| Organic light-emitting diodes（OLEDs）have the advantages of lightness,flexibility,low power consumption,high contrast,etc.,and have been initially applied to flat panel displays,white light lighting,automotive taillights and other fields.In the past thirty years,many different approaches have been taken to improve the performance of OLED devices in the development of new materials and the design of device structures.However,at present,the efficiency and stability of blue,especially saturated deep blue OLED,lag far behind the red and green materials.Before realizing the large-scale commercialization of OLED to replace the existing mainstream liquid crystal display technology,this thorny problem must be completely solved.Compared with red and green emitting materials,blue light materials have higher energy,can be used as an excitation light source or the host material generates light of other colors through energy transfer,and the rational use of blue emitters can simplify the device structure.In addition,the higher the purity of blue emitters,the wider the color gamut of the display,and the more conducive to reducing OLED power consumption.Therefore,high-efficiency saturated deep blue materials play a very important role in the industrialization process of OLED.Up to now,most of the high-efficiency blue emitters are doped devices,and the preparation of non-doped blue devices is beneficial to simplifying the device preparation process and reducing production costs.Phenanthroimidazole group possesses high fluorescence quantum yield（PLQY）and near-ultraviolet emission,which is suitable for constructing blue and deep blue emitting materials.However,its potential in blue and deep blue fields has not been fully exploited,and there is still room for further improvement of device performance.Therefore,this paper prepares a series of blue to deep blue emitting materials with phenanthroimidazole as the core,systematically studies the relationship between molecular structure and photoelectric properties,and strives to achieve high efficiency and low roll-off non-doped deep blue OLED.The specific research content of the paper is as follows:1.With phenanthroimidazole as the donor,3,5-difluorobenzene,trifluoromethylbenzene and benzonitrile groups as the acceptor,a benzene bridge was introduced at the C2 position of phenanthroimidazole to link the donor and receptor to construct D-π-A type molecules PPIM-22F,PPIM-23F and PPIM-2CN.At the same time,three isomers PPIM-12F,PPIM-13F and PPIM-1CN were constructed by connecting the donor and acceptor at the N1 position,and the effects of acceptor strength and modification sites on the material properties were explored.It is found that:due to the moderate torsion angle in the C2 direction and longer conjugation,the introduction of the same acceptor at the C2 position makes the molecular emission color redshift more and PLQY higher than that at the N1 position;due to the large torsion angle between the phenanthroimidazole plane and the N1 substituent group,connecting a benzonitrile with strong electron-absorbing ability at this site will produce an excited state characteristic dominated by the charge transfer state,which increases the non-radiative transition rate and decreases PLQY;in general,due to the high solid-state fluorescence efficiency,the device electroluminescence performance of phenanthroimidazole C2 position modified weak acceptor molecules PPIM-22F,PPIM-23F and PPIM-2CN are better than that of N1 position modified molecules PPIM-12F,PPIM-13F and PPIM-1CN.Among them,the non-doped device based on PPIM-22F shows the best electroluminescence performance in these compounds,with a maximum external quantum efficiency of 7.87%and a color coordinate of（0.16,0.10）.This work provides a new idea for the next step of the design of blue phenanthroimidazole derivatives.2.In order to further weaken the composition of the charge transfer state in the molecule,a relatively neutral biphenyl group was used instead of the acceptor group in the previous chapter,and the PPIDPhmolecule was constructed by linking with phenanthroimidazole at the C2 position by benzene bridge.At the same time,in order to further blue shift the emitting color,sp3 hybrid carbon,oxygen and sulfur atoms were introduced to interrupt the molecular conjugation to prepare the molecules PPIDPhC,PPIDPhO and PPIDPhS molecules,and the influence of this molecular design strategy on the structure and photoelectric properties of the material was explored.All four molecules show high solid-state luminescence efficiency,among which PPIDPhnon-doped films has the highest PLQY of 83.51%.Compared with the emission peak of 454nm of PPIDPhnon-doped films,the emitting color of their non-doped films blue shifts to 433 nm,437 nm and 440 nm,respectively,and the corresponding color coordinates of the non-doped devices are all 0.08,which belongs to saturated blue light emission,and the maximum external quantum efficiencies are 8.56%,7.89%and 8.46%,respectively.The blue non-doped device based on PPIDPhexhibits the best performance,the maximum external quantum efficiency is 10.33%,and the efficiency roll-off is only 7.4%at the brightness of 1000 cd m-2,showing excellent device stability with color coordinates of（0.15,0.10）.3.Two fluorene derivatives with large steric hindrance,9,9-dimethylfluorene and9,9-diphenylfluorene,were selected as substituents,and the compounds PPI-2-DMF and PPI-2-DPF were obtained by connecting with phenanthroimidazole at the C2position of fluorene through a benzene bridge.In order to further enhance the rigidity of the molecule,the 9,9’-spirodifluorene group was used to link with phenanthroimidazole through the C3 position to synthesize PPI-3-SBF molecules.It is found that only the C-H···πinteraction between molecules exists in the solid stacking of the three molecules,which helps to inhibit the aggregation-induced quenching in the solid stacking and obtain high solid-state fluorescence efficiency.The PLQYs of PPI-2-DMF,PPI-2-DPF and PPI-3-SBF are 57.56%,67.22%and 66.05%,respectively.The theoretical calculation results show that since the PPI-3-SBF molecule adopts the C3-position ligation method,its terminal benzene ring does not participate in conjugation,the conjugate length is the shortest,and its non-doped thin film emission peak is 434nm.The maximum external quantum efficiencies of the non-doped devices prepared by PPI-2-DMF and PPI-2-DPF are 7.57%and 8.48%,the half-peak width are 70 nm and68 nm,respectively,and the color coordinates are both（0.16,0.09）.Benefiting from the stronger molecular rigidity and the shortest conjugate length,the half-peak width of the non-doped device of PPI-3-SBF is reduced to 60 nm,the color coordinate y value is reduced to 0.07,which is located in the deep blue light field,and the maximum external quantum efficiency is 8.41%.4.The results of the first three parts indicate that relatively neutral groups with a certain steric hindrance have more advantages in constructing efficient deep blue emissing materials.Therefore,the m-terphenyl group was selected to link with phenanthroimidazole through benzene bridge to construct PPITPhmolecule.Its solid-state PLQY is 77.37%.The non-doped device based on PPITPhexhibits the maximum external quantum efficiency of 11.83%with color coordinates of（0.15,0.07）.At the brightness of 1000 cd m-2(10000 cd m-2),the external quantum efficiency remained at10.17%（7.50%）,which is the most efficient non-doped deep blue device reported with a brightness of 1000 cd m-2.Electroluminescence transient decay test and theoretical calculation show that the hot exciton mechanism is the main source of triplet excitons.It realizes high efficiency,low efficiency roll-off non-doped deep blue light devices at high brightness.Furthermore,phosphorescent-doped device with PPITPhas the host and PO-01 as the guest was prepared.The external quantum efficiency can reach up to32.02%,and it still maintain 31.17%at brightness of 1000 cd m-2,exhibiting a low efficiency roll-off,which showing the good application ability of PPITPhphosphorescent host material. |
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