## 3 Optical Modeling Results
### Planar CIGS Solar Cells
As will be shown in Figure 3, most of light are absorbed near the ZnS/CIGS interface and a part of light reflected from the Mo bottom contact layer is negligible. Although the 2 \(\upmu\)m thickness of the CIGS layer is greater than the coherence length of sunlight (\(\sim\)0.6 \(\upmu\)m),  
Figure 2: **(a)** Schematic diagram of the planar CIGS solar cell together with the spectral irradiance of the AM 1.5 sunlight ranging between 300 and 1100 nm. Both the 3 mm cover glass and 0.5 mm EVA layers, the thicknesses of which are greater than the coherence length of sunlight (\(\sim\)0.6 \(\upmu\)m), are treated as incoherent. The remaining layers are considered as coherent. Because the glass cover and the EVA flattening layers are assumed to be transparent, both layers are modeled 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) when the ADETAM is applied to calculate the effect of the incoherent front glass and EVA layers in the simulation. **(b)** Complex refractive index spectra of the materials used in the CIGS solar cell. it can be treated as a coherent layer because no optical interference effect is observed in the light absorption profiles of the CIGS layer.The optical modeling of the planar CIGS solar cell is numerically performed based on the two-dimensional (2D) FEM, where the RF module of COMSOL Multiphysics is used as a simulation platform [32]. The wave equation is numerically solved at very tiny meshes of the simulation domains with appropriate boundary conditions. We use the port boundary condition at the uppermost boundary of the air domain, where a plane wave having the wavelength of \(\lambda\) and the incidence angle of \(\theta_{0}\), is generated for either s and p polarization. The wavelength-dependent input power of the incident plane wave for each polarization is determined by (1/2)S(\(\lambda\))d\(\lambda\), where S(\(\lambda\)) indicates the spectral irradiance of the AM 1.5 sunlight shown in Figure 2a. Because the AM 1.5 sunlight is unpolarized, a factor of 1/2 is included to represent the incident optical power for each polarization. In this simulation, the spectral irradiance of the AM 1.5 sunlight between 300 and 1100 nm is divided with the wavelength spacing of d\(\lambda\) = 10 nm. The port boundary condition applied to the lowermost boundary suppresses the artificial reflection at the bottom boundary. In addition, the Floquet periodic boundary conditions are used at the leftmost and rightmost boundaries of the simulation domain. To show how the light absorption takes place within the planar CIGS solar cell, two FEM simulation results, calculated at the single wavelength of 400 nm, are represented in Figure 3. Here, spectral 2D coherent calculation results of the electric field amplitude and optical absorption are obtained at the wavelength of 400 nm when the equispaced phase thicknesses in the phase-shift layers are zero (\(b_{1}^{1}=b_{2}^{1}=0\)). 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 polarizations. The 2D spatial profiles of the electric field amplitude have a tilted wavy pattern, which is ascribed to the interference between the obliquely incident and reflected lights. On the other hand, the spatial profiles of optical absorption depend only on the longitudinal z-direction and have no spatial dependence in the transverse x-direction, which results from the fact that optical power only flows into the longitudinal z direction. When the ADETAM is applied to the incoherent modeling of a mixed coherent-incoherent multilayer, the accuracy of the calculation results converges if the number of the equispaced phase thicknesses is greater than five [20; 21]. In this calculation, we set the number of equispaced phase thicknesses in the phase-shift layer to be ten.