Crystalline silicon solar cells

Single-Crystal Silicon. Silicon is still the dominant material in photovoltaic. It has good efficiency, which is 25% in theory and 15% in actual practice. Silicon photovoltaic devices are made from wafers sliced from single crystal silicon ingots, produced in part by CVD (see Ch. 8, Sec. 5.1). However, silicon wafers are still costly, their size is limited, and they cannot be sliced to thicknesses less than 150 im. One crystalline silicon wafer yields only one solar cell, which has an output of only one watt. This means that such cells will always be expensive and can only be used where their high efficiency is essential and cost is not a major factor such as in a spacecraft applications. 

The optical properties of a-Si H are of considerable importance, especially for solar-cell applications. Because of the absence of long-range order, the momentum k is not conserved in electronic transitions. Therefore, in contrast to crystalline silicon, a-Si H behaves as though it had a direct bandgap. Its absorption coefficient for visible light is about an order of magnitude higher than that of c-Si [74]. Consequently, the typical thickness (sub-micrometer) of an a-Si H solar cell is only a fraction of that of a c-Si cell. 

Gangopadhyay, U. Kim, K. Mangalaraj, D. Junsin, Y. 2004. Low cost CBD ZnS antireflection coating on large area commercial mono-crystalline silicon solar cells. Appl. Surf. Sci. 230 364-370. 

Because of its indirect bandgap, bulk crystalline silicon shows only a very weak PL signal at 1100 nm, as shown for RT and 77 K in Fig. 7.9. Therefore optoelectronic applications of bulk silicon are so far limited to devices that convert light to electricity, such as solar cells or photodetectors. The observation of red PL from PS layers at room temperature in 1990 [Cal] initiated vigorous research in this field, because efficient EL, the conversion of electricity into light, seemed to be within reach. Soon it was found that in addition to the red band, luminescence in the IR as well as in the blue-green region can be observed from PS. 

Australia, and scaled up by BP Solar in Spain, the heterojunction with intrinsic thin layer (HIT) cells developed by Sanyo by replacing the diffused P-doped emitter with an amorphous silicon layer and the back contact cells developed by Stanford University for use in concentrator technology and now converted to a large area for flat plate use. All three use single-crystalline silicon, while the majority of screen-printed cells use multicrystalline silicon wafers. 

While silicon is not the ideal solar cell material, it currently dominates the solar PV market due to its prevalence in the microelectronics industry. Crystalline silicon (c-Si) is an inorganic semiconductor, in which the valence-band maximum and conduction-band minimum are not directly aligned in Uspace, making c-Si an indirect bandgap material. The indirect nature of the bandgap in c-Si means that a considerable change in momentum is required for the promotion of an electron from... 

Germane is used, along with silane, SiH4, to make amorphous or crystalline silicon solar cells having an extended solar energy absorption range to increase conversion efficiency. 

The specific application of a material generally determines the particular structure desired. For example, hydrogenated amorphous silicon is used for solar cells and some specialized electronic devices (10). Because of their higher carrier mobility (see Carrier Transport, Generation, and Recombination), single-crystalline elemental or compound semiconductors are used in the majority of electronic devices. Polycrystalline metal films and highly doped polycrystalline films of silicon are used for conductors and resistors in device applications. 

Crystalline solar cells are heavy and expensive to manufacture. However, their efficiency in converting sunlight has historically been superior to thin-film. Crystalline cells are constructed with silicon semiconducting materials. 

Thin-film solar shingles used on residential roofs today are only about 5 to 8% efficient. The efficiency of solar towers is also about 5%. Photovoltaic (PV) cell efficiencies range from about 5% for amorphous silicon (A-Si) designs, 9% to 10% for CdTe modules, and 13% to 16% for crystalline silicon modules. SEGS efficiencies are between 10 and 25%. 

Fig. 15.12. Daily variation in electrolytic hydrogen production rate (1), the solar array temperature (2), and radiation power density (3). Single crystalline silicon solar cells, SPE electrolyzer location, Cape Canaveral, Florida. The time scale denotes minutes elapsed from 5 a.m. (Reprinted from Yu. I. Khar-kats, Electrochemical Storage of Solar Energy, in Environmental Oriented Electrochemistry, C. A. C. Sequeira, ed., Fig. 5, p. 477, copyright 1994. Reproduced with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netherlands.)...

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