# High-Performance Color-Converted Full-Color Micro-LED Arrays
## 3 Results and Discussion  
Figure 2 details the transmittance of a 10 \(\upmu\)m-thick photo-curable acrylic, where the thickness depends on the spin-coating rate of the photo-curable acrylic and the UV-visible absorption and photoluminescence (PL) spectroscopy for green and red dyes. Figure 2a,b indicates that the photo-curable acrylic had high transmittance in the visible range and the thickness could be increased by decreasing the spin-coating rate. For CCLs, we initially attempted to mix conventional phosphors such as \(\beta\)-SiAlON:Eu\({}^{2+}\) and CaAlSiN\({}_{3}\):Eu\({}^{2+}\) for green and red emissions. However, in terms of agglomeration of phosphor in acrylic, selectively patterning on blue LEDs was not possible, as shown by the Supplementary Figure S1a. This result was obtained due to the strong absorption of UV light by the aggregated phosphor particles. Therefore, understanding the dispersion of color-conversion materials was important. We determined that nano-size organic materials were suitable for mixing with photo-curable acrylic, as shown in Supplementary Figure S1. With patterned CCLs, it is possible to fabricate full color arrays as shown in Supplementary Figure S2. Before mixing the green and red nanosize dyes in photocurable acrylic, we tried to measure transmittance of the photocurable acrylic. The transmittance was almost 100% in the visible range. This result indicated that the photocurable acrylic could not absorb green and red light produced by color conversion. The thickness of acrylic materials could be increased by decreasing the spin coating rate. To confirm the possibility of color conversion of green and red dyes by blue light, we tried to measure the absorption and PL spectra using a 365 nm excitation. The result shows that blue light could be absorbed in green, and that green dyes and green and blue dyes emitted different peaks at 550 nm and 620 nm, respectively, as shown in Figure 2c,d. These results indicate that the blue and green dyes can be used as the color-conversion layer for full-color micro LEDs based on color-converted blue LED. Figure 1: The schematic diagram of full-color micro LEDs based on photo-curable color-conversion materials. To identify the optimum conditions for CCL, we attempted to determine the optimum mixing ratios between photocurable acrylic and dye, and thickness. To find the optimum mixing ratio between acrylic and dye, the conversion efficiency of blue to red or green light was measured as shown in Figure 3a,b. With increasing the mixing rate to 0.6 wt %, the conversion efficiency was improved by 47% and 35% for green and red dyes, respectively. However, when mixing rate was further increased, conversion efficiency was significantly decreased, owing to the aggregation of dye. The aggregation of dye can be attributed to the excimer formation and concentration induced quenching. [16] To find optimum thickness for color conversion, we tried to measure the PL spectra of green and red dyes, depending on the thickness of the mixture of photocurable acrylic and green/red dyes. Figure 3c,d shows the PL spectra of green and red dyes with the emission of blue LED. This spectrum shows that the blue leakage was decreased while green or red emission was increased with increasing thickness of the color-conversion layer. The full with half maximum (FWHM) of green and red dye was 78 nm and 52 nm, respectively. Figure 3e,f shows the normalized PL intensity of blue light and green or red light for the green and red dyes, respectively. When the thickness of CCLs is 3 \(\upmu\)m, there was leakage of blue light as shown in Figure 3c,d. When increasing the thickness of the color-conversion layer, leakage of blue light was significantly decree sed and green or red emission was increased as shown in Figure 3e,f. We found that the color-conversion efficiency of red dyes was slightly lower than that of green dyes. This result can be attributed to low absorbance of the green dye of blue light compared to that of green dye.