Amorphous Silicon, Microcrystalline Silicon, and Thin-Film Polycrystalline Silicon Solar Cells
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Microcrystalline Silicon, and Thin-Film Polycrystalline Silicon Solar Cells
Ruud E.I. Schropp, Reinhard Carius, and Guy Beaucarne Abstract Thin-film solar cell technologies based on Si with a thickness of less than a few micrometers combine the low-cost potential of thin-film technologies with the advantages of Si as an abundantly available element in the earth’s crust and a readily manufacturable material for photovoltaics (PVs). In recent years, several technologies have been developed that promise to take the performance of thin-film silicon PVs well beyond that of the currently established amorphous Si PV technology. Thin-film silicon, like no other thin-film material, is very effective in tandem and triple-junction solar cells. The research and development on thin crystalline silicon on foreign substrates can be divided into two different routes: a low-temperature route compatible with standard float glass or even plastic substrates, and a high-temperature route (⬎600⬚C). This article reviews the material properties and technological challenges of the different thin-film silicon PV materials.
Introduction About 30 years ago, the first thin-film silicon solar cell based on hydrogenated amorphous silicon (a-Si:H) was reported.1 Since then, research and development (R&D) efforts have led to single-and and multijunction solar cells and large-area modules based on a-Si:H and related alloys (e.g., a-Si1–xGex :H). Such modules are now established on the market. Even though this disordered (noncrystalline) material lacks high charge carrier mobility and diffusion length because of bond angle and bond length distortion, the very effective passivation of potential defects by hydrogen, which occurs when hydrogen bonds to unpaired electrons associated with the defects, still makes this material attractive for large-area thin-film electronics. For example, in addition to the
application in solar cells, a prominent application is in thin-film transistors for flat panel displays. Other applications include color sensors and scanners. All of these applications take advantage of the great flexibility of this material, which can be deposited at low temperature and, because of its noncrystalline nature, can be stacked without severe constraints. Doping of a-Si:H is achieved during film growth by adding dopant gases; this method avoids high-temperature processes and post-treatments. Because of the larger bandgap of a-Si:H (1.7–1.9 eV) compared with crystalline silicon (c-Si, 1.1 eV), higher open-circuit voltages are achieved in solar cells, but the photocurrent is limited by insufficient absorption of the solar spectrum. A narrower optical
MRS BULLETIN • VOLUME 32 • MARCH 2007 • www.mrs.org/bulletin
gap is obtained by alloying of the silicon with germanium. Modules based on stacked multijunction cells (a-Si : H/ a-SiGe:H/a-SiGe:H) have long demonstrated the highest confirmed record efficiencies of this materials class. Because light-induced degradation of a-Si:H and a-SiGe:H is considered to set limits on the efficiency of t
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