Optical Enhancement by Textured Back Reflector in Amorphous and Nanocrystalline Silicon Based Solar Cells
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Optical Enhancement by Textured Back Reflector in Amorphous and Nanocrystalline Silicon Based Solar Cells Baojie Yan1, Guozhen Yue1, Chun-Sheng Jiang2, Yanfa Yan2, Jessica M. Owens1, Jeffrey Yang1, and Subhendu Guha1 1 United Solar Ovonic LLC, 1100 West Maple Road, Troy, MI, 48084 2 National Renewable Energy Laboratory, 1617 Cole Blvd, Golden, CO, 80401
ABSTRACT We present the results of systematic studies of optical enhancements by textured Ag/ZnO back reflectors in a-SiGe:H and nc-Si:H solar cells. First, the back reflector materials were characterized by AFM, XRD, and TEM. The results showed that the ZnO layers deposited by sputtering exhibit an increased surface texture with the deposition temperature and film thickness. The material structure showed a strong (002) preferential orientation. The large columnar crystal structure determines the surface texture. Second, the solar cell performance was correlated to the back reflector structure. We found that with a thin ZnO layer, a textured Ag layer results in more light scattering than a flat Ag layer. However, when a thick ZnO layer is used, a flat Ag layer can produce similar or more light scattering than a textured Ag layer. Third, we developed a method to estimate the optical enhancement for a-SiGe:H and nc-Si:H solar cells on various structures of Ag/ZnO back reflectors. Comparing the quantum efficiency data from solar cells made using the same recipe but one on a flat stainless steel substrate and another on a textured Ag/ZnO BR substrate revealed that the optical enhancement for the long wavelength light can be as high as 20 to 30. Compared to the theoretical value of 4n2, there is still scope for further improvement. INTRODUCTION High efficiency solar cells need not only high quality active semiconductor layers, but also inactive contact layers such as top transparent contact and bottom back reflector. Al/ZnO and Ag/ZnO back reflector coated stainless steel substrates are normally used for high efficiency a-Si:H, a-SiGe:H, and nc-Si:H nip solar cells [1]. In recent years, the light trapping effect by textured back reflectors has received great attention and becomes a critical topic in the thin film solar cell community [2-4] as evidenced by this symposium. At United Solar, we continue to optimize the back reflector properties because any extra enhancement of light trapping will result in an improvement of cell efficiency [5]. In order to reduce the absorption in the substrate and enhance the reflection, metal layers such as Al and Ag are used as a back reflector, and the metal surface is textured for enhanced light scattering. In addition, a dielectric layer such as ZnO is used for additional texturing and more efficient light scattering. Furthermore, it has been reported that a flat metal layer and a textured ZnO layer may be the best combination because the flat metal surface can reduce the plasmon absorption at the metal/dielectric interface [6]. Although a significant amount of work has been done on the optimization of the light trapping
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