| Recently, solution-processed single-layered PhOLEDs provide an economically attrative alternative to vacuum-deposition multilayered ones. In view of this, most single-layer devices have been developed containing both hole-transporting and electron-transporting materials as dopants. However, these doping systems were inferior in phase stability, processability, and repeatability, which could result in degradation. To overcome these problems, non-doped devices should be fabricated. Compared with polymers and small molecules, dendrimers are believed to also have the advantage of being solution processable, and incorporate the control over the optoelectronic properties by careful design. Moreover, encapsulation effect from the periphery dendrons can effectively reduce the strong concentration quenching of emitting cores. Nevertheless, most of the iridium dendrimers suffer from poor performances in single-layer devices, indicating the severe carrier imbalance due to the hole-type dendrons. And only a few studies have reported the utilization of electron-deficient groups to modify iridium dendrimers in single-layer devices, indicating that the preparation of high generation iridium(?) dendrimers combined with balanced charge transport remains a synthetic challenge and major obstacle to their application. The main content of this dissertation is as follows:1. A new series of green emitting iridium(?) complexes have been designed and synthesized, in which we chose phenyl benzimidazole-ligand homoleptic iridium(?) complex as chromophore, diphenylamine-based dendrons as encapsulating groups. It was demonstrated that the second generation dendrimers G2 and G2Cz have improved thermal stability than the first generation dendrimer G1. The Td values of G2 and G2Cz increase to 571? and 609?, respectively. XRD analysis proved that the diphenylamine-based dendrons have strong effect on restrain the aggregation. Both G2 and G2Cz are amorphous. The neat film photoluminescence quantum yield (PLQY) of the dendrimer increases from 7% of G0,28% of G1, to 50% of G2Cz and 40% of G2 with the increasing molecular sizes. It is clear that T-T annihilation was suppressed in the second-generation dendrimers. The cyclic voltammetry results revealed that the complexes have appropriate HOMO energy levels. Furthermore, the more oxidation states of G2 and G2Cz might be anticipated to make it superior to G1 and G0 in hole injection and transporting. The self-host single-layer devices based on G2 and G2Cz have been fabricated by spin-coat, where PEDOT:PSS/ITO was used as anode and Cs2CO3/Al was used as cathode. The maximum current efficiency(?c,max) of 14.02 cd A-1 and 18.35 cd A-1 have been realized, which are among the highest ever reported for single-layer self-host green PhOLEDs. That is said that the good balanced charge injection/transporting capacities could be obtained by simply changing the location of the substituents on the ligands of phosphorescent complexes. By introducing the dendrons into the para-site of the phenyl segment of the 2-phenylbenzimidazole ligand, the flower bouquet-shaped iridium dendrimers have been obtained, and the performance of devices could be improved greatly.2. This is the first report of the second-generation homoleptic iridium(?) dendrimer containing electron-transporting diphenylphosphoryl (DPPO) units. A novel bipolar iridium dendrimer, namely G2PO, has been synthesized by surface group engineering on diphenylamine dendrons. It was demonstrated that the second generation dendrimer has improved thermal stability and the Td values of G2PO increases to 490?. XRD analysis proved that DPPO units have strong effect on restrain the aggregation; therefore morphologically stable and uniform amorphous films could be formed. The electronic structure of the excited state is independent of the attached surface groups. The interactions between the emissive iridium(?) complex cores in the solid state have been reduced because of the higher steric hindrance of DPPO relative to carbazole dendrons. For the same reason, the neat film PLQY of the dendrimer'increases from 50% of G2Cz to 65% of G2PO accordingly. The cyclic voltammetry and theoretical calculation results revealed that the complexes have appropriate HOMO energy levels, and the incorporation of the electron-accepting DPPO functionalities would benefit the electron transport in the EML. Ket-/Ket+ of G2PO (2.44) is almost identical to Ket-/Ket+ of G2Cz (2.37). It indicates that both of them show the bipolar charge-transport properties. Meanwhile, they possess the similar degree of charge balance with a slight preference for electron or hole transport, respectively. The self-host single-layer device based on G2PO has been fabricated by spin-coat, where PEDOT:PSS/ITO was used as anode and Cs2CO3/Al was used as cathode. The maximum current efficiency(?c,max) of 21.6 cd A-1 has been realized, which is among the highest ever reported for single-layer self-host green PhOLEDs. y of the device based on G2PO is calculated as 0.55 according to its maximum EQE, indicating a medium balance of carrier injection/transportation. In addition, the similar ? of G2Cz and G2PO are precisely consistent with the almost identical values of Ket-/Ket+ and Ket-/Ket+ discussed above. This may be useful for future optimization of iridium dendrimers in silico.3. A new series of second-generation bipolar iridium(?) dendrimer containing electron-transporting diphenylphosphoryl (DPPO) units and hole-transporting carbazole units have been designed and synthesized. It was demonstrated that the dendrimers have improved thermal stability than the complexes using aliphatic chain as dendrons, and the Td values increase to 430?-450?. It means that the decomposition of the aliphatic moieties at lower temperature can be efficiently prevented by end-capping with rigid moieties. XRD analysis proved that the dendrimers have excellent film forming ability. Steady-state and transient-state spectroscopy were used to investigate the energy-transfer mechanism. The result indicates that an efficient energy transfer occurred from the excited singlet state of the dendrons to the iridium(?) center due to the light-harvesting antenna effect of the dendritic wedges. Almost complete energy transfer was observed in the films of dendrimers because of the intermolecular energy transfer might be more efficient than those in dilute solutions, indicating the high light-harvesting efficiencies in the EML of self-host devices. The electronic structure of the excited state is independent of the dendrons which attached to the cores through a flexible spacer. The neat film PLQY increases from 45% of one-position modified dendrimers to 75% of double-position counterparts. The cyclic voltammetry results revealed that the complexes have appropriate HOMO energy levels, and the electrochemical property can be kept almost unaffected after the introduction of the dendrons through a flexible chain. Bearing high steric hindrance with balanced charge transport, good electroluminescence (EL) performance can be expected in the single-layer self-host PhOLEDs. |
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