## 3 Optical Modeling Results
### Surface-Textured CIGS Solar Cells  
Figure 6a shows a schematic diagram of the surface-textured CIGS solar cell including the front incoherent encapsulation of the cover glass/EVA layers. The interfaces of the EVA/AZO/ZnS/CIGS multilayer are assumed to be randomly textured with the root-mean-square (RMS) surface roughness of \(\sigma_{\text{RMS}}\), which enhances light absorption caused by the scattering effect. The thickness of each layer, along with its complex refractive index, is the same as that of the planar CIGS solar cell in Figure 2. Figure 6b shows incoherent multiple optical reflections inside the planar cover glass layer. A 2D FEM simulation is performed in the surface-textured CIGS solar cell with the same boundary conditions used for the planar CIGS solar cell. To investigate how the degree of the surface roughness affects the absorption characteristics of the surface-textured CIGIS solar cell, calculation results are obtained at four values of \(\sigma_{\text{RMS}}\) = 25, 50, 75, and 100 nm. Because the 3 mm cover glass and the 0.5 mm EVA flattening layers are transparent, these incoherent layers are modelled as the 1 \(\upmu\)m thick initial layer plus the respective phase-shift layer whose thicknesses \(b_{1}\) and \(b_{2}\) are determined based on Equation (4) in the ADETAM-based calculation. To show how the light absorption takes place within the surface-textured CIGS solar cell, two FEM simulation results, calculated at the single wavelength of 400 nm, are represented in Figure 7. Here, spectral 2D coherent calculation results of the optical absorption in the surface-textured CIGIS solar cell are obtained at the wavelength of 400 nm when the equispaced phase thicknesses in the phase-shift layers are zero. The RMS surface roughness is set to be \(\sigma_{\text{RMS}}\) = 100 nm. The 400 nm input plane wave with the surface power density of 5.57 W/m\({}^{2}\) has the incidence angle of 45\({}^{\circ}\) for both s and p polarizations. To clarify the spectral optical absorption profile near the ZnS/CIGS interface, all the calculation results are shown near the ZnS/CIGS interface. The absorption distributions of the surface-textured CIGS solar cell are not uniform due to the optical  
Figure 6: (**a**) Schematic diagram of the surface-textured CIGS solar cell. Because the 3 mm cover glass and the 0.5 mm EVA flattening layers are transparent, they are modelled as the 1 \(\upmu\)m thick initial layer plus the respective phase-shift layer whose thicknesses \(b_{1}\) and \(b_{2}\) are determined based on Equation (4) in the ADETAM-based calculation. The interfaces of the EVA/AZO/ZnS/CIGS multilayer are assumed to be randomly textured with the RMS surface roughness, which varies as \(\sigma_{\text{RMS}}\) = 25, 50, 75, and 100 nm. (**b**) Incoherent multiple optical reflections inside the planar incoherent cover glass layer to determine the minimum reflectance of the surface-textured CIGS solar cell. The reflectance from the cover glass to the air is \(R_{g-a}\) and the reflectance from the cover glass to the EVA is \(R_{g-EVA}\). The refraction angle at the cover glass is \(\theta_{1}\) when the incidence angle of sunlight is \(\theta_{0}\) in the air. scattering, which is different from those of the planar CIGS solar cell in Figure 4. At each combination of equispaced phase thicknesses, FEM-based coherent numerical results of the optical absorption profiles such as those in Figure 7 are calculated at each combination of the wavelength, polarization, and RMS surface roughness. In the ADETAM-based calculation, a total of \(10^{2}=100\) optical absorption profiles are averaged at each mesh point. In addition, at each RMS surface roughness, ADETAM-based calculation results are averaged over three different arbitrarily-textured surfaces with the same RMS roughness to simulate CIGS solar cells with random surface-texture.