| Triplet energy level and carrier injecting/transporting properties are not only two decisive factors for the performances of electrophosphorescence hosts, but also interrelated contradictories in the molecular design. Among all kinds of hosts, phosphine oxide (PO)-based hosts arouse great interest due to their prominent advantages, and is expected to be one of the most important approaches for overcoming the drawbacks of blue electrophosphorescence, such as high driving voltages and poor efficiency stabilities. PO moieties can not only block the conjugation extension and preserve high triplet energy levels (Ti), but also effectively polarize molecules and enhance carrier injecting/transporting ability. However, the advantages of PO hosts are far from being completely utilized because of limitated structural designs.This thesis focused on the aryl PO compounds, and presented two efficient strategies to obtain hige Ti, namely "multi-insulating linkage" and "short-axis substitution". With the guidance of these strategies, a series of template PO molecules were synthesized. Consequently, we modified the templates with hole-transporting moieties, such as carbazolyl and triphenylamine groups to construct "multi-functionalized" molecules. The optoelectronic properties and electroluminescent (EL) performance of the hosts were finely tuned through adjusting the type, number and substituted position of the functional groups.1. We modified diphenyl ether with diphenylphosphine oxide (DPPO) moieties to configure "multi-insulating linkage" molecules. Due to the multi-insulate effect of-O-and P=O linkers, a high Ti (3.0eV) of the original DPEPO is realized. The combined effect of electron-donating-O-and electron-drawing P=O induces its fully separated HOMO and LUMO electron clouds. Through the further intruduction of hole-transporting groups, the hole-transporting abilities of the derivatives are improved greatly. Low turn-on voltage (2.6V) and high effiencies (21.4cd A-1for current efficiency (C.E.),21.0lm W-1for power efficiency (P.E.) and12.0%for external quantum efficiency (E.Q.E.)) of the blue-doping (FIrpic) device based on DPEPOCz were realized.2. We demonstrated the strategy of "short-axis substitution" through ortho-linkage of dibenzofuran and DPPO moieties with the characteristics:(1) blocking the conjugation and restaining the influnce on Ti;(2) more effectively polarizing the molecules to enhance the electron-transporting capabilities. Tis of the original DBFxPO are as high as3.15eV. The corresponding blue PHOLEDs based on the monosubstituted DBFSPO showed superior performance with low turn-on voltage (2.6V) and low efficiency roll-offs (4%for external quantum efficiency from100to1000cd m-2). Moreover, without any additional enhancing technologies, of the white prototype PHOLEDs based on DBFSPO host achieved extremely low driving voltages (less than3.4V at1000cd m-2), high efficiencies (more than30lm W-1for P.E. and12%for E.Q.E.), and low efficiency roll-offs (only14%and3%for P.E. and E.Q.E. at1000cd m-2, respectively). These results are among the best results reported so far and demonstrated single-host based WOLED having both high efficiencies and extremely low operating voltages at practical luminescence. Tis of multi-functionalized derivatives remain higher than2.8eV, accompanied with the enhanced and balanced carrier injection and transportation. The EL performances of corresponding devices were also improved, such as ultralow turn-on voltage of2.4eV.3. Dibenzothiophene was used as the chromophore to construct PO hosts with DPPO moieties on the basis of short-axis substitution. The investigation of the influnce of different chromophores on the performance of hosts was performed. Owing to the good hole-transporting ability of dibenzothiophene, HOMO of DBTxPO was increased for0.4eV and the energy gap was decreased for0.3eV, compared with DBFxPO. Therefore, DBTxPO were endowed with stronger and more balance carrier-transporting abilities. The corresponding devices realize the excellent performances. The blue device based on DBTDPO showed the lowest turn-on voltage of2.4V, and the driving voltages at100and1000cd m-2were as low as2.8and3.2V. The highest luminance of14780cd m-2and the maximum effciencies of28.8cd A-1for C.E.,32.2lm W-1for P.E. and14.3%for E.Q.E. were realized. The device also showed excellent efficiency stability, the percentages of efficiency roll-off (at1000cd m-2) were only5.2%for C.E.,12.7%for P.E. and5.6%for E.Q.E..Through the persistent effords, we not only demonstrated two effective strategies of carrier-transporting high energy-gap hosts, but also achieved several low-voltage driving PHOLEDs with state-of-art performances. Therefore, our work provided the theoretical guidence for molecular design and the potential structure condidates for the future investigation of host materials. |
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