Development of a novel fluorinated n-nc-SiO:H material for solar cell application

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In this paper we report the development of fluorinated nanocrystalline SiO:F:H materials prepared by Plasma Enhanced Chemical Vapour Deposition (PECVD) technique, the characteristics of which are better than previously developed material like ZnO, n-μc-SiOx:H for application as back reflector for a-Si (single and multi-junction) solar cells. For a good Back Reflector the material should have low refractive index which increases reflectance of the layer.Earlier we have developed n-type microcrystalline hydrogenated silicon oxide (μc-SiO:H) material by the RF-PECVD method (13.56 MHz) from source gas mixture of silane (SiH4), carbon dioxide (CO2), hydrogen (H2) having the refractive index of 2.1. By adding fluorine to the gas mixture the refractive index is reduced to 1.70 which will make the material a better reflector for a-Si solar cells.The material has been characterized in detail by the following studies, viz: (1) structural studies by HRTEM, AFM, Raman and FTIR spectroscopy and (2) optoelectronic properties using temperature dependent conductivity measurements and (3) Ellipsometry and UV-VIS spectrum analysis etc.The optimized material shows superior properties than μc-SiO:H thin film. The refractive index is 1.85, E04 = 2.61 eV, lateral electrical conductivity is 10−8 S cm−1 and Raman crystallinity is Xc = 48.0%.These tailored properties make this material most suitable for use as Back Reflector Layer (BRL) in a-Si solar cells, and also as Internal Reflection Layer (IRL) in Micromorph solar cells.