• **The absorbent layer: Cu (In, Ga) Se2**

It is a semiconductor obtained by combining elements from groups I-III-VI2 of the periodic table (**Table 2**) and has a chalcopyrite crystal structure. Its standard thickness is between 1.5 - 3 μm. This tetragonal structure can be described as a stack of two Zinc Blende structures in which the tetrahedral sites are occupied by atoms of group VI (Se) (anions) and the other sites are occupied in an orderly manner by atoms of groups I (Cu) and III (In) (cations). CIGSe is a solid solution of the semiconductor materials CuInSe2 and CuGaSe2 which have direct gaps of 1.06 eV and 1.7 eV respectively.

$$\text{The ratio}: \mathcal{X} = \frac{[\text{Ga}]}{[\text{Ga}] + [\text{In}]} \tag{2}$$

*<sup>χ</sup><sup>e</sup>* <sup>¼</sup> <sup>4</sup>*:*<sup>6</sup> � <sup>1</sup>*:*15667*<sup>x</sup>* <sup>þ</sup> <sup>0</sup>*:*03333*x*<sup>2</sup> (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

*Thin-Film Solar Cells Performances Optimization: Case of Cu (In, Ga) Se2-ZnS*

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 standard substrate used to make CIGSe cells is soda lime glass. It's benefit

*3.1.4 Investigations on the ZnS – Cu (In, Ga) Se2 interface: Highlighting of the surface*

Between the ZnS and the absorber, a layer called OVC (Ordered Vacancy Compound) has been identified. Investigations carried out within recent works identify his properties and found that they were similar to that of a Surface Defects Layer (SDL). Ouédraogo et al. [24] and Tchangnwa et al. [25, 26], highlighted the beneficial effect related to the existence of the SDL on cell performance. In fact, the presence of an Indium-enrich micro-layer which exhibits an N-type conductivity (N-SDL), at the top of the P-type conductivity CIGSe material, is responsible of the existence of that defect state layer. That leads to a discontinuity at the band gap level at the interface. It has a positive effect since it enhance the transport of the charge carriers through the junction. Another interpretation is given for the existence of that defect state layer from other authors [27], that is, it results from a copper-poor film at the top of the CIGSe material. Both interpretations complete each other, since the N-SDL is almost located within the CIGSe layer, but exhibits different opto-electrical, electronic and structural properties. As a consequence, at the P-N interface between the N-type ZnS and the P-type CIGSe materials, an homojunction is formed. That is, the N-type SDL is almost fully integrated within the CIGSe structure. That explains why in our structure, we will model our P-N heterojunction (the ZnS-CIGSe junction) as two different interfaces: the ZnS/SDL interface and SDL/CIGSe interface, each of them exhibiting different properties.

The photovoltaic effect is based on the properties of semiconductor materials. Indeed, the latter are capable of absorbing photons of frequency *ν* whose energy is:

effects were described above in §2.2. Its thickness varies from 1 to 3 mm.

that will be considered in this study.

*DOI: http://dx.doi.org/10.5772/intechopen.93817*

• **Substrate: Soda-lime glass**

*defect layer (SDL)*

**3.2 General principle**

**93**

*3.2.1 Mechanism of photovoltaic conversion*

the interface.

• **Rear contact: Molybdenum (Mo)**

determine the rate of Gallium atoms that replace Indium atoms in the structure. The value of the band gap and the electrical susceptibility of the material vary as a function of *x* between the values of pure CIS and pure CGS according to the following empirical laws [21]:

$$E\_{\rm g} = 1.06 + 0.39238\varkappa + 0.24762\varkappa^2\tag{3}$$

