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

卤化铅钙钛矿（LHP）是公知的材料，最近在光电子学中获得了相当大的关注。特别地，组成CsPbX3（其中X=Cl、Br、I）的LHP纳米晶体作为发光材料对于发光二极管中的潜在应用是非常有吸引力的。它们将本体LHP的优点（特别是它们的缺陷容限、溶液可加工性和带宽可调谐性）与胶体量子点的众所周知的特征（如具有窄发射带宽的高光致发光量子产率以及尺寸和组成可调谐的颜色）结合联合收割机。在钙钛矿发光二极管（PeLED）商业化之前要解决的基本问题是LHP在电流下的稳定性差。在常规PeLED中，电子注入LHP层导致元素铅的不可逆形成和LHP结构的破坏。此外，在LHP的高离子迁移率提出的问题，在PeLED的电流的一个显着的比例是由离子提供的，这使得没有贡献的电致发光和LHP结构上的额外的不稳定的影响。我们解决这些固有的问题PeLED通过使用有机π系统作为表面配体的纳米晶体。因此，它们实现了三个目的：1）通过饱和表面缺陷，它们增加了纳米晶体的PLQY。2)通过诱导表面偶极子，它们增加了LHP层的功函数，从而增加了负电荷下的稳定性。3)刚性结构与有机π系统的良好电荷载流子导电性相结合抑制离子扩散并增加总电流的电子部分。该项目研究了这一策略对PeLED的影响，涵盖了完整的RGB配色方案，并在外部量子效率、亮度和长期稳定性方面优化了器件。计算建模伴随着材料开发并指导其优化。

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.