Singlet and triplet harvesting enable efficient NIR-II quantum-dot electroluminescence

Colloidal quantum dots (CQDs) are promising materials for constructing 'second-window' near-infrared (1000-1700 nm) light-emitting diodes (NIR-II LEDs), but their practical application has been hampered by low film external quantum efficiency (EQE). Here, we report a chemical strategy that incorporates photoactive fluorophores-spanning fluorescence, phosphorescence and thermally activated delayed fluorescence-into CQD films to boost NIR-II emission. Energy transfer from fluorophores (via both singlet and triplet pathways) raises the photoluminescence quantum efficiency of CQD to 85% beyond 1000 nm. As a result, these composite films power NIR-II LEDs with a record EQE of 25.3% for emission of >1000 nm, the highest among all LEDs with emission of >1000 nm. We further demonstrate the scalability of the approach by fabricating large-area (30 mm × 30 mm) NIR-II LEDs with uniform high performance.