Studies On Fabrication Of Organic Light-Emitting Diodes Based On Aggregation-Induced Emission Luminogens

Nowadays,organic light-emitting diodes(OLEDs),which integrate the advantages of low cost,light weight,thinness,flexibility,high efficiency,and ultra-high contrast,have huge application prospects in full-color displays and lighting sources.In the development of OLEDs,the exploration of new organic light-emitting materials and the development of device preparation techniques are indispensable.Among them,luminescent materials with aggregation-induced emission(AIE)properties are of the potentials to fundamentally solve the problem of concentration quenching.Combining AIE materials with thermally activated delayed fluorescence(TADF)is a wise strategy for the development of novel robust luminescent materials.However,there exists some unexplored field about AIE-OLEDs,such as exploring the structure-property relationship of AIE-TADF luminogens,and also,the application of AIE materials in other aspects of OLEDs.Last but not least,OLEDs architectures for display applications(i.e.,top-emitting OLEDs)should be explored.Therefore,the main research contents of this thesis are summarized as follows:In Chapter 2,based on the benzoyl(BP)as the electron acceptor(A),the fluorene derivatives and the 9,9'-dimethylacridine(DMAC)group as the electron donors(D/D'),AIE-TADF luminogens of DMF-BP-DMAC and DPF-BP-DMAC with asymmetric D-A-D'structure are constructed.The new luminogens are systematically characterized,which exhibit higher quantum yields and delayed lifetime in neat film than in solution,showing prominent aggregation-induced delayed fluorescence(AIDF)nature.DMF and DPF correspond to 9,9'-dimethylfluorene and 9,9'-diphenylfluorene moieties,respectively.The DPF moiety shows greater steric hindrance,which endows DPF-BP-DMAC higher photoluminescence quantum yield in neat film,and thus better performance of non-doped OLED,with maximum external quantum efficiency(EQE),current efficiency(CE),and power efficiency(PE)of 14.4%,42.3cd A^–1,and 30.2 lm W^–1 respectively.These results indicate that the bulky donor in D-A-D'molecular structure can enhance the photoluminescence quantum yields to some extents.In Chapter 3,on the basis of the previous chapter,the more rigid 9,9-spirobifluorene(SBF)is selected to construct the delayed fluorescence luminogen,namely SBF-BP-DMAC,which presents an even higher photoluminescence quantum yield as we systematically characterized.SBF-BP-DMAC also shows prominent AIDF property,with even higher photoluminescence quantum yield in neat film.The non-doped OLED based on SBF-BP-DMAC shows electroluminescence(EL)efficiencies of 67.2 cd A^-1,65.9 lm W^-1 and 20.1%,while the doped OLEDs show better EL efficiencies of 62.3–79.1 cd A^–1,40.8–70.7 lm W^–1 and 20.0–24.5%.What is more,SBF-BP-DMAC can also work as a host in orange phosphorescent OLEDs(Ph OLEDs),which reaches EL efficiencies up to 88.0 cd A^-1,108.0 lm W^-1 and 26.8%.On the basis of Ph OLEDs,a color-stable warm-white OLED is then fabricated,with EL efficiencies of69.3 cd A^-1,45.8 lm W^-1and 21.0%.We also study the mechanism in the SBF-BP-DMAC-hosted OLEDs,which indicates the exciton recombination and energy transfer process.This chapter expands the multifunctional application of AIE materials in OLEDs,explaining that the weak intermolecular interaction of AIE materials makes it not only a high-efficiency luminogen,but also an excellent host material.In Chapter 4,DMAC-BP is still used as the main D-A skeleton in the molecule,and carbazole-based moieties with stronger electron-donating ability are introduced on the other side of the molecular structure.m CP-BP-DMAC and DCB-BP-DMAC,with bluish-green emission,perform well in nondoped and doped devices with maximum EQEs of 21.0%and26.6%,respectively.Utilizing these two materials as the host and low-concentration phosphor as guest,single-emitting-layer warm-white OLEDs are obtained,giving rise to better EQE(23.6%)and higher color rendering index(70)compared to those obtained in Chapter 3.Also,the white OLEDs show stable EL spectra under large range of driving voltage.Therefore,via modulating the electron-donating ability of the weak donor,the emission color of the D-A-D'-structure luminogen can be adjusted.In Chapter 5,based on the aggregation enhanced delayed fluorescence(AEDF)luminogen BDMAC-XT,a fully fluorescent white electroluminescent(EL)devices without a spacer are developed.The maximum forward-viewing EL efficiencies are 18.64%,45.46 cd A^–1 and 44.17lm W^–1,and maintained at 17.86%,43.53 cd A^–1 and 35.99 lm W^–1 under the brightness of 1000cd m^–2;the EL spectra are stable in broad voltage range,with CIE coordinates of(0.334,0.444),and CRI of 76.The CRI can be increased to>80 by slightly sacrificing device efficiency.The results indicate that the rational design of doping concentration is crucial to exciton recombination in OLEDs.In Chapter 6,based on m CP-BP-DMAC,DCB-BP-DMAC and an AIE blue emitter(TPE-TAPBI),we explored the fabrication process of top-emitting OLEDs(TEOLEDs)devices.Sky-blue TEOLEDs were prepared based on m CP-BP-DMAC and DCB-BP-DMAC,and the maximum EQEs of the non-doped and doped devices are as high as 22.9%and 30.5%,respectively.TPE-TAPBI-based tandem OLEDs are successfully fabricated using a non-doped charge generation unit.Then,the tandem structure is applied to TEOLEDs to obtain saturated blue emission with high color purity.Although the deep-blue tandem TEOLEDs don't show satisfied EL efficiencies,we can further improve the EL performance of TEOLEDs in the future.