Surface ligand engineering-assisted CsPbI3 quantum dots enable bright and efficient red light-emitting diodes with a top-emitting structure

All-inorganic CsPbX3 (X = Cl, Br, I) quantum dots (QDs) are promising materials for light-emitting diodes (LEDs), but the long insulating ligands such as oleic acid (OA) and oleylamine (OLA) on the QD surface limit the device performances. They reduce the electrical conductivity of QDs, while their loss deteriorates the photoluminescence and stability. To overcome these issues, octylphosphonic acid (OPA) with a shorter carbon chain was added in the synthesis of perovskite QDs to partially replace OA ligands. Owing to the strong interaction between OPA and Pb atoms, the undercoordinated Pb atoms on the surface of CsPbI3 QDs can be effectively passivated by OPA, leading to improved fluorescence quantum efficiency (close to unity) and solution stability. Furthermore, the electrical conductivity of QD films was also enhanced from 5.3 x 10(-4) to 1.1 x 10(-3) S/m by using shorter chain OPA ligands. Finally, highly efficient red perovskite QD LEDs with peak external quantum efficiency (EQE) of 12.6% and maximum luminance of 10171 cd m(-2) were achieved in a top-emitting device structure, a nearly 3-fold enhancement in luminance compared with pristine CsPbI3 QDs based LEDs.