Efficient and stable PEDOT:PSS-free quantum dot light-emitting diodes enabled by bidentate self-assembled monolayers

The development of stable and efficient hole injection layers (HIL) is crucial for advancing quantum dot light-emitting diodes (QLEDs). Self-assembled monolayers (SAMs) are promising candidates due to their ability to tailor interfacial properties through molecular design. We propose and validate a design strategy that integrates strong bidentate anchoring with a hole-transport moiety in a single molecule to create an ideal SAM-based HIL. Following this strategy, a novel bidentate molecule, (3,3′-((4-(diphenylamino)phenyl)azanediyl)dibenzoic acid, DA), is designed. Its dual carboxyl groups ensure robust anchoring on indium tin oxide, forming a uniform monolayer, while the triphenylamine units provide excellent hole-transport capability. Critically, the electron-withdrawing carboxyl groups stabilize the highest occupied molecular orbital level of DA, enabling favorable energy alignment with common hole transport materials for efficient injection. Consequently, red QLEDs based on the DA SAM achieve not only a high peak external quantum efficiency of 18.80 % but also an extended T ₉₅ lifetime of 138 h, significantly outperforming devices with commercial HILs or no HIL. This work validates the molecular design strategy and establishes DA SAMs as a superior HIL for efficient QLEDs.