Submonolayer Quantum Dot Light-Emitting Devices Enable Efficient Sub-Bandgap-voltage Emission

Quantum dot light-emitting diodes (QLEDs) with high external quantum efficiency (EQE) are attractive candidates for wearable integrated displays. However, the pursuit of high power efficiency—especially high EQE at low operating voltages-has long been hampered by interlayer charge injection barriers and inefficient intralayer charge transport. Here, we report a rational strategy to construct a submonolayer (sub-ML) quantum dot (QD) emissive layer (EML) using Langmuir-Blodgett assembly. We fabricate a closely packed monolayer of QD/NaYF4 hybrid nanoparticles (NPs), wherein each QD is precisely positioned to establish direct contact with both the electron transport layer (ETL) and hole transport layer (HTL), thus eliminating vertical charge transport between adjacent QDs. Critically, partial replacement of QDs with insulating NaYF4 NPs introduces localized electrostatic fields via embedded equivalent nanocapacitors, which substantially enhances the hole injection level. The sub-ML QLEDs realize efficient sub-bandgap-voltage emission, delivering a practical luminance of 585 cd m−2 at a low driving voltage of 1.8 V. These devices exhibit a maximum external power efficiency of 23.7