Geometry Analysis of Surface Anti-reflection Structures for Solar Cells

抗反射在太陽能電池領域中是一重要的課題，而表面粗化為其中一種方法，在電池的上表面蝕刻出週期性的幾何結構，且其大小需大於光波長數倍，主要的效果為增加光在結構中的反射次數，進而使光入射的機會增加，減少光在電池上表面的反射率。 在本論文中，我們針對兩種主要的粗化結構，V型溝槽及正金字塔陣列作分析，推導出能描述反射次數和路徑的分析式。其中光源採平行光，並考慮垂直入射和傾斜入射結構，在傾斜入射V型溝槽時再細分成平行週期與平行溝槽傾斜兩種情況，推導過程除了使用幾何光學外，也引入座標轉換。而從V型溝槽的分析中，我們更發現正金字塔陣列可由V型溝槽來近似，使得正金字塔陣列的分析式因此簡化，只需要平行週期與平行溝槽傾斜入射V型溝槽的線性組合即可。 我們也從分析式計算出減少的反射率，並與商業軟體TracePro作比較，驗證分析式的正確性，再與未作表面粗化的平板結構作比較，從中可確定使光線平行溝槽傾斜入射V型溝槽為抗反射效果最好的方式，能有效降低光在太陽能電池上表面的反射率，且高傾斜入射角的反射率也能降至5%以下。最後再將分析式擴展到與折射率隨光波長變化的intrinsic silicon材質，以及計算加入抗反射薄膜後的反射率，並從中得到了最佳的薄膜厚度。Surface texturing is a method to make anti-reflection in solar cells. To form geometric structure on top surface of solar cells by etching can increase number of reflections and reduce reflectivity. In addition, the scale of structure must be several times larger than light wavelength. In this paper, we analyze two texturing structures of V-groove and upright pyramid array. And we derive analytical formula which can describe number of reflections and optical path. In analyzation we use collimated light to be light source and let rays incident texturing structures normally and obliquely. For oblique incidence on V-groove, we define two conditions according to plane-of-incidence of incident lights which are lying in x-y and y-z plane. Above analyzations we use geometric optics and coordinates transformation of theories. In analytical process, we know that upright pyramid array can be formd by V-groove approximately. This idea let we simplify the analyzations of upright pyramid array. We also calculate reduct reflectivity from analytical formula. And we compare its solutions with simulation results of TracePro to check the accuracy of analytical formula. We get the results that oblique incidence on V-groove in y-z plane has less reflectivity and it is below 5% as larger tilted angles. Finally, we extend analytical formula to other complex conditions. Like intrinsic silicon which refractive index changes with light wavelength and adding thinfilm on surfaces of texturing structure. We can also get optimal thickness by calculating reflectivity of adding thinfilm