Dipole Engineering through the Orientation of Interface Molecules for Efficient InP Quantum Dot Light-Emitting Diodes

Area of Science:

• Materials Science
• Optoelectronics
• Nanotechnology

Background:

• Indium phosphide (InP) quantum dots (QDs) offer heavy-metal-free, efficient, size-tunable light-emitting diodes (QLEDs).
• Replacing organic hole-injection layers (HILs) with inorganic ones can improve QLED stability.
• Current inorganic HILs exhibit poor hole injection due to shallow work functions.

Purpose of the Study:

• Investigate work function tuning of nickel oxide (NiO) HILs using self-assembled molecules (SAMs).
• Determine the impact of SAM molecular orientation and dipole on NiO work function.
• Optimize HILs for efficient hole injection and high QLED performance.

Main Methods:

• Density functional theory (DFT) simulations to model SAM interactions with NiO.
• Near-edge X-ray absorption fine structure (NEXAFS) to analyze molecular orientation.
• Fabrication and characterization of InP QLEDs with modified NiO HILs.

Main Results:

• SAM molecular orientation significantly influences NiO work function tuning, more than the intrinsic molecular dipole.
• Nitro group (NO2)-terminated SAMs, oriented parallel to NiO, showed limited tuning and caused luminescence quenching.
• Trifluoromethyl group-terminated SAMs, with an angled orientation, improved hole injection without quenching.

Conclusions:

• SAM molecular orientation is critical for optimizing inorganic HILs in QLEDs.
• Angled SAMs with trifluoromethyl groups enhance hole injection and prevent quenching in InP QLEDs.
• Achieved a record 18.8% external quantum efficiency (EQE) for InP QLEDs with inorganic HILs.