## 3 Optical Modeling Results  
### Planar CIGS Solar Cells  
Figure 2a shows a schematic diagram of the planar CIGS solar cell used in this calculation [24]. The sunlight with the incidence angle of \(\theta_{0}\) is incident from the air to the CIGS solar cell, which can be considered as a mixed incoherent-coherent system. The spectral irradiance of sunlight is assumed to follow the air mass (AM) 1.5 sunlight with a total power of S\({}_{0}\) = 100 mW/cm\({}^{2}\), which is shown in inset of Figure 2a. Because sunlight is unpolarized, the incidence optical powers of s- and p-polarized light are equally distributed at each incidence angle of \(\mathbf{\theta_{0}}\). The CIGS solar cell is composed of a 3 mm cover glass at the top, a 0.5 mm EVA layer as a surface flattening layer, 50 nm aluminum-doped zinc oxide (AZO) as a TCO, 10-nm zinc sulfide (ZnS) as an n-doped layer, 2 \(\upmu\)m CIGS as a p-doped active layer, and 400 nm molybdenum (Mo) as a bottom contact layer [24]. The bottom glass substrate and EVA layer are not included in this calculation because they have no effect on the optical and electrical characteristics of the CIGS solar cell. Figure 2b shows the complex refractive index spectra of the materials used in the CIGS solar cell, which are taken from the literature (CIGS, ZnS, AZO, and Mo) [25; 26; 27]. It is well known that the complex refractive index of semiconductor materials depends on doping concentration. In particular, the variation of Al dopant concentration significantly changes the complex refractive index of AZO thin films [28; 29]. In this study, the donor concentration of the ZnS and acceptor concentration of the CIGS are set to be \(5\times 10^{16}\) cm\({}^{-3}\) and \(3\times 10^{16}\) cm\({}^{-3}\), respectively [24]. According to the complex refractive index spectra of the AZO shown in Figure 2b, the Al concentration of the AZO is speculated to be 2 wt% in reference to the measured complex refractive index spectra of the AZO in [28]. In this calculation, the front encapsulation layers of the cover glass and EVA are assumed to be transparent at the wavelength range between 300 and 1100 nm. In general, the cover glass and EVA has high absorption at the wavelength range between 300 and 350 nm [30; 31]. However, our assumption that the cover glass and EVA are transparent between 300 and 350 nm will not significantly change the calculation results of optical absorption at the ZnS/CIGS active layers, where the generated electron-hole pairs contribute to the short-circuit current density (J\({}_{\text{SC}}\)). The first reason is that the AZO, of which complex refractive index spectrum is shown in Figure 2, also has very high extinction coefficient at the wavelength range between 300 and 350 nm. Hence, most of incident sunlight within these wavelength ranges is absorbed at the AZO TCO, which results in very low external quantum efficiency at the wavelength range between 300 and 350 nm [24]. Secondly, the amount of the AM1.5 sunlight irradiance at the wavelength range between 300 and 350 nm, shown in the inset of Figure 2a, is relatively lower than that of the AM1.5 sunlight irradiance at the other wavelengths, which indicates that the contribution of J\({}_{\text{SC}}\) generated at the wavelength range between 300 and 350 nm is relatively small. Both the 3 mm cover glass and 0.5 mm EVA layers, the thicknesses of which are greater than the coherence length of sunlight (\(\sim\)0.6 \(\upmu\)m), are treated as incoherent. Because the cover glass and the EVA flattening layers are assumed to be transparent, both layers are modeled as the 1 \(\upmu\)m thick initial layer plus the respective phase-shift layer whose thicknesses \(b_{1}^{q_{1}}\) and \(b_{2}^{q_{2}}\) are determined based on Equation (4) when the ADETAM is applied to calculate the effect of the incoherent front glass and EVA layers in the simulation. The remaining layers from the AZO TCO to the Mo bottom contact are considered as coherent.