3 S/cm even if the optical energy gap exceeds 3.2
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