## 4 Discussion
Because the thickness of the EVA layer (0.5 mm) was much larger than the coherence length of sunlight (-0.6 \(\upmu\)m), the EVA layer should also be considered optically incoherent. The EVA layer was assumed to be transparent with a constant refractive index of 1.49 between 300 and 1100 nm [19]. According to the optical modeling based on the ETAM, the incoherent cover glass and EVA layers can be modeled as an initial 1-\(\upmu\)m layer plus the EPL layer, respectively. The optically incoherent characteristics of the cover glass/EVA layer can then be considered by averaging the coherent simulation results over all combinations of the equispaced phase thicknesses in two incoherent layers [19]. For instance,  
Figure 8: EPL-thickness dependence of (**a**) absorptivity in the CIGS absorber layer \(A_{CIGS}(\lambda,t_{b})\) and (**b**) external quantum efficiency (EQE). ETAM-applied calculation results were obtained by averaging 10 coherent calculation results over the equispaced phase thicknesses. the ETAM-applied optical power dissipation, which includes the effect of the incoherent cover glass/EVA layer, can be obtained by  
\[Q^{\text{ETAM}}(\overrightarrow{r},\lambda)=\frac{1}{M\times N}\sum_{b=1}^{N} \sum_{a=1}^{M}Q(\overrightarrow{r},\lambda,t_{a},t_{b}), \tag{21}\]  
where \(M\) and \(N\) indicate the number of equispaced phase thicknesses in the EVA and cover glass layers, respectively. If the number of equispaced phase thicknesses was set to 10 for both incoherent layers, the ETAM-applied optical power dissipation can be calculated by averaging the coherent calculation results \(Q(\overrightarrow{r},\lambda,t_{a},t_{b})\) over 100 combinations of the equispaced phase thicknesses in two incoherent layers. Figure 10: Modified schematic diagram of CIGS solar cell, where a 0.5-mm EVA encapsulation layer was added into the multilayer structure of the CIGS solar cell in Figure 1. Because the thickness of the EVA layer (0.5 mm) was larger than the coherence length of sunlight (\(\sim\)0.6 \(\upmu\)m), the EVA layer should also be considered optically incoherent. Because both a cover glass and EVA have no absorption, two incoherent layers can be modeled as a 1-\(\upmu\)m initial layer plus the EPL, whose respective thicknesses of \(t_{a}\) and \(t_{b}\) can be determined based on Equation (17) to include the effect of the incoherent cover glass/EVA layers on the optical characteristics based on the ETAM. Figure 9: (**a**) ETAM-applied calculation results of absorptivity in the entire multilayer \(A\), absorptivity in the CIGS absorber layer \(A_{CIGS}\) and EQE. (**b**) ETAM-applied spatial profiles of the optical power dissipations in the longitudinal z-direction at several wavelengths. Figure 11 shows the calculation results of the ETAM-applied absorptivity spectra and J-V characteristics of a CIGS solar cell having only a cover glass (Figure 1) or a cover glass/EVA as an incoherent layer (Figure 10). In Figure 11a, the absorptivity spectra calculated by the GTMM show a good agreement with those obtained by the ETAM, which verifies the validity of the calculation results obtained using the ETAM. There was a slight difference between the two calculation results of absorptivity because the difference in the refractive index between the cover glass (1.55) and EVA (1.49) was very small. Hence, the calculated values of J\({}_{\rm SC}\) between the cover glass/AZO/ZnS/CIGS/Mo (-34 mA/cm\({}^{2}\)) and cover glass/EVA/AZO/ZnS/CIGS/Mo (-33.91 mA/cm\({}^{2}\)) structures were very close and deviate by 0.26%. This indicates that the addition of the incoherent EVA encapsulation layer into the multilayer structure of the CIGS solar cell in Figure 1 will provide little change in either the optical or electrical calculation results shown in Figures 3-7, where only a cover glass was considered as the incoherent layer.