介電反射器太陽能電池與超穎材料完美吸收器之光學特性研究

Abstract

為了有效降低太陽能電池的成本，最直接的作法就是「薄」，亦即減少半導體層的厚度。但減少半導體層的厚度，將會使太陽能電池對入射光之吸收率不足。因此，光捕捉技術對薄膜太陽能電池而言，更為重要。過去，電漿性薄膜太陽能電池曾被認為能有效地提升薄膜太陽能電池的吸收率與轉換效率。其中，物理機制為表面電漿共振，造成在金屬表面會有侷域性近場增強，因此大量地光子在金屬表面被散射、吸收，使得鄰近金屬表面的半導體層得以獲益。對薄膜矽太陽能電池而言，因矽在近紅外線波段吸收微弱，所以一般都設計在此波段產生共振，進而提升矽的吸收效率。然而，因光子亦會被金屬所吸收，往往半導體層的吸收率提升相當有限。此外，表面電漿共振只會在特定波長發生，所以無法全面性地提升半導體層在近紅外線波段的吸收率。後來，有部分研究提出以全由介電材料所構成之太陽能電池取代電漿性太陽能電池。 最近三年內，奈米光學成為了相當熱門的研究題目，由其是超穎材料完美吸收器。超穎材料是一種人造材料，是以金屬─介電質薄膜交替堆疊而成。在此結構之下，超穎材料在某一方向上之有類似金屬的介電性質，而在其它方向上，則有類似介電質的介電性質。如此在自然中相當罕見的介電特性，會使得結構的光子能態密度大幅增加，使超穎材料結構有寬頻譜的吸收效果。在適當的圖案設計之下，超穎材料結構可以有極寬頻譜且超高吸收率的吸收特性。更重要的是，其吸收頻譜可調性高，透過改變其結構週期、金屬膜厚度、介電膜厚度等，得以使超穎材料結構在任何波段中，如可見光、近紅外線、中遠紅外線等，有先前所言的吸收效果。如此空前的吸收特性使超穎材料有相當大的潛力，得以應用在各種不同的領域當中，太陽能電池亦不例外。 在本文第三章，將會模擬使用銀反射器與一維銀光柵陣列的薄膜矽結構與使用C-DBR與一維銀(或二氧化鈦)光柵陣列的薄膜矽結構的電磁特性，並討論兩者吸光特性有何差異。 在第四章，會模擬並最佳化二維梯形超穎材料完美吸收器以及二維垂直側壁超穎材料完美吸收器之吸收特性，並討論與比較此兩種結構作為完美吸收器之優劣。 在第五章，會對超穎材料應用至薄膜太陽能電池之可行性作一總結，同時，與其它在本文中所提及的其它結構作一完整的比較。

The thin-film solar cell is a direct solution for the low-cost photovoltaics application. However, insufficient light absorption because of lack of semiconductor layer thickness is a critical issue. In other words, light trapping technology is definitely essential for thin-film solar cells to increase light absorption of semiconductor. Plasmonic solar cells were once potential solutions to achieve high-efficiency thin-film solar cells for surface plasmon resonance (SPR) of metallic gratings at near infrared. Nevertheless, metallic loss would depress further enhancement and SPR only works at specific resonant wavelengths. Therefore, some research proposes all-dielectric solar cells to substitute for plasmonic solar cells. Over the past three years, nanophotonic optics draw significant attention, especially Hyperbolic metamaterial(HMM). HMM is an artificial media created by sub-wavelength structure with metal-dielectric alternating layers. Under the novel design, HMM has metal-like permittivity in one direction and dielectric-like permittivity in another direction, and this unnatural property will lead to dramatically increase of the photonic density of state. By appropriate pattern design for HMM, it can achieve an ultrahigh absorptance and an ultrabroadband absorption spectra. More importantly, the absorption profile of HMM is tunable through changing structure parameters, like thickness of metal/dielectric layers, structure period etc. Such unprecedented property makes HMM become the most potential material for various application, like thermal emitters, thermophotovoltaics, potentially photovoltaics and so on. In this work, we will simulate metal reflector solar cells with 1D Ag grating and C-DBR(Composite Distributed Brag Reflector) solar cells with 1D Ag/TiO2 grating to compare the absorption efficiency between all-dielectric solar cells and plasmonic solar cells. There will be a detailed discussion for these two different solar cells in chapter 3. In chapter 4, we will simulate and optimize 2D tapered HMM structure proposed by other scholars, and 2D straight-sidewall HMM structure at the same time. Then, the further discussion on merits and drawbacks of these two structure as a perfect absorber is included in chapter 4 too. In chapter 5, we will summarize the feasibility of HMM structure for thin-film solar cells application.