CdTe Thin-Film Cells

The largest thin-film solar cell supplier is First Solar, manufacturing thin-film cells of CdTe. According to the New York Times [80], First Solar, an American firm based in Tempe, Arizona, has signed an agreement with the Chinese government for a 2 GW photovoltaic farm to be built in the Mongolian desert.

The photovoltaic farm of area 25 square miles is part of a 11.9 GW renewable energy park to be built at Ordos City in Inner Mongolia. The overall project is to include 6.95 GW of wind power, 3.9 GW of photovoltaic power, and 0.72 GW of solar

thermal farms. Further to the plan for Ordos City are biomass operations, fueled by organic materials like wood chips and straw for 0.31 GW, and 70 MW from hydro storage, a load-balancing technology that uses off-peak power to pump water to a high reservoir from which it can be released to turn turbines at peak demand periods.

First Solar is likely to build a plant in China to make thin-film solar panels. According to the article, the 2 GW solar farm as built in China is likely to cost significantly less than the $5-$6 billion if it were built in the United States. It is commented that the CdTe solar cells of First Solar are less efficient than the standard crystalline silicon solar cells made by companies like Suntech, but they are signif­icantly less expensive to build. First Solar has also recently agreed to provide 1.1 GW to two California utilities from three big solar farms. It is commented in this article that the Chinese project is atypical oflarge solar projects, which have generally been awarded to solar thermal technology, which deploys mirrors to heat a liquid to create steam that drives an electricity-generating turbine, rather than to straight photovol­taic projects. It is pointed out in this comparison that the straight photovoltaic projects generally have fewer environmental impacts, such as requiring cooling water, and can be brought online faster than solar thermal plants.

Basic forms of CdTe cell construction are shown in Figure 6.15. The junction is between n-type CdS and p-type CdTe.

CdTe is a direct bandgap material with high absorption of light of energy beyond the bandgap about 1.45 eV. Although Cd is toxic, the devices contain thickness of only a few micrometers of CdTe and the toxic material is encapsulated to as not to present an external hazard. The material is very favorable it seems from the point of view of cost of fabrication. It appears that CdTe cells are leaders in the competition for large and cost-competitive photovoltaic installations.

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glass substrate

Figure 6.15 Two types of CdTe solar cells [81]. In both cases, the structure is built starting from the glass substrate, and methods including CSS (closed space sublimation) and chemical bath deposition are used to deposit succeeding layers. In these devices, the CdTe film is

Подпись: back contact О З c о u e 2 n CdTe ZnxCd,.xS/CdS Zn?Sn04 (ZTO) Cd2Sn04 (CTO) glass substrate 1.5-3 gm in thickness, is deposited using CSS especially for large-area devices, and is subject to an anneal with CdCl2 or other Cl- containing compound to promote grain growth after fabrication. Device efficiencies are in the range 10-16%, panel record efficiency is 12.8%.

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6.5 Dye-Sensitized Solar Cells | 153


Updated: October 27, 2015 — 12:10 pm