coating on top of the CIS layer, although other materials can be used. Zinc, molybdenum, tin, and aluminum are used in these cells as components of the layers. Figure 2-5 Comparison of Conversion Efficiency of Solar Cell Materials (Goetzberger and Hebling, 2000 and Haug and Zurich, 2003). (Note: material efficiencies are shown relative to the optimal efficiencies as a function of wavelength for the solar spectrum in space (air mass zero, AM0) and on the earth’s surface (air mass 1.5, AM1.5).) The material parameters and the technical design of the solar cell determine the efficiency of the energy conversion process. The first silicon solar cells developed had a conversion efficiency of about 6% and were treated mostly as laboratory curiosities. By 1993, the typical conversion efficiency for commercial silicon cells had increased to between 10 and 13% (Moller, 1993). The first thin-film cells had efficiencies of 8 to 12% (Partain, 1995). 2-5
Overview of the Photovoltaic Industry Table 2-1 lists the module size, conversion efficiency, and power produced for various thin-film modules in 1995. Conversion efficiencies of both silicon and thin-film cells continued to increase as shown in Figure 2-6 for the years from 1975 to 2000. Theoretical research on thin-film solar devices indicates that the materials could achieve conversion efficiencies of up to 25% under laboratory conditions (Moller, 1993). A new concept that may substantially increase efficiencies of cells is to use stacks of interconnected cells, which could achieve efficiencies up to 41.9 percent for tandem or two cells, and 50 percent for more cells (Goetzberger et al., 2003). Use of concentrator systems and devices to track the sun provide another method for increasing efficiency of PV systems (e.g., up to 34 percent for terrestrial concentrator GaAs/Ge cells (Spectrolab, 2003)). Advantages of thin-film solar cells are that they are generally cheaper to produce than standard silicon solar cells, have less material requirements, and can be deposited on many different substrates. Another advantage is that a variety of thin-film deposition techniques, which offer greater flexibility for thin-film preparation, are available. Table 2-1 Characteristics of Example Thin-Film Solar Cell Modules Company Thin-Film Material Area (sq. cm) Efficiency (%) Power (W) Astro Power Polycrystalline Silicon 3984 9.7 37.8 BP Solar Cadmium Telluride 706 10.1 7.1 Fuji Two Terminal Tandem - Amorphous Silicon Cell 1200 10.05 NA Kaneka Single Junction - Amorphous Silicon Cell 100 10.3 NA Kaneka Two Terminal Tandem - Amorphous Silicon Cell 100 10.1 NA Matsushita Battery Cadmium Telluride 1200 8.1 9.73 Photon Energy* Cadmium Telluride 3323 6.4 21.3 Photon Energy* Cadmium Telluride 832 8.1 6.8 Sanyo Single Junction - Amorphous Silicon Cell 100 12 NA Siemens Solar Copper Indium Diselenide 3883 9.7 37.8 Siemens Solar Copper Indium Diselenide 938 11.1 10.4 Reference: Partain, 1995 NA = not available.