Enhancement of Color Conversion Efficiency of Hybrid GaN-Based Nano-Structured Light-Emitting Diodes Consisting of Nanocrystal Quantum Dots
|關鍵字:||氮化鎵發光二極體;奈米結構;量子點;轉換效率;螢光共振能量轉移;GaN light emitting diodes;Nano-structured;Quantum dots;color conversion efficiency;Förster resonance energy transfer|
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.