College of Science and Engineering, James Cook University, Townsville, QLD 4811, Australia.
Centre for Organics Photonics & Electronics, School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia.
J Chem Phys. 2019 Mar 7;150(9):094110. doi: 10.1063/1.5083639.
Ir(ppy):CBP blends have been widely studied as the emissive layer in organic light emitting diodes (OLEDs), yet crucial questions about charge transport within the layer remain unaddressed. Recent molecular dynamics simulations show that the Ir(ppy) molecules are not isolated from each other, but at concentrations of as low as 5 wt. % can be part of connected pathways. Such connectivity raises the question of how the iridium(iii) complexes contribute to long-range charge transport in the blend. We implement a kinetic Monte Carlo transport model to probe the guest concentration dependence of charge mobility and show that distinct minima appear at approximately 10 wt. % Ir(ppy) due to an increased number of trap states that can include interconnected complexes within the blend film. The depth of the minima is shown to be dependent on the electric field and to vary between electrons and holes due to their different trapping depths arising from the different ionization potentials and electron affinities of the guest and host molecules. Typical guest-host OLEDs use a guest concentration below 10 wt. % to avoid triplet-triplet annihilation, so these results suggest that optimal device performance is achieved when there is significant charge trapping on the iridium(iii) complex guest molecules and minimum interactions of the emissive chromophores that can lead to triplet-triplet annihilation.
铱(ppy)：CBP 混合物作为有机发光二极管 (OLED) 的发射层已得到广泛研究，但有关该层内电荷输运的关键问题仍未得到解决。最近的分子动力学模拟表明，铱(ppy)分子彼此之间并非孤立存在，而是在低至 5wt.%的浓度下就可以成为相互连接的通路的一部分。这种连通性引发了一个问题，即三价铱络合物如何在混合物中促进长程电荷输运。我们实施了一个动力学蒙特卡罗输运模型来探测客体浓度对电荷迁移率的影响，结果表明，由于陷阱态数量的增加，在约 10wt.%的铱(ppy)浓度下出现了明显的最小值，这些陷阱态可能包括混合物薄膜中相互连接的络合物。最小值的深度取决于电场，并由于客体和主体分子的不同电离势和电子亲和力导致电子和空穴的不同俘获深度而在电子和空穴之间有所差异。典型的客体-主体 OLED 采用低于 10wt.%的客体浓度来避免三重态-三重态湮灭，因此这些结果表明，当铱(iii)络合物客体分子上存在显著的电荷俘获，并且发光发色团之间的相互作用最小，从而导致三重态-三重态湮灭时，器件性能最佳。
