Simulation of Rough Surface of CIGS (CuInGaSe) Solar Cell by RCWA (Rigorous Coupled Wave Analysis) Considering the Incoherency of Light

Abstract

The surface of semiconductor solar cells, such as a-Si or CIGS (CuInGaSe) solar cells is not flat but textured in the microscopic domain. With textured surfaces, the optical reflectivity of a solar cell is different from that of flat surfaces in the wavelength region. In this paper, the effects of a textured surface on a CIGS solar cell are presented by RCWA (Rigorous Coupled Wave Analysis) method. The effect of incoherent light is also considered by RCWA with a Fourier analysis while conventional optical simulation uses the input light on the solar cell as coherent light. Using experimental results, the author showed that the RCWA method with a Fourier analysis is a proper method to simulate the optical properties of CIGS solar cells.

Effects of Incoherent Front Cover Glass on Current-Voltage Characteristics of Cu(In,Ga)Se2 Solar Cells: Investigation into Calculation Accuracy for Cover Glass Modeled as Optically Coherent or Incoherent

“…3 of 16 Figure 1 shows a schematic diagram of the multilayer structure of the CIGS solar cell used in this study [17,19]. It comprises a cover glass (3 mm) at the top, aluminum-doped zinc oxide (AZO) (50 nm) as a TCO, zinc sulfide (ZnS) (10 nm) as an n-doped layer, CIGS (2 μm) as a p-doped layer and molybdenum (Mo) (400 nm) as a bottom contact layer.…”

“…The 3-mm cover glass, the thickness of which is larger than the coherence length of sunlight (~0.6 µm), is considered as incoherent, and the remaining layers are modeled as coherent. Because the 3-mm cover glass has no absorption, the incoherent cover glass can be modeled as a 1-µm initial cover glass layer plus an equispaced phase layers (EPLs) whose thickness t b is determined based on Equation (17) when the equispaced thickness averaging method (ETAM) is applied to calculate the effect of the incoherent front glass layer in the simulation.…”

“…If the number of equispaced phase thicknesses was set to 10 for both incoherent layers, the ETAM-applied optical power dissipation can be calculated by averaging the coherent calculation results Q( 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]. Because the thickness of the EVA layer (0.5 mm) was much larger than the coherence length of sunlight (~0.6 μm), the EVA layer should also be considered optically incoherent.…”

“…Here, the optical behavior of the electric field amplitude in the coherent multilayer was described by the 2 × 2 TMM, and the optical behavior of the electric field intensity in the incoherent layer was modeled by the 2 × 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].…”

“…By comparing the J-V characteristics of the CIGS solar cell calculated between coherently and incoherently modeled cover glass layers, we calculated the percent of deviation of the short-circuit current density (J SC ) and open-circuit voltage (V OC ), which can be considered to be the inaccuracy of the calculation when a thick cover glass layer is inappropriately modeled as a coherent layer. Figure 1 shows a schematic diagram of the multilayer structure of the CIGS solar cell used in this study [17,19]. It comprises a cover glass (3 mm) at the top, aluminum-doped zinc oxide (AZO) (50 nm) as a TCO, zinc sulfide (ZnS) (10 nm) as an n-doped layer, CIGS (2 µm) as a p-doped layer and molybdenum (Mo) (400 nm) as a bottom contact layer.…”