Theoretical Study Of Carbazole And Phosphine Oxide Derivatives As Ambipolar Hosts For Blue Phosphorescent Organic Light Emitting Diodes

Research and development of blue phosphorescent organic light-emitting diode(PhOLEDs) have lagged behind the red and green phosphorescent devices, which hasrestricted the development high quality information display and white light illumination. Theunsatisfactory performance of blue phosphorescent devices lies in the lack of efficient bluephosphorescent materials. At home and abroad in recent years, the OLED developers havecarried out extensive and in-depth research on blue phosphorescent material and theseresearch achievements vigorously push forward the blue and white light device. However, therealization of commercial production still have quite a distance since the luminescenceefficiency of blue phosphorescent materials is still a problem to be solved in the future. Inaddition, blue phosphorescent materials put forward strict requirements to other parts of thedevice including: host material, charge transport layer, and blocking layer. Especially for thehost material, its lowest triplet energy should be higher than that of phosphorescent materials(i.e., greater than2.65eV), to confine the triplet exciton to guest phosphor and further torealize undergo phosphorescence radiation. Compared with the host materials for red andgreen phosphors, efficient ambipolar host materials for blue phosphors are still scarce,because it is difficult for a material to achieve a trade-off between a wide triplet energy andgood charge injection property. An efficient design strategy is to connect the donor andacceptor groups by a non-conjugated linkage, which can not only efficiently improveelectron/hole injection and transport properties by tuning the respective HOMO and LUMO,but also maintain high triplet energy due to localization of the triplet exciton on the donor oracceptor group.Our goal is, through quantum-mechanical investigations on electronic structures andproperties for the host molecules, to rationalize the experiment report, and to further predictthe host performance of the designed material. In additional, we constructed the appropriatecombination of host and guest molecules for efficient energy transfer. We hope to provide atheoretical guide for design and improvement of blue-light host materials. Our work mainlyincludes four aspects as follows:1. We theoretically studied a series of hybrid host molecules (PO(S)-PhCBZs) throughdifferent linkage modes between phenylcarbazole (PhCBZ) and phosphine oxide/sulfide(PO/PS) moieties. The results indicate that the different linkage modes can influencedistributon and level of the lowest unoccupied molecular orbital (LUMO), and thus improvethe injection abilities of carriers from neighboring layers in different degrees, whereas havinglittle influence on the highest occupied molecular orbitals (HOMOs) and the triplet excitons. The HOMO energies and high triplet energies (ET) are therefore well kept up for all systems.For all hybrid PO(S)-PhCBZs, the difference between the S1and T1energies can be logicallyemployed as an index to evaluate the performance of host materials. Especially PO(s)-Phs(PO linked to the phenyl of PhCBZ) exhibit an enhancement in electron injection and tripletenergy, and thereby have great potential as ambipolar host for application in high efficientblue light emitting diodes.2. Based on our previous work, we theoretically studied a series of novel phosphineoxide (PO)/phenylcarbazole (PhCBZ) hybrids (1-4) which incorporate the PO moiety linkedto the CBZ through phenyl bridge and investigated the influence of various ratios and linkingmodes of carbazole and PO moieties on the electronic structures and the properties as hostmaterials. In addition, we chose reference guest and predict the host-guest emissionmechanisms via estimating the S1and T1energies of host and guest molecules. The resultsindicated that the LUMO spreads over the PO-phenyl center due to the strongelectron-withdrawing inductive effect of PO on phenyl of PhCBZ moieties while the HOMOand triplet excition is localized at only PhCBZ moiety with the increase of peripheral CBZunits. Besides, the electrostatic effects between the CBZ units and PO-phenyl center have theinfluence on the HOMO and LUMO levels. So the balance between electron and holeinjection is achieved by various ratios and linking modes of carbazole and PO moieties. Fromthe viewpoint of match of S1/T1energies between host and guest,1-4may exhibithigh-efficiency performance in deep blue FIr6-based PhOLEDs.3. We theoretically designed a series of ambipolar host materials (1-7) which incorporatephosphine oxide and carbazole groups to the two ends of diphenyl (DP)-like bridges by para-and meta-connections, respectively. Density functional theory calculations were performed toinvestigate the influence of altering the position of N atom or increasing the number of Natom at DP-like bridge of these molecules on electronic structures and properties, and furtherto predict their performances as host materials in organic light-emitting diodes. Theinvestigated results show the HOMOs and LUMOs of1-7, distributed at phenylcarbazole andDP-like bridge, are responsible for hole and electron injection properties, respectively. Thedifference in the energies of HOMOs or LUMOs for1-7may be derived from differentdegrees of conjugation effect, inductive effect, or electrostatic induction with altering DP-likebridges of1-7. On the other hand, altering DP-like bridges bring great effect on triplet excitondistributions, and consequently different triplet energies. The different singlet/triplet energiesfor1-7make hosts1-7suitable for four reference guests with green/deep-blue light whenconsidering match of host and guest in singlet/triplet energies for efficient energy transfer.4. We studied the electronic structures of a series of fluorene derivatives (p/mPODPFsand p/mPOAPFs) using density functional theory calculations and investigated theirperformances as host materials in organic light-emitting diodes from three aspects, i.e. triplet energy, ability of charge injection from neighboring organic layer or electrode, and match ofthe hosts and the reference guests (FIrpic and FCNIr) for efficient energy transfer. From theinvestigated results, we deduced that pPODPF and pPOAPF are suitable for sky-blue FIrpicdue to feasible F rster/Dexter energy transfers from pPODPF/pPOAPF to FIrpic, whichagrees well with the experimental results. Furthermore, the higher external quantum efficiencyof pPOAPF-based device than that of the pPODPF-based device in experiment is attributed tothe matching S1energies between pPOAPF and FIrpic as well as good hole/electron injectionabilities. By contrast, for the designed mPOAPF, PO creates a pulling effect on the mainbackbone, although the triplet exciton is not distributed at PO group itself, which leads tolocalization of triplet exciton at triphenylamine and thus high triplet energy. mPOAPF isexpected as high-efficiency ambipolar host material for deep-blue PhOLEDs.