## 3 Simulation Results  
The optical modeling of the CIGS solar cell in Figure 1 was conducted based on the two-dimensional (2D) FEM, which was implemented using the RF module of COMSOL Multiphysics [20]. The complex refractive index spectra of the materials, which were taken from [21, 22, 23], are shown in Figure 2. In the FEM simulation, wave equations shown in Equation (1) were numerically solved at very small meshes of the calculation domains with appropriate boundary conditions. We use the port boundary condition at the uppermost boundary of the air domain to generate a plane wave with a wavelength of \(\lambda\), which was normally injected into the multilayer structure of the CIGS solar cell. In addition, the Floquet periodic boundary conditions were used at the leftmost and rightmost boundaries of the simulation domain. If the refractive index of the transparent incoherent cover glass was \(n_{g}\), the equispaced phase thickness of the EPL can be determined by  
\[t_{b}=\frac{\lambda}{2n_{g}}\frac{(b-1)}{N},\;\text{b}=1,\;2,\;\cdots,\;(N-1),N. \tag{17}\]  
When the ETAM is applied to the incoherent modeling of a planar organic solar cell, the accuracy of the calculation results converge if the number of the equispaced phase thicknesses is greater than five [18]. In this calculation, we set the number of equispaced phase thicknesses to 10. Hence, the corresponding equispaced phase thicknesses of the EPL were  
\[t_{1}=0,t_{2}=\frac{\lambda}{20n_{g}},t_{3}=\frac{2\lambda}{20n_{g}},\;\cdots, \;t_{9}=\frac{8\lambda}{20n_{g}},t_{10}=\frac{9\lambda}{20n_{g}}. \tag{18}\]  
Figure 3 shows the calculated 2D spatial profiles of the electric field amplitude, time-averaged Poynting vector, and optical power dissipation when the equispaced phase thickness of the EPL was set to \(t_{1}=0\) nm. All calculation results were obtained at a wavelength of \(\lambda=400\) nm, where the extinction  
Figure 2: Complex refractive index spectra of the materials used in the CIGS solar cell, of which the real and imaginary parts are (**a**) the refractive index (**b**) extinction coefficient, respectively. coefficient of the CIGS material was the largest, as shown in Figure 2b. In Figure 3, all calculation results depend only on the longitudinal z-direction and have no spatial dependence of the transverse x-direction, which results from the Floquet periodic boundary condition applied to the leftmost and rightmost boundaries in the optical simulation domain. In Figure 3b, the time-averaged Poynting vector remains unchanged at the transparent cover glass layer, but continues decreasing at the AZO, ZnS and CIGS layers owing to the gradual light absorption at these layers. Because of the very high absorption coefficient at the 2-\(\upmu\)m thick CIGS layer, the optical power dissipation in Figure 3c was significant near the ZnS/CIGS interface and there was no light absorption near the CIGS/Mo interface. Hence, the CIGS layer can be optically modeled as a coherent layer although its 2-\(\upmu\)m thickness was larger than the coherence length of sunlight. To investigate how the light absorption within the CIGS solar cell was changed depending on the equispaced phase thickness of the EPL, the spatial profiles of the optical power dissipations were calculated at 10 different equispaced phase thicknesses of \(t_{1}\), \(t_{2}\), \(\cdots\) and \(t_{10}\) when the wavelengths of sunlight were 400 and 800 nm. Because the 2D calculation results obtained through the optical modeling, as shown in Figure 3, depend only on the longitudinal z-direction owing to the periodic boundary condition applied to the transverse x-direction, the calculated spatial profiles in Figure 4 were plotted in the longitudinal z-direction. In Figure 4a, the overall exponentially decaying shapes of the optical absorption calculated at different thicknesses of the EPL were comparable to each other except for their relative magnitudes. This was ascribed to the difference in the optical phase change induced by the respective thickness of the EPL in the cover glass layer.