以閃耀光柵與背面電場改善薄膜結晶矽太陽電池效率

Improving Thin-Film Crystalline Silicon Solar Cell Efficiency with Back Surface Field and Blaze Gratings

吳信毅(Shin-Yi Wu)

指導教授 ： 邱奕鵬

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2010年)

https://doi.org/10.6342/NTU.2010.03339

摘要

本論文中，我們利用程式建立太陽電池的數值模擬，模擬包含了基於波動光學的光吸收以及基於半導體電學的載子傳輸問題，並使用背面電場和閃耀光柵有效地改善薄膜結晶矽太陽電池的效率。
首先我們使用有限元素法以及名為PC1D的程式基於半導體電學計算太陽電池中的能帶結構和載子的傳輸，在一般的幾何光學假設下，可計算得光電流和元件效率。由模擬可知，使用背面電場可以有效的增加薄膜結晶矽太陽電池的效率，而且當元件有背面電場和不錯的光侷限能力時，可以使得元件厚度小於擴散長度時有最大的效率，因此可獲得最佳的元件厚度。此外，背面電場可以使得元件在厚度較小時，大範圍的波段有最大的載子收集效率，因此有效的增加短路電流，並且當厚度變小時，可以減小飽和電流密度，進而增加開路電壓。
接下來，我們使用有限元素法以及嚴格耦合波分析法基於波動光學計算太陽電池中的光吸收，並利用有效的光捕捉結構增加光在太陽電池中的吸收，在光捕捉結構中，我們提出在元件背面加上閃耀光柵，因閃耀光柵可以提供較大的高階繞射效率，因此可大幅增加光在元件中的等效路徑，進而增加光在元件中的吸收。另外，我們也模擬了已發表期刊的光捕捉結構與我們所提出的結構作比較，由結果可知，我們的光捕捉結構確實能較大地增加元件效率。又光柵所產生的繞射角度可由光柵週期決定，而繞射效率可由光柵的幾何形狀以及材料決定，所以我們可以藉由改變閃耀光柵的結構參數達到增加長波長波段的吸收，接著在理想的太陽電池電壓電流特性曲線方程式以及合理的數值假設下，可以利用模擬獲得的光吸收計算得到元件效率，由模擬結果，我們得到了最佳化的閃耀光柵的結構參數，並比較了有無閃耀光柵下太陽電池的光吸收與效率的增益，此外，閃耀光柵確實能明顯的增加薄膜結晶矽太陽電池光捕抓的能力以及大幅的改善長波長區域的光吸收。
最後我們利用程式建構結合波動光學和半導體電學的模擬，並用來計算太陽電池加上光柵的光電流和效率，接著我們分別考慮元件加上背面電場以及最佳化閃耀光柵的例子，並計算太陽電池所獲得的效率增益，由模擬結果可知，背面電場與閃耀光柵都可有效的改善薄膜結晶矽太陽能電池的效率，而背面電場和閃耀光柵分別主要地改善了元件的開路電壓和短路電流。

關鍵字

結晶矽太陽電池 ； 薄膜太陽電池 ； 背面電場 ； 光捕捉結構 ； 閃耀光柵

並列摘要

In this thesis, we use package softwares to develop numerical simulations of solar cells. The built numerical models evaluate the photon absorption based on wave optics and carrier transport behavior based on semiconductor physics. Based on the established model, we improve the efficiency of thin-film crystalline (TF-cSi) solar cells with the incorporation of back surface field (BSF) and optimized blaze grating structures.
First, we use finite element method (FEM) and a program called PC1D based on semiconductor physics to calculate the energy level distribution and carrier transport in solar cells. According to general geometric optics, photon-generated current and cell efficiency can be obtained by these programs. In the simulation, the application of BSF significantly increases the efficiency of TF-cSi solar cells. Moreover, the cell with BSF and efficient light-confinement mechanism has the maximal efficiency while the cell thickness is shorter than the diffusion length of carriers. Therein, the optimal thickness of cSi solar cells can be obtained. In addition, the cell with BSF has the maximal carrier collection efficiency in a wide wavelength range when cell thickness is thinner. Hence, short-circuit current density can be enhanced. The open-circuit voltage of devices is also increased due to the decreasing of saturation current density when reducing the cell thickness.
Next, we use FEM and rigorous couple-wave analysis (RCWA) method based on wave optics to calculate the photon absorption of silicon layer. With these simulation tools, we try to enhance the photon absorption by efficient light-trapping structure. For this light-trapping structure, blaze grating is used at the back surface. It is known that blaze grating has larger diffraction efficiency for high order diffraction, which results in larger equivalent optical path length in the silicon layer. Therefore, the photon absorption of devices is enhanced. In addition, we simulate a solar cell with a light-trapping structure which has been proposed in the literature and compare this structure with ours. According to the results, our structure has larger cell efficiency enhancement. Since the diffraction angle and efficiency of the reflective light depend on the grating period and the geometric form of gratings, respectively, it is feasible to enhance the absorption in the long wavelength range by varying the structural parameters of blaze gratings. With the introduction of $J-V$ characteristic of an ideal solar cell and reasonable physical parameters, the photon absorption is converted into the cell efficiency. With our calculation, the optimized structural parameters of blaze grating is obtained, and the photon absorption enhancement and the efficiency enhancement between cells with and without blaze grating are calculated and compared. Furthermore, the blaze grating greatly enhances the light-trapping ability of the TF-cSi solar cells and shows a significantly improvement of the photon absorption in the long wavelength range.
Finally, we use a program to develop the simulation that combines wave optics and carrier transport. We calculate the cell efficiency and the photon-generated current in TF-cSi solar cells with blaze grating. Then, we consider the case including both the optimized blaze grating and BSF and calculate the efficiency enhancement of these devices. In our simulation, both BSF and blaze grating can significantly improve the efficiency of TF-cSi solar cells. In addition, BSF and blaze grating mainly enhance the open-circuit voltage and short-circuit current density, respectively.

並列關鍵字

Crystalline silicon solar cells ； thin-film solar cells ； back surface field ； light-trapping structure ； blaze grating

參考文獻

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