The Study On The Energy Transfer Mechanism Of Thermal Activation Delayed Fluorescence Luminescence Layer

Organic light-emitting diodes(OLEDs)are the most promising flat panel display technology due to their lightweight,thin,fast response time,and high refresh rate.Thermally activated delayed fluorescence(TADF)material is the third-generation OLED light-emitting material.Compared with traditional phosphorescent materials,it does not need to introduce precious metals to utilize triplet excitons.TADF materials typically have smaller Δ Est,and they can convert the triplet excitons into singlet excitons through the reverse intersystem crossing(RISC)process with the help of thermal activation.Therefore,TADF materials have shown great potential in the field of flat panel displays,and their light-emitting devices have been widely developed and applied.The lightemitting layer is the core part of OLED devices and the key factor affecting the performance of OLED light-emitting devices.It is necessary to study the working principle of the luminescent layer thin film of TADF devices to develop and design more efficient new OLED devices.This article used molecular dynamics to simulate the amorphous film structure of the luminescent layer and studied the energy transfer process and luminescence mechanism of exciton in the luminescent thin film of TADF devices by combining quantum mechanics and molecular mechanics.Our research provides theoretical guidance for designing and fabricating TADF luminescent devices with excellent performance.The main work of this paper is as follows:1.The effect of the donor groups of TADF molecules on exciton quenching processWe investigated the exciton quenching processes of two TADF molecules(PXZTRZ and DMAC-TRZ)with different donor functional groups.The amorphous structures of two non-doped films were obtained by molecular dynamics simulation.We used the ONIOM model to calculate the excited state properties of molecules in the amorphous films.We analyzed the interaction between molecules with the help of the energy molecule based on the molecular force field(EDA-FF)and the independent gradient model(IGMH).The research results indicate that PXZ-TRZ exhibits stronger π-πinteractions between molecules when forming non-doped thin films compared to DMACTRZ.This π-π interaction will inhibit molecular vibrations,limit the geometric structure deformation of the ground state and excited state of the molecule,and make the molecule have smaller recombination energy of the excited state.Besides,π-π interactions will strengthen exciton coupling between molecules.Therefore,for light-emitting devices made with PXZ-TRZ,more excitons in the luminescent layer will be quenched due to exciton energy transfer,resulting in poorer device performance compared to DMAC-TRZ.This work provides theoretical insights for understanding the influence of molecular structure on exciton quenching.2.The effect of substitution positions of host molecules on the energy transfer processWe investigated the exciton energy transfer process of two luminescent thin films formed by host molecules with isomeric functional group positions(2,6-2Cz BN and 3,5-2Cz BN)and the same guest luminescent molecule(4t Cz BN).First,we calculated the electronic structure properties of the three molecules using the density functional theory(DFT)and time-dependent density functional theory(TD-DFT).The singlet excitons are distributed in both carbazole(CZ)and benzonitrile(BN)fragments,and the triplet excitons are mainly distributed in benzonitrile fragments.We obtained the amorphous morphologies of the two luminescent thin films by molecular dynamics.We analyzed the intermolecular interactions between host and guest molecules and the radial distribution functions.The results showed that the device performance was better when 3,5-2Cz BN was used as the host material.The stronger intermolecular interactions,especially between the benzonitrile fragments(where the triplet excitons are mainly located)between host and guest molecules,would improve the exciton coupling value of the energy transfer between the host and guest molecules.The faster energy transfer process plays a vital role in improving devices’ external quantum efficiency(EQE).In addition,we also found the interaction between the host and guest molecules could inhibit the molecular vibration relaxation,limit the geometric structure deformation between the excited and ground states,and reduce the reorganization energy of the energy transfer process.The smaller reorganization energy is beneficial to energy transfer between the host and the guest molecules.In this work,we revealed the influence of the structural differences of the host molecules on the exciton energy transfer process between host and guest molecules.It provided theoretical guidance for understanding exciton energy transfer in luminescent thin films and designing efficient host materials.3.The influence of TADF molecular concentration on device performanceThe concentration of the guest TADF molecules in the luminescent thin film significantly impacts the performance of OLED devices,such as the external quantum efficiency(EQE)and emission wavelength.To further understand the relationship between the concentration and the device’s performance,we investigated the phenomenon that the exciton quenched and the wavelength occurred redshift in luminescent layer films with different guest concentrations in this work.The host material is CBP,and the guest material is ρCNQ-TPA in this host-guest system.The research results indicated that the interaction between guest molecules would strengthen as the concentration of the guest molecules increased,leading to the aggregation of guest molecules and the exciton quenching phenomenon.The interaction between guest molecules will enhance the short-range intermolecular energy transfer,especially in the Dexter energy transfer process.The critical distance of the Dexter energy transfer process increases with the enhancement of intermolecular interaction.In addition,we found that an increase in the concentration of guest molecules leads to stronger intermolecular charge transfer states(A-D…A-D),which is the main reason for the red shift of the luminescence wavelength.This work reveals the mechanism by which the concentration of guest molecules causes a rollover in device efficiency and a redshift in luminescent wavelength,providing a theoretical basis for regulating guest doping concentration and designing high-performance TADF luminescent devices.