Low-Temperature Cross-Linkable Hole Transport Materials for Solution-Processed Quantum Dot and Organic Light-Emitting Diodes with High Efficiency and Color Purity

Photothermally Cross-Linkable Polymeric Hole Transport Material Functionalized with Azide for High-Performance Quantum Dot Light-Emitting Diodes

Poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl)))] (TFB) is a widely used hole transport material (HTM) in quantum dot light-emitting diodes (QLEDs). However, TFB-based solution-processed QLEDs face several challenges, including interlayer erosion, low hole mobility, shallow energy level of the highest occupied molecular orbital, and current leakage, which compromise the device efficiency and stability. To overcome these challenges, bromine and azide-based photothermally cross-linkable TFB derivatives, i.e., TFB-Br and TFB-N3, were designed and synthesized. TFB-N3 photothermally cross-linked under 254 nm ultraviolet light at 140 °C exhibited excellent solvent resistance within 30 s. Furthermore, the photothermally cross-linked TFB-N3 formed a compact three-dimensional (3D) network in QLEDs, enhancing hole transport and reducing the leakage current. Moreover, the HOMO energy level in photothermally cross-linked TFB-N3 decreased to -5.39 eV from that in TFB (-5.30 eV), reducing the hole transport energy barrier. Thus, the charge balance in the quantum dot (QD) layer was enhanced, and the current leakage was reduced, improving the overall QLED performance. The photothermally cross-linked TFB-N3-based QLEDs achieved a maximum external quantum efficiency of 19.53%, i.e., 61% higher than that of devices using TFB. Moreover, the T90 lifetime of the photothermally cross-linked TFB-N3-based QLEDs was 4.49 times longer than that of TFB-based devices. The proposed strategy demonstrates that incorporating azide groups into polymeric HTMs can considerably enhance their hole transport and solvent resistance and reduce leakage current, improving QLED efficiency and stability.

Low-Temperature Cross-Linked Hole Transport Layer for High-Performance Blue Quantum-Dot Light-Emitting Diodes

Quantum-dot light-emitting diodes (QLEDs), a kind of promising optoelectronic device, demonstrate potential superiority in next-generation display technology. Thermal cross-linked hole transport materials (HTMs) have been employed in solution-processed QLEDs due to their excellent thermal stability and solvent resistance, whereas the unbalanced charge injection and high cross-linking temperature of cross-linked HTMs can inhibit the efficiency of QLEDs and limit their application. Herein, a low-temperature cross-linked HTM of 4,4'-bis(3-(((4-vinylbenzyl)oxy)methyl)-9H-carbazol-9-yl)-1,1'-biphenyl (DV-CBP) with a flexible styrene side chain is introduced, which reduces the cross-linking temperature to 150 °C and enhances the hole mobility up to 1.01 × 10-3 cm2 V-1 s-1. More importantly, the maximum external quantum efficiency of 21.35% is successfully obtained on the basis of the DV-CBP as a cross-linked hole transport layer (HTL) for blue QLEDs. The low-temperature cross-linked high-mobility HTL using flexible side chains could be an excellent alternative for future HTL development.

Photothermal Synergic Cross-Linking Hole Transport Layer for Highly Efficient RGB QLEDs

Developing an insoluble cross-linkable hole transport layer (HTL) plays an important role for solution-processed quantum dots light-emitting diodes (QLEDs) to fabricate a multilayer device with separated quantum dots layers and HTLs. In this work, a facile photothermal synergic cross-linking strategy is simultaneous annealing and UV irradiation to form the high-quality cross-linked film as the HTL without any photoinitiator, which efficiently reduces the cross-linking temperature to the low temperature of 130 °C and enhances the hole mobility of the 3-vinyl-9-{4-[4-(3-vinylcarbazol-9-yl)phenyl]phenyl}carbazole (CBP-V) thin films. The obtained high-quality cross-linked CBP-V films exhibited smooth morphology, excellent solvent resistance, and high mobility. Moreover, the high-performance red, green, and blue (RGB) QLEDs are successfully fabricated by using the photothermal synergic cross-linked HTLs, which achieved the maximum external quantum efficiency of 25.69, 24.42, and 16.51%, respectively. This work presents a strategy of using the photothermal synergic cross-linked HTLs for fabrication of high-performance QLEDs and advancing their related device applications.