Iridium(Ⅲ) Complexes And Host Materials For Phosphorescent Organic Light-Emitting Diodes

The application of Organic Light-Emitting Diodes (OLEDs) in display field and lighting industry is just taking off, which is the highlight in organic opto-electronic research. In comparison with fluorescent materials, the phosphorescent emitters can harvest both singlet and triplet excitons and the internal quantum efficiency of the phosphorescent OLEDs (PhOLEDs) is possible to achieve a theoretical limit of unity. Therefore, the phosphorescent emitters and devices come into the focus of OLEDs. The phosphorescent emitters are typically doped in a certain host materials in phosphorescent OLEDs. As a result, both phosphors and hosts play essential role in determining the device performance. In this thesis, a series of cyano-decorated bipolar hosts and oligo-carbazole based solution-processible hosts were developed. In addition, a group of RGB iridium complexes incorporating diphenylphosphoryl unit at the cyclometalating ligands, and a group of multi-fluorinated iridium complexes, were designed and synthesized for application as highly efficient emitters in single-color and white PhOLEDs. The structure/property/device performance relationship was investigated. Dendritic iridium complexes were also synthesized and solution-processed white OLEDs with mixing blue-, green-and red-phosphor were fabricated. The details are as follows:(1) Solution-processible small-molecular host materials:Two novel molecules, CMP and CPMP, were synthesized by attaching oligo-carbazole and pyrazole to the2,2’-dimethylbiphenyl core in a symmetric and asymmetric way, respectively.These molecules exhibit high triplet energy and glass transition temperatures and improved film-forming capability. The solution-processed phosphorescent devices using CMP and CPMP as hosts exhibited good performance.(2) CN-decorated bipolar host materials:The CN-decorated carbazole, dibenzoforan or dibenzothiophene was used as n-type unit to construct a series of bipolar host materials in combination with the carbazole as p-type group and the phenylene as the molecular core. Their bipolar feature was confirmed by theoretical calculation and single-carrier devices study. These materials exhibitedhigh efficiencies and slow efficiency roll-off in their hosted blue PhOLEDs with Firpic as emitter.The devices with o-CzCzCNand m-CzSCN as hosts achievemaximum external quantum efficiency of21.0%and23.3%, respectively, among the best ever reported for blue OLEDs. It was proved that the ortho-linkage of the p-type and n-type groups on the phenylene core is more favorable for balanced charge transportation and device efficiency than the meta-linkage.(3) Diphenylphosphoryl-decorated Iridium Complexes:A series of diphenylphosphoryl-substituted iridium complexes, Ir-1-Ir-4, were synthesized by attaching diphenylphosphoryl unit to the pyridine unit of the cyclometalating ligands. In comparison with their parent complexes, the LUMO energy levels of Ir-1-Ir-4shift downwards and their phosphorescence spectra show bathochromic shift. Theoretical calculation supports the effect of diphenylphosphoryl unit on the narrowed bandgaps and bathochromic shifts. The monochromatic devices were fabricated using each iridium complex as doped emitter. And two-components white device with Firpic and Ir-4as dopants was also obtained. All these devices delivered good performance.(4) Multi-fluorinated iridium complexes:A series of multi-fluorinated tris-cyclometalated iridium complexes,3F-6F, were synthesized. The fluorine atom number on the ligand was varied to tune the optoelectronic properties of the resultant iridium complexes. The single crystal X-Ray diffraction analysis confirmed their meridional conformation. With successive increasing the fluorine number, their HOMO levels were regularly decreased and the UV-vis absorption and phosphorescence spectra showed the regular hypochromatic shift. The average lengths of coordination bonds of4F and5F are shorter than those of3F and6F. Highly efficient phosphorescent OLEDs were fabricated using3F-6F as emitters. Particularly,4F-based device achieved the maximum efficiencies of106.2cd A-1and29.5%. Furthermore, the efficiencies were still as high as90cd A"1and25%at a brightness of10000cd m-2, being the highest efficiencies at such high brightness for green PhOLEDs reported so far.(5) Carbazole-based dendritic iridium complexes:The dendritic iridium complexes were developed by incorporating the1st or2nd generation oligo-carbazole dendrons into the ligands of RGB iridium complexes via methylene bridge. In comparison with their parent complexes, the solubility and film-processing ability were largely improved. And the methylene bridge can inhibit the bathochromic shift for these complexes. The performance of GIFirpic-based devices fabricated by either spin-coating or vacuum-deposited was superior to the corresponding Firpic-based devices. Furthermore, a solution-processed white OLED with CIE (0.30,0.38) was realized by using Firpic,(G2ppy)2Ir(acac) and (G2piq)2Ir(acac) as emitters.