Nanotech 2011

Comprehensive Study on Reflectance of Si3N4 Subwavelength Structures for Silicon Solar Cell Applications Using 3D Finite Element Analysis

Z-L Lu
National Chiao Tung University, TW

Keywords: antireflection coating, Si3N4 SWS, 3D FE analysis, cylinder, right circular cone, square pyramid, transverse electric mode, incident angle, perfect matched layer


Solar cell is one of the most promising renewable energy technologies in order to relieve the impact of the climate change. In semiconductor based solar cells, electron-hole pairs are generated via absorption of impinging photons. Due to high refraction index of semiconductor materials, especially silicon, the incident sunlight power is largely reflected back, resulting in the reduction of light absorption and poor energy conversion efficiency. Antireflection coating (ARC) is mounted over absorption layers to lower reflectance of solar cells. Bases on the theory of impedance matching, single layer and multilayer of antireflection layer are proposed for reduced reflectance property; however, the resulting reflectance spectra meet the demand only within a narrow spectral domain. Sub-wavelength structure, whose dimensions are much smaller than the wavelengths of light, with ARC on the surface of solar cells can substantially improve the capability of light trapping, then achieving the enhanced efficiency, which was studied both numerically and experimentally recently [1-3]. Figs. 1(a) to 1(d) exhibit the solar cell panel with Si3N4 SWS and with single layer of antireflection (SLAR), fabricated by our group [4]. Compared to Si solar cell with SLAR, the efficiency of solar cell with Si3N4 SWS is better shown in the inset tableau of Fig. 1 [4]. Yet, the morphological effect on reflectance property is obscure. In this work, 3D FE analysis for the reflectance of Si3N4 SWS with three types of structural shapes was conducted for quantitative understanding of reflectance property. Proper selection on the boundary conditions can alleviate the computational load from simulating a whole ARC. The reflectance of Si3N4 SWS with shapes of cylinder, right circular cone, and square pyramid on the Si substrate is thus calculated. The analysis of reflectance spectrum with wideangle incidences of electromagnetic wave and the average reflectance with various heights are presented.

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