Theoretical Insight On Molecular Orientation Of Thermally Activated Delayed Fluorescent Molecules In Vacuum Deposition

Since the first report of organic light-emitting diode(OLED)by Ching Tang and co-workers in 1987,OLED materials have attracted wide attention due to their high efficiency,ultra-thin,high brightness,low power consumption and good flexibility.In 2012,Adachi and co-workers successfully prepared thermally activated delayed fluorescent materials(TADF),which achieved nearly 100%internal quantum efficiency of electroluminescence.The energy gap(?E_ST)between singlet state and triplet state of TADF is very small,which makes it easy for triplet excitons to efficiently convert to singlet state and then radiate to ground state,so as to achieve 100%exciton utilization.Although numerous TADF materials with high quantum efficiency have been reported,the efficiency roll off of OLED devices based on TADF emitters is always large,and the EQE value of such devices is still facing challenges and bottlenecks,especially the blue light-emitting materials.It is reported that the horizontal orientation of TADF emitters will affect the external light out-coupling efficiency,thus improving the EQE of TADF-OLED devices.Due to the limitation of experimental conditions,however,it is difficult to understand the micro process and influencing factors of TADF molecular orientation from the molecular level,so it's hard to know the influence of these micro factors on the EQE of TADF molecular devices.Therefore,it is very critical to reveal the micro mechanism and influencing factors of TADF molecular horizontal orientation.In order to explore these problems,this paper will focus on the following three aspects:1.Theoretical insight into molecular orientation for Thermally Activated Delayed Fluorescence Emitters in vacuum depositionIn order to explore the micro mechanism of TADF molecular horizontal orientation,the vacuum deposition of indolocarbazole-isomer derivatives,as highly emissive emitters for TADF-OLEDs,was simulated by use of all atomistic molecular dynamics(MD)simulations and quantum mechanical analysis.It is shown from the results that the molecular orientation in vacuum deposition is determined by the intermolecular?-?stacking.There are the strong van der Waals interactions(mainly dispersion)between emitters and substrate due to the intermolecular?-?stacking.The van der Waals forces lead to the formation of an energetically stable molecular conformation,which makes emitters as stable as possible at the horizontal orientation relative to substrate.The compact?-?stacking of emitters on the substrate is responsible for emitter-substrate weak interactions.Furthermore,theoretical optical properties of indolocarbazole-isomer derivatives are in good accordance with experiments.In this work,we revealed the microscopic mechanism of molecular orientation of TADF molecules in vacuum deposition,that is,the horizontal orientation mainly depends on the degree of?-?stacking between emitters and substrate,which provides a theoretical insight for understanding the molecular orientation in vacuum deposition.2.Effect of temperature and host molecules on molecular orientation of TADF emitters in vacuum depositionTo further explore the objective factors affecting the molecular horizontal orientation of TADF,the vacuum depositions of two rod-like blue TADF emitters were simulated by use of all atomistic molecular dynamics to investigate the effect of substrate temperature and guest-host doping ratios on the molecular orientation of emitters.As shown,the molecular orientation of emitters relative to substrate is greatly affected by the Van der Waals interactions between emitter and substrate,and the substrate temperature and guest-host doping ratios of emitters can considerably regulate the Van der Waals forces.Controlling the substrate temperature below the glass temperature of substrate and the moderate high guest-host doping ratios are beneficial to the horizontal arrangement of emitters relative to the substrate,and thus to the improvement of the photoelectric performance of emitters.This work has theoretical significance for understanding the origin of molecular orientation of emitters and designing high performance emitter materials.3.Controlling the horizontal orientation of thermally activated delayed fluorescent molecules by weak hydrogen bondsIn order to further adjust the horizontal orientation of TADF molecules,we used the classical TADF molecule(PXZ-TRZ)as the research object,strategically used pyridine group,designed a molecule based on pxz-trz,and simulated the vacuum deposition process of these two kinds of TADF molecules with good linear structure by using all atom molecular dynamics method.The results show that the TADF molecules between layers mainly rely on van der Waals interaction generated by?-?stacking,so as to maintain the relative horizontal orientation,which is also the essence of molecular horizontal orientation.However,TADF molecules in the same horizontal plane can rely on the weak hydrogen bonds between molecules to obtain better horizontal orientation.In this work,we have regulated the horizontal orientation of TADF molecules by hydrogen bonding,and revealed the micro mechanism of hydrogen bonding regulating the horizontal orientation of TADF molecules,which provides theoretical guidance for the design of efficient and reasonable TADF materials.