In this study, we present the structural schematic of a 5 µm blue micro RCLED array with staggered quantum wells (QWs) grown on a 2-inch polar c-plane (0001) GaN epitaxial wafer, as shown in Fig. 1(a). Through bandgap engineering with staggered QWs arrangements, the overlap of wave functions can be enhanced, thereby achieving improved efficiency and wavelength stability for micro LEDs. Figure 1(b) shows the reflectance spectrum of the structure's DBR (NP-DBR) and the top DBR (Ta2O5/SiO2 DBR). The refractive indices of GaN and NP-GaN are approximately 2.39 and 1.7, respectively. The upper and lower DBRs together form a Fabry-Perot cavity, used for filtering the wavelength of emitted light.[3, 4, 25] After the device fabrication, optical properties were measured with a spectrometer (Maya 2000 Pro), where the emitted light was collected by an integrating sphere. Figure 1(c) displays the electroluminescence (EL) emission spectrum, showing that increasing the input current from 26.5 A/cm² to 159.2 A/cm² led to a peak wavelength shift of about 6.5 nm. This shift places it within the blue spectrum region, belonging to the blue light spectrum. In terms of FWHM, it expanded by 0.204 nm. These results confirm that the blue micro RCLEDs we prepared exhibit excellent wavelength stability, showing smaller wavelength shifts and narrower FWHM. Both characteristics are advantageous for subsequent applications in full-color displays.
In the field of QDs color conversion technology, the incorporation of nanoscale scattering particles significantly enhances light conversion efficiency and display performance. These minute particles scatter incident light, extending the light's path within the quantum dot layer. This extension increases light absorption and conversion efficiency. Furthermore, nano-scattering particles contribute to the uniformity of the light source and enhance the purity and saturation of colors. This improvement is crucial for the development of high-performance displays and optical devices. Commonly used nanoscattering particles include titanium dioxide (TiO2)[13, 22], zirconium dioxide (ZrO2)[26, 27], silica (SiO2)[23], and aluminum oxide (Al2O3)[28], among others. These particles are selected for their high refractive index, chemical stability, and favorable optical properties, which effectively scatter visible light and boost the luminous efficiency of QDs. They are extensively utilized to enhance the CCL of QDs, providing more uniform light output and improved color performance. The careful selection and application of these nano-scattering particles, based on their specific physical and chemical characteristics, are pivotal in various QD color conversion systems. By precisely engineering the size, shape, and distribution of the particles, the optical performance of the QD CCL can be optimized further. This optimization leads to a more efficient and stable photoelectric conversion effect. In previous research, we explored the advantages of adding TiO2 scattering particles in QDPR using simulation software and confirmed that the addition of an appropriate amount of scattering particles can help improve the LCE. [13, 21–23] In this study, we specifically delve into the reliability aspect. Figure 2 compares green QDPR thin films without scattering particles, with ZrO2 added, and with TiO2 added, respectively. The testing conditions were set at 25°C and 50% humidity, and all samples were left unsealed and stored for 30 days. The measurement results showed that the QDPR with added TiO2 demonstrated superior reliability. This could be due to TiO2 having a higher refractive index compared to ZrO2, which may offer better performance in enhancing the optical properties of the QD thin films. This optimized optical performance could, to some extent, offset any minor stability issues that might arise during long-term storage.
To further reduce the blue light leakage phenomenon in QD CCLs, this research explores the best color choice for a CF to be added to the surface of QD CCLs. It also compares the differences in LCE brought about by the introduction of DBR, as depicted in Fig. 3. Figure 3(a) displays the simulated emission spectrum of RGY QDPR along with the transmission spectra of RGY filters and DBR using LightTools (8.6) illumination design software. The thickness of the filter is 0.6 µm, and the DBR is 6 µm thick. From the figure, it is observed that DBR provides the best transmission for RGY and can effectively filter blue light, with yellow CF being the next best option. Green CF shows poorer transmission for the red light spectrum, and red CF has very low transmission for green and yellow light bands. To reduce manufacturing costs and time, yellow CF is the preferred choice. Given the higher production cost and greater thickness of DBR, which is disadvantageous for use in small-sized devices, the yellow CF, with its relatively better performance, was chosen. Figure 3(b-e) present the simulated and actual measured spectra of RG QDPR using RGY CF. It is observed that, compared to samples without CF, the addition of CF, although reducing emission intensity, is very effective in suppressing blue light leakage. The difference in emission intensity between using yellow CF and RG CF is not significant. Therefore, yellow CF was ultimately selected as the blue light filtering layer for QD CCLs.
While blue light micro RCLEDs have already demonstrated excellent wavelength stability, for high-end display products pursuing high color purity, it is crucial to minimize any slight differences. Fortunately, the emission wavelength of QDs is very stable and does not vary with minor differences in the wavelength of the provided light source. However, leakage of the excitation light source can cause a shift in chromaticity, and it is essential to avoid this phenomenon. We used LightTools (8.6) to simulate the wavelength shift situation of blue light micro RCLEDs with added QDPR when changing the output power (0.25 ~ 3.5 W). Figure 4 compares the impact of adding a CF on the chromaticity coordinates of blue light micro RCLEDs under varying excitation power. Without CF, as the current gradually increases, the blue light micro RCLED experiences a Quantum Confined Stark Effect (QCSE), leading to a blue shift in wavelength.[25, 29] The leaked blue light affects the overall color purity, causing a shift in CIE coordinates, as shown in Fig. 4(a). Conversely, with the addition of a yellow CF, blue light leakage is suppressed, and the chromaticity coordinates remain almost unchanged, as indicated in Fig. 4(b), due to the absence of QCSE impact. Notably, after adding a yellow CF, the color purity of RGY QDPR significantly increases, with RGY improving by 14.7%, 21.5%, and 15.3%, respectively. Such results demonstrate extremely high wavelength stability, beneficial for the development of high-end display products. Moreover, the study also compared the wavelength shift of commercial micro LEDs under varying driving currents for RGB, as shown in Fig. 4(c). From the figure, it is observed that commercial micro LEDs are severely affected by QCSE, exhibiting significant wavelength shifts, with a substantial blue shift at high driving currents. This is highly disadvantageous for applications, making the use of QDPR CCL a crucial step in driving the development of future display technologies.
Following previous research that determined the selection of yellow pigment for the CF, QDPR CCL were fabricated for further study and analysis. Figure 5 shows the emission spectrum of RGY QDPR + YCF CCL. Optical properties were measured using a spectrometer (Maya 2000 Pro), with the wavelengths of RGY QDPR + YCF CCL being 629 nm, 537 nm, and 565.3 nm, respectively. The Photoluminescence Quantum Yield (PLQY) was measured using a commercial device developed by Otsuka Tech Electronics Co., Ltd. Electronics (TQ-10), with the PLQYs of RGY QDPR + YCF CCL being 78.0%, 66.6%, and 73.5%, respectively. Due to the self-aggregation effect of QDs, the uniformity of illumination in yellow light photolithography processes for QDs is usually poor.[13] By adding nano-scattering particles into the QD ink, the light scattering effect is amplified, increasing the possibility of blue light stimulating the QDs. Moreover, the addition of TiO2 can maintain the positioning of QDs to prevent the self-aggregation effect after the spin-coating process, thus enhancing color performance. To analyze the color performance of the emitted light images, the fabricated RGY QDPR CCL was measured using a Fluorescence Optical Microscope (FLOM). The FLOM images were converted to grayscale and divided into 300 x 300 pixels using MATLAB. The uniformity of illumination is defined as the ratio between the minimum value and the average value of the illumination pixels. The illumination uniformity of RGY QDPR was 98.6%, 96.7%, and 73.8%, respectively. As a result, the self-aggregation phenomenon of QDs was effectively suppressed, approaching high color purity, which is very promising for display technology.
The foundational research previously conducted led to the fabrication of a full-color micro RCLED array with individual pixel dimensions of 5×5 µm2 and a spacing of 1 µm, resulting in an 8 × 8 array. Figure 6(a) displays the spectral figure of the full-color micro RCLED at a driving power of 2.5 W. Figure 6(b) examines the impact of adding a yellow CF on the color gamut area. Within the CIE-1931 chromaticity diagram, comparing the national television standards committee (NTSC) and Rec. 2020 chromaticity space standards, the color gamut areas without CF were 55.9% and 41.8%, respectively. With the addition of the yellow CF, the color gamut areas increased to 106.08% and 79.20%, respectively. This enhancement shows that adding a yellow CF brings the color gamut area closer to the Rec.2020 standard due to the yellow pixels' ability to accurately render specific colors, particularly in terms of brightness and color temperature. Such an improvement is crucial for displays seeking a high color gamut as it helps reduce the energy consumption needed to achieve the same visual effects. Furthermore, incorporating yellow sub-pixels allows for easier regulation of white light. Compared to merely mixing red and green light to produce yellow, using yellow pixels provides a brighter yellow without increasing power consumption. This can enhance the energy efficiency of the overall system, especially for displaying brighter images or videos. Additionally, it's noteworthy that a layer of Al2O3 was applied on top of the QDPR CCL array using low-temperature ALD technology. Past research has proven the effectiveness of using ALD passivation protection technology to coat QD surfaces with oxides.[13, 14] This method effectively protects QDs from any changes due to high temperatures and material characteristics, as well as prevents moisture and oxidation. The developed full-color micro RCLED significantly improved light leakage and utilized RGBY four-color pixels, showcasing a full-color display with low energy consumption, high color purity, and a wide color gamut area.[3, 5, 9, 24]