## 1 Introduction  
Thin-film solar cells based on a Cu(In,Ga)Se\({}_{2}\) (CIGS) absorption material have been intensively investigated as an auspicious alternative to traditional silicon-based solar cells owing to their low cost and high efficiency [1; 2; 3]. Because of their high optical absorption coefficient, CIGS solar cells with a wide absorption spectrum and high stability reached a power conversion efficiency of 19.9% in 2008 [1] and beyond 20% in more recent years [2; 3]. In general, a CIGS solar cell consists of multiple sub-micrometer layers, which range from a metal back contact to transparent conducting oxide (TCO) top contact layers. In the module configuration, millimeter-thick front encapsulation and cover glass layers are added on the TCO top contact layer to implement glass-glass encapsulating technology, which can provide a higher mechanical durability and operational stability [4]. Because the coherence length of sunlight is approximately 0.6 \(\upmu\)m [5] (preserving the phase information of the optical field), the multilayer structure of a CIGS solar cell module can be treated as mixed coherent and incoherent layers in optical modeling. Inside the coherent multilayer structure,the thickness of which is much less than the coherence of sunlight, the optical interference effect plays an important role in the optical characteristics of the reflectivity and absorptivity [6]. By contrast, light experiences a random phase change at each coherence length, such that the optical interference effect, on average, disappears in the incoherent layer [7; 8]. Thus, this mixed coherent-incoherent multilayer structure of a CIGS solar cell greatly affects the absorption characteristics and has to be thoroughly considered in optoelectronic modeling. There have been several studies conducted on the optoelectronic modeling of a CIGS solar cell, which have provided the current-voltage (J-V) characteristics obtained through the combination of optical and electrical simulations [9; 10; 11; 12; 13]. However, the previous optoelectronic modeling results of CIGS solar cells have not included the incoherent front encapsulation or cover glass layers in the optical modeling. Instead, they assumed sunlight to be directly incident from the air into the multilayer structure of the CIGS solar cell. The analytical formulation of a \(2\times 2\) transfer matrix method (TMM) based on the thin-film optics was used to calculate the optical interference effect within the coherent multilayer structure, which ranged from the TCO top contact layer to the metal back contact layer [11; 12; 13]. Thus, the previous optoelectronic modeling methods were insufficient when considering the realistic aspect of the optical and electrical characteristics of a CIGS solar cell because the optical effect of the incoherent front layer was not considered. According to thin-film optics, the absorption spectrum of the mixed coherent-incoherent layer structure of organic solar cells was calculated based on the generalized transfer matrix method (GTMM). Here, the optical behavior of the electric field amplitude in the coherent multilayer was described by the \(2\times 2\) TMM, and the optical behavior of the electric field intensity in the incoherent layer was modeled by the \(2\times 2\) intensity matrix formulation [14; 15]. In numerical methods, such as rigorous coupled-wave analysis (RCWA) and finite element method (FEM), coherent simulation results (obtained through the numerical solution of the wave equation in a small mesh or grid) were averaged out to provide the incoherent characteristics of light propagation in a mixed coherent-incoherent multilayer structure [16; 17; 18]. The effects of the incoherent front layer on the reflection spectrum of the CIGS solar cell were calculated based on the spectral averaging method in combination with the RCWA [16; 17]. We recently used the equispaced thickness averaging method (ETAM) to efficiently calculate the effect of the incoherent front layer on the reflection spectra of a CIGS solar cell [19].