Air stable eco-friendly quantum dots with a light-mediated photoinitiator for an inkjet printed flexible light emitting diode

Printed quantum dot (QD) light emitting diodes (QLEDs) over a large scale have received much interest in the last decade owing to the demand for the next generation of self-emissive large-area displays. To realize printing technology for QLEDs, a pioneering study suggested using a co-solvent system or a polymer additive in CdSe-based QLEDs. However, limitations on printed QLEDs with eco-friendly QDs, including the selection of ink solvent, environmental degradation of QDs in the air by temperature and O-2/H2O level, and viscosity, make it difficult to achieve higher luminescence and external quantum efficiency (EQE) than that of ionic-bonded Cd-based QDs. Herein, we design an air-processable and stable ink with a photoinitiator (PI) mediating cross-linkage between eco-friendly QDs for inkjet-printed QLEDs. Once QD inks with a PI are deposited on the desired surface, their film polymerizes in situ through radical formation induced by ultraviolet (UV) exposure. Cross-linking reactions between ligands in the QDs reduce the distances between them, leading to flattening of the surface and enhancement of environmental stability in air. Printed InP-based green QLEDs demonstrated maximum luminescence values of 3600 cd m(-2) at 10 V on ITO/glass for the very first time. Finally, large-scale InP red/green/blue QLEDs directly printed with a bird image were fabricated on the flexible substrate.