## 4 Electrical Modeling Results
### Electrical Modeling Procedure
Comparison of the J-V Characteristics without and with the Inclusion of the Incoherent Front Encapsulation Layers  
To investigate the effect of the front incoherent encapsulation layers on output performance of the surface-textured CIGS solar cell, the angular dependences of reflectance spectrum and J-V curves are calculated and compared between the planar and the surface-textured CIGS solar cells without and with the inclusion of the two front encapsulation layers, as shown in Figure 9. In Figure 9a,c, the multilayer structures of the planar and surface-textured CIGS solar cells are the same as those in Figures 2a and 6a except the incoherent front encapsulation layers. In this case, sunlight is assumed to be directly incident from the air to the AZO/ZnS/CIGS/Mo layer, all of which is treated coherently. The device structures of the planar and surface-textured CIGS solar cell with two front encapsulation layers are the same as those in Figures 2a and 6a. The RMS surface roughness of the surface-textured structure is set to be \(\sigma_{\text{RMS}}=100\) nm, which show the minimum reflectance in Figure 8. Here, we will focus on how the absorption and short-circuit current enhancement of the surface-textured CIGS solar cell over the planar CIGS solar cell can be changed whether the front incoherent encapsulation layers are included or not in the optical modeling. Figure 10 shows the calculated reflectance spectra of the surface-textured CIGS solar cell without and with the inclusion of the front incoherent layers when the incidence angle of the AM 1.5 sunlight is \(0^{\circ}\), \(30^{\circ}\), \(45^{\circ}\), and \(60^{\circ}\). For reference, the reflectance spectra of the planar CIGS solar cell are also calculated. According to the Fresnel equation, the reflectance of the planar interface between two media is proportional to the refractive-index difference of the two media [35]. In reference to the complex refractive index spectra of the materials in Figure 2b, the refractive index of the cover glass, which is very close to that of the EVA, is smaller than that of the AZO, ZnS, or CIGS. Because of the optical interference effect in the AZO/ZnS/CIGS/Mo multilayer, the reflectance of the planar CIGS solar cell without the front encapsulation layers can be lower than that of the CIGS solar cell with the front encapsulation layers in a relatively narrow spectral range around 500 nm. However, overall  
reflectance of the planar CIGS solar cell with the front encapsulation layers is lower than that of the CIGS solar cell without the front encapsulation layers. This is ascribed to the fact that the refractive-index difference between the air and the cover glass is less than the refractive-index difference between the air and the AZO TCO. \[J_{p}(\overrightarrow{r})=-q\mu_{p}p(\overrightarrow{r})\
abla V( \overrightarrow{r})-qD_{p}\
abla p(\overrightarrow{r}), \tag{20}\]0 Calculated reflectance spectra of the surface-textured CIGS solar cell without and with the inclusion of the front incoherent layers. The RMS surface roughness of the surface-textured structure is set to be \(\sigma_{\text{RMS}}=100\) nm. For reference, the reflectance spectra of the planar CIGS solar cell are also calculated. The incidence angle of the AM 1.5 sunlight is \(\theta_{0}=\) (**a**) 0\({}^{\circ}\), (**b**) 30\({}^{\circ}\), (**c**) 45\({}^{\circ}\), and (**d**) 60\({}^{\circ}\), respectively. \[J_{p}(\overrightarrow{r})=-q\mu_{p}p(\overrightarrow{r})\
abla V( \overrightarrow{r})-qD_{p}\
abla p(\overrightarrow{r}), \tag{20}\]1 Four different device structures to investigate the effect of the front incoherent encapsulation layers on output performance of the planar and surface-textured CIGS solar cells. The RMS surface roughness of the surface-textured structure is \(\sigma_{\text{RMS}}=100\) nm. The angular dependences of reflectance spectrum and J–V curves are calculated and compared in the surface-textured CIGS solar cell (**c**) without and (**d**) with the inclusion of the two front encapsulation layers.