Ultrathin (0.5 pm) films of copper indium diselenide (CIS), whose band gap energy is around 1 eV, absorb virtually all h ■ n > EG photons; CIS also exhibits high stability under the effects of light. But unfortunately indium is a very rare element. Figure 3.53 shows the cross-section of a thin-film CIS solar cell.
The band gap energy of CIS can be substantially altered by replacing a portion of the indium (In) with gallium (Ga) and a portion of the selenium (Se) with sulphur (S). Replacing 10 to 20% of the indium with gallium yields CuInGaSe2 (CIGS), whose band gap energy is around 1.1 eV (roughly the same as c-Si). The open-circuit voltage of CIGS (around 500 to 720 mV) is also considerably higher than with pure CIS cells (400 to 550 mV). CIGS cells also exhibit a better fill factor FF. According to [3.11], small-scale CIGS cells (AZ — 1 cm2) attain Zpv — 19.4% efficiency. Commercial CIS modules from vendors such as Wurth Solar (and, previously, Shell Solar) exhibit cell efficiency zpv ranging from 10.5 to 12.5%. The interconnections and an electron microscope cross-section for a CIGS solar cell (light incident from above) are shown in Figures 3.54 and 3.55 respectively.
Many vendors make CIS and CIGS solar cells using a thin intermediate cadmium sulphide (CdS) film, which, owing to the presence of toxic cadmium, is also environmentally problematic. However, the search for cadmium substitutes is ongoing. With their relatively low band gap energy, CIS and CIGS are highly suitable for use as back-side solar cells behind a high-band-gap-energy front cell in tandem arrays (see Section 220.127.116.11, Figure 3.35 and Figure 3.36).
Substrate (Glass. Metal Foil polymer)