In many of the semiconductors that are suitable for solar cell manufacturing, only a film 0.5 to 5 pm thick is needed for full absorption of photons where h ■ n > EG. This massively reduces material and thus energy costs, if the semiconductor properties needed for solar cell functionality can be realized in such thin films. Against this backdrop, the developmental efforts of many researchers and vendors centre around thin-film solar cells, by virtue of the cost reduction potential they offer [3.3], [3.6], [3.7].
Among the semiconductors that are suitable for the manufacture of thin-film solar cells are amorphous silicon (a-Si), CdTe, CuInSe2 (CIS), Cu(In, Ga)Se2 (CIGS), CuInS2, CdS/Cu2S and GaAs. Using high-efficiency light-trapping techniques and suitable back-contact designs, researchers and vendors also seek to create on suitable substrates polycrystalline thin-film solar cells that likewise use far less material (maximum 10 pm). For thin-film solar cells, high base-material costs are less of a factor and the semiconductor material quality standards are lower. Moreover, the efficiency of such cells under low-irradiance conditions often falls off less rapidly than is the case with crystalline silicon solar cells, and in some cases is even somewhat higher than under elevated irradiance conditions.
In thin-film solar cells, ultrathin layers of suitable semiconductor material are vapour deposited onto inexpensive backings made of glass, metal, ceramic, plastic or the like. Such backings are referred to as substrates when they face away from the light and transparent superstrates when they face the light. The various layers are deposited successively from the gas phase at relatively low temperatures (only a few 100 °C), such that considerably less energy is expended than for the manufacture of crystalline solar cells. If the manufacturing workflow is deftly structured, the requisite series connections can also be realized for the various solar cells while deposition is being carried out. Since deposition is an automatic and monolithic process, as for the production of integrated circuits, the thin-film solar cell manufacturing process lends itself to substantial cost savings and energy efficiency measures far more readily than is the case with crystalline technology. Thin-film solar cells can also be realized as tandem cells or triple cells (see Section 184.108.40.206), which in principle allow for optimized use of solar energy (see Figure 3.34). However, considerable progress will need to be made in order for thin-film solar cells to become commercially available and provide stable efficiency amounting to, say, 15 to 20%, as is the case with today’s monocrystalline solar cells.
In this section, we will first discuss in somewhat greater detail amorphous silicon thin-film solar cells, as they are composed of environmentally-friendly materials and are already available from various vendors. This will be followed by a very brief discussion of other materials.
Figure 3.46 Amorphous silicon containing mainly hydrogenated broken bonds (DB stands for dangling bond, i. e. a non-hydrogenated bond) [3.6]. Reproduced by permission of Arvind Shah