**4.2 Influence of the doping concentration of the CIGSe layer on the performance of the cell**

Recent works with the CdS as buffer layer [3] has shown that, beyond a concentration of holes *NA* <sup>¼</sup> <sup>0</sup>*:*8 1014*cm*�3, the overall parameters of the cell degrade considerably. Ouédraogo et al. [19] have shown in their work that beyond *NA* <sup>¼</sup> 1016*cm*�<sup>3</sup> the voltage reaches saturation independently of the thickness of the absorber layer. The resulted obtained are plotted on the **Figure 3** below. From these plots, we can see that, varying *NA* influences the performances of the cell. For a fixe value of*tCIGSe*, *VOC*, increases significantly with the increase of *NA*. If we decide as well to increase *tCIGSe*,

<sup>2</sup> SCAPS-1D: Solar Cell Capacitance Simulator in One Dimension. Free software developed by M. Burgelman, Nollet and Degrave from the University of Gent in Belgium in 2000.

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

**Figure 2.** *Influence of ZnS χe, on the fill factor (FF) and efficiency.*

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

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 proportional to hole concentration and to the square root of hole concentration (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 intrinsic doping level of CIGSe is *NA* <sup>¼</sup> <sup>10</sup><sup>15</sup>*cm*�3.

$$\ln \left( V\_{oc, ZCE} \right)\_{dom} = \frac{E\_{\rm g}}{q} - \frac{nkT}{q} \ln \left( \frac{1}{J\_{ph}} \* \frac{qD\_n N\_C N\_V}{L\_n N\_A} \right) \tag{24}$$

increases, the voltage ð Þ *Voc*,*ZCE dom* tends to reach critical value materialized as

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

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

**4.3 Effects of the thickness of CIGSe on cell performance**

There are two main areas of interpretation,

• the first for 100 *nm* <*tCIGSe* <500 *nm*, and

• the second for 500 *nm* <*tCIGSe* < 2500 *nm*.

The results obtained are in good agreement with those reported by Ouédraogo and co-workers [23] who were investigating on a CIGSE-based cell structure with CdS as buffer layer. We bring out from these results that both the choice of the buffer layer material and its thickness are not limiting input parameters for hole doping level within the absorber layer. Th high density of recombination mechanisms and a weak collection mechanism of free charge carriers can explain very well the decreasing of short-circuit current (*JSC*). For a doping level of more than 10<sup>15</sup> *cm*�3, within the CIGSe material, the output parameters of our device are not

In this subsection, we are investigating how the global output parameters of

The simulations are carried considering the optimized properties of the other layers. From literature, the commonly used thickness for absorber layer is within the range [2000: 3000 nm]. Since we are on the way of reducing materials quantities, we will observe the performances of our cell with CIGSe thickness ranged from 100 nm to

Vital information came out from the results obtained. The global performances of the cell (*JSC*, *VOC*, FF, *η*) are improved significantly when the thickness of the absorber layer *tCIGSe* is gradually increased.*JSC* is the most sensible parameter, the values of 20*:*65*mA=Cm*<sup>2</sup> and 30*:*31*mA=Cm*<sup>2</sup> are recorded for *tCIGSe* values of 100 nm and 500 nm respectively. That is a jump of about 16*mA=Cm*<sup>2</sup> (**Figure 4(b)**). In fact, at the absorber and Molybdenum junction, the rate of backward recombination mechanisms are reduced significantly because of the Space Charge Region which is completely localized within the active material layer. A similar situation is observed with the plot of the open-circuit voltage, which value increases from 0.65 V for 100 nm of absorber thickness, to 0.76 V for 1000 nm of thickness respectively. That is a gain of 0.13 V! (**Figure 4(a)**). That is not all, the efficiency increased from 10.78% to 24.31%, a jump of almost 14% (**Figure 4(d)**). A value of 83.4% was recorded for the Fill Factor (**Figure 4(c)**). Those results are just impressive. Let us remember that we are undertaking our calculations based on optimized values of properties of the other materials that constitute our structure. These are reported in

For a thickness of the absorber layer between 500 nm and 2500 nm, we record very good value of our cell output performance. Of course, for a thicker CIGSe layer, the probability of absorbing a wide range of light Spectrum is higher and thus, and since the materials properties have been optimized, the quantum efficiency will

Pogrebjak and co-workers [7, 32] who worked on the influence of temperature on CIGSe-based ultra-thin solar cells and on nano-scale technologies, obtained results which are in good agreement with those obtain during our simulations. The slight difference in results can be explained by global condition of calculation such

be higher too. From **Figure 4(a)**, we can easily notice how remarkably *VOC*

the device is affected when the thickness of the active layer is a variable.

saturation.

enhanced.

3000 nm.

the **Table 4** below.

**101**

increases with absorber layer thickness.

#### **Figure 3.**

*Influence of the acceptor density NA on the cell parameters for different CIGSe thickness values. (a) Open circuit voltage (VOC), (b) short-circuit current density (JSC*Þ*, (c) fill factor (FF), and (d) efficiency (η).*

where *Dn* is the electron scattering coefficient, *NC* and *NV* the state densities in the conductance and valence bands, *Ln* the electron scattering length, *NA* the density of acceptor states in the CIGSe layer. From this relation when *NA*

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

increases, the voltage ð Þ *Voc*,*ZCE dom* tends to reach critical value materialized as saturation.

The results obtained are in good agreement with those reported by Ouédraogo and co-workers [23] who were investigating on a CIGSE-based cell structure with CdS as buffer layer. We bring out from these results that both the choice of the buffer layer material and its thickness are not limiting input parameters for hole doping level within the absorber layer. Th high density of recombination mechanisms and a weak collection mechanism of free charge carriers can explain very well the decreasing of short-circuit current (*JSC*). For a doping level of more than 10<sup>15</sup> *cm*�3, within the CIGSe material, the output parameters of our device are not enhanced.

#### **4.3 Effects of the thickness of CIGSe on cell performance**

In this subsection, we are investigating how the global output parameters of the device is affected when the thickness of the active layer is a variable. The simulations are carried considering the optimized properties of the other layers. From literature, the commonly used thickness for absorber layer is within the range [2000: 3000 nm]. Since we are on the way of reducing materials quantities, we will observe the performances of our cell with CIGSe thickness ranged from 100 nm to 3000 nm.

There are two main areas of interpretation,


Vital information came out from the results obtained. The global performances of the cell (*JSC*, *VOC*, FF, *η*) are improved significantly when the thickness of the absorber layer *tCIGSe* is gradually increased.*JSC* is the most sensible parameter, the values of 20*:*65*mA=Cm*<sup>2</sup> and 30*:*31*mA=Cm*<sup>2</sup> are recorded for *tCIGSe* values of 100 nm and 500 nm respectively. That is a jump of about 16*mA=Cm*<sup>2</sup> (**Figure 4(b)**). In fact, at the absorber and Molybdenum junction, the rate of backward recombination mechanisms are reduced significantly because of the Space Charge Region which is completely localized within the active material layer. A similar situation is observed with the plot of the open-circuit voltage, which value increases from 0.65 V for 100 nm of absorber thickness, to 0.76 V for 1000 nm of thickness respectively. That is a gain of 0.13 V! (**Figure 4(a)**). That is not all, the efficiency increased from 10.78% to 24.31%, a jump of almost 14% (**Figure 4(d)**). A value of 83.4% was recorded for the Fill Factor (**Figure 4(c)**). Those results are just impressive. Let us remember that we are undertaking our calculations based on optimized values of properties of the other materials that constitute our structure. These are reported in the **Table 4** below.

For a thickness of the absorber layer between 500 nm and 2500 nm, we record very good value of our cell output performance. Of course, for a thicker CIGSe layer, the probability of absorbing a wide range of light Spectrum is higher and thus, and since the materials properties have been optimized, the quantum efficiency will be higher too. From **Figure 4(a)**, we can easily notice how remarkably *VOC* increases with absorber layer thickness.

Pogrebjak and co-workers [7, 32] who worked on the influence of temperature on CIGSe-based ultra-thin solar cells and on nano-scale technologies, obtained results which are in good agreement with those obtain during our simulations. The slight difference in results can be explained by global condition of calculation such

as external temperature, incident light power, and defect states density input

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

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

Running several calculations allowed us to detect the critical values of some properties values beyond which the device is no more stable, even if the efficiency is higher. This is for example 2750 nm for absorber layer thickness or an intrinsic doping level of more than 1015. Since we manufacture our solar cell device based on the model proposed in this work and watching out not to have these highlighted critical values, our device will definitely work in good condition. The **Table 5** above reports the best performance of our cell obtained during all our calculations.

This chapter focused on enhancing efficiencies of solar cell devices working on a CIGSe-based second – generation ultra-thin model, and using Zinc Sulfide (ZnS) as a window layer. Vital information is found when investigating the influence of layers and interfaces properties on output parameters of the device. The challenging part is not the use of the promising ZnS material itself, but it is to find through literature and recent works, the key values of the ZnS properties in a preferential crystallographic orientation, that allow to obtain better performances and also the good choice of materials that make up the other layers. Starting on that point, the following cell *(Ni/Al)/MgF2/ZnO: B/i-ZnO/ZnS/CuInGaSe2/Mo/Substrate* has been modeled and simulations were ran from version 3.3 of the SCAPS-1D software. The benefits associated with the existence of the Surface Defects Layer (SDL) on the device stability have been highlighted. The Blende structure of Zinc Sulfide material (ZnS) forms a more stable lattice matching with CIGSe absorber layer chalcopyrite structure. That is why most of the key values of its intrinsic properties are obtained

from that orientation, especially its band gap *Eg*\_*ZnS* ¼ 3*:*68*eV*, its electrical susceptibility *χ<sup>e</sup>* ¼ 4*:*3*eV*, its dielectric constant *ε<sup>r</sup>* ¼ 8*:*3 according to simulation results. After running numerous simulations, very promising performances are recorded, a conversion efficiency of 26.30% and a fill factor of 85.14%. Going further in research, some may obtain even more interesting results by directing the work towards implementation of additional manufacturing technologies, including the use of antireflective coatings and the texturization of the inner back layers.

Fridolin Tchangnwa Nya\* and Guy Maurel Dzifack Kenfack

\*Address all correspondence to: nyafridolin@yahoo.fr

provided the original work is properly cited.

Department of Physics, Faculty of Science, University of Maroua, Maroua,

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

values.

**5. Conclusion**

**Author details**

Cameroon

**103**

#### **Figure 4.**

*Influence of the thickness of the absorber layer on the output parameters of the cell for tZnS* ¼ *5 nm NA* ¼ 1015*cm*�<sup>3</sup>*. (a) Open circuit voltage (VOC), (b) short-circuit current (JSC), (c) fill factor (FF), and (d) efficiency (η).*


#### **Table 4.**

*Optimized value of some layers/interface properties used for simulation.*


#### **Table 5.**

*Output parameters of our photovoltaic device.*

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

as external temperature, incident light power, and defect states density input values.

Running several calculations allowed us to detect the critical values of some properties values beyond which the device is no more stable, even if the efficiency is higher. This is for example 2750 nm for absorber layer thickness or an intrinsic doping level of more than 1015. Since we manufacture our solar cell device based on the model proposed in this work and watching out not to have these highlighted critical values, our device will definitely work in good condition. The **Table 5** above reports the best performance of our cell obtained during all our calculations.
