Development of optimized n-μc-Si:H/n-a-Si:H bilayer and its application for improving the performance of single junction a-Si solar cells

Abstract

Hydrogenated microcrystalline silicon (μc-Si:H) films play different types of roles in the fabrication of single and multi-junction amorphous silicon (a-Si) solar cells and also act as an active layer in microcrystalline cells which forms the bottom cell of micromorph solar cells. The detailed study of doped microcrystalline layers have not been done properly and there is uncertainty about the role of n-microcrystalline layer on the performance of single junction a-Si solar cells (Poissant et al., 2003). In this paper we have reported the results of deposition of n-type hydrogenated microcrystalline silicon (n-μc-Si:H) layers with different deposition parameters. The deposition has been done by Plasma Enhanced Chemical Vapor Deposition (PECVD) technique at 13.56MHz frequency. The detailed characterization of the films include the following: (1) electrical properties by activation energy, dark and photo conductivity, (2) optical properties like band gap and E04 (3) structural studies which include crystalline fraction by Raman spectroscopy and grain size by X-ray diffraction and AFM, FE-SEM, TEM studies. The interpreted results are also presented.The n-μc-Si:H layer has been introduced as the n-layer of single junction p–i–n structure a-Si solar cells. By various techniques the optimum use of n-μc-Si:H layer for enhancing the performance of a-Si solar cells has been done. It has been found that by using suitable bilayer of n-μc-Si:H layer/(thin) n-a-Si:H layer it is possible to increase the fill factor of the solar cell by 5%. This may be due to the higher conductivity of the n-μc-Si:H layer and the quality of intrinsic layer. It is also possible to increase the open circuit voltage by 3.3%. The maximum efficiency obtained without back reflector is 9.46% that is about 14.3% higher than that obtained by using n-a-Si:H layer as n-layer in the solar cells.