production requires only half as many process steps, with simplified materials handling.

Thin-film technology appears to hold greater promise for cost reduction, which has led to research by several laboratories over the past two or three decades. Some of the results in efficiency improvement of small laboratory research-size cells, typically of the size of 1 cm2, are shown in Figure G-15. Research cell efficiencies as high as 21.5 percent for copper-indium (gallium)-diselenide (CIGS) are reported (Ullal et al., 2002). Similarly, high efficiency of 16.5 percent has been reported for CdTe research cells. Amorphous silicon is deposited by using silane (SiH4) and hydrogen mixtures. In laboratory-scale cells of amorphous silicon, the highest efficiencies obtained are about 12 percent.

One big challenge for thin-film solar cells is to overcome the large drop in efficiency from the laboratory-scale cell to that of a real module. For example, commercial modules of CdTe and CIGS have efficiencies in the range of 7 percent to 12 percent (as compared with laboratory-scale cell efficiencies of 16.5 percent and 21.5 percent). Similarly, commercial amorphous silicon modules have efficiencies less than 10 percent (Shah et al., 1999). The drop in efficiency as cell size is increased is substantial. Attempts are being made to increase the efficiency of amorphous and microcrystalline silicon cells by making dual and triple junction cells (Yang et al., 1997). This change leads to multiple layers, each having a different optimum band gap. However, the deposition of multiple layers increases the processing steps and therefore the cost. A final note is that amorphous silicon modules, when exposed to sunlight, undergo light-induced degradation, operating thereafter at a lower, stabilized efficiency (Shah et al. 1999; Staebler and Wronski, 1977).

In spite of its promise, the thin-film technology has been unable to reduce the cost of solar modules, owing to low deposition rates that have led to low capital utilization of expensive machines. The yields and throughputs have been low. These plants need better inline controls. In recent times, owing to manufacturing problems, some corporations have shut down their thin-film manufacturing facilities. Clearly, easier and faster deposition techniques leading to reproducible results are needed. Also, deposition techniques that would not result in a substantial drop in efficiency from laboratory scale to module scale are required.

Today there is no one clear “winner technology.” More than a dozen firms produce solar modules. Even the largest of these firms do not have world-class, large-scale produc-

FIGURE G-15 Best research cell efficiencies for multijunction concentrator, thin-film, crystalline silicon, and emerging photovoltaic technologies. SOURCE: National Renewable Energy Laboratory.



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