*3.1.2 ZnS material global properties*

### *3.1.2.1 ZnS crystallographic properties*

The crystallography of compounds II-VI6 to which Zinc Sulfide belongs poses some problems because of the polymorphism of these compounds. They can have crystallographic structures of two main types: the cubic structure of the sphalerite type (Zinc Blende), and the hexagonal structure of the Wurtzite type. The cubic structure (Zinc Blende) is stable at room temperature (27°C), while the hexagonal structure is more stable at very high-top temperatures of around 1020°C [10]. Indeed, with a lattice parameter *aCIGS* ¼ 0*:*58 *nm* for the CIGSe chalcopyrite structure and *a*0\_*ZnS* ¼ 0*:*541 *nm* respectively*a*0\_*ZnS* ¼ *b*0\_*ZnS* ¼ 0*:*3811*nm* for ZnS in its cubic and hexagonal structure respectively, CIGSe forms a better lattice agreement with ZnS in its sphalerite structure which is furthermore its stable structure [11].

A deposition technique by ALD [12] or by laser sputtering [11] would make it possible to obtain a layer of ZnS having a crystallographic orientation preferentially sphalerite.


**Table 1.** *Chemical elements of the Mendeleev table of columns II and VI [13].*

#### *3.1.2.2 ZnS Opto-electrical properties*

The spectrum of white light extends from the ultraviolet characterized by short wavelengths (λ < 380 nm) to the infrared characterized by long wavelengths (λ > 780 nm) through the visible spectrum whose wavelengths are between 380 nm < λ < 780 nm. The absorption spectrum of ZnS is an important element that characterizes its absorption power when subjected to illumination. It is all the more significant as it extends over a frequency band that is difficult to absorb by other materials. The important element to characterize it is the value of the band gap of the material. The energy value of the band gap, *Eg* of ZnS can be determined by an optical approach through its absorption spectrum. The formula that describes light absorption ability of a material given his band gap is well highlighted by Rayan, Elseman and co-workers [14–16] and is given by:

$$\left(ab\nu\right)^{2} = A\left(h\nu - E\_{\text{g}}\right) \tag{1}$$

• **Second window layer: i-ZnO**

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

*One-dimensional structure of the cell.*

• **First window layer: ZnO: B**

• **The buffer layer: ZnS**

attributed to it:

interface;

**91**

ambient conditions [20].

**Figure 1.**

Zinc oxide is a semiconductor belonging to group II-VI with a number of properties that make it a material widely used in several fields, these are its piezoelectricity, its wide band gap and its intrinsic doping type N. It can exhibit an N-type doping process, for example by adding the atoms of Al, Ga and B and it also exhibits an N-type conductivity due to structural defects. Its forbidden band is 3.3 eV and can vary by adding Mg, Cd or S atoms. It can crystallize according to the Zinc Blende, wurtzite or diamond structures. Only the wurtzite structure is stable under

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

Most CIGSe-based solar cells use Aluminum-doped Zinc oxide (ZnO: Al) as the Transparent Conductive Oxide (TCO). Within the framework of this work we opt for ZnO doped with boron (ZnO: B) as OCT. Boron doping would be more beneficial for solar cells. Its standard thickness varies between 450 and 1400 nm [18].

Its standard thickness varies between 40 and 60 nm. It has an N-type conductivity and its gap is greater than that of the absorber layer. Three roles are mainly

• **An electrical role**: it adapts the width of the forbidden band between the absorber and the window layer and limits the recombination of carriers at the

• **An optical role**: due to its wide forbidden band (*Eg* = 3.68 eV), it makes it possible to absorb the maximum of the light spectrum in the region not

absorbed by the active layer and thus minimizes optical losses;

This formula is only valid for authorized direct transitions. *α* is the absorption coefficient, *A* is a constant to be determined, ɦ is Planck's constant and *ν* is the frequency of the incident photon.

Literature tells us that ZnS is a semiconductor with a wide band gap (> 3.5 eV). This wide band gap gives it high optical transparency in the visible and infrared regions of the solar spectrum and a high absorption coefficient in the ultraviolet region. Its transmission spectrum is recorded between 400 and 850 nm [17], which allows the transmission of photons of higher energy, thus increasing the absorption of the light spectrum in the absorber layer.

It is also a direct gap semiconductor. Indeed, the maximum of the valence band and the minimum of the conduction band are found at the centre of the Brillouin zone (where *K* ! ¼ 0). The transitions are made from band to band without the

intervention of phonons, therefore without loss or dissipation of energy in thermal form [18].

The refractive index of ZnS is 2.41 to 0.5 μm and 2.29 to 1.1 μm in depth [19]. Remember that the electronic structures of Sulfur and Zinc are:

$$Zn: [Ar] \text{4d}^{10} \text{Sr}^2 \text{ S}: [Ne] \text{3d}^2 \text{\textdegree} p^4$$

The 3*p* states of Sulfur form the valence band, the 5 *s* states of Zinc constitute the conduction band. This gives ZnS a wide forbidden band. This wide band gap makes it a very promising material for optoelectronic and solar applications. Its forbidden band is between 3.68 eV and 3.9 eV depending on whether we are in a cubic or hexagonal structure. This band gap value may vary depending on the preparation method and the doping rate [19]. We can also find from the literature that it has a relatively high exciton binding energy (34 *meV*); its structure exhibits a better lattice matching with absorbers having energy bands in the range (1,2–1,5 *eV*) [13] and finally it has a high electrons mobility (165 *cm*2/*V. s*).

#### *3.1.3 Structure of our solar cell*

In its most common structure, a CIGSe-based cell is formed by a stack of several thin film materials deposited successively on a substrate. Let us consider the following structure:

(Ni/Al)/MgF2/ZnO: B/i-ZnO/ZnS/CuInGaSe2/Mo/SLG (**Figure 1**).

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

**Figure 1.**

*One-dimensional structure of the cell.*
