Large-area Deposition of Amorphous Silicon Alloys Using a Roll-to-roll Operation
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Large-area Deposition of Amorphous Silicon Alloys Using a Roll-to-roll Operation Subhendu Guha and Jeffrey Yang United Solar Ovonic Corporation, 1100 W. Maple Road, Troy, MI 48084, U.S.A. ABSTRACT Large-area deposition of thin-film amorphous silicon alloy triple-junction solar cells on lightweight and flexible stainless steel substrate is described. The proprietary roll-to-roll operation enables continuous depositions of sophisticated multi-layer structures. The deposition methods include sputtering and plasma-enhanced chemical vapor depositions. Spectrumsplitting triple-junction solar cell design, manufacturing processes, and product applications are presented. INTRODUCTION
Recent growth in the electronic industry has necessitated the development of large-area devices for a variety of applications. Display panels are becoming larger and larger. So are the solar panels that are covering roofs of large commercial buildings. At United Solar Ovonic, we use a roll-to-roll deposition process where solar cells are deposited onto rolls of stainless steel 1.5 miles long, 14 inches wide, and 5 mil thick. The solar cell structure consists of two layers of back reflector that are sputtered, nine layers of active material that are Grid Grid deposited by plasma-CVD and a single layer of anti-reflection coating that is ITO sputtered. In order to provide product p3 consistency, a great deal of in situ diagnostic is carried out during all the i3 a-Si alloy processes. In this paper we discuss the manufacturing method for the n3 production of the solar cells and their p2 application.
i2 a-SiGe alloy
SOLAR CELL DESIGN In order to obtain high efficiency with good stability, we have pioneered [1] the triple junction cell structure where cells of different bandgaps are stacked together (figure 1). The top cell, which captures the blue photons, uses a-Si alloy with an optical gap of ~1.8 eV for the intrinsic (i) layer. The i layer for the middle cell is an amorphous silicon-germanium (a-SiGe)
n2 p1 i1 a-SiGe alloy n1 Zinc Oxide Silver Stainless Steel Figure 1. Triple-junction cell structure.
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Wash Machine
Back Reflector Deposition Machine
Amorphous Silicon Alloy
Anti Reflection Coating Deposition Machine
Deposition Machine
Figure 2. Schematic diagram of the roll-to-roll machines. alloy with about 10-15% Ge. The optical gap is ~1.6 eV, which is ideally suited for absorbing the green photons. The bottom cell captures the red and infrared photons and uses an i layer of a-SiGe alloy with about 40-50% Ge, corresponding to an optical gap of ~1.4 eV. Light that is not absorbed in the cells gets reflected from the silver/zinc oxide (Ag/ZnO) back reflector, which is usually textured to facilitate light trapping. The cells are interconnected by the heavily doped layers that form tunnel junctions between adjacent cells. MANUFACTURING A roll-to-roll automated process [2] has been developed for manufacturing solar cells on stainless steel. Rolls of stainless steel, a mile and a half (2500 m) long, 14 in (36 cm) wide, and 5 mi
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