介電與金屬散射器在奈米光學之應用

Abstract

能量在奈米粒子之間的傳遞對於大部分的光學元件，如：太陽能電池、發光二極體、奈米粒子波導…等是非常重要的，首先在本論文中將討論透過各種不同的設計和最佳化，如：殼層包覆(core-shell wrapping)、超結構(supercells)等，相較於一般的奈米粒子鍊，可以大幅降低在奈米粒子間能量傳遞的消耗，增加能量傳遞的效率；並可發現在個別的最佳化結構下，介電奈米粒子的能量傳遞效應會比金屬奈米粒子要來的好。

第二部分將介紹介電奈米粒子在抗反射層上的應用，藉由系統性的探討各種結構，可以發現氮化矽-二氧化鈦或二氧化矽-二氧化鈦的推疊結構能提供非常低的反射率，但卻能擁有平坦的表面來降低表面複合速率以及開路電壓的衰減。透過漸變折射率(graded index)的米氏散射(Mie scattering)，可以獲得近百分之0.25的反射率，且總厚度也只有279.8奈米左右，遠低於傳統奈米針(nanotip)1.6 微米的厚度，而在有低反射率的特性下，也可同時提供良好的光捕捉(light trapping)能力

The energy transfer between nano-particles is of great importance for, solar cells, light emitting diodes, nano-particle waveguides (NPWG), and other photonic devices. This thesis will show through novel design and algorithm optimization, the energy transfer efficiency between plasmonic and dielectric nano-particles can be greatly improved. Using versatile designs including core-shell wrapping, supercells, and dielectric mediated plasmonic scattering. The energy attenuation can be drastically reduced over the baseline dielectric nano-particle chain. In addition, it is also found that the dielectric nano-particle chains can actually be more efficient than the plasmonic ones, at their respective optimized geometry.

Second, the anti-reflection coating(ARC) based on dielectric nano-particles has been recently proposed as a new way to achieve the low reflectance required for solar cell front surfaces. In this scenario, the Mie modes associated with the dielectric nano-particles are utilized to facilitate photon forward scattering. In this thesis, versatile designs together with systematically optimized geometry are examined, for the ARCs based on dielectric scatters. It is found that the Si3N4-TiO2 or SiO2-TiO2 stack is capable of providing low reflectance while maintaining a flat and passivated ARC-semiconductor interface which can be beneficial for reduced interface recombination and prevent VOC degradation associated with topography on the active materials. At the ultimate design based on mixed graded index(GI) Mie-scattering, the averaged reflectance can be as low as 0.25%. On the other hand, the mixed GI Mie design preserves a flat and passivated ARC-silicon interface, with total thickness reduced to 279.8nm compared to 1.6m for silicon nanotips. In addition, it is also shown that the mixed GI Mie design can still provides light trapping capability while providing much lower reflectance compared to TiO2 anti-reflection coating.