以光場增強效應提升薄膜太陽能電池吸收效率研究

Abstract

本二年期研究計畫主要是針對光電太陽能元件的光學特性之基礎研究，共包括四個研究方向： (一) 進行光 學特性的基礎研究，以增進對太陽能電池中，光場增強效應(Light Trapping) 之深入了解。 (二) 針對寬頻介電質 反射鏡(dielectric broad band reflecting mirror)做深入研究，透過電磁理論找出適合應用於太陽能電池的反射鏡結 構與形狀，並將介電質反射鏡實際應用於太陽能光電元件並計算其產生的效率提升效果。 (三) 進行隨機幾何形 狀應用於太陽能電池底部反射鏡(back reflector) 之研究，將利用基因演算法來進行幾何形狀之最佳化，決定最適 合運用在太陽能電池的幾何形狀。(四) 提出新式太陽能電池結構，利用周期性寬頻高效率光場增強陣列光子晶 體，或是將光譜分離式共振腔結構應用於多接面太陽能電池上(high-efficiency resonant-cavity-like structures for multi-junction or multi-band solar cell application)，相對於隨機幾何繞射結構，周期性的結構可提供可重複且穩定 的效率轉換。 本計畫將從計算薄膜太陽能電池的能帶結構以及特徵模式(wave characteristic mode)開始，去釐清太陽能電池 中的波導效應，明確找出可提供光場增強效應(light trapping effect)給寬頻光譜的方法，並了解其中之物理意義。 此外，此計畫也將探討寬頻介電質反射鏡在太陽能電池中的應用。這一方面的研究主要會先從計算單獨一個寬 頻介電質反射鏡的穿透和反射頻譜開始，接著，再把介電質反射鏡整合進入太陽能電池的結構之中。轉換效率 的提升，將以積分量子效率和頻率響應來衡量。對於寬頻介電質反射鏡的幾何參數之最佳化，將會是本次研究 的重點，而最佳化對於轉換效率的提升，也會被仔細評估。另外，我們也將研究隨機幾何形狀光柵的可行性， 藉由基因演算法、頻率響應、和功率頻譜密度，來探討光柵的空間頻率分佈(spatial frequency) 和其與效率提升 間的關聯。最後，將評估利用寬頻周期性光學結構，來實現具有極佳光場增強效應的前瞻太陽能光電元件之可 行性，這包括光子晶體的結構、寬頻透涉/波導/反射結構、和光譜分離式共振腔型態的結構。 預期本計畫的執行將可得到下面的成果：(I)證實/示範運用奈米光學模擬可以有效的提升太陽能電池的設計 (II)透過有效的光場增強效應，太陽能電池的吸收與轉換效率可以得到顯著的提升，(III)透過特徵模式的分析、 透射/反射/吸收頻譜的計算、和繞射級數的計算，提升對於太陽能電池基礎物理的了解，尤其是光子在元件中折 射/反射/繞射的現象，(IV)前瞻光學共振腔型態元件的提案，將提供未來高效率太陽能電池的設計藍圖

This research proposal is mainly for the fundamental research of photovoltaic device optics. The goal can be separated into four parts (I) Fundamental understanding of light trapping phenomena in photovoltaic cells. (II) Search of dielectric broad band reflecting mirrors, and incorporate these mirrors into photovoltaic cells (III) Improved randomized grating structures for high efficiency solar cells, through genetic algorithm optimization, and based on the physics studied in goal (I). And finally (IV) The search/design of periodic structures for photovoltaic cells, which may provide reproducible efficiencies, and make possible ultra-efficient resonant-cavity-like structures for multi-junction or multi-band solar cell applications. The research will begin with the calculation of bandstructures and characteristic modes of planar solar cell structures. The waveguiding effect can thus be clarified and the guideline for light trapping of different wavelengths should be provided and underlying physical reasons will be pointed out. Secondly broad band dielectric mirror is studied by calculating transmission and reflecting spectrum of a standalone mirror structures, and then the solar cell structure with dielectric mirror will be studied by integrated efficiency/spectral response calculation. Optimization of the geometry parameters will be conducted and improvement in efficiency will be assessed. Survey of practical material system for dielectric mirror and choice of material will also be presented. Thirdly, randomized gratings is investigated through genetic algorithm optimization, spectral response, and power spectral density to study the grating spatial frequency distribution and its relation to the efficiency improvement. Finally, the possibility of utilizing broad band light trapping structures with periodic arrays for solar application is carefully discussed, including photonic crystal structures, broad band transmission/waveguiding/reflecting structures, and spectrally separated resonant-cavity-type structures. The success of this project will contribute in (I)Demonstration of using nano photonic modeling approach to design solar cell optical structures. (II)Improvement of solar cell absorbance and thus conversion efficiency by proper light trapping method and design. (III) Improving the fundamental understanding of electrodynamic phenomenon inside solar cells by eigen mode analysis, transmission/reflection/absorption calculation, and diffraction order analysis. (IV)New/Novel/Promising optical cavity structures proposal provides directions for future high efficiency solar cell design.