## 1 Introduction  
Thin-film solar cells have been greatly studied as one of the promising alternatives to traditional silicon-based solar cells due to their potential to low cost and high efficiency [1]. Thin-film solar cells have been fabricated in various absorption materials such as amorphous silicon, organic compound, and Cu(In, Ga)Se\({}_{2}\) (CIGS) [2; 3; 4]. Among them, the CIGS solar cell has reached a power conversion efficiency of beyond 20% owing to high optical absorption coefficient, wide absorption spectrum, and long material stability [5]. Furthermore, it has reached a power conversion efficiency of 23.35% with double buffer layers [6]. Generally, thin-film solar cells, including the CIGS solar cell, comprise thin multilayer on the order of sub-micrometer and a thick substrate on the order of one millimeter. Because of the optical interference effect, the absorption efficiency of the thin-film solar cell is greatly affected by the nano-meter-order thickness of the multilayer structure. Thus, optimizing the thickness of each thin layer based on accurate optical modeling is crucial to maximize the optical absorption in the active region and to improve the power conversion efficiency. In the optical modeling of the thin-film solar cell, the coherence length of sunlight (-0.6 \(\upmu\)m) [7], within which the phase information of sunlight is preserved, should be carefully considered. The sub-micrometer multilayer, covering an absorptive active region,a carrier transport layer, and a transparent electrode, can be modeled as optically coherent layers, where their reflectance and absorbance are affected by optical interference effect [3]. In contrast, the millimeter-order thick substrate has to be treated as an optically incoherent layer, within which the phase of light is randomly changed at each coherence length and optical interference effect disappears due to the averaging of optical responses over the random phase changes [8; 9]. To calculate the reflectance or absorbance of this mixed coherent-incoherent thin-film solar cells, various analytical methods have been used for planar structures. In the generalized transfer matrix method (GTMM), the propagation behavior of the electric field intensity in planar incoherent layers was separately described by matrix formalism similar to the transfer matrix method (TMM) used in coherent planar multilayers [10; 11]. In the case of the random phase method (RPM), coherent calculation results obtained by the TMM were averaged over thousands of random phase shifts added to a selected incoherent layer [12]. To reduce the computation time, coherent simulation results with respect to a set of equidistant phase shifts added to an incoherent layer were averaged in the equidistant phase method (EPM) [13]. Using the GTMM, our group successfully calculated and analyzed the optical absorption characteristics of a planar thin-film organic solar cell having an incoherent thick glass substrate in both normal and oblique incidence of sunlight [14; 15]. Light absorption efficiency of planar thin-film solar cells can be improved by use of surface-textured structures, of which absorption characteristics cannot be calculated based on the above-mentioned analytical methods applied to planar structures. In this case, the numerical methods of the rigorous coupled wave analysis (RCWA) [16], the finite-difference time-domain (FDTD) [17; 18], and the finite element method (FEM) [19; 20; 21] should be used, where wave equations are directly solved in a small mesh or grid for optical modeling. Because these numerical methods provide only coherent simulation results, an additional averaging procedure such as the so-called \"spectral averaging method\" (SAM) [9], the first-principle calculation [22], and \"one-pass coherent calculation\" [23] is required to include the incoherent characteristics of light propagation. However, these methods have very complicated mathematical and computational procedures, which result in a long computation time and implementation complexity [21].