# CZTSe-Based Solar Cell Performance Improvement Using the CSLO Technique
## 3 Results and Discussion  
In this section, to perform the numerical simulations of the device characteristics, a Silvaco simulator was used to extract the electrical characteristics and optical behaviour of the proposed solar cell. Moreover, the impact of the carrier lifetime and thickness on the performance of the solar cell was studied. As shown in Table 2, different cases with different thicknesses and concentrations of ITO and absorber layers were analyzed. According to the efficiency results (shown in the tenth row of Table 1), by increasing the doping concentration of the absorber layer, the efficiency was significantly improved. Thus, in comparison to cases 1 and 2, where all parameters except the concentration were constant, increasing doping from \(1\times 10^{15}\) to \(1\times 10^{18}\) resulted in improved efficiency from 18.86% to 32%. Indeed, by increasing the doping concentration of the absorber layer (case 2), the width of the depletion region declined. Therefore, the short circuit current (Jsc) decreased slightly, since fewer carriers were collected at the contacts as a result of a reduction in carrier transmission. At the same time, the open circuit voltage (Voc) increased because of the lower recombination rate of the electron and holes than the rate in case 1, resulting in an amplification of the efficiency. More holes and fewer electrons in the absorber layer caused a change in the recombination rate. It is important to choose the doping concentration of the absorber precisely when the dopant concentration is so high, as more photons will be absorbed in the buffer layer instead of the absorber layer, and efficiency will be decreased. Higher doping concentrations lead to lower carrier mobility, higher leakage, and increased scattering. As such, the optimum doping concentration values were compared for different cases in Table 2. \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Parameters** & **Cu\({}_{2}\)ZnSiSe\({}_{4}\)** & **Cu\({}_{2}\)ZnGeSe\({}_{4}\)** & **CZTS** & **CZTSe** & **CdS** & **ZnO** & **ITO** \\ \hline Band gap (eV) & 3 & 2.1 & 1.5 & 1.04 & 2.4 & 3.3 & 3.6 \\ \hline Electron affinity (eV) & 2.4 & 3.4 & 4.1 & 4.15 & 4.5 & 4.6 & 4.2 \\ \hline Dielectric permittivity & 7.5 & 6.8 & 7 & 10 & 10 & 9 & 10 \\ \hline Electron mobility (cm\({}^{2}\)/Vs) & 60 & 60 & 60 & 100 & 100 & 100 & 100 \\ \hline Hole mobility (cm\({}^{2}\)/Vs) & 20 & 20 & 20 & 25 & 10 & 10 & 10 \\ \hline Effective conductance & & & & & \(2.2\times 10^{18}\) & & \\ band density (cm/s) & & & & & & & \\ \hline Effective valence band & & & & & & & \\ density (cm/s) & & & & & & & \\ \hline Defect density (cm\({}^{-3}\)) & \(1\times 10^{13}\) & \(1\times 10^{13}\) & \(1\times 10^{13}\) & \(1\times 10^{13}\) & \(6\times 10^{16}\) & - & - \\ \hline \end{tabular}
\end{table} 
Table 1: Standard material parameters used in the solar cell design shown in Figure 1.  
In addition, by decreasing the thickness of the ITO layer and using the CSLO technique to make p+pn junction (case 3), the efficiency was again increased. Accordingly, it is believed that the efficiency of solar cells depends on the alignment band of the hetero-interface, and that the layer-thickness selection caused more light absorbance and less recombination of electrons and holes, as demonstrated in Table 2. Furthermore, the performance of the CZTS-based, Ge-based Cu\({}_{2}\)ZnGeSe\({}_{4}\), and Si-based Cu\({}_{2}\)ZnSiSe\({}_{4}\) Kesterite solar cells made with the proposed CSLO technique was analyzed. As illustrated in Table 2, the Ge-based Cu\({}_{2}\)ZnGeSe\({}_{4}\) solar cell achieved the best optical and electrical results because of the Ge's properties, such as having the best direct energy bandgap among the CZTS-based and Si-based Cu\({}_{2}\)ZnSiSe\({}_{4}\) solar cells, providing an over 70 percent efficiency and fill factor, which represent the ratio of the maximum power to the minimum power (Pmin) and the ratio of the theoretical power to the maximum power (Pmax), respectively.