低維度材料與光電元件之整合應用

Abstract

低維度奈米材料由於其具有獨特的材料特性而在近年來引起相當大的科學關注。一般所討論到的低維度奈米材料包含零維材料(例如量子點)、一維材料(例如奈米線)、以及二維材料(例如石墨稀或過渡金屬硫族化合物)......等。首先，量子點在最近幾年來迅速的成長，因為其有特殊且優良的物裡和化學性質，並開始在光電子產業應用上展露頭角，例如應用在發光二極體、背光模組、光偵測器、或生物光學應用。另一方面，雖然過渡金屬硫族化合物已被研究幾十年以上，但最近在奈米尺度下對此材料的特性表徵和元件製作讓科學家重新看到此種層狀二維材料在微電子與光電領域發展的機會，因此大量的科學家正瘋狂投入其中。在此篇論文裡，我們利用量子點的光子下轉換效應，來設計新穎高效率之混合型量子點敏化太陽能電池；或是利用量子點結合紫外光微發光二極體陣列，製做無彩色濾光片且可全彩顯示出光之微米顯示器；最後在二維材料的研究上，我們利用酸處理的方式來改善化學氣相沉積法(CVD)合成之缺陷單層二硒化鉬(MoSe2)的材料品質，有效提高樣品的品質與改變參雜濃度。 第一部分，我們在這裡討論了多種提高太陽光譜利用與改善太陽能電池效率之方法，並針對光子下轉換機制做深入的研究。隨著量子點製造技術的提高、改善本身的發光量子效率，在此我們結合製作量子點敏化太陽能電池可有效的觀察到光子下轉換機制在太陽能電池上的影響。此設計可藉由轉換紫外光至可見光再讓太陽能電池吸收，提高對紫外光區間的光生載子收集與增加對紫外光頻譜的利用。利用多層量子點結構配合脈衝噴塗技術精準控制量子點的使用量，並最佳化量子點的使用濃度與發光波長選擇。 在第二部分中，我們討論利用量子點結合微發光二極體陣列來做顯示器。由於傳統液晶顯示器顯示反應速率慢，且當背光源光線經過液晶模組與彩色濾光片時，大部分的光都會被吸收損耗，利用效率極低。因此我們利用霧化噴塗系統分別噴塗三原色量子點於可獨立操控點亮之紫外光微發光二極體陣列上，可以製做出不需液晶模組與彩色濾光片之微米顯示器。同時巧妙利用紫外光布拉格反射鏡增加紫外光的利用率且提高量子點的發光效率。透過量子點所製作的全彩微顯示器可大幅的提升顯示器的色域，使色域達到NTSC 152 %的卓越表現，可提供更生動、飽和的色彩，此種量子點微顯示器未來極有潛力可應用於穿戴式顯示裝置上。 在論文的第三個部分中，我們嘗試利用酸處理來改善CVD成長之二維過渡金屬硫族化合物的缺陷問題。發現利用氫鹵酸適當處理過渡金屬硫族化合物樣品可使得光致螢光光譜強度增加30倍以上，因此我們利用變溫光致螢光光譜量測來觀察exciton和trion的發光情形。從實驗結果可以發現氫鹵酸處理造成二硒化鉬樣品的材料缺陷可大幅減少且增加p型參雜。由此可知此研究內容提供進一步了解與控制單層過渡金屬硫族化合物的光學特性之可能。 本論文能提供研發應用量子點整合高效率太陽能電池、微型二極體陣列顯示器、以及二維材料缺陷處理的相關研究以之參考。

Low dimensional materials have attracted considerable scientific attention because of the unique material properties when the size of materials is in the scale of nanometer. The low dimensional materials includes zero-dimensional materials, such as quantum dots (QDs), one-dimensional materials, such as nanowire, or two-dimensional materials, like graphene or transition metal dichalcogenides (TMDCs). In recent years, the QDs rapidly developed because of exhibiting superior physics and chemicals properties and there are many applications of QDs in optoelectronic devices, such as light-emitting diodes, backlight modules, photo-detectors, and bio-photonics. On the other hand, recent advances in nanoscale materials characterization and device fabrication have opened up new opportunities for two-dimensional layers of thin TMDCs in nanoelectronics and optoelectronics. In this dissertation, we use the luminescent down-shifting effect of QDs to design a novel and highly efficient hybrid QDs solar cells and to combine with ultra-violet micro LED arrays to be a display application without color filters. Then, for the two-dimensional materials, we utilize simple hydrohalic acids treatment to be an effective strategy for promoting the excitonic emission of defective monolayer TMDCs. In the first part of this study, we propose several methods to utilize the solar spectrum in order to increase the power efficiency of solar cell. Among the all mechanisms, down-shifting effect is investigated in detail. While the enhancement of solar cell efficiency was not clearly observed in the past, the advances in quantum dot fabrication have brought strong response out of the hybrid platform of a quantum dot solar cell. This hybrid design effectively boosts photon harvesting at long wavelengths while enhancing the collection of photogenerated carriers in the ultraviolet region. A multiple layer structure is proposed and demonstrated. With the help of pulse spray system, precise control can be achieved. The optimization properties of concentration and the emission wavelength of colloidal quantum dots also be investigated. Second, the conventional liquid crystal displays have much energy loss, which is due to the light absorption from backlight module and color filter. Colloidal quantum dots which can emit red, green, and blue colors are incorporated with a micro-LED array to demonstrate a feasible choice for future display technology. We utilize Aerosol Jet printing system to spray quantum dots onto the micro light-emitting diode array. The ultra-violet LEDs are used in the array to excite the red, green and blue quantum dots on the top surface. To increase the utilization of the UV photons, a layer of distributed Bragg reflector was laid down on the device to reflect most of the leaked UV photons back to the quantum dot layers. Moreover, the full-color micron display that is manufactured by quantum dots can improve the color gamut of the display. The color gamut NTSC reached the ultrahigh value about 152%. In the third part, here we report an effective strategy for promoting the excitonic emission of defective monolayer TMDCs by simple hydrohalic acids treatment (HBr, HCl, and HI). The chemical treatment can effectively enhance the photoluminescence intensity of atomically thin MoSe2 for more than 30 times. We further invest the behaviours of exciton and trion PL in the treated TMDCs by temperature dependence measurement, and observe a significant suppression of trap state exciton, which lead to a promoted exciton and trion emission in defective TMDCs through the p-doping process. Our results suggest that the HBr treatment not only reduces the n-doping in MoSe2 but also reduces the structural defects. The results provide further insights of the control and tailoring optical properties of monolayer TMDCs. Finally, the output of this dissertation provided a great help on enhancing the light absorption and improving the power conversion efficiency of solar cells, demonstrating a high vivid, saturated full colors quantum dot micro display technologies, and working on the acids treatment for the TMDCs to reduces the n-doping in MoSe2 and the structural defects.