Improved Stability of Quantum Dot Light-Emitting Diode with Alternatingly Doped Structure of Hole Transport Layer

Quantum dot light-emitting diodes (QLEDs) have garnered attention as next-generation light-emitting diodes (LEDs) due to their tunable spectra, wide color gamut, high color purity, cost-effective processing, and excellent optoelectronic properties. While extensive research has improved the performance of InP-based QLEDs, their stability needs further enhancement to compete with organic LEDs. InP quantum dots face challenges such as imprecise synthesis and weak electron stress, leading to lower exciton stability compared to Cd-based QDs. Therefore, improving the chemical and device engineering of InP QDs is essential for enhancing the stability of QLEDs.<br/>Using a thick hole transport layer is an effective method for enhancing the lifetime and stability of QLEDs. However, increasing HTL thickness also increases resistance. Furthermore, the underlayer washing out during solution processing complicates the use of the thick HTL. Typically, doped charge transport layers using transition metal oxides such as MoO₃, WO₃ or organic molecules with high electron affinity are employed to increase the conductivity of the organic charge transport layers. Additionally, delta doping has been investigated as a method to precisely control the doping profile by inserting thin dopant layers at the nanometer scale in organic LEDs.<br/>In this study, we propose a delta-doping structure with high conductivity and thick, solution-processed HTL to enhance the lifetime of QLEDs. The structure consists of alternating stacked layers of N4,N4′-di(naphthalen-1-yl)-N4,N4′-bis(4-vinylphenyl)biphenyl-4,4′-diamine (VNPB) and phosphomolybdic acid (PMA). We investigated the conductivity enhancement mechanism in alternatingly doped structure and examined the effect of HTL thickness on QLED performance and operational lifetime. This research represents a significant advancement in the commercialization of QLEDs, with substantial implications for new technologies and applications.