• **Radiative recombination**

It takes place by the direct transition of an electron from the conduction band to the valence band. The energy of the transition is released as a photon. If the trap levels created by defect states are close to the middle of the forbidden band, the rate of radiative recombination and the lifetime of electron can be given by [21]:

$$R\_R \approx \frac{n - n\_0}{\tau\_{R, n}} \tag{19}$$

with

Auger recombination coefficient.

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

it a little further in the third section.

may have this situation:

nesses *ti*�*ZnO*; *tZnS* and *tSCR*.

previous work reports [27].

**97**

*<sup>τ</sup><sup>A</sup>*,*<sup>n</sup>* <sup>≈</sup> <sup>1</sup> *p*2 <sup>0</sup>*Cp*

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

*τ<sup>A</sup>*,*<sup>n</sup>* is the lifetime of the electrons, *p*<sup>0</sup> the hole density at equilibrium, *Cp* is the

We recall these mathematical expressions to highlight the influence of doping process on recombination mechanism rate at a certain point. We will more explain

Most of the time, defect states are a result of the way the processes of preparation or deposition of layer sheets are performed. They act on material properties, mainly on material electrical properties the same way as for adding impurities. Generally, the main goal is to improve charge carriers transport within the material or to reduce recombination mechanism rate. However, as we will see a little further, they have detrimental effects on the cell performances. Recalling what has been said in **§3.1.6**, and because of the highlighted SDL, at the heterojunction level, we will

In the **§3.1.5**, we highlighted one of the numerous positive roles of the ZnS on

• The negative charges concentration located in the Space Charge Region (SCR),

where *Qn* is the surface charge in the depletion zone of the boron-doped ZnO window layer, q is the elementary charge, *Nw*, *Nb* and *Na* are the doping concentrations in the i-ZnO, ZnS and CIGSe top film part layers with respective thick-

Introducing a defect state within a material crystallographic structure, is generally materialized by adding a negative charge. Let say, this situation happens at the ZnS/SDL sub-heterojunction section, to compensate the negative charge added due to defect states, we should reduce the width of the SCR in the CIGSe top film part. That action we will increase the recombination rate at the ZnS-SDL level and negatively affect the cell output parameters. This interpretation is confirmed by

*Qn* þ *qti*�*ZnONw* þ *qtZnSNb* ¼ *qNatSCR* (23)

that is almost localized within the top film part of the CIGSe layer is compensated with the positive charges concentration on the other side, resulting from top to bottom of ZnO: B, i-ZnO and ZnS materials.

That is, at equilibrium, it can be described by the following equation:

enhancing the global cell performance, that is "it protects the surface of the absorber during the deposition of the ZnO layer, which can cause defects on the surface of the CIGSe". Therefore, a good choice of the deposition process can significantly reduce these defects density at ZnS/SDL sub-heterojunction. If we consider an ideal case, where the preparation and the deposition processes are perfect, that means with no defect reported within the sub-heterojunction, then we

*3.3.2 Investigations on the effects of defect states on cell performances*

investigate these effects on two interfaces: ZnS/SDL and /CIGSe.

• **Investigation on the ZnS/SDL sub-heterojunction.**

(22)

with

$$\tau\_{\text{R,n}} = \frac{1}{p\_0 \mathcal{C}\_r} \tag{20}$$

and,

*τRad*,*<sup>n</sup>* is the lifetime of the electrons, *Cr* is the radiative recombination coefficient, *p*<sup>0</sup> the hole density at equilibrium.

However, this mechanism is not always so harmful on the performance of the cell because in fact, it is possible that the photon released during this mechanism is reabsorbed and thus forming another electron-hole pair. Indeed, the energy value of the emitted photon is close to that of the band gap.

#### • **Auger recombination**

These are direct carrier band-to-band transfers. It could be an electron or a hole. However, instead of being emitted as a photon, the energy is transferred to another carrier of the same type as thermal energy. The latter will return to its initial state by interacting with the crystal lattice, it will therefore emit a phonon [21]. The rate of radiative recombination can be given by:

$$R\_A \approx \frac{n - n\_0}{\tau\_{A, n}} \tag{21}$$

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

with

$$
\tau\_{A,n} \approx \frac{1}{p\_0^2 C\_p} \tag{22}
$$

*τ<sup>A</sup>*,*<sup>n</sup>* is the lifetime of the electrons, *p*<sup>0</sup> the hole density at equilibrium, *Cp* is the Auger recombination coefficient.

We recall these mathematical expressions to highlight the influence of doping process on recombination mechanism rate at a certain point. We will more explain it a little further in the third section.
