Xiamen research team achieves ultra-efficient and bright MicroLED

According to news on June 3, Xie Rongjun, Huang Kai, Xuan Tong and others from Xiamen University demonstrated green and red quantum dot luminescent microspheres, which have both high color conversion efficiency and excellent PL stability, thereby achieving ultra-efficient and bright RGB Micro LEDs.

It is reported that Micro LED has the characteristics of ultra-high resolution, ultra-high brightness, fast response speed, high contrast, and low power consumption.

However, in the process of miniaturizing traditional inorganic III-V semiconductor LEDs to microscale (≤50µm), they are faced with technical and performance challenges such as sharp decline in green and red LED radiation efficiency, difficulty in mass transfer, and mismatch of red, green, and blue (RGB) pixel driving voltages, which have seriously affected their commercialization process.

In order to solve these problems, red and green quantum dots (QDs) with nanoscale size, high efficiency, narrow-band emission, and high color purity are used as color conversion materials, and combined with blue micro-LEDs, full-color micro-LED displays can be achieved. This method promises to simplify mass transfer, be easy to drive the circuit, and be low-cost.
< br /> However, due to the low extinction coefficient of quantum dots in the blue region and poor light extraction efficiency (LEE) of quantum dots, the micro-LED converted by quantum dots has serious blue light leakage and low luminous efficiency, so the performance of the quantum dot color conversion layer (CCL) is still low.

Recently, blue light leakage can be effectively suppressed by embedding quantum dots into nanoporous GaN and adding color filters (CF) or distributed Bragg reflectors (DBR) to the device to absorb or reflect the residual blue light that the quantum dots cannot absorb.

However, these methods inevitably increase power consumption, reduce viewing angles, and increase the surface temperature of micro-LED displays. Furthermore, for conventional QD displays, the QD CCLs are sandwiched between two water-oxygen barrier layers up to 260 microns thick to increase the reliability of the display. This strategy is not suitable for micro-LEDs because the aspect ratio would increase significantly, which makes small pixels (
Although thinner and more stable quantum dot pixels can be prepared by coating silica shells, alumina shells, or siloxane ligands without the use of water- Oxygen barrier layer, thereby improving the PL stability of quantum dots, but due to the damage to the quantum dot surface and lower blue absorption rate due to silane hydrolysis, surface ligand modification or atomic layer deposition (ALD), the color conversion performance of quantum dots is greatly reduced and non-radiative recombination is increased.

Therefore, constructing quantum dot materials with both excellent color conversion efficiency and excellent PL stability is the key to realizing high-reliability full-color micro-LEDs.

Dendritic mesoporous silica spheres (dms) have the characteristics of adjustable diameter, rich mesopores, high refractive index, excellent optical transmittance and strong chemical stability. Therefore, encapsulating quantum dots in the mesopores of DMS can improve the color conversion performance and reliability of quantum dots.

The spatial confinement of mesopores reduces the reabsorption of quantum dots and separates quantum dots from water and oxygen. In addition, the optical microcavity formed between the mesopores and the filler improves the local light field of excitation light and emission light. However, there are several key issues that need to be solved, such as i) how to suppress the shedding of organic ligands and reduce damage to the quantum dot surface during the sealing process; ii) How packaging structure affects color conversion performance.

The research team used a wet method to synthesize cadmium-based QD@dMS@polyoctadecene (PMAO)@SiO2 (QD@dMS@PMAO@SiO2) luminescent microspheres with an average diameter of 220 nm, in which PMAO serves as a bridge between the quantum dots and the SiO2 shell. PMAO can not only inhibit the shedding of ligands, but also inhibit the hydrolysis of 3-aminopropyltriethoxysilane (APTES), thereby destroying the surface of quantum dots.

The results show that the microsphere has high external photoluminescence quantum efficiency (EQY) and excellent PL stability against blue light, heat and water and oxygen. Combining finite difference time domain (FDTD) with experimental results reveals the improvement mechanism of color conversion performance. Finally, the maximum external quantum efficiencies (EQEs) of green and red micro-LEDs reached 40.8% and 22.0%, respectively. They are further integrated with thin film transistor (TFT) backplanes to achieve high pixel resolution and high brightness micro-LED displays.