Low-cost semiconductor cells with efficiency approaching 20% have evolved making use of alloys of selenides of copper, indium, and gallium (CuIn1_xGaxSe2, known as “CIGS”). These crystalline alloys are typically P-type semiconductors with appropriate bandgaps, high absorption coefficients, and quite large diffusion lengths. The cell is typically in the form of a thin N-layer placed near the front of an “absorber” of CIGS with thickness about 1 pm. The PN junction, as in Figure 6.10a, sends electrons and holes in opposite directions. The efficiency limit in these cells seems to be about 20%, with 15% available in preliminary large-scale production. This is in the same range as polycrystalline Si cells, but it appears that the CIGS cells are easier to fabricate on large scale and can be much cheaper to produce. These cells are made commercially by Avancis, Nanosolar, Honda Soltec, Solar Frontier, Wurth Solar, and Global Solar.
The historical origin of this line of CIGS cells may have been the CdS/Cu2S cell formed by chemical surface treatment of CdS. This cell fell out of favor partly because of the toxic nature of Cd.