The 1.46 eV band gap energy of cadmium telluride is very close to maximum theoretical efficiency (see Figure 3.25), and this material also lends itself very well to the manufacture of thin-film solar cells. Unlike amorphous silicon, cadmium telluride exhibits excellent stability and no degradation secondary to the effects of light. Figure 3.52 shows a cross-section of a cadmium telluride thin-film solar cell, whose active films are only a few micrometres thick. Such cells have attained up to 16.7% efficiency in the lab [3.2] and are relatively easy to manufacture. US-based First Solar makes a large number of commercial cadmium telluride modules measuring 120 cm ■ 60 cm with power outputs of 60, 62.5, 65, 67.5, 70, 72.5, 75 and 77.5 Wp, efficiency ranging from цм = 8.3 to 10.8%, and fill factors FF ranging from 62 to 68%. Hence the consequent cell efficiency is likely to be around 1% higher for each such module. The production costs for these solar cells, as of December 2009, was just under $0.9 per Wp (€0.6 per Wp) and are likely to decrease. As with other thin-film solar cells, the efficiency of cadmium telluride cells decreases more slowly in the presence of low irradiance than is the case with crystalline silicon solar cells.
However, cadmium (Cd) is ecologically unfriendly, and even if only very thin cadmium films are needed, some cadmium solar cells may not lend themselves to proper disposal as hazardous waste at
—-transparent front contact
—– metallic back contact
—– glass substrate
the end of their service lives; this in turn will result in some of the highly toxic cadmium ending up in the environment without having been disposed of properly. That said, according to [Mar03], cadmium telluride is highly stable. Moreover, as cadmium telluride only breaks down at temperatures exceeding 1000 °C, it is not released into the environment in case of fire [DGS05].