Surface Ligand-Mediated Carrier Balance for Stable Inverted Red InP Quantum Dot Light-Emitting Diodes

Abstract

Indium phosphide (InP)-based quantum dots (QDs) are cadmium (Cd)-free emitters for display applications, but their operational stability is limited by surface defects and imbalanced carrier injection. Here, we introduce a [2-(9H-carbazol-9-yl)ethyl]phosphonic acid (2PACz)-mediated surface modification strategy for efficient and stable quantum dot light-emitting diodes (QLEDs). 2PACz forms a mixed-ligand surface by partially replacing native oleic acid ligands and binding to under-coordinated surface sites, thereby reducing trap-assisted nonradiative recombination. In parallel, 2PACz-mediated energy-level modulation facilitates hole injection and suppresses excessive electron injection, improving carrier balance in inverted red InP-based QLEDs. The resulting devices achieved a peak external quantum efficiency of 23.7% and an extrapolated half-lifetime of 217.8 h at an initial luminance of 1000 cd m⁻². Transient and chemical analyses reveal that 2PACz treatment stabilizes carrier dynamics and suppresses interfacial degradation under electrical stress. This work shows that surface ligand design can improve operational stability of Cd-free QLEDs by coupling QD surface passivation with carrier balance control.