Mixed-ligand engineering of quasi-2D perovskites for efficient sky-blue light-emitting diodes

Achieving efficient blue emission with spectrum stability in perovskite light-emitting diodes (PeLEDs) is a great challenge, although the external quantum efficiency (EQE) of PeLEDs for red and green emission has exceeded 20%. Here, we report efficient sky-blue PeLEDs with high brightness based on mixed-ligand confined quasi-2D perovskites. By detailed characterization of the optical and physicochemical properties of n-butylammonium (BA) or 2-phenylethylammonium (PEA)-based perovskites with various alkyl ammonium bromides as co-ligands, it is found that the molecular size and charge on the amino-groups of the co-ligand hold the key to constructing effective mixed-ligand systems. Accordingly, dimethylammonium (DMA) is an ideal co-ligand in both BA- and PEA-based quasi-2D perovskites. Thanks to its appropriate molecular size, DMA could help to confine the diameter of perovskite nanocrystals (NCs) and improve their dispersity in organic ligands. Besides, DMA is prone to disperse among PEA or BA molecules to cap the perovskite synergistically, and the phase of n = 1 caused by excess co-ligands could be effectively avoided benefiting from the relatively weak coordination ability of DMA due to the relatively small positive charge on amino-groups. The optimized PeLEDs based on BA(2)DMA(1.6)Cs(2)Pb(3)Br(11.6) and PEA(2)DMA(1.2)Cs(2)Pb(3)Br(11.2) exhibit color-stable sky-blue (490 nm) and bluish green (499 nm) emission with a record maximum luminance of 2825 and 7760 cd m(-2) respectively. The applicability of this approach in the red emission region is also demonstrated, which provides a versatile strategy for full spectrum modulation.