Design, Synthesis And Optoelectronic Properties Of Blue And Yellow/Orange Phosphorescent Materials And Host Materials

Organic light-emitting diodes (OLEDs) offer a bright future in flat panel display and solid lighting sources. Phosphorescent organic light-emitting diodes (PhOLEDs) can approach100%internal quantum efficiency by utilizing both singlet and triplet excitons of phosphorescent emitters. To fabricate good-performance PhOLEDs, a phosphorescent heavy metal complex as triplet emitter is usually doped into an organic small molecule or polymer matrix, named host materials, to reduce aggregation quenching and triplet-triplet annihilation. Therefore, the design and synthesis of proper host materials and guest emitters are important to realize the commercialization of PhOLEDs. Recently, green and red PhOLEDs have commercialized one after another, but highly efficient and stable blue PhOLEDs remain to be developed because of the lack of suitable host materials and guest materials. Meanwhile, to achieve high-performance two-color (blue-yellow/orange) based white PhOLEDs, the development of yellow/orange phosphorescent emitters is still on its way.The reseach of the thesis is focusing on the design and synthesis of host/guest materials for blue and yellow/orange PhOLEDs. Some new strategies and practically useful host/guest materials for blue and yellow/orange PhOLEDs are developed.The main contents and results are as follows:In Chapter1:The historic development of OLEDs/PhOLEDs is outlined. And finally, the design ideas of the thesis is given.In Chapter2:Three novel fully ortho-linked oligo(9,9'-spirobifluorene)s are synthesized from the intermediate4,4'-dibromo-9,9'-spirobifluorene. The glass transition temperatures of the three oligomers signficantly increase with the enlargement of the molecular system. But their absorption and emission peaks show no red shift and their high triplet energies remain invariant. The devices with trimer as host material and FIrpic or Ir(ppy)3as guest emitter showed maximum external quantum efficiencies of11.6and17.3%for blue and green electrophosphorescence, respectively.In Chapter3:Two new and simple bipolar host materials, POBPCz and POBPtCz, based on the biphenyl molecule are synthesized and characterized. The molecular structures of POBPCz and POBPtCz show twist configuration due to the steric effect, and thus minimize the n conjugation of biphenyl molecule. The hole-transporting carbazole group and the electron-transporting diphenylphosphine oxide group in the two molecules still retain their individual attributes on account of their different spatial orientations. Both POBPCz and POBPtCz show high triplet energies above2.9eV. The FIr6-based blue device A with POBPCz as host achieved a maximum current efficiency of40cd A"1, a maximum power efficiency of36lm W"1, and a maximum external quantum efficiency of19.5%.In Chapter4:Two new and simple bipolar host materials, POBPDPA and POBPmDPA are synthesized and characterized. Due to the strong electron-donating ability of diphenylamine, both POBPDPA and POBPmDPA show higher HOMO energy levels and narrowed HOMO-LUMO energy gaps than their carbazole-counterpart. POBPDPA and POBPmDPA exhibit triplet energy levels around2.72eV. The blue PhOLEDs using FIr6as the dopant and POBPDPA/POBPmDPA as the host materials achieved a maximum current efficiency of36cd A"1, a maximum power efficiency of39lm W"1and a maximum external quantum efficiency of18.8%.In Chapter5:Three blue/green iridium complexes based on the skeleton of FIrpic molecule are synthesized and characterized by introducing the F, Cl and Br atoms to the4-position of pyridine ring in FIrpic. The introduction of F atom cause the emission of FIrpic blue-shift while the Cl and Br atoms cause the emission of FIrpic red-shift. The sky-blue PhOLEDs using the F-substituted molecule (4-F-FIrpic) achieved a maximum current efficiency of29cd A", a maximum power efficiency of29lm W"1and a maximum external quantum efficiency of14.6%with a good CIE coordinate of (0.15,0.28).In Chapter6:Two new blue iridium complexes, POFIrpic and SOFIrpic, have been developed by introducing the polarized P=O/S=O moieties into the3'-position of the2-(2',4'-difiuorophenyl) pyridine (dfppy) to enlarge the energy gap. The two phosphorescent dyes show blue emission with the maximum emission around460nm and high quantum yields of ca.50%in solution. The single-layer solution-processed blue PhOLEDs based on POFIrpic showed a maximum current efficiency of11.1cd A-1and a maximum EQE of7.1%. The single-layer trichromatic white PhOLEDs employing POFIrpic as the blue component exhibited a maximum current efficiency of25cd A-1and a maximum EQE of15%.In Chapter7:Three yellow iridium complexes based on the skeleton of (Bt2Iracac molecule are synthesized and characterized by introducing the F, CI and Br atoms to the4-position of benzene ring in (Bt)2Iracac. The Cl and Br atoms affect the HOMO/LUMO energy levels and the energy gaps. But the Cl and Br-substituted molecules show almost identical maximum emission as (Bt)2Iracac molecule. The Cl-substituted molecule [(4-Cl-Bt)2Iracac] exhibited obvious improvement in the efficiencies of PhOLEDs, which showed a maximum current efficiency of50.7cd A-1, a maximum power efficiency of28.4lm W-1and a maximum external quantum efficiency of17.3%. At the same conditions, the (Bt)2Iracac-based device showed only28.4cd A"1,19.9lm W1and EQE of9.8%.In Chapter8:Two new orange iridium complexes, EtPylracac and SFPylracac, have been developed and characterized by employing the new heterocyclic ring of thieno[3,2-c]pyridine as the ligand. EtPylracac and SFPylracac show maximum emission of588and592nm, with quantum yields of10and13%respectively. The single-layer solution-processed orange PhOLEDs based on EtPylracac showed a maximum current efficiency of13.4cd A"1and a maximum EQE of11.2%, with a CIE coordinate of (0.618,0.376), which is close to the saturated red coordinate of (0.68,0.32).