The scientific research team of Henan University has made progress in the research of direct photopatterning of ultra-bright and stable quantum dot LEDs using small molecule cross-linking agents.
Colloidal quantum dots have unique advantages, including high color purity, tunable emission spectrum, and high luminescence quantum yield, making them promising luminescent materials for various optoelectronic device applications. In recent years, many high-performance, stable red, green, and blue quantum dot light-emitting diodes have been developed, laying the foundation for the manufacture of full-color active matrix QLED displays. For AMQLED displays, pixelation of the quantum dot layer is essential. Common quantum dot patterning methods include inkjet printing, transfer printing, photolithography, liquid transfer, electrophoretic deposition and interface assembly. Each technology involves different fabrication.
manufacturing processes and have specific advantages and disadvantages, resulting in different results in terms of production efficiency, patterning resolution, pixel quality and device compatibility. For example, while inkjet printing has the advantages of high material utilization and mask-less processing, it struggles to produce uniform patterns with ultra-high resolution (<5 µm). Liquid transfer is considered a promising method to achieve high-resolution quantum dot patterning, but still suffers from low throughput and poor pattern fidelity.Among various candidate methods, photolithography is considered a promising quantum technology due to its advantages such as high throughput, ultra-high resolution, and compatibility with mass production.
Point patterning method. Typically, traditional photolithography techniques require the use of sacrificial layers to achieve high-resolution quantum dot patterns. However, this method faces challenges such as complex processes, incomplete removal of the sacrificial layer, poor quantum dot pattern fidelity, and significant quantum dot fluorescence loss. To overcome the shortcomings of traditional photolithography, a method for direct photopatterning of inorganic nanomaterials based on photosensitive additives was proposed. This method has been gradually expanded into several types, including in situ ligand exchange, ligand stripping, ligand cleavage, ligand-additive cross-linking, ligand-ligand cross-linking, and polymer-additive cross-linking. These direct optical lithography methods broaden the realization of quantum dot patterning, allowing the maximum external quantum efficiency of patterned or cross-linked quantum dot light-emitting diodes to exceed 20%. However, the performance of devices fabricated by photolithography still cannot meet the requirements of practical applications. The maximum operating brightness is still less than 200,000 cd/m², and most T95 lifespans are less than 10,000 hours, indicating that there is still a significant gap compared with traditional QLED devices.There are many reasons for this situation. For example, patterning methods using in-situ ligand exchange, stripping, or cleavage can easily cause many defects on the surface of quantum dots, damaging their fluorescence properties. Ligand cross-linking strategies often rely on specific functional ligands, which limits their general applicability. In addition, the gas released during the cross-linking process will reduce the film quality of the functional layer and adversely affect the performance of the electroluminescent device. The use of functional polymers for cross-linking can alleviate the above-mentioned ligand-related problems. However, high molecular weight polymer molecules tend to disperse unevenly in nonpolar quantum dot solutions, leading to compatibility issues and rapid deterioration of device performance. In addition, these functional polymer materials often suffer from batch-to-batch inconsistencies during their synthesis and purification processes, which can lead to severe
This compromises batch-to-batch repeatability of device performance and increases the difficulty of mass production.To achieve ultrahigh brightness and stable photopatterned devices, it is crucial to integrate highly compatible quantum dot-crosslinker blends into the original QLED structure. We discovered a self-crosslinking small molecule material, CBP-V, that is compatible with our synthesized CdZnSe. /ZnSeS/ZnS quantum dots exhibit excellent compatibility at the molecular level. This material shows the potential to implement a direct photolithography process that can meet the photolithography requirements while maintaining the electroluminescent properties of the quantum dots. Previous studies have shown that its molecular structure is excellent.
The terminal unsaturated carbon-carbon double bonds can generate active free radicals at high temperatures and initiate cross-linking reactions. However, precisely controlling submicron patterns through high-temperature thermal cross-linking is challenging because heat easily diffuses and this high-temperature treatment reduces the luminescence properties of quantum dots. Recently, Wu et al. used a photothermal synergistic strategy to cross-link CBP-V, lowering the annealing temperature to 100°C. Nonetheless, this approach requires the use of silicon templates to assist in the formation of self-assembled quantum dot patterns prior to cross-linking, which imposes stringent process requirements, limits throughput, and complicates large-scale production.Introduction by Shen Huaibin, Wang Shujie, Zou Shijie and others from Henan University
A light-induced cross-linking method is proposed to form an effective cross-linked CBP-V network at room temperature by adding a photoinitiator. The network firmly anchors the quantum dots, allowing them to form precise patterns after development. Characterization techniques such as FTIR spectroscopy were used to further elucidate the chemical mechanism of cross-linked network formation. This photolithographic strategy eliminates the negative factors that compromise QLED device performance, including high temperatures, ligand stripping, and material decomposition. As a result, it maintains the luminescent properties of quantum dots and enables the development of ultra-bright, stable QLEDs using direct optical lithography. Furthermore, the hybrid luminescent layer composed of cross-linked CBP-V network and CdZnSe/ZnSeS/ZnS quantum dots exhibits suitable energy level alignment in the classic QLED structure. Ultimately, a high-resolution quantum dot pattern with a feature size of 2 µm (equivalent to approximately 6350 pixels per inch) was achieved. The cross-linked/developed device demonstrated a record brightness of 1,167,822 cd/m², a maximum external quantum efficiency of 18.47%, and a long T95 lifetime of 12,860 hours at 1000 cd/m² via a simple glass sheet encapsulation method. These
The results represent a significant advance for quantum dots toward next-generation displays, particularly for ultra-high-brightness applications.
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