Design and Fabrication of Sub-wavelength Structures on Silicon Nitride for Solar Cells
Sahoo, Kartika Chandra
Chang, Yi Edward
|關鍵字:||太陽能電池;次波長;氮化矽;樣品試製;奈米;刺激;Solar Cell;Sub-wavelength Structure;Silicon Nitride;Fabrication;Nanocone;Simulation|
In this dissertation, we numerically study the reflectance of sub-wavelength structures on silicon nitride for solar cell application. Based on the numerical study, we develop a fabrication method to form the sub-wavelength structures on silicon nitride surface for solar cells. Since silicon nitride is a well known antireflection coating used in semiconductor industry, we explore the texturization on silicon nitride antireflection coating and its optical properties. The main motivation behind this lies in the fact that the sub-wavelength structures will act as a second antireflection coating layer with an effective refractive index so that the total structure can perform as a double layer antireflection coating layer. Thus, we could cost down the deposition of second antireflection coating layer can be saved with better or comparable performance as that of a double layer antireflection coating solar cell. In this study, we calculate the spectral reflectivity of pyramid-shaped silicon nitride sub-wavelength structures. A multilayer rigorous coupled-wave approach is advanced to investigate the reflection properties of silicon nitride sub-wavelength structure. We examine the simulation results for single layer antireflection and double layer antireflection coatings with sub-wavelength structure on silicon nitride surface, taking into account effective reflectivity over a range of wavelengths and solar efficiency. The results of our study show that a lowest effective reflectivity of 3.43% can be obtained for the examined silicon nitride sub-wavelength structure with the height of etched part of silicon nitride and the thickness of non-etched layer of 150 nm and 70 nm, respectively, which is less than the results of an optimized 80 nm silicon nitride single layer antireflection coating (~ 5.41%) and of an optimized double layer antireflection coating with 80 nm silicon nitride and 100 nm magnesium fluoride (~5.39%). 1% cell efficiency increase is observed for the optimized Si solar cell with silicon nitride sub-wavelength structure, compared with the cell with single layer silicon nitride antireflection coatings; furthermore, compared with double layer antireflection coated solar cell, the increase is about 0.71%. The improvement on the cell efficiency is mainly due to lower reflectance of silicon nitride sub-wavelength structure over a wavelength region from 400 nm to 600 nm that leads to lower short circuit current. Based upon our theoretical calculation of improved efficiency of silicon solar cell with silicon nitride sub-wavelength structures, we have developed a simple and scalable approach for fabricating sub-wavelength structures on silicon nitride by means of self-assembled nickel nano particle masks and inductively coupled plasma ion etching. The size and density of nickel nano particles are controlled by the initial thickness of nickel film that will be annealed to form the nano-particles on the silicon nitride film deposited on the silicon substrate. Inductively coupled plasma etching time is responsible for controlling the height of the fabricated silicon nitride sub-wavelength structure on silicon substrate. Nevertheless, the surface profile of a sub-wavelength structure is strongly dependent on the conditions of the reactive ion etching process. So, we have also investigated the effect of inductively coupled plasma etching conditions on the profile of fabricated sub-wavelength structure on Silicon nitride antireflection coating layers. At last, we succeeded in fabrication of nanopillar structures and nanocone structures on silicon nitride surface by one step and two step inductively coupled plasma etching methods. The relationship of etching time with structure height and average reflectance spectra has been drawn. In summary, design and fabrication of sub-wavelength structures on silicon nitride antireflective surface was investigated for the first time. The structure height and non-etched part of silicon nitride has been optimized for lowest effective reflectance by theoretical calculation using rigorous coupled wave analysis method. Also the shape effect has been studied theoretically. Based on theoretical results, the nanopillar and nanocone structures on silicon nitride surface have been fabricated successfully using self-assembled nickel nano clusters and inductively coupled plasma etching method. The achieved low reflectance is believed to be useful to improve the efficiency of solar cells. Also, the preliminary results for a silicon solar cell has been obtained using silicon nitride sub-wavelength structure, which shows a great promise in improvement of efficiency compared with a single layer antireflection coating.
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