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

卤化铅钙钛矿（LHP）是一种众所周知的材料，近年来在光电子学领域引起了相当大的关注。特别是组成CsPbX3 （X=Cl, Br， I）的LHP纳米晶体作为发光材料在发光二极管中的潜在应用非常有吸引力。他们结合了大块LHP的优点-特别是其缺陷容错性，溶液可加工性和带宽可调性-以及胶体量子点的众所周知的特征，如高光致发光量子产率，窄发射带宽以及尺寸和组成可调的颜色。在钙钛矿发光二极管（PeLEDs）商业化之前需要解决的一个基本问题是lhp在电流下的稳定性差。在传统的等离子体发光二极管中，向LHP层注入电子会导致元素铅的不可逆形成和LHP结构的破坏。此外，LHP中的高离子迁移率带来了一个问题，即PeLED中相当大比例的电流是由离子提供的，这对电致发光没有贡献，并且对LHP结构有额外的不稳定作用。我们通过使用有机pi系统作为纳米晶体的表面配体来解决pled的这些固有问题。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，增加了LHP层的功函数，从而提高了负电荷下的稳定性。3)有机pi体系的刚性结构与良好的载流子电导率相结合，抑制了离子扩散，增加了总电流的电子部分。该项目研究了这种策略对pled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的发展并指导其优化。

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.