奈米結構發光二極體與奈米量子點複合物顏色轉換效率研究

Abstract

奈米量子點由於尺度非常微小，因此在特性上也與塊材半導體有著明顯著不同。量子點螢光亮度強、光穩定性佳、可藉由調控粒徑大小來改變螢光波長涵蓋所有可見光波段、螢光波形狹窄且對稱，因此近年來膠狀量子點發光材料成為新一代受矚目的研究對象，且被視為取代螢光粉從而製造出高色純度的發光二極體的優先選擇。然而目前量子點的能量轉移效率還非常的低，使用一般的發光二極體並沒辦法有效的激發複合物量子點，因此在本論文中，我們提出結合具有奈米結構的發光二極體與量子點複合物，除了增加發光二極體本身的光萃取以利激發更多量子點，另外利用奈米結構可以增加一非輻射電偶極交互作用路徑將能量傳遞給量子點，以達到提升量子點的色轉換效率。本論文將分為兩部份，第一部利用將平面結構的發光二極體製作成奈米柱結構與硒化鎘量子點結合，提高其轉換效率。不過由於鎘為有毒之重金屬，侷限了量子點在生醫與其他方面的應用。為了讓量子點能應用於各個領域，無重金屬鎘之磷化銦量子點由於可調變至近紅外光波長而成為新一代研究對象。此外由於奈米柱彼此之間不相連，電流無法均勻通過整個二極體，因此在第二部分中我們使用了磷化銦量子點，並將發光二極體改為奈米洞的結構而達到更好的顏色轉移效率，使量子點與二極體複合物能有更廣泛的用途。

In recent year, colloidal nanocrystals quantum dots (NQDs) have attracted intensive attention, owing to the brighter emission and photo-stability. The emission color of colloidal NQDs can be easily tuned from the visible to the near-IR range of the electromagnetic spectrum through changing their size or shape which provide the possibility for high power efficiency, flexible, low-systems-cost, large-area, and exceptional color optoelectronic devices. Therefore, hybrid NQD–GaN light emitting diodes (LEDs) become capable candidates for highly efficient multicolor lighting. However, there have various energy-loss steps in the transfer process, such as light-scattering from the NQDs and waveguide leaky mode losses, which will decrease the efficiency of radiative energy transfer and make it relatively low (<10%). In this thesis, we introduce a non-radiative energy transfer methodology which can enhance the conversion efficiency of the NQDs. We use nano-imprint technique combined with photolithography to fabricate two kinds of nano-structured LEDs. We then deposit different kinds of QDs on the nano-structured LEDs to study the enhanced color-conversion efficiency and proposed the possible mechanisms. In the first part, micro-cavity with nano-rods light emitting diodes (MCNR-LEDs) have been fabricated by nano-imprint lithography. After that, pulsed spray method has been used to deposit CdSe quantum dots (QDs) on the top of MCNR-LEDs to obtain hybrid MCNR-LEDs and QDs composites. Therefore, the distance between CdSe QDs and multiple quantum wells (MQWs) can be shorten by using this method, which generates the so called Fro ̈ster resonance energy transfer (FRET) process. This non-radiative energy transfer is very effective and able to enhance the color-conversion efficiency. Time-resolved photoluminescence (TRPL) was used to proof the non-radiative energy transfer phenomenon by the relationship between collected photons and time. The electroluminescence (EL) measurement shows that the color-conversion efficiency enhancement has been improved up to 12.4%. In the second part, because CdSe QDs contain a heavy toxic metal, they are not suitable for in vivo clinical application, and may pose risks to human health as well as the environment. To overcome these challenges, we used non-Cd QDs – InP QDs as a substitute. Furthermore, to make more homogeneous current distribution and enhance non-radiative energy transfer from nitride active layers to InP QDs, nano-cavities light emitting diodes (NC-LEDs) were proposed and fabricated. Similar to the first part, we used TRPL to confirm the existence of non-radiative energy transfer. In addition, NC-LEDs exhibit 14.29% color-conversion efficiency enhancement. The higher enhanced conversion efficiency between NC-LEDs and InP QDs can be attributed to the appearance of FRET. It is believed that our newly designed hybrid nano-structured LEDs and QDs with high-performance color-conversion should be very useful for practical applications in solid-state lighting, displays, lasers and many other optoelectronic devices.