利用新穎材料提升氮化鎵太陽能電池的效率

Abstract

在本論文中，我們針對氮化鎵太陽能電池的反射率高於20%而導致效率不佳的問題來做一個改良，另一部份是針對氮化鎵太陽能電池短波段的入射光被表面的透明導電層所吸收而導致短波段光都浪費掉的問題來做一個改善，主要是針對這兩個問題而改進進而提升我們氮化鎵太陽能電池的效率。 本論文第一部分是在討論如何改善反射率高的問題，很多文獻顯示，我們如果在表面製作奈米結構可以達到降低反射率的效果。所以我們製作具有奈米結構的PDMS模，這種模製程可以大面積而且便宜方便，不需要像其他做奈米結構要在真空下面進行，而且我們發現這樣製作出來的PDMS模具有很好的抗反射的效果，而且不只這樣，這種模也具有讓光散射的效果，讓光在電池裡行徑路徑比較長，有利增加我們效率。之後，我們將此種具有降低反射效果的PDMS模應用在我們氮化鎵太陽能電池上，並針對元件之反射率、外部量子效應、光電轉換效率等來進行分析，並以變角度反射率量測來證明次波長結構在變角度下仍然有不錯的抗反射效果，因此我們觀察到全波段量子效率的提升，將此具有PDMS模的電池相較於沒有PDMS模的電池，提高了7.2%光電流增加與光電轉換效率7.1%的增強，有效驗證了PDMS模對於氮化鎵太陽能電池光電轉換效率之提昇是有幫助的。 本論文第二部分是在針對氮化鎵太陽能電池短波段的入射光被表面的透明導電層所吸收而導致短波段光都浪費掉的問題來做一個改善。因為短波長紫外光區域具有全太陽頻譜7%，如果我們可以好好利用這些光會對我們效率有很大的幫助。 所以我用了量子點(quantum dot)，我們選用400nm 的CdS 量子點，這種量子點會吸收在400以前的光進而再放射出400nm左右的可見光，可以被我們氮化鎵太陽能電池所吸收利用，我們是在做好的元件上面點上我們的量子點讓它在表面是一層膜，他就可以在ITO吸收短波長之前吸收我們的短波長的光，而放射出ITO吸不到的波段給我們電池使用，這樣就不會有之前提到的浪費的能量。之後我們將有點量子點的電池相較於沒有量子點的，提高了6.3%光電流增加與光電轉換效率7.2%的增強，有效驗證了量子點對於氮化鎵太陽能電池光電轉換效率之提昇是有幫助的。

The InGaN/GaN multiple quantum well solar cell was considered for future generation of photovoltaic applications due to the high efficiency. However, due to the high reflectance, and the high absorption by ITO at short wavelength region. The efficiency is limited. How to improve light harvesting is a very important issue in high efficiency InGaN/GaN multiple quantum well solar cell. Antireflection and photo collection are key techniques for efficiently harvesting solar photon. For the first problem, Several studies of nanostructure offer ultra-low reflectivity, another feature of nanostructure is diffraction of light, which get longer optical path length and then further enhance the light absorption. Here we use textured PDMS film to solve the problem. The advantages of using PDMS film are the low-cost, non-vacuum system and simple process (only spin coating and imprinting needed). PDMS film provides a refractive index gradient to serve as an anti-reflection layer. Extra benefits of light trapping and scattering can be added when the film is stamped with high texture pattern. We demonstrated the textured PDMS film is useful in harvesting solar photon and enhancing the power conversion efficiency of InGaN/GaN multiple quantum well solar cell. Compared with a flat InGaN/GaN multiple quantum well solar cell, the power conversion efficiency achieved 7.1% enhancement, and the Jsc achieved 7.2% enhancement respectively. Second part we will solve problem for the high absorption by ITO at short wavelength region. So the UV region light loss in the process. We use quantum dot(QD) to solve the problem. This CdS QD layer is capable of converting ultraviolet photons (λ<400nm) to the visible blue band. So we can spread the quantum dots on the cells, it can absorb the UV region light before ITO layer and converting to visible region light that ITO can not absorb. In this way, we can solve the problem mentioned before, just by adding some CdS quantum dots material. We demonstrated the quantum dots is serve as the down conversion center of the light in InGaN/GaN multiple quantum well solar cell. Compared with a InGaN/GaN multiple quantum well solar cell without quantum dots, the power conversion efficiency achieved 7.2% enhancement, and the Jsc achieved 6.3% enhancement respectively.