Xie Rongjun, Huang Kai, Xuan Tongtong, and others from Xiamen University demonstrated green and red quantum dot luminescent microspheres with high color conversion efficiency and excellent PL stability, enabling ultra-efficient and bright RGB MicroLEDs.
MicroLEDs are reportedly characterized by ultra-high resolution, ultra-high brightness, fast response, high contrast, and low power consumption.
However, the miniaturization of traditional inorganic III-V semiconductor LEDs to microscale (≤50 μm) faces technical and performance challenges, including a sharp decline in the radiative efficiency of green and red LEDs, difficult mass transfer, and mismatched driving voltages for red, green, and blue (RGB) pixels, severely hindering their commercialization.
To address these issues, nanoscale-sized, highly efficient, narrow-band emission, and high-color-purity red and green quantum dots (QDs) were used as color conversion materials. Combined with blue MicroLEDs, they enable full-color MicroLED displays. This approach promises simplified mass transfer, easier driving circuits, and low cost.
However, due to the low extinction coefficient of quantum dots in the blue region, their light extraction efficiency (LEE) is poor, resulting in significant blue light leakage and low luminous efficiency in micro-LEDs converted from quantum dots. Consequently, the performance of quantum dot color conversion layers (CCLs) remains low.
Recently, efforts have been made to effectively suppress blue light leakage by embedding quantum dots into nanoporous GaN and adding color filters (CFs) or distributed Bragg reflectors (DBRs) to the device to absorb or reflect the residual blue light that the quantum dots cannot absorb.
However, these approaches inevitably increase power consumption, reduce viewing angles, and increase the surface temperature of micro-LED displays. Furthermore, for conventional QD displays, the QD CCL is sandwiched between two water and oxygen barrier layers up to 260 microns thick to improve display reliability. This strategy is unsuitable for micro-LEDs because the aspect ratio increases significantly, making patterning small pixels (<50?m) difficult.
Although thinner and more stable quantum dot pixels can be fabricated by coating with silica shells, alumina shells, or siloxane ligands, eliminating the need for a water-oxygen barrier layer and thus improving the PL stability of quantum dots, surface damage and low blue absorbance caused by silane hydrolysis, surface ligand modification, or atomic layer deposition (ALD) significantly reduce the color conversion performance of quantum dots and increase non-radiative recombination.
Therefore, constructing quantum dot materials with both excellent color conversion efficiency and excellent PL stability is key to achieving highly reliable full-color micro-LEDs.
Dendritic mesoporous silica spheres (DMS) feature tunable diameter, abundant mesopores, a high refractive index, excellent optical transmittance, and strong chemical stability. Therefore, encapsulating quantum dots within the mesopores of DMS can improve the color conversion performance and reliability of quantum dots.
The spatial confinement of the mesopores reduces reabsorption of quantum dots and isolates them from water and oxygen. Furthermore, the optical microcavity formed between the mesopores and the filler improves the localized light field of both excitation and emission light. However, several key issues remain to be addressed, including i) how to suppress organic ligand shedding and minimize damage to the quantum dot surface during the encapsulation process; and ii) how the encapsulation structure affects color conversion performance.
The research team used a wet method to synthesize cadmium-based QD@dMS@polymaleic anhydride octadecene (PMAO)@SiO2 (QD@dMS@PMAO@SiO2) luminescent microspheres with an average diameter of 220 nm. PMAO served as a bridge between the quantum dots and the SiO2 shell. PMAO not only inhibited ligand shedding but also suppressed the hydrolysis of 3-aminopropyltriethoxysilane (APTES), which damages the quantum dot surface.
The results showed that the microspheres exhibited high external photoluminescence quantum efficiency (EQY) and excellent PL stability against blue light, heat, and water oxygen. Finite-difference time-domain (FDTD) analysis combined with experimental results revealed the mechanism underlying the improved color conversion performance. Ultimately, the maximum external quantum efficiencies (EQEs) of the green and red micro-LEDs reached 40.8% and 22.0%, respectively. They are further integrated with thin-film transistor (TFT) backplanes to enable micro-LED displays with high pixel resolution and high brightness.
