Qu Boning, Ding Kan, Sun Kai, Hou Shaocong, Morris Steven, Shtein Max, Forrest Stephen R
Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States.
Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, United States.
ACS Nano. 2020 Oct 27;14(10):14157-14163. doi: 10.1021/acsnano.0c07017. Epub 2020 Oct 5.
Fast deposition of thin films is essential for achieving low-cost, high-throughput phosphorescent organic light-emitting diode (PHOLED) production. In this work, we demonstrate rapid and uniform growth of semiconductor thin films by organic vapor phase deposition (OVPD). A green PHOLED comprising an emission layer (EML) grown at 50 Å/s with bis2-(2-pyridinyl-)phenyl-iridium(III) (Ir(ppy)(acac)) doped into 4,4'-bis(-carbazolyl)-1,1'-biphenyl (CBP) exhibits a maximum external quantum efficiency of 20 ± 1%. The morphology, charge transport properties, and radiative efficiency under optical and electrical excitation of the PHOLED EML are investigated as functions of the deposition rate both experimental and theoretical approaches. The EML shows no evidence for gas phase nucleation of the organic molecules at deposition rates as high as 50 Å/s. However, the roll-off in quantum efficiency at high current progressively increases with deposition rate due to enhanced triplet-polaron annihilation. The roll-off results from accumulation of stress within the PHOLED EML that generates a high density of defect states. The defects, in turn, act as recombination sites for triplets and hole polarons, leading to enhanced triplet-polaron annihilation at high current. We introduce a void nucleation model to describe the film morphology evolution that is observed using electron microscopy.
快速沉积薄膜对于实现低成本、高通量的磷光有机发光二极管（PHOLED）生产至关重要。在这项工作中，我们展示了通过有机气相沉积（OVPD）实现半导体薄膜的快速且均匀生长。一种绿色PHOLED，其发射层（EML）以50 Å/s的速率生长，双2-(2-吡啶基)-苯基-铱(III)（Ir(ppy)(acac)）掺杂在4,4'-双咔唑基-1,1'-联苯（CBP）中，其最大外量子效率为20±1%。通过实验和理论方法，研究了PHOLED EML在光学和电激发下的形貌、电荷传输特性以及辐射效率与沉积速率的关系。在高达50 Å/s的沉积速率下，EML没有显示出有机分子气相成核的证据。然而，由于三重态-极化子湮灭增强，高电流下量子效率的滚降随着沉积速率逐渐增加。这种滚降是由PHOLED EML内应力积累导致产生高密度缺陷态引起的。这些缺陷反过来又充当三重态和空穴极化子的复合位点，导致高电流下三重态-极化子湮灭增强。我们引入了一个空穴成核模型来描述使用电子显微镜观察到的薄膜形貌演变。
