Luminescence Mechanism And Molecular Design Of White Organic Light-Emitting Diodes

White Organic Light-Emitting Diodes(WOLEDs)have received widespread attention due to their great potential application in display devices,solid-state lighting and molecular sensors.The typical method for the preparation of WOLEDs is based on emitters with two complementary(blue-orange or blue-yellow)or three primary(blue,green,and red)colors which could cover the whole visible light range from 380 to 780 nm.However,due to the relatively narrow emission spectrum of light-emitting materials,most WOLEDs are usually stacked devices with multiple light-emitting materials,which leads to some problems such as unstable light color,complex production process and high production cost.White light materials based on single components with dual emission characteristics can effectively solve this problem.The new kind of materials could switch reversibly between blue prompt fluorescence(PF)and orange(or yellow)thermally activated delayed fluorescence(TADF),and the full-width half-maximum coverage of the emission spectrum up to 150 nm,thus the two wavelengths they emit can cover the visible light region.TADF material has a small energy gap between the first singlet excited state(S₁)and the first triplet excited state(T₁),thus the triplet excitons can be effectively up-converted to singlet excitons,thereby achieving 100%exciton utilization efficiency.Since the number of TADF white light materials with dual emission is still quite limited so far,and they are also rarely applied in WOLEDs due to the unclear light-emitting mechanism.Therefore,it is of great significance to theoretically study the luminescence mechanism of white organic light-emitting diodes and design effective white TADF molecules.In this paper,the luminescence properties of a series of TADF molecules are studied based on first-principles calculations,and theoretical molecular design of TADF molecules for white emission is also performed.The bicolor switching and the luminescence mechanism of TADF molecules are studied based on the calculation of the potential energy curves and the decay rates of excited states.The donors that can favor molecules generate single and dual conformations are classified by studying their influence on the conformation of the ground state and the excited state.The influence of the substitution in donors on the luminescence properties is also investigated.Based on the theoretical study of TADF molecules designed,molecules with potential application in WOLEDs are predicted.The main research work of this paper is as follows:(1)Study on the mechanism of delayed fluorescence and molecular design of white organic light-emitting materials are performed.The PTZ-TTR and PTZ-Ph-TTR with white light emission are studied,and two stable conformations are found.It is found that the PTZ-TTR prefers to exist in quasi-axial conformation,which induces that the excited states of the quasi-equatorial conformation can only be obtained from the excited state transformation of the quasi-axial conformation.That is also the reason that the orange emission from quasi-equatorial excited states is weak than that emission from quasi-axial conformation.On the other hand,PTZ-Ph-TTR can exist in both two conformations in the ground state.Therefore,the orange light emission from the excited states of the quasi-equator conformation can be directly excited or transformed from the quasi-axial conformation in the excited state.The results show that the up-conversion from T₁ to S₁ can be achieved in the quasi-equator conformation,and it can also occur during the conformation transform process.In addition,by comparing the emission characteristics of PTZ-TTR in toluene and aggregate state,the environmental effects are revealed.At last,the influence of the substitution at the donor on the light-emitting properties is studied,and it is found that the systems with the six-membered heterocyclic unit in the donors tend to generate dual conformations.Donors with strong electron-donating ability are favorable to the compensatory emissions needed in WOLEDs.In addition,a series of TADF molecules with different donors are designed based on the TTR unit,and DMAC-t Bu-TTR and PXZ-Ph-TTR are proposed as potential emitters with dual emission used in WOLEDs.(2)Design of TADF molecules with white emission.A series of TADF molecules with potential dual emission are designed based on the systematical study of three TADF molecules including PTZ-BP,FCO-Cz S and PTZ-BPN with reversible two-color switching behavior.By investigating the geometric structure,potential energy curve and decay rate,the luminescence mechanism of three TADF molecules in toluene is studied.It is found that the quasi-axial conformation is responsible for blue PF and the quasi-equatorial conformation is responsible for orange TADF.Three up-conversion mechanisms are proposed:(1)conversion from T₁ to S₁ for quasi-equatorial conformation(eq^T₁?eq^S₁);(2)eq^T₂?eq^S₁;(3)T₂ to S₁ up-conversion during the conformation transformation process.The influence of donor substitution on luminescence characteristics is studied,the donors with single-conformation and dual-conformation are classified,which provides an innovative perspective for adjusting single emission to dual emission to achieve single-molecule white light emission.Besides,a series of molecules based on PTZ and PXZ are designed,and some molecules with potential application for white emission are predicted.There are five chapters in this paper.The first chapter is the introduction,which summarizes the research progress of organic light-emitting diodes,device structures and working mechanism,and briefly describes the research progress of light-emitting materials and single-molecule organic white light materials.Chapter 2 briefly describes the theoretical methods used in this thesis,including Density Functional Theory(DFT),Time-Dependent Density Functional Theory(TD-DFT),combined quantum mechanics and molecular mechanics(QM/MM)method,and theoretical methods for calculating radiation transition rate and non-radiation transition rate.Chapters 3 and4 show specific results and discussions on the study of TADF molecules with dual emission.The mechanism of delayed fluorescence in white organic light-emitting diodes and the design of molecular donors are presented in Chapter 3.Based on the study of the mechanism of TADF emitters used as white light emission,some TADF molecules with dual emission which shows potential application as WOLED emitters are theoretically designed in Chapter 4.The last chapter gives the summary of the study results and prospects for the work.