## 4 Discussion  
To investigate which wavelength affects the variation of J\({}_{\rm SC}\) in Figure 7 further, we calculate the EPL-thickness dependence of absorptivity in the CIGS absorber layer and external quantum efficiency (EQE), which are shown in Figure 8. The absorptivity in the CIGS absorber layer was obtained by integrating the optical power dissipation in the longitudinal z-direction over the CIGS absorber layer and was written as  
\[A_{\rm CIGS}(\lambda,t_{b})=\frac{1}{S(\lambda)}\iint\limits_{\rm CIGS}Q( \overrightarrow{r},\lambda,t_{b})d\overrightarrow{r}. \tag{19}\]  
The EQE was defined as the ratio of the number of charge carriers collected by the solar cell to the number of incident photons shining on the solar cell outside. In this calculation, the EQE spectra was expressed as  
\[EQE(\lambda,t_{b})=\frac{J_{\rm SC}(\lambda,t_{b})/q}{S(\lambda)/(hc/\lambda)}, \tag{20}\]  
Figure 7: (**a**) Calculated J–V characteristics of a CIGS solar cell depending on the equispaced phase thickness of the EPL along with the ETAM-applied calculation result. None of the calculation results of the J–V characteristics can be distinguished due to the extremely small difference among the eleven calculation results in the J–V scale. (**b**) Close view of the J–V characteristics near the zero applied voltage. The maximal percent deviation of J\({}_{\rm SC}\) calculated between the coherent and ETAM-applied models was \(\pm\) 0.54%, the value of which can be understood to be the calculation inaccuracy when the thick cover glass layer was inappropriately modeled as the coherent layer. where \(J_{SC}(\lambda,t_{b})\) indicates the EPL-thickness-dependent spectral short-circuit current density, which was obtained by the numerical solution of the coupled drift-diffusion and continuity equations when the incident optical power was set to the spectral irradiance of AM 1.5 sunlight \(S(\lambda)\). In Figure 8, the ETAM-applied calculation results were obtained by averaging 10 coherent calculation results over the equispaced phase thicknesses. According to the calculated EQE spectra in Figure 8b, the variation of the EQE with respect to the EPL thickness was noticeable around the wavelength range between 600 and 1000 nm. Figure 9a shows the ETAM-applied calculation results of absorptivity in the entire multilayer structure, the absorptivity in the CIGS absorber layer and the EQE, where a significant reduction of the EQE was observed within the relatively short (300-450 nm) and long (1000-1100 nm) wavelength ranges. First, the reduction of the EQE between 300 and 450 nm was ascribed to the optical absorption in the AZO-based TCO layer, which has a high extinction coefficient within the short wavelength range, as shown in Figure 2b. Hence, the difference in the absorptivity between the entire multilayer structure and the CIGS absorber layer becomes very high within the wavelength range of 300-450 nm. This fact was also confirmed by the ETAM-applied spatial profiles of the optical power dissipations shown in Figure 9b, where the optical power dissipation at the AZO layer was observed at wavelengths of 300, 350 and 400 nm. Second, the difference between the absorptivity and EQE in Figure 9a increases as the wavelength increases from 500 to 1100 nm. As shown in Figure 9b, this can be explained by the fact that the optical absorption, taking place farther away from the interface between the p-doped CIGS and n-doped ZnS layers at a longer wavelength, contributes less to the electric current at a longer wavelength due to the low diffusion length of the minority electron carriers [26]. In the actual module configuration of the CIGS solar cell, the front encapsulation layer, made of ethylene vinyl acetate (EVA), lies between the front cover glass and the TCO top contact layer [4]. Figure 10 shows a modified schematic diagram of the CIGS solar cell, where a 0.5-mm EVA encapsulation layer was added into the multilayer structure of the CIGS solar cell shown in Figure 1[17; 19].