Exciton Behavior In Organic Light Emitting Devices:Theoretical Study From Single Molecule To Aggregate

OLED materials have been widely studied and initially used in the products and our daily life because these materials had advantages such as the low cost,flexibility,easily-controlled preparation,and changeable optical properties through group modification compare with traditional inorganic LED materials.Over the past few decades,OLED materials have developed rapidly,both in their emission range(red,blue,green,white)and in terms of stability and durability.In addition,the external quantum efficiency has also been significantly improved.But in recent years,OLED materials with traditional research ideas have been successfully commercialized,and their properties have been almost completely explored.However,the improvement of existing materials and the demand for new materials have been driving researchers to seek innovation.Therefore,the design and development of new materials requires breakthroughs from more perspectives.The OLED device has a multi-layer structure.In order to improve its optical performance,it is necessary to thoroughly study the working mechanism of each part and improve its performance respectively.This paper mainly focuses on the most important parts of its layered structure: the organic lightemitting layer and the charge transport layer.Through theoretical calculations,the relationship between the structure of the material and its properties is systematically explored,fundamentally provide an anchor for the research of light-emitting devices.Which will provide theoretical basis and design ideas for material design and property prediction,and promote the development of organic light-emitting devices.This paper consists of five chapters.The first chapter is the introduction,which introduces the development process and structure of OLED,briefly summarizes its advantages,achievements and development potential,and explains the importance of its research.The second chapter is the part of theoretical basis and calculation method,it covers the basis of quantum mechanics and quantum chemical calculation methods,the basic principles and formulas used in the electron radiation and non-radiative transition processes involved in the luminescence process,the principles and formulas of the quantum tunneling model in the charge transport process,and new background charge method.The third chapter to the fifth chapter is the main part of the article.Systematic exploration of materials in organic light-emitting devices,gradually expanding from ideal single-molecule models to real multi-molecular systems.Through the in-depth study and discussion of the molecular structure and its optoelectronic properties in different research systems,the theoretical exploration of its mechanism which is closer to the real environment is carried out.The main research contents are as follows:1.Starting with the single-molecule model,a new design idea for combining graphene quantum dots and organometallic complex light-emitting materials in the light-emitting layer is proposed and the luminescence mechanism of these materials is discussed in depth.Through the calculation of the geometry,molecular orbitals,absorption-emission properties,and characteristics of radiative and nonradiative transitions,a series complexes with excellent optical properties are obtained.At the same time,the size and edge shape of graphene quantum dots have a decisive influence on the photophysical properties of the material,and the corresponding explanations are given through theoretical calculations.The complexes of protuberance type and arc type complexes have the potential as excellent light-emitting materials,the flat type and lateral type complexes belong to the modified products of graphene quantum dot structure.Moreover,the lateral type materials has a triplet ground state,which has great application prospects in other fields.2.Based on the excited state properties of the luminescent materials in the singlemolecule system,the carrier mobilities of the macroscopic multi-molecular system were simulated,and the corresponding relationship between the crystal structure and the material properties was expounded.The effects of different crystal structures(including ?-stacking and herringbone arrangements)on charge mobilities are revealed,through the theoretical studies of the reorganization energies,transfer integrals,and the degree of overlap of frontier molecular orbitals for a series of BOXD crystals.More indepth study found that the intermolecular distance determines the transfer integral in ?stacking.The transfer integral of the herringbone arrangement will usually be smaller than that of the ?-stacking,and its decisive factor is the dihedral angle.3.Based on the results of the multi-molecular carrier mobility calculation,the analysis of the luminescence properties in the crystalline aggregation system were continued.For such systems with excessively large number of atoms,the concept of the effect of space point charges is introduced into the model,and a new calculation method—the background charge method is innovatively involved to analysis that how the crystal structure in crystalline materials affects its luminescence properties.The advantages of this method and the accuracy of the calculation results are illustrated by calculating the spectral characteristics,light emission efficiency and other properties of two types of metal Ir complex luminescent materials.This method can more quickly and accurately simulate the spectral characteristics of crystal materials,and reduce the calculation error.In addition,we found that the crystal structure also affects the intramolecular charge transfer characteristics,which changes the vibrational mode of the molecule and ultimately affects the luminous efficiency of the material through a nonradiative process.