利用光子晶體增加矽串疊型太陽能電池之效率

Abstract

隨著溫室效應越來越嚴重，綠能已成為未來重要的議題之一，而其中太陽能電池具有非常大的潛力。第二代薄膜型太陽能電池改善了太陽能電池製造費用昂貴的問題，但也犧牲了光電轉換的效率。目前有許多針對第二代薄膜型太陽能電池的相關研究，不過還是有很多改進的地方。 有很多方法可以改善薄膜型太陽能電池的效率，使用串疊型太陽能電池可以將吸光效率提升至二到三倍甚至更高。本篇論文探討如何再進一步的提升串疊型太陽能電池的吸光效率。我們所模擬的對象為矽串疊型太陽能電池，其上主動層為氫化非晶矽、下主動層為氫化微晶矽，適當設計兩材料中間的反射層可提高矽串疊型太陽能電池的總體吸收效率。首先我們預想中間反射層的反射頻譜的形狀為帶通濾波器時會有很好的效果，經過模擬以後證實不但可以提升矽串疊型太陽能電池的效率還減少了上主動層的厚度。在實作結構中，我們選用光子晶體當作中間反射層，因為其具有類似帶通濾波器的反射頻譜。在本論文中我們探討了一維光子晶體和二維光子晶體對矽串疊型太陽能電池的影響。 在一維光子晶體方面，我們以多晶氧化鋅和單晶氧化鋅交疊成十週期的高低折射率堆疊結構來改善矽串疊型太陽能電池的效率。固定氫化微晶矽的厚度為1.5 μm，將一維光子晶體各層厚度及氫化非晶矽的厚度利用基因演算法做最佳化來求出最大的吸光效率。在二維光子晶體方面，我們將透明的導電層銦錫氧化物中間挖圓形的空氣柱形成二維週期折射率分布的結構。二維光子晶體的好處是可以形成更大的折射率反差而有更大頻寬及峰值的反射率頻譜，不過也會形成更大的波瓣。改良方法是在二維光子晶體後面加一層銦錫氧化物，調整其厚度可以降低波瓣。固定氫化微晶矽的厚度為1.5 μm，將二維光子晶體的週期及銦錫氧化物的厚度做最佳化來求出最大的吸光效率。 我們利用傳遞矩陣法模擬利用一維光子晶體和二維光子晶體提升矽串疊型太陽能電池的吸光效率。一維光子晶體最佳化後可以提升13.5%的效率。二維光子晶體可以提升13.4%。二維光子晶體還有一項優點，當入射光不是垂直入射時，使用二維光子晶體作中間反射層的矽串疊型太陽能電池吸光效率較一維光子晶體還高。本論文所提供的計算流程及方法不僅可以明顯的提升雙層串疊型太陽能電池的吸光效率，還可以推廣至三層或四層不同材料的組合上，因此有很大的應用價值。

In recent years, the green house effect has made the environment worse. Hence, the green energy became one of the most important issues. Solar energy is one of the most potential green energy. The 2nd generation thin-film solar cell solves the high-cost problem for the fabrication of conventional solar cells. However, the efficiency of the thin-film solar cells is still low. Even there are many research activities focusing on enhancing the efficiency of the thin film solar cells, it still can be improved. There are many ways to enhance the efficiency of thin-film solar cells. Using tandem solar cell structure, we can increase the absorptance of solar cell by 2 to 3 times or more. In this thesis, the enhancement of the efficiency of silicon tandem solar cells is studied. The top cell is a-Si:H and the bottom cell is μc-Si:H. A filter between these two materials is designed. From the simulation result, it is found that a band-pass filter can increase the aborptance of both the top and bottom cells. Besides, the thickness of the top cell can be reduced. Photonic crystals have the similar reflective spectrum as band-pass filter. In this study, 1D and 2D photonic crystals enhance the absorptance of silicon tandem solar cells are demonstrated. First, poly-ZnO and mono-ZnO are used as materials of 1D photonic crystals. There are one poly-ZnO layer and one mono-ZnO layer in a period and there are 10 periods in the simulation. Fixing the thickness of μc-Si:H as 1.5 μm, the thickness of 1D photonic crystals of all layers and a-Si:H are optimized by the genetic algorithm. Regarding 2D photonic crystals, using air cylinder within ITO, the contrast of the refractive indices is higher than 1D photonic crystals. This leads to wider and higher bandwidth in the reflection spectrum. However, the side-lobe is also high. To improve it, an ITO layer is added below the 2D photonic crystals. Similarly, fixing the thickness of μc-Si:H as 1.5 μm, the period of 2D photonic crystals and the thickness of a-Si:H are optimized. Besides, from the simulation result, 2D photonic crystals have better performance than 1D photonic crystals when the light is not normal incident. The transfer matrix is used to simulate the enhancement of the efficiency of silicon tandem solar cells by 1D and 2D photonic crystals. With 1D photonic crystals, the efficiency of silicon tandem solar cell is increased by 13.5%. With 2D photonic crystals, the efficiency of silicon tandem solar cell is increased by 13.4%, but it still can be improved. The concept and process proposed in this thesis for enhancing the absorptance of silicon tandem solar cell can also be applied to the tandem solar cells with three or more layers and with different materials.