用於太陽電池之氮化矽次波長抗反射結構設計與樣品試製

Design and Fabrication of Sub-wavelength Structures on Silicon Nitride for Solar Cells

Kartika(Sahoo, Kartika Chandra)

指導教授 ： 張翼

國立交通大學/工學院/材料科學與工程系所/博士(2009年)

https://doi.org/10.6842/NCTU.2009.01115

摘要

本論文首先以數值模擬計算太陽電池之氮化矽次波長抗反射結構。根據理論的分析，吾人進而發展樣品試製方法去製作氮化矽次波長結構。氮化矽是一種常用於半導體上的反射結構層材料，吾人藉此次波長結構當成第二抗反射層，使得完整樣品的結構為雙層抗反射層。此種雙層抗反射結構成本低並具有不錯的太陽電池的光、電特性。

模擬上我們是使用嚴謹的波偶合法進行反射率計算，研究上吾人是對一理想化的金字塔型的氮化矽次波長抗反射結構進行反射率對照射波的計算。其中單層抗反射層、雙層抗反射層皆一一加以分析比較，進而找出較佳的幾何結構比，及其伴隨的有效反射率。藉由理論分析的結果，吾人運用自組裝鎳奈米粒子以及誘導式電漿離子偶合蝕刻提出一個簡單且可控制比例的製程步驟。藉此法在矽基板上以鎳薄膜厚度去控制鎳奈米粒子的大小與密度。由於次波長結構的表面形成與反應式離子蝕刻的條件密切相關，因此研究上也探討誘導式電漿離子偶合蝕刻製程條件對於氮化矽次波長抗反射結構成長的影響。吾人藉由實驗已找出蝕刻時間與結構幾何比例以及反射率的關係。

總之，本研究已經完成適用於太陽電池之氮化矽次波長抗反射結構的理論與實驗研究。未來可以研究不同次波長結構形狀的製程以及反射率之相關議題。

關鍵字

太陽能電池 ； 次波長 ； 氮化矽 ； 樣品試製 ； 奈米 ； 刺激

並列摘要

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.

並列關鍵字

Solar Cell ； Sub-wavelength Structure ； Silicon Nitride ； Fabrication ； Nanocone ； Simulation

參考文獻

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