# Mini-LED and Micro-LED: Promising Candidates for the Next Generation Display Technology
## 3 Micro-LED
### Color Conversion Full-Color Display
#### 3.2.2 Quantum Dots
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Color of QD**} & **Working Distance** & **Carrier Gas Flow Rate** & **Sheath Gas Flow Rate** & **Stage Speed** \\  & **(mm)** & **(sccm)** & **(sccm)** & **(mm/s)** \\ \hline Red & 1 & 83 & 17 & 10 \\ Green & 1 & 72 & 15 & 10 \\ Blue & 1 & 66 & 11 & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Parameters of the AJ system for RGB QDs [48]. Table reproduced with permission from Optical Society of America. Figure 14: Process flow of the full-color micro-LED display. (**a**) The structure of the UV micro-LED array. (**b**) Aligning the PR mold to the micro-LED array. (**c-e**) Consequently jetting the RGB QDs inside the PR mold window to form full-color pixels [48]. Figure reproduced with permission from Optical Society of America. Figure 15: The image and the schematic illustration of the AJ system [47; 48]. Figure reproduced with permission from Optical Society of America. Moreover, the coffee ring effect Figure 17a [49] that the suspended particles are driven to the edge and deposited in a ring-like pattern can also be resolved by the PR mold. Figure 17c demonstrates the image of the coffee ring effect when QD drops directly deposited on a GaN substrate without the PR mold. The principle and the image of the PR mold to eliminate the coffee ring effect are shown in Figure 17b,d, respectively. With the help of the PR mold, the thickness of the liquid within a cell would be equalized, which leads to a similar speed at which the liquid will evaporate, preventing the QD particles from being driven to the perimeter or middle site by the solution. The UV micro-LED array emits the 395 nm light which can excite the RGB QDs deposited on the PR mold. However, due to the thin thickness of the QD layer, the UV photons from the underneath LED cannot be absorbed completely [50]. Therefore, in order to increase the UV light utilization, and avoid bio-logical damage, a DBR structure has been designed for the micro-display. As shown in Figure 18a, the DBR provides 90% of reflectance at 395 nm and high transmission (around 90%) at RGB bands. as shown in Figure 18b, a pronounced UV peak (at 395 nm) is an indicator of less-efficient pumping, while the RGB signals are weak without the DBR. After the DBR is added, the strong  
Figure 16: (**a**) Microscope image and (**b**) fluorescence microscopy image of the RGB QDs on the PR mold that is above micro-LEDs. (**c**) The cross-talk effect of the QD pixels without the PR mold [48]. Figure reproduced with permission from Optical Society of America. Figure 17: (**a**) Principle of the coffee ring effect. (**b**) Principle of reducing the coffee ring effect via the PR mold. (**c**) The coffee ring effect can be observed without the PR mold. (**d**)The PR mold confines the QD drops and resolve the coffee ring effect [48]. Figure reproduced with permission from Optical Society of America. reflection at UV band successfully suppressed the 395 nm peak and increase the visible intensities by 194% (blue), 173% (green) and 183% (red). In addition to exciting QDs for full-color display via UV micro-LEDs, [PERSON] et al. fabricated monolithic RGB micro-LEDs, using GaN-based 451 nm blue micro-LEDs to individually excite red and green QDs [50]. A black matrix photoresist with light-blocking capability is spun onto the micro-LEDs to improve the pixel CR and color purity by suppressing the crosstalk among the RGB light. A hybrid Bragg reflector (HBR) is deposited on the bottom side of the substrate to reflect the RGB light and the light output intensity, while a DBR with high reflection for the blue light is deposited on the top side of the QDs to further improve the color purity of the red and green light. Finally, the output intensities of red and green lights of the micro LEDs with HBR and DBR can be enhanced by about 27%. These researches above achieve remarkable performance in full-color displays based on photoluminescence performance of QDs.