利用三維空氣空缺增強氮化銦鎵多重量子井太陽能電池之光吸收

Abstract

氮化銦鎵太陽能電池是有潛力可以作為高轉換效率的太陽能電池。然而，由於晶格不匹配引起的應力會造成極化電荷場，抵抗內建電場的強度，降低電池的效率。另外，我們利用多重量子井的太陽能電池取代傳統高銦含量的主動厚吸收層，以提升結晶的品質。但是量子井的主動吸收層很薄，會使得吸收的入射光不足。因此，減少應力在電池中的影響，並且增加光性的吸收，是我們改善的目標。從之前的研究中，空氣空缺結構在發光二極體裡已經有很好的效果。因此，我們將利用低成本的方法製作三維陣列大面積的空氣空缺的結構應用於太陽能電池中。 在第一部分中，我們使用FDTD法模擬光波在空氣空缺太陽能電池的分佈。從場效圖，我們可以發現空氣空缺層提供有效的高反射和散射，使得更多的光反射到主動吸收層。和沒有空氣空缺的結構相比，在穩態下有空氣空缺的元件在模擬計算的吸收提高了12.2％。所以我們發現如果我們製作更多的空氣空缺層，則會讓第一層沒有反射的光再經由第二層或第三層反射。因此我們製作了三層、兩層和單層的空氣空缺基板和一般的基板做比較。 從SEM圖，我們成功的結合奈米小球微影技術和磊晶技術製作出三層、兩層和單層的空氣空缺基板。從反射率的頻譜可以發現，有空氣空缺的基板反射率高於沒有空氣空缺的基板，隨著空氣空缺層數目的提升，越來越多的光從基板反射出來，所以反射率提升。在不同角度入射光的反射頻譜我們可以發現，有空氣空缺的基板在大角度的反射率都有提升，而且光在不同角度下反射率的差異也變小，使得光在各個角度的反射率更均勻。 我們將空氣空缺基板製作成有p-n接面的元件。利用拉曼光譜去分析應力釋放的狀況，從位移的頻譜可以發現應力釋放的效果，減少極化效應對內建電場的影響，對於元件的電性會有很好的效果。接著再分析入射光吸收頻譜，三層空氣空缺的元件在主動吸收層吸收波段內有最高的吸收值。再根據入射光吸收頻譜對三層、兩層和單層的空氣空缺元件計算而得的短路電流，和沒有空氣空缺的元件相比分別達到了6.75％、18.1％和24.1％提升。這個結果符合我們的模擬結果。因此，三維空氣空缺的基板有助於作為高效率的氮化銦鎵太陽能電池。

InGaN solar cell is nominated to be future generation of high power conversion efficiency photo-voltaic devices. However, the induced strain due to lattice mismatch will degrade the performance of cell. In addition, we utilize multiple quantum wells solar cell to instead of conventional bulk intrinsic layer to achieve high crystal quality in high indium content. But the thickness of quantum well is so thin that insufficient light absorbed by active absorption layer. Therefore, reducing the stress in the epitaxial wafer and enhancing light harvesting in the active layer are the main targets that we are going to improve. From previous research, air-void structure had the advantages in LED applications. Therefore, we demonstrate the air-void structure would benefit in photo-voltaic devices, and the air-void structures are improved to three-dimension array with large area by low-cost process. At the first part, we use finite-difference time-domain (FDTD) method to simulate optical distribution in our device. From the wave-field diagram, we can find that the air-void layer provides high reflection and scattering for transparent light from the active layer. Due to more light bounce back to the active layer, the simulated absorption of the air-void u-GaN device was enhanced by 12.2% compared to no air-void one in the steady state. Therefore, we demonstrate if we fabricate more air-void layer, they will provide more reflectance light. We fabricated triple, double and single three-dimensional air-void layer in the u-GaN substrate in practical. From the SEM images, we successfully fabricated triple, double and single air-void layer in the u-GaN substrate by combining nano-sphere lithography and epitaxial techniques. As the air-void u-GaN substrate, the reflectance is higher than no air-void u-GaN substrate and the reflectance arises up with increasing number of air-void layer in reflectance spectrum. Because air-void provides refractive index mismatch causing high reflection and nano-structure provides light scattering effect. The air-void substrate showed the omni-directional reflectance enhancement in angle-dependent reflectance spectrum. Thus, the air-void u-GaN substrate can reflect the light which incident from different angles. As the p-i-n layers grown on the air-void substrate, we can find the strain released in Raman spectrum. If we reduce strain effect, the piezo-electric polarization would decrease result in increasing the carrier collection. Furthermore, according to the absorption spectrum, triple air-void u-GaN device has highest absorption in the active absorption wavelength region compare than others. The estimated Jsc of single, double and triple air-void devices calculated from absorption spectrum achieved 6.75%, 18.1% and 24.1% enhancement, respectively. The result is corresponding to our simulation. Therefore, air-void u-GaN substrate provides not only strain released but also broadband enhancement in reflection which can be applied in high efficiency InGaN solar cell.