## 1 Introduction
Recently, we proposed a so-called \"equispaced thickness averaging method\" (ETAM) as a simple and efficient optical modeling method to calculate the incoherent characteristic of surface-textured thin-film solar cell [20]. In the numerical simulation of the ETAM, a millimeter-order incoherent layer could be converted into a micrometer-order coherent layer with a set of equispaced phase thicknesses and corresponding coherent simulation results were averaged. Using the ETAM, we successfully calculated the effect of the three successive incoherent layers on the absorption characteristics of a surface-textured CIGS solar cell in only normal incidence of sunlight [21]. The above-mentioned numerical modeling methods to consider the mixed coherent-incoherent characteristics of surface-textured thin-film solar cells have been performed under the condition that sunlight is normally incident from the air, which did not reflect a realistic deployment situation of thin-film solar cells. In a general deployment situation such as a rooftop application, the incidence angle of sunlight gradually varies during the daytime such that an optimized device structure of the surface-textured solar cell has to be determined in the consideration of the variation in the incidence angle and polarization of sunlight [15]. Moreover, in a realistic module configuration, millimeter thick front encapsulant and cover glass layers, which can provide a higher mechanical durability and operational stability, are deposited on the top contact layer. These front optically-incoherent encapsulation layers will affect the light absorption and power conversion efficiencies of surface-textured thin-film solar cell, which also has to be considered to obtain the accurate optical modeling results. However, the effects of both incoherent encapsulation layers and oblique sunlight incidence have not yet been simultaneously considered on the optical modeling of surface-textured CIGS solar cells. In this paper, we calculate the optical and current-voltage (J-V) characteristics of surface-textured CIGS solar cells by simultaneously considering the front incoherent encapsulation layers and oblique sunlight incidence. We propose an angle-dependent equispaced thickness averaging method (ADETAM) that can calculate the incident-angle-dependent optical characteristics of the mixed coherent-incoherent solar cell by modifying the currently-used ETAM only applicable to the normal sunlight incidence. The layer structure of the CIGS solar cell includes two successive front encapsulation layers of a cover glass and an ethylene vinyl acetate (EVA), whose respective thicknesses are greater than the coherence length of sunlight (\(\sim\)0.6 \(\upmu\)m). We verify the calculation accuracy of the ADETAM in a planar CIGS solar cell through a comparison with exact analytical results obtained by the GTMM. In a surface-textured CIGS solar cell, the reflectance spectra and J-V characteristics depending on the incidence angle and polarization of sunlight are calculated and compared when two successive encapsulation layers are included or not in the simulation. According to the calculation results, the output performance enhancement of the surface-textured CIGS solar cell over the planar CIGS solar cell can be over-predicted when the thick encapsulation layers are not considered in the optical modeling.