## 3 Simulation Results
Figure 4: Calculated spatial profiles of the optical power dissipations in the longitudinal z-direction \(Q(z,\lambda,t_{b})\) with respect to 10 different equispaced phase thicknesses of \(t_{1}\), \(t_{2}\), \(\cdots\) and \(t_{10}\). The calculation results were obtained at wavelengths of **(a)** 400 and **(b)** 800 nm. To consider the effect of the incoherent cover glass, the ETAM-applied optical power dissipation \(Q^{ETAM}(z,\lambda)\), obtained by averaging the 10 coherent calculation results, was designated by the red dotted line. The final step of the optical modeling was to calculate the spatial profile of the generation rate of the electron-hole pair, which was one of the input parameters of the continuity equation in the electrical modeling. According to Equations (7) and (8), the generation rate of the electron-hole pair was obtained by integrating the optical power dissipation weighted by the spectral irradiance of AM 1.5 sunlight, as shown in Figure 6a. Figure 6b shows the calculated spatial profiles of the generation rate in the longitudinal z-direction \(G(z,t_{b})\) with respect to 10 different equispaced phase thicknesses of \(t_{1}\), \(t_{2}\), \(\cdots\) and \(t_{10}\). To include the incoherent effect of the thick cover glass, the ETAM-applied generation rate in the longitudinal z-direction \(G^{ETAM}(z)\) was also plotted in the red dotted line. The overall exponentially decaying shapes of the generation rates in Figure 6b were comparable to those of the optical power dissipations in Figure 4. In comparison with the aggregate sums of the generation rates between the coherent and ETAM-applied models, the deviation was calculated to be between \(-0.67\%\) and \(+0.9\%\). This value was much smaller than the maximal absolute deviations calculated under the optical power dissipations shown in Figure 4a (10.22%) and the absorptivity in Figure 5 (10.44%), which results from the significant dependence of both the wavelength of sunlight and the thickness of the EPL. By contrast, the generation rate corresponds to the accumulated sum of the optical power dissipation within the wavelength region ranging between 300 and 1100 nm. Thus, the generation rate calculated at each equispaced phase thickness was less sensitive to the change in wavelength, which causes a reduction of the percentage variation of the electron-hole pair generation rate. To obtain the J-V characteristics of a CIGS solar cell, the coupled drift-diffusion and continuity equations, shown in Equations (9)-(16), were numerically solved through the use of the semiconductor module of COMSOL multiphysics [20]. In the 2D simulation domain of the CIGS solar cell in Figure 1, we only consider the regions containing the ZnS and CIGS layers for the electrical modeling. Both the bottom cathode terminal of the Mo/CIGS interface and the top anode terminal of the AZO/ZnS interface were assumed to be ideal Schottky contacts. In accordance with the Floquet periodic boundary condition in the optical modeling, the periodic conditions were used at the leftmost and rightmost boundaries of the simulation domain in the electrical modeling. The FEM-based electrical modeling was performed under the same mesh configuration as the FEM-based optical modeling to minimize the error of a mismatch. The simulation parameters used in the electrical modeling are shown in Table 1[24]. Figure 5: Calculation results of the absorptivity spectra \(A(\lambda,t_{b})\) depending on 10 different equispaced phase thicknesses of \(t_{1}\), \(t_{2}\), \(\cdots\) and \(t_{10}\). The ETAM-applied absorptivity spectrum \(A^{ETAM}(\lambda)\) was obtained by averaging 10 coherent calculation results over the equispaced phase thickness. The absorptivity spectrum calculated by another analytical model of the generalized transfer matrix method (GTMM) shows a good agreement with that obtained based on the ETAM, which verifies the validity of the calculation results obtained using the ETAM.