Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA.
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA.
Adv Mater. 2017 Oct;29(40). doi: 10.1002/adma.201701987. Epub 2017 Sep 11.
Multiexcited-state phenomena are believed to be the root cause of two exigent challenges in organic light-emitting diodes; namely, efficiency roll-off and degradation. The development of novel strategies to reduce exciton densities under heavy load is therefore highly desirable. Here, it is shown that triplet exciton lifetimes of thermally activated delayed-fluorescence-emitter molecules can be manipulated in the solid state by exploiting intermolecular interactions. The external heavy-atom effect of brominated host molecules leads to increased spin-orbit coupling, which in turn enhances intersystem crossing rates in the guest molecule. Wave function overlap between the host and the guest is confirmed by combined molecular dynamics and density functional theory calculations. Shorter triplet exciton lifetimes are observed, while high photoluminescence quantum yields and essentially unaltered emission spectra are maintained. A change in the intersystem crossing rate ratio due to increased dielectric constants leads to almost 50% lower triplet exciton densities in the emissive layer in the steady state and results in an improved onset of the photoluminescence quantum yield roll-off at high excitation densities. Efficient organic light-emitting diodes with better roll-off behavior based on these novel hosts are fabricated, demonstrating the suitability of this concept for real-world applications.
多激发态现象被认为是有机发光二极管中两个紧迫挑战的根源；即效率下降和降解。因此，非常需要开发新的策略来降低重载下的激子密度。在这里，通过利用分子间相互作用，显示出热活化延迟荧光发射器分子的三重态激子寿命可以在固态中进行操纵。溴化主体分子的外部重原子效应导致自旋轨道耦合增加，这反过来又提高了客体分子的系间穿越速率。通过组合分子动力学和密度泛函理论计算证实了主体和客体之间的波函数重叠。观察到较短的三重态激子寿命，同时保持高的光致发光量子产率和基本不变的发射光谱。由于介电常数增加导致的系间穿越速率比的变化导致在稳态下发射层中的三重态激子密度几乎降低了 50%，并且在高激发密度下导致光致发光量子产率滚降的起始改善。基于这些新型主体制造了具有更好滚降行为的高效有机发光二极管，证明了该概念适用于实际应用。
