# Micro-LEDs, a Manufacturing Perspective
## 4 Full-Color Schemes
Phosphors, which are typically micrometer-sized powders, have the advantages of high quantum yield (exceeding 80%), high thermal stability (with temperature up to 150 \({}^{\circ}\)C), high chemical stability (moisture resistance), and stable quantum yield and spectral properties under continuous irradiation with high light flux and fast luminescence decay. Consequently, they are widely used in white LEDs for general lighting [60; 61; 62]. The shortcomings, however, are strong light scattering and poor beam collimation due to their large size [61]. These shortcomings lead to low color uniformity which becomes more pronounced for micro-LED displays in which the phosphor size is comparable to that of small micro-LEDs. Reducing the phosphor size down to the nanometer scale has the potential to enhance luminance homogeneity due to reduced light scattering [63]. However, it also results in a reduction in quantum efficiency. QDs have been proposed as an alternative method for full-color micro-LED displays due to their excellent properties, which include high quantum efficiency, large absorption cross-section, and tunable emission wavelength [62]. As compared to typical phosphor particles, the size of colloidal QDs lies in the range of nanometers. Therefore, enhanced color uniformity via QDs-based color-conversion can be expected. White light generation and good luminous efficacy of radiation have been demonstrated in 128 \(\times\) 128 full-color micro-LED arrays with a pixel pitch of 40 \(\upmu\)m, by combining UV micro-LEDs with RGB CdSe QDs deposited via aerosol jet printing [56]. For commercialization, some critical factors in QDs, such as photo-stability and thermal quenching, would have to be addressed. To ensure the color purity and contrast ratio, the optical cross-talk effect resulting from the inaccurate distribution of color converters must be minimized. It has been reported that for a 128 \(\times\) 128 micro-LED array with 35 \(\upmu\)m \(\times\) 35 \(\upmu\)m pixel size, cross-talk resulting from the overflow of QDs during solvent evaporation still caused color shift, even though the aerosol jet printing technique with optimized parameters was employed [64]. To reduce cross talk, [PERSON] et al. have developed a lithographically fabricated photoresist mold consisting of a window for QD jetting and a light blocking wall, as shown in Figure 7[64]. Using the window-confined photoresist mold led to successful suppression of the overflow of the QDs resulting in a clear boundary between the pixels. With silver coatings on the sidewall of the mold, light leakage was also prevented and, hence, a near zero cross-talk was achieved. In order to increase the utilization of light from UV micro-LED array, and avoid biological damage, a distributed Bragg reflector (DBR) structure has been implemented on the QDs-depositedLED array, which enhanced the QD emission intensity by 23%, 32%, and 5% for red, green, and blue, respectively [64]. In another study, [PERSON] et al. have used three strategies to improve pixel contrast ratio and color purity of monolithic RGB micro-LEDs based on blue LEDs and red and green QDs, which include a black matrix photoresist with light-blocking capability to block light leakage from the sidewalls, a DBR with high reflectivity at blue light on the top of red and green QDs/micro-LEDs and a hybrid Bragg reflector (HBR) structure on the bottom for multiple reflection of the blue light into the color conversion layer [65]. Consequently, the contrast ratio of the micro-LEDs was improved from 11 to 22, and the red and green light output intensities of the micro LEDs were enhanced by about 27%. To improve light efficiency and eliminate color crosstalk of color-converted micro-LED displays, [PERSON] et al. have proposed a funnel-tube array design in color-converted micro-LED displays with color filter [66].