Optimizing Charge Injection Balance in InP-Based Quantum Dot Light-Emitting Diodes Via Precise Control of Hole Transport Layer Cross-Linking Density

초록

Quantum dot light-emitting diodes (QLEDs) have attracted considerable attention as next-generation light-emitting devices capable of achieving high emission efficiency based on their organic-inorganic hybrid architecture. However, the environmental degradation and performance loss arising from the vulnerability of the organic layers within the device to oxygen and moisture remain a major factor limiting long-term device stability. To address this issue, we introduce VNPB (N-4,N-4 '-di(naphthalen-1-yl)-N-4,N-4 '-bis(4-vinylphenyl)biphenyl-4,4(')-diamine), a cross-linkable hole transport layer (HTL) material, into InP-based QLEDs and systematically investigate the effect of the cross-linking density, controlled by the post-annealing temperature, on the electrical and optical properties of the devices. VNPB contains terminal vinyl groups that enable the formation of an intermolecular covalent network during thermal annealing, thereby providing excellent solvent resistance and structural stability that suppress the re-dissolution of the underlying layer during multilayer solution processing. By varying the post-annealing temperature of the VNPB film from 160 to 190 degrees C, we elucidated the effect of the cross-linking density on the performance of InP-based QLEDs. Rinsing tests confirmed that the cross-linking density increased progressively with annealing temperature, forming a robust network. Device analysis showed that the 170 degrees C condition achieved a maximum EQE of 5.55% and the optimal operational lifetime (T-70 = 45.1 h). In contrast, devices annealed at 180-190 degrees C exhibited increased surface roughness and local current response. Together with the larger voltage variation observed under constant-current operation, these results suggest increased morphology-induced localized conduction and electrical instability, resulting in non-uniform charge transport and charge imbalance. This study demonstrates that controlling the cross-linking density of the hole transport layer can serve as an important processing strategy for regulating charge injection balance and recombination behavior in solution-processed QLEDs.

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