Solar cell hydrogenation

Good solar cell results have been obtained from cells of materials, including polycrystaUine silicon, amorphous silicon—hydrogen (a-Si H) alloys, Cu S—CdS, CuInSe2—CdS, and CdTe. 

In most cases, CVD reactions are activated thermally, but in some cases, notably in exothermic chemical transport reactions, the substrate temperature is held below that of the feed material to obtain deposition. Other means of activation are available (7), eg, deposition at lower substrate temperatures is obtained by electric-discharge plasma activation. In some cases, unique materials are produced by plasma-assisted CVD (PACVD), such as amorphous siHcon from silane where 10—35 mol % hydrogen remains bonded in the soHd deposit. Except for the problem of large amounts of energy consumption in its formation, this material is of interest for thin-film solar cells. Passivating films of Si02 or Si02 Si N deposited by PACVD are of interest in the semiconductor industry (see Semiconductors). 

In many cases, the deposited material can retain some of the original chemical constituents, such as hydrogen in siUcon from the deposition from silane, or chlorine in tungsten from the deposition from WCl. This can be beneficial or detrimental. For example, the retention of hydrogen in siUcon allows the deposition of amorphous siUcon, a-Si H, which is used in solar cells, but the retention of chlorine in tungsten is detrimental to subsequent fusion welding of the tungsten. 

Limitations of Plasma CVD. With plasma CVD, it is difficult to obtain a deposit of pure material. In most cases, desorption of by-products and other gases is incomplete because of the low temperature and these gases, particularly hydrogen, remain as inclusions in the deposit. Moreover, in the case of compounds, such as nitrides, oxides, carbides, or silicides, stoichiometry is rarely achieved. This is generally detrimental since it alters the physical properties and reduces the resistance to chemical etching and radiation attack. However in some cases, it is advantageous for instance, amorphous silicon used in solar cells has improved optoelectronic properties if hydrogen is present (see Ch. 15). 

Pandey RN, Misra M, Srivastava ON (1998) Solar hydrogen production using semiconductor septum (n-CdSe/n and n-U02/Ti) electrode based photoelectrochemical solar cells. Int J Hydrogen Energy 23 861-865... 

Chandra Babu KS, Pandey RN, Srivastava ON (1995) Photoelectrochemical semiconductor septum (CdSe/Ti and Ti02/Ti) solar cells in relation to hydrogen production. Int J Hydrogen Energy 20 771-775... 

Deposition of hydrogenated amorphous silicon employing the VHF PECVD technique (typical frequency range 20-110 MHz) has been reported to yield an increase in deposition rate by one order of magnitude over the conventionally used frequency of 13.56 MHz [16,146,250,280], without adversely affecting material quality [183, 280, 475]. This is of great importance for lowering the production cost of fl-Si H solar cells. 

The different types of quinones active in photosynthesis are being used as electron acceptors in solar cells. The compounds such as Fd and NADP could also be used as electron/proton acceptors in the photoelectrochemical cells. Several researchers have attempted the same approach with a combination of two or more solid-state junctions or semiconductor-electrolyte junctions using bulk materials and powders. Here, the semiconductors can be chosen to carry out either oxygen- or hydrogen-evolving photocatalysis based on the semiconductor electronic band structure. 

Heller, A., Hydrogen evolving solar cells, Science, 223(4641), 1141,1984. 

Borrell, L., Cervera-March, S., Gimenez, J., Simarro, R., and Andujar, J.M., A comparative study of CdS-based semiconductor photocatalysts for solar hydrogen production from sulphide + sulphite substrates, Solar Energ. Mater. Solar Cells, 25, 25, 1992. 

Kobayakawa, K., Miura, T., Suzuki, A., Sato, Y., and Fujishima, A., Enhancement of the photocatalytic activity of CdS powder for hydrogen evolution from aqueous sulfide solution by heat treatment with KC1, Solar Energ. Mater. Solar Cells, 30, 201,1993. 

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