Single-Crystal Silicon: Photovoltaic Applications

Extract

The vast majority of solar cells used in the field are based on single-crystal silicon. There are several reasons for this. First, by using this material, photovoltaic manufacturers can benefit from the economies of scale of the much larger microelectronics industry, where crystalline silicon also dominates. Since lower-quality silicon is acceptable for solar cells, cell manufacturers are able not only to benefit from large production volumes, but also to use off-grade material. The relatively high efficiencies that result from this material, its excellent reliability in the field, its almost complete lack of environmental problems, and recent laboratory progress strengthen the position of this technology.

Although there had been earlier work with what might be termed “cast multicrystalline silicon,” the first efficient crystalline silicon cells were reported in 1954. These cells displayed energy conversion efficiencies of about 6%, a substantial improvement over what had been previously demonstrated. Performance improved rapidly through the 1950s, with efficiencies up to about 15% demonstrated in the early 1960s. By this time, their use on spacecraft had become the major commercial purpose of the cells, with reliability a more important attribute than efficiency. The technology then stabilized for a decade. In the early 1970s, another burst of activity pushed the performance of laboratory cells to about 17%. At about this time, a simple processing sequence was developed, based on screen printing the metal contacts to the cell. Anisotropic chemical etching of the silicon to produce surface texture to reduce reflection loss was also implemented, to give the cell structure shown in Figure 1.