Optical Modeling of a-Si Solar Cells
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ABSTRACT We describe applications of PV Optics to analyze the behavior of a metallic back-reflector on an a-Si solar cell. The calculated results from PV Optics agree well with the measured data on solar cells. Several unexpected results obtained from these calculations are qualitatively explained. INTRODUCTION Modem high-efficiency amorphous-Si solar cells use fairly complex optical designs. Typically, these cells may have three junctions and as many as 12 layers of metal, dielectric, and semiconductor materials. Furthermore, they use many design features that are very difficult or even impossible to handle by a simple optical analysis. These features include (i) nonplanar interfaces, (ii) a combination of thick and thin layers, (iii) multiple semiconductors of different optical properties, and (iv) dielectric and metal coatings. More mature numerical analysis tools are needed to deal with the complicated optical design and analyses of these cells and modules. We have recently developed an optical software package, PV Optics that can handle many aspects of cell and module design. This paper describes some results of applying PV Optics to the design of a-Si solar cells. The intent of this short paper is not to deal with a comprehensive cell design, but to identify features of the software that can address unique cell-design issues. BRIEF DESCRIPTION OF PV OPTICS PV Optics is a software package developed for the design of thick as well as thin multilayer solar cells and modules. It can handle multilayer structures of highly absorbing semiconductor and metallic layers with planar and nonplanar interfaces. It uses combined features of ray and wave optics selected by coherence criteria. This package outputs a variety of data in a form that can be easily used by a design engineer. These data include plots of reflectance, transmittance, absorbed photon flux and its distribution within each layer, and the Maximum Achievable Current Density (MACD) from each active layer. MACD is the value of the photocurrent if each absorbed photon produces an electron-hole pair and all of the generated carriers are collected. Further description of the package is given elsewhere [1,2]. Here we will primarily use MACD as a measure of the cell performance. RESULTS AND DISCUSSION We first describe some capabilities of PV Optics that make it well suited for a-Si solar cell analysis and design. We show that if the optical parameters of a cell are known, the theory can yield results that are in good agreement with the experimental data. Next, we discuss examples related to back-reflector design, and arrive at some physical insight into the back-reflector optics. 755
Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
The examples described here are not meant to be exhaustive, but are selected to illustrate mechanisms that must be considered in analysis of back-reflector design of thin devices. 1. Comparison of theora and experiment Previous publications have shown that for thick, wafer-based Si solar cells, the calculated r
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