**Table 2.**

*Thin-Film Solar Cells Performances Optimization: Case of Cu (In, Ga) Se2-ZnS DOI: http://dx.doi.org/10.5772/intechopen.93817*

$$\chi\_{\epsilon} = 4.6 - 1.15667x + 0.03333x^2 \tag{4}$$

The choice of the value of *Eg* (and therefore of *χe*) depends on several factors.

The best yields are obtained with a value of the band gap *Eg* of about 1.2 eV. This corresponds to a Gallium concentration level close to [Ga] = 30%. This is the value that will be considered in this study.

#### • **Rear contact: Molybdenum (Mo)**

The back contact here is a thin layer of Molybdenum (Mo) which is 300 nm thick (the standard thickness is between 0.3-1 micron). It has the ability to form ohmic contact with CIGSe [23]. Indeed, Mo can react with selenium (Se) during the deposition of CIGSe to form MoSe2. Consequently, the CIGSe/Mo structure then becomes *CIGSe/MoSe2/Mo* with a thickness of MoSe2 of about 10 nm. MoSe2 is a semiconductor with a gap of 1.41 eV and its existence has the effect of giving an ohmic behavior to the CIGSe/Mo hetero-contact, while reducing recombination at the interface.

### • **Substrate: Soda-lime glass**

The standard substrate used to make CIGSe cells is soda lime glass. It's benefit effects were described above in §2.2. Its thickness varies from 1 to 3 mm.
