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

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

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

In the **§3.1.5**, we highlighted one of the numerous positive roles of the ZnS on 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 may have this situation:

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

$$q\_n + qt\_{i-ZnO}N\_w + qt\_{ZnS}N\_b = qN\_at\_{SCR} \tag{23}$$

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 thicknesses *ti*�*ZnO*; *tZnS* and *tSCR*.

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 previous work reports [27].


**Table 3.**

*Reports of recent studies on the effect of defect states on the cell performances.*

## • **Investigation on the SDL/CIGSe sub-heterojunction**.

Recent studies were carried out on the effects of the density of defect states on the global performance of CIGSe/CdS-based solar cells and their conclusions are summarized within the **Table 3** above [27, 29].

then for the same value of *NA*, *VOC* will increase. This observation is the same with efficiency (**Figure 3(d)**). Conversely, *JSC* (**Figure 3(b)**) and the fill factor (FF) (**Figure 3(c)**) decrease significantly with the increase of *NA*. By setting the value of *NA*, and by varying *tZnS*, the output parameters of the cell are almost constant. When the thickness of the absorber layer is less than 250 nm, the performances of the cell are

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

But a significant increase on the output results is observed for a doping level *NA* <sup>¼</sup> <sup>10</sup><sup>15</sup>*cm*�<sup>3</sup> and with a thickness of the absorber layer no more than 0.5 <sup>μ</sup>m. For *NA* ∈ 10<sup>14</sup> : 10<sup>15</sup>*cm*�<sup>3</sup> � �, and for *tCIGSe* < 2500 nm, the parameters of the cell are globally interesting because *VOC* < *VSAT* (**Figure 3(a)**), where*VSAT* is the saturation voltage of our device. In addition, the short-circuit current is at its maximum value *JSC* ¼ *J*ð Þ *SC max* (**Figure 3(b)**) and the values of the fill factor (**Figure 3(c)**) *FF* ≈85%. This observation is important because it will allow to circumscribe the optimal value of *NA*. For*NA* >1015, the overall performance of the cell increases significantly. Believing that the best performances can be obtained with a high level of intrinsic doping recorded in the absorber layer is an utopia; two factors limit that way of thinking: The quality factor in **Figure 5(c)** decreases significantly and characterizes the poor quality and instability of the solar device in question; For *NA* <sup>¼</sup> 1016*cm*�<sup>3</sup> and considering *tCIGSe* > 500 nm, saturation is automatically reached. Thus, for *tCIGSe* <sup>¼</sup> <sup>500</sup>*nm*, we have *<sup>V</sup>*ð Þ *OC SAT* <sup>¼</sup> <sup>0</sup>*:*8*V*; *JSC* <sup>¼</sup> <sup>34</sup>*:*<sup>39</sup> *mA=cm*2, *<sup>η</sup>* <sup>¼</sup> <sup>22</sup>*:*27%, and *FF* ¼79.47%. let us recall the mathematical expressions given in §3.3.1, those are Eq. (20) and Eq. (22). According to Eq. (20), when we add the holes density *P,* the lifetime of negative charge carriers decreases since the latter is inversely propor-

tional to hole concentration and to the square root of hole concentration

intrinsic doping level of CIGSe is *NA* <sup>¼</sup> <sup>10</sup><sup>15</sup>*cm*�3.

ð Þ *Voc*,*ZCE dom* <sup>¼</sup> *Eg*

(Eq. (22)). As a conclusion, the benefits of higher doping for P-type conductivity materials are limited by the Auger and radiative mechanisms. Moreover, for *NA* ∈ 10<sup>15</sup> : 4*:*1015*cm*�<sup>3</sup> � � the performances of the cell are globally very interesting. This would probably justify why Daouda et al. [23] obtained a good yield (18.6%) by working with*NA* <sup>¼</sup> <sup>7</sup>*:*10<sup>15</sup>*cm*�3. However, they quickly reached saturation as soon as*tCIGSe* > 1000 nm. The analysis of the different graphs shows that the optimal

> *<sup>q</sup>* � *nkT q*

ln <sup>1</sup> *Jph* <sup>∗</sup> *qDnNCNV LnNA*

(24)

!

not influenced with variation of *NA* in particular FF and *η:*

*Influence of ZnS χe, on the fill factor (FF) and efficiency.*

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

**Figure 2.**

**99**

#### **3.4 Modeling and calculation tools**

We have modeled our solar cell device by using the version 3.3 of the onedimensional numerical simulation software SCAPS-1 D<sup>2</sup> [30].
