基于N-杂卡宾铱配合物的深蓝光磷光材料的设计及其性能研究

Deep-blue organic light-emitting diodes has been one of the most popular research topics in recent years. N-heterocyclic carbene based iridium complexes could achieve deep-blue phosphorescence through raising their lowest unoccupied molecular orbitals (LUMO). However, high energy levels of their LUMOs increase the energy barriers for electron transporting, resulting in a non-balanced distribution of charge carriers inside the emissive layers. Meanwhile, the photoluminescence quantum yields of these complexes are relatively low. Therefore, the deep-blue organic light-emitting diodes by using these complexes as emitters exhibit relatively inferior performance. To solve these problems, in this project, the groups with σ-electron withdrawing properties and intraligand lock are introduced into the chemical structures of iridium complexes. The former is to reduce the interaction between ligand and iridium so that the LUMO level is decreased, tuning their energy levels. The intraligand lock is to control the planarity of the ligand and the rigidity of the complex, giving an increased quantum yield. First, we will synthesize these N-heterocyclic carbene based iridium complexes in the project. Then, the influence of chemical structures of the ligands on the properties, such as coordination bonds, the highest occupied molecular orbitals/LUMO/triplet energy levels, photoluminescence, et al will be fully investigated. Lastly, by choosing the excellent iridium complexes as emitters and exciton blocking materials, the deep-blue light-emitting devices will be fabricated. And we will find how the chemical structures of emitters and architectures of devices affect their properties of electroluminescence. This project aims to developing new N-heterocyclic carbene based iridium complexes and designing the architectures of devices according to the properties of these iridium complexes, so as to achieve highly efficient deep-blue OLEDs.

深蓝光有机发光二极管是近几年的研究热点之一。N-杂卡宾铱配合物通过提高最低未占分子轨道 (LUMO)，增加材料能级带隙，实现深蓝光。然而，高LUMO增加电子传输的能垒，导致载流子非平衡地分布在发光层内，加之材料发光量子效率低，使其深蓝光发光二极管的性能不尽人意。为了解决如上问题，本项目拟在卡宾配体上增加σ电子攫取基团和设计配体“锁”。前者旨在削弱配体与铱的相互作用，有利于降低LUMO，调控能级；后者为了控制配体的平面性和配合物的分子刚性，增加量子效率。合成本项目涉及的配合物，并研究配体化学结构对配位键形成、配合物的最高占有分子轨道/LUMO/三线态能级、光致发光性能等的影响；选用合适的配合物作为发光材料和激子阻挡材料，制备发光器件并研究配合物对电致发光性能的影响。本项目旨在发展新的N-杂卡宾铱配合物并根据其性能特点设计合理的器件结构，以实现高效的深蓝光有机发光二极管。

N-杂卡宾铱配合物通过提高最低未占分子轨道（LUMOs）从而实现宽带隙的蓝色磷光。然而，高LUMO能级导致向发光层注入电子困难，发光层内载流子注入不平衡，加之其荧光量子产率（QY）低，使基于N-杂卡宾铱配合物的蓝色磷光有机二极管（PHOLEDs）效率较低。本项目从铱配合物化学结构设计出发，建立化学结构与材料光物理性能和轨道能级（包括最高占据分子轨道（HOMO）、LUMO及三线态）之间的相互关系，实现具有高效、稳定三线态发射的和能级可调控的蓝色磷光材料。结合器件工程，实现发光层内平衡的载流子注入，实现高性能的蓝色PHOLEDs。1）添加吸电子基团于贡献HOMO能级轨道一端，实现QY超过60%、发射峰位于460-470 nm、具有稳定HOMO/LUMO能级和稳健三线态的磷光铱配合物。其蓝色PHOLED发光亮度超过10000 cd m^-2、外量子效率（EQE）超过20%。2）通过叠层器件结构设计解决发光层和电子传输层界面上的电子堆积，实现了EQE为32.3%、CIE色坐标为（0.15，0.10）的高性能纯蓝色PHOLEDs。同时，在贡献LUMO能级轨道一端添加吸电子基团，实现QY超过45%、发射峰位于700 nm、具有稳健三线态的近红外磷光铱配合物，其PHOLED器件EQE（~10%）在当年均处于该领域的领先水平。此外，上述能级调控策略同样可用于构建高性能量子点发光二极管（QLEDs）。通过设计聚合物化学结构、量子点核/壳结构及氧化锌表面，依次实现具有稳定HOMO能级的空穴传输聚合物、具有“空穴注入跳板”的量子点、具有较少表面缺陷的电子传输材料。这些策略有利于实现发光层内载流子平衡的注入和减小氧化锌表面缺陷对发光层激子的猝灭，从而明显提升QLEDs的性能表现。综上，通过建立材料化学结构与发光性能、能级之间的相互关系，结合器件工程，实现高性能的发光材料和实现发光层内平衡的载流子注入是实现高效率PHOLEDs和QLEDs的一种重要策略。