Electroluminescent perovskite nanocrystals - From tailor-made assemblies to optoelectronic properties

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Lead halide perovskites (LHP) are well-known materials that have recently gained considerable attention in optoelectronics. Particularly LHP nanocrystals of the composition CsPbX3 (with X=Cl, Br, I) are very attractive as luminescent materials for potential applications in light-emitting diodes. They combine the advantages of bulk LHP – notably their defect tolerance, solution processability, and band-width tunability – with well-known features of colloidal quantum dots, like high photoluminescence quantum yield with narrow emission bandwidth as well as size and composition tunable colors. A fundamental problem to be addressed before the commercialization of perovskite light-emitting diodes (PeLEDs) is the poor stability of LHPs under an electric current. In conventional PeLEDs, the injection of electrons into the LHP layer leads to the irreversible formation of elemental lead and the destruction of the LHP structure. In addition, the high ion mobility in LHPs poses the problem that a significant proportion of the current in a PeLED is provided by ions, which makes no contribution to electroluminescence and has an additional destabilizing effect on the LHP structure.We address these inherent problems of PeLEDs by using organic pi-systems as surface ligands on the nanocrystals. As such, they fulfill three purposes: 1) By saturating surface defects, they increase the PLQY of the nanocrystals. 2) By inducing a surface dipole, they increase the work function of the LHP layer and thus the stability under negative charging. 3) The rigid structure in combination with the good charge carrier conductivity of the organic pi-systems suppresses ion diffusion and increases the electronic part of the total current. The project investigates the impact of this strategy on PeLEDs, covering the complete RGB color scheme and optimizing the devices in terms of external quantum efficiency, luminance and long-term stability. Computational modeling accompanies the material development and guides its optimization.