Amorphous semiconductors

The key determinants of future cost competitiveness of a-Si H PV technology are a-Si H deposition rates, module production yields, stabilized module efficiencies, production volume, and module design. Reported a-Si H deposition rates vary by more than a factor of 10, but most researchers report that the high quaUty films necessary for high stabilized efficiencies require low deposition rates often due to high hydrogen dhution of the Si (and Ge) source gases (see Semiconductors, amorphous). 

Figure 4.89 Spectral absorption a v) of some semiconductors. Amorphous silicon a-SiH with indirect absorption transitions shows a smaller slope da/dv of the curve a(v) while semiconductors with direct absorption (InP, GaAs or CuInSe2) have a much steeper slope...

Non-ideal semiconductors. For non-ideal semiconductors (amorphous, partially crystalline, or other defects that lead to localized states in the band gap) illumination can lead to many different types of electron transitions as illustrated in Fig. 5 from the valence band to the conduction band (ideal case), and transfer involving localized states. I.e., electron transition can take place at photon energies smaller than the band gap. Electron transition leading to trapped electrons in the localized states can only contribute to the photocurrent if they can reach the conduction band, or alternatively the underlying metal or the electrolyte. For this, trapped electrons have different possibilities ... 

Tauo J (ed) 1974 Amorphous and Liquid Semiconductors (New York Plenum)... 

Silicon is prepared commercially by heating silica and carbon in an electric furnace, using carbon electrodes. Several other methods can be used for preparing the element. Amorphous silicon can be prepared as a brown powder, which can be easily melted or vaporized. The Gzochralski process is commonly used to produce single crystals of silicon used for solid-state or semiconductor devices. Hyperpure silicon can be prepared by the thermal decomposition of ultra-pure trichlorosilane in a hydrogen atmosphere, and by a vacuum float zone process. 

Crystalline tellurium has a silvery-white appearance, and when pure exhibits a metallic luster. It is brittle and easily pulverized. Amorphous tellurium is found by precipitating tellurium from a solution of telluric or tellurous acid. Whether this form is truly amorphous, or made of minute crystals, is open to question. Tellurium is a p-type semiconductor, and shows greater conductivity in certain directions, depending on alignment of the atoms. 

D. Emin, in P. G. LeComber and J. Mort, eds.. Electronic and Structural Properties of Amorphous Semiconductors Academic Press, Inc., New York, 1973, Chapt. 7. 

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). 

StiU another method used to produce PV cells is provided by thin-fiLm technologies. Thin films ate made by depositing semiconductor materials on a sohd substrate such as glass or metal sheet. Among the wide variety of thin-fiLm materials under development ate amorphous siUcon, polycrystaUine sUicon, copper indium diselenide, and cadmium teUuride. Additionally, development of multijunction thin-film PV cells is being explored. These cells use multiple layers of thin-film sUicon alloys or other semiconductors tailored to respond to specific portions of the light spectmm. 

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