Amorphous Semiconductor Thin Film

A silicon solar cell is a solid state semiconductor device that produces DC (direct current) electricity when stimulated by photons. The three most readily available types of silicon solar cells are the single crystal cell, the poly crystal cell and the vapor deposition type, often called amorphous or thin film cell. 

Another important strategy is the adaptation of efficient PV semiconductor thin-films and nanostructures for effective use in PEC applications. Recent research in this area has focused on material classes with inherent bandgap tuning capabilities such as the amorphous silicon compounds (including silicon carbides and nitrides) [109, 112-114, 131, 132], and polycrystalline copper chalcopyrite compounds [133-137]. 

Teteris J. (2002). Holographic recording in amorphous chalcogenide semiconductor thin films, JOAM Vol.4, No. 3,687. 

The average intermolecular distance of 0.7-1 nm is about as large as for an amorphous organic semiconductor thin film or a molecular-dispersion polymer-type organic semiconductor. On the other hand, liquid crystal molecules aggregate by self-assembly, and the intermolecular distance is reduced to 0.4-0.6 nm in the... 

Spin coating of purified CP(2H) followed by heating at 170-200 °C gives an insoluble crystalline semiconductor thin film of TBP(2H). Spun-cast films of the precursor exhibit amorphous, insulating behavior upon a thermal annealing either in vacuum or under N2, and the amorphous films are converted into polycrystalline films of TBP(2H) with crystal sizes exceeding 1 xm [126]. Observed mobility of the devices exceeds 10 cm /Vs with appropriate process, device structure, and on/off current ratios exceeding 10 (Fig. 32) [126]. 

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. 

The thermal decomposition of silanes in the presence of hydrogen into siUcon for production of ultrapure, semiconductor-grade siUcon has become an important art, known as the Siemens process (13). A variety of process parameters, which usually include the introduction of hydrogen, have been studied. Silane can be used to deposit siUcon at temperatures below 1000°C (14). Dichlorosilane deposits siUcon at 1000—1150°C (15,16). Ttichlorosilane has been reported as a source for siUcon deposition at >1150° C (17). Tribromosilane is ordinarily a source for siUcon deposition at 600—800°C (18). Thin-film deposition of siUcon metal from silane and disilane takes place at temperatures as low as 640°C, but results in amorphous hydrogenated siUcon (19). 

The thickness of a photovoltaic cell is chosen on the basis of its ability to absorb sunlight, which in turn depends on the bandgap and absorption coefficient of the semiconductor. For instance, 5 nm of crystalline silicon are required to absorb the same amount of sunlight as 0.1 nm of amorphous silicon and 0.01 nm of copper-indium diselenide. Only MBE and MOCVD are capable of producing such extremely thin films.i l... 

As described earlier, the covalently bonded hydrogen, by passivating dangling bond defects and removing strained weak Si—Si bonds from the network, dramatically improves the semiconducting quality of amorphous silicon. Hence without the presence of hydrogen, effective amorphous semiconductor devices such as solar cells or thin film transistors would not be possible. Unfortunately, low defect density, high electronic quality... 

Nomura, K. Ohta, H. Takagi, A. Kamiya, T. Hirano, M. Hosono, H. 2004. Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors. Nature 432 488-492. 

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