## 3 Simulation Results
The ETAM-applied optical power dissipation, which includes the optically incoherent characteristics of the cover glass layer, lies in the middle of the 10 coherent calculation results. If the cover glass was considered optically incoherent, all the spatial profiles of the optical power dissipations calculated at 10 different equispaced phase thicknesses will be the same as the ETAM-applied calculation result. By comparing the aggregate sums of the optical power dissipations distributed at the CIGS layer between the coherent and ETAM-applied models, we obtain a deviation within the range of \(-9.65\%\) to \(+10.22\%\), which can be considered as the inaccuracy of the calculation when a thick cover glass was inappropriately modeled as the coherent layer. Figure 3: Calculated 2D spatial profiles of (**a**) the electric field amplitude, (**b**) time-averaged Poynting vector and (**c**) optical power dissipation at a wavelength of \(\lambda\) = 400 nm. The equispaced phase thickness of the EPL was set to \(t_{1}\) = 0 nm. The variations in the optical power dissipations depending on the equispaced phase thickness of the EPL were also calculated in Figure 4b when the wavelength of sunlight was 800 nm. The overall shapes of the calculation results in Figure 4b are comparable to those in Figure 4a. When the aggregate sums of the optical power dissipations positioned at the CIGS layer were compared between the coherent and ETAM-applied models, a deviation of \(-9.60\%\) to \(+10.17\%\) was obtained. It was noticeable that the calculated optical power dissipations in Figure 4b decrease more slowly from the ZnS/CIGS interface toward the CIGS layer than the calculated optical power dissipations in Figure 4a. This can be explained by the fact that the extinction coefficient of the CIGS layer, which can be seen in Figure 2b, decreases as the wavelength increases from 400 to 800 nm. Figure 5 shows the calculation results of the absorptivity spectra depending on 10 different equispaced phase thicknesses of \(t_{1}\), \(t_{2}\), \(\cdots\) and \(t_{10}\) along with the ETAM-applied absorptivity. According to Equations (4) and (6), the absorptivity corresponds to the accumulated sum of the optical power dissipation in the longitudinal z-direction at a given wavelength. As indicated in Figure 5, the absorptivity spectrum calculated at each equispaced phase thickness shows a respective ripple. This results from the fact that the wave vector of \(k_{0}=2\pi/\lambda\) and the complex refractive index in Figure 2 depend on the wavelength of light, which causes a wavelength-dependent optical phase change and an optical interference effect. Owing to the additional optical phase change induced by the different thicknesses of the EPL \(t_{b}\) in Figure 1, the spectral positions of the peaks and valleys were shifted in the absorptivity spectra calculated at different equispaced phase thicknesses of \(t_{1}\), \(t_{2}\), \(\cdots\) and \(t_{10}\). However, these ripples in the absorptivity spectra disappear in the calculation results obtained by the ETAM, where the calculation results of 10 coherently calculated absorptivity spectra were averaged to include the optically incoherent characteristics of the cover glass layer. For comparison, the absorptivity was also calculated based on the GTMM, which was considered another analytical model to include the incoherent effect of the cover glass layer. The numerical results based on the ETAM were in agreement with the analytical results based on the GTMM, which verifies the validity of the calculation results obtained using the ETAM. In addition, the ETAM-applied absorptivity spectrum has the average value among the 10 coherently modeled absorptivity spectra. At a wavelength of 400 nm, as shown in Figure 5, the variation of the coherently modeled absorptivity in reference to the ETAM-applied absorptivity was observed to be within the range of \(-9.52\%\) to \(+10.44\%\), which was close to the range of the deviation of the optical power dissipation shown in Figure 4a.