Theoretical Investigation Of Non-conjugated Polymer Host And Thermally Activated Delay Fluorescence

Organic/polymer light-emitting diodes (OLED) have been widely applied in flat-panel displays and solid-state lighting, the OLED materials study has attracted much attention. According to the spin statistics, the internal quantum efficiency (ηint) of fluorescent OLED is limited at 25%. The phosphor of transition metal complex could harvest both singlet and triplet excitons, the ηint of phosphorescent OLED (PhOLED) achieve 100%. ηint of the emerging thermally activated delay fluorescence (TADF) OLED also could achieve 100%. In PhOLED, the guest is dispersed in host which has high triplet energy (ET) to prevent reverse energy transfer from guest to host. While, development of an efficient host is a challenging task because high ET and good charge transport ability are required. In TADF-OLED, it is necessary that the materials which possess a small singlet and triplet energy different (△EST). And the mechanism of charge and energy transfer between host and guest still unresolved. In this article, a series polymer host and D-A-D-type TADF emitters are designed and predicted the application prospect in OLED by Density Functional Theory (DFT) and Time-dependent (TD) DFT calculation. We hope our study could provide instructive information for experimental and theoretical investigation in host and TADF materials. This article is divided into the following five specific components:First, this part introduces the research status of organic electroluminescent materials, components and working principle. The relationship of between structure and performance, design principle of phosphorescence and thermally activated delay fluorescent materials would be introduced in detail. And the application and development prospect of the electroluminescent materials were expected, and points out the research significance.Second, we detailed introduction the methods which applied in our study, such as Molecular Orbital Theory, DFT, TD-DFT, Nature Bond Orbital and light principle.Third, a series of poly(4,4-Vinyltriphenylamine) based non-conjugated polymer as hosts are designed and studied by DFT. The results show that the substituent has a great influence on the properties of polymer. The parent directly linked para-carbazole, β-pyrrole and triphenylamine are favorable to hole injection. The large changes of structural parameters between the lowest triplet state and ground state can cause the decrease of ET. In addition, the energy transfer mechanism between host and guest is thermodynamically feasible, and host-guest model indicate that a good charge transfer can be achieved between host and guest. In addition, the N atom of parent molecule linked para-carbazole substituent shows a great potential for the green PhOLED.Forth, a series of carbazole incorporated C/Si substituents as host molecules are designed and studied. The DFT calculations are performed to investigate the influence substituent on energy and charge transfer, and predict the application in PLED. The results show that carbazole linked to C/Si atoms series (cbz-sub) exhibits high ET. And the cbz-sub series possesses strong molecular orbital interactions and suitable singlet and triplet energy difference. Further investigation show that the Si atom could inhibit the charge and exciton delocalization and insert methyl and tert-butyl at carbazole in cbz-ph-sub series could improve the ET and show obvious intramolecular charge transfer character. Energy matching of host and guest molecules shows that the high device performance can be expected when Si is the central atom of substituent in cbz-sub structure hosts are combined with blue guest material used in PLEDs.Fifth, A series of donor-acceptor-donor type compounds containing 9,9-dimethyl-9,10-dihydroacridine and diphenylsulphone as thermally activated delayed fluorescence emitters are designed and investigated, their broad application prospect in organic light emitting diodes are predicted by density functional theory (DFT). The results show that the orbital interaction of the adjacent atom between acceptor and donor is an important factor to influence the singlet-triplet energy difference. The effective intermolecular singlet-singlet, singlet-triplet and triplet-triplet energy transfer from host to emitters which donor and acceptor linked by C-N bond would be formed. The para- and meta- linked compounds exhibit blue emission and ortho- linked compound show green emission in these emitters.