應用仿生抗反射結構於高效率三五族多接面太陽能電池

Abstract

我們成功利用了奈微米球微影術(Nanosphere lithography, NSL)製作大面積氮化矽基的次波長抗反射層結構應用在多接面三五族太陽能電池上，並利用乾式非等向性反應式離子蝕刻的參數調整達到形貌控制的效果進而比擬蛾眼結構，比傳統四分之一波長厚度的抗反射層更具有寬頻譜範圍的抗反射能力，除了500nm~700nm為傳統單層抗反射層的優勢之外，都可普遍使反射率大幅降低。由於在紫外光與近紅外光的抗反射特性，應用在磷化銦鎵/砷化鎵/鍺三接面太陽能電池上，相較於沒有製作抗反射層以及傳統單層抗反射層的太陽能電池元件，分別提升了26.74%以及3.48%的光電轉換效率。 本篇論文的第一部分討論如何製作仿生氮化矽基的次波長抗反射結構。我們採用旋塗法搭配奈微米球微影術這種大面積、便宜又快速的方法，加上非等向性蝕刻達到控制次波長形貌的效果，並從量測發現其具有優異的全波段的抗反射特性，以及利用RCWA設計與實驗結果相符的模型，進一步預測在製程可行的範圍，對不同的結構高度、球徑作抗反射波段及反射率的模擬。在第二部分中，我們將此種次波長結構成長於磷化銦鎵/砷化鎵/鍺太陽能電池的表面做為抗反射層，並自元件表面的反射率、外部量子效應、光電轉換效率等來進行元件特性的分析，並以變角度的效率量測證明次波長結構針對大角度入射光時仍能具有優異的抗反射能力；然而磊晶片中的窗口層與氮化矽間的差值形成了不可抗的極限，於是進一步利用高密度電漿化學氣相沉積矽統成長折射係數更高的富矽氮化矽，將其光學參數代入RCWA軟體作初步的模擬預測，結果仍受限於材料在吸光波段吸收係數的提高。

We have successfully fabricated large-scale SiNx-based sub-wavelength antireflective structures (SWS) for a Ga0.5In0.5P/GaAs/Ge triple-junction solar cell employing the polystyrene nanosphere lithography. The sidewall profiles of sub- wavelength structures were controlled by adjusting anisotropic etching parameters to mimic moth-eye structures. Compare to the conventional single-layer anti-reflective coating (SL ARC) which shows suppressed reflectance in 500 nm~700 nm wavelengths, the SWS ARC shows a broadband spectral response of reflection characterisitcs. Both the reflectance spectroscopy and external quantum efficiency measurements confirm the improved optical absorption in the ultraviolet/blue and near-infrared wavelengths. Hence, the conversion efficiency of the GaInP/ GaAs/Ge triple-junction solar cell is enhanced by 26.74% and 3.48% due to much improved current-matching, compare to cells without an ARC and SL ARC, respectively. In the first section of my thesis, I discuss how to fabricate biomimetic SiNx antireflective structures. An optimized spin coating method was employed for the large-area deposition of polystyrene nano-spheres, followed by anisotropic reactive-ion etching of the SiNx layer. Broadband reflectance spectra of SiNx SWS were obtained using an UV/Visible/NIR spectrophotometer with a built-in integrating sphere. Calculations based on a rigorous coupled-wave analysis not only verified the broadband antireflection characteristics, which were consistent with the measured data, but also served as an optimization tool for the implementation of sub-wavelength antireflective structures in multi-junction solar cells. In the second session, the Ga0.5In0.5P/GaAs/Ge solar cell without any ARC, with SL ARC and SiNx-SWS were characterized under the AM 1.5g illumination condition and by external quantum efficiency (EQE) analysis. The omnidirectional property of SWS was demonstrated via angle-resolved current-voltage measurement. The refractive index difference between SiNx (~1.8) and Al0.5In0.5P(>3) ultimately limited the anti-reflectance characteristics of SiNx structures. Further improvements may be achieved with high-refractive index materials, such as Si-rich nitride (n>2.3), but currently are limited by the increase of absorption coefficient in the UV wavelengths.