Amorphous Si Solar Cells

High solar-energy conversion efficiency. A high efficiency equal to that of the amorphous Si solar cell has been obtained as a laboratory development and efficiencies greater than 10% might be possible. 

Fig. 5.25. Plots of (a). /sc(l7oc) and (b) dark current J([(V) at different temperatures. In good quality single crystal Si solar cells /sc and / j are equal if Voc = V. In amorphous Si solar cells /c >> /(I proving that dark current increases on illumination [146].

The cell efficiency of a single crystalline Si solar cell reaches 18- 0 % in the mass production line. The poly crystalline and cast Si solar cell shows 15-18% on average. The cell efficiency of amorphous Si solar cells (a-Si) is 8-9 %. Silicon solar cell generates electric power of direct current with about 1 V, a few combinations of which are suitable to apply to water-electrolysis. Therefore, if Si solar cell is combined with SPE, the system efficiency will be 10 % in practical use. This value is the highest among the systems which produce hydrogen with use of renewable energies as will be described here in-below. 

Commercialization of amorphous silicon solar cells started in 1980 when Sanyo introduced calculators powered only by small solar-cell panels (total area 5 cm2). Shortly thereafter, Fuji Electric also started producing a-Si H solar cells for calculators. As of 1983, a-Si H photovoltaic devices are produced for several other applications such as photodetectors, power supplies for watches, and NiCd battery chargers. Before the end of 1984 one may see a-Si H solar panels used in larger-scale applications such as irrigation and remote electrification. 

As mentioned in Section 9, the highest conversion efficiency observed to date for an amorphous silicon solar cell is 10.1 % (Catalano et al, 1982). The theoretical limit for the conversion efficiency of a single-junction a-Si H cell can be estimated to be —20%. This follows from an upper limit of — 22 mA cm-2 for JK as determined from optical absorption data (optical path length — 2 fim), and from upper limits of —1.0-1.05 V for and -0.86 for the fill factor (Tiedje, 1982). 

Experiments show that in high quality Si solar cells the superposition principle is valid to a good approximation. In CdS/Cu2S, amorphous Si [139] and in polymer solar cells some of these approximations are grossly violated. As an example consider the effect of series resistance. In the presence of the series resistance Rs the dark current is given by,... 

The fabrication of high quality doped a-Si H films is desirable for many technical applications. In particular, amorphous silicon solar cells rely on highly conductive and layers with go photovoltaic properties (Carl-... 

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. 

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. 

FIG. 72. Schematic cross-section of (a) a single junction p-i-n o-Si H superstrata solar cell and (b) a tandem solar cell structure. (From R. E. I, Schropp and M. Zeman. "Amorphous and Microcrystalline Silicon Solar Cells—Modeling, Materials and Device Technology," Kluwer Academic Publishers, Boston, 1998, with permission.)... 

Silane decomposes to its elements at above 400°C. Process (1) is known as direct thermal decomposition, and produces either amorphous or polycrystalline Si (function of reaction temperature and other processing parameters), and is commonly used, for instance, in the solar cell industry to reduce silane to silicon. 

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