Design, Synthesis And Properties Of Novel Host And Guest Materials For Phosphorescent Organic Light-emitting Diode

Organic light-emitting diodes (OLEDs) have attracted much attention from bothacademic and industrial communities for their potential practical applications indisplays and solid state lighting. Compared with fluorescent OLED, phosphorescentOLED has been the mainstream OLED technology because of the higher luminousefficiency. To the phosphorescent OLED devices, in addition to the device structure, thekey OLED materials are another important factor affecting the performance of OLEDdevices. Wherein the organic layer materials (including the hole transport layer material,electron transport layer material, and light-emitting layer material), especiallylight-emitting layer materials (including host material and dopant material) are ofcrucial importance for the efficiency and lifetime of the phosphorescent OLED.Therefore, the efforts in searching better key OLED materials have never beenexhausted. Herein, the OLED light-emitting layer materials are mainly focused on,including a series of novel bipolar triazole/carbazole binary system as well as a noveltriazole/carbazole/phosphine oxide ternary system host materials and a series of bluephenyl imidazole-based iridium(III) phosphorescent dopant materials. Otherwise, anovel triazole/phosphine oxide binary system electron transport material was alsodeveloped. Follows are the details:1. Combining the carbazole group with good hole-transporting ability and thetriazole group with excellent electron-transporting ability, four noveltriazole/carbazole bipolar host materials were designed and synthesized. Thesehost materials are all confirmed by NMR and MS. These new materials werefound to exhibit wide energy gaps (E_g:3.29–3.52eV), high triplet energies(E_T:2.56–2.76eV), high thermal stability (T_d:426–454oC), highglass-transition temperatures (T_g:116–156oC) and excellent film formingproperty. Electron-only (EOD) and hole-only (HOD) devices of them were in the current densities between EOD and HOD, such evidently proves thebipolar transporting property of these materials. Green and blue PHOLEDdevices fabricated with them as host materials showed better performancesthan those with traditional host material (such as CBP and mCP). The highestcurrent efficiency of32.7and21.1cd/A for green and blue devices wereobtained, respectively. The structure-property relationship also provides auseful reference for the design of other new types of host materials.2. Ternary bipolar host material TCPO has been designed and synthesized. Basedn the previous work of the triazole/cabazole hybrid host materials, we chose adiphenylphosphine oxide as a third functional group in TCPO.Diphenylphosphine oxide, which has a strong electron-withdrawing ability,can improve the electron transporting ability of TCPO. Compared with othergroups, diphenylphosphine oxide dose not extend the conjugation, which willnot decrease the triplet energy of the core. With these properties, high tripletenergy (E_T:3.06eV) and excellent bipolar properties were obtained. Bluephosphorescent OLEDs using TCPO as host materials showed very highefficiency of41.6cd/A. Orange and green phosphorescent OLEDs usingTCPO as host have also been fabricated, which showed good performances,indicating the TCPO could be a universal host material.3. Blue homoleptic iridium complexes using phenyl-imidazole derivatives asligands have been synthesized. Photophsycal, thermal and electrochemicalproperties have been investigated. These four complexes showed blueemissions around474nm and high quantum yield (≥48%), which could beused as blue dopants in phosphorescent OLEDs. High current efficiency of61.5cd/A (EQE23.1%) was achieved by using the iridium complex as dopant,wich is the highest efficiencies ever reported on homoleptic blue iridiumcomplexes. The rules between the ligands and the properties have also beendiscovered, which could be used as a guide for designing deep blue dopantsfor phosphorescent OLEDs.4. Electron transporting material (E_TM) triazole/diphenylphosphine oxide-basedTPO has been designed and synthesized. By combining two strongelectron-withdrawing groups, excellent electron transporting abilities could be expected in TPO. Compared with a structurally similar electron transportingmaterial TAZ, TPO showed much higher glass transfer temperature and betterelectron transporting ability, which could be attributed to thediphenylphosphine oxide group. Green phosphorescent OLED using TPO asan E_TM showed better performances than that of traditional E_TMs (TPBI,BCP and TAZ). The turn-on voltage decreased and the power efficiencydoubled, compared with TPBI, BCP and TAZ-based devices. These resultsimplied the diphenylphosphine oxide group could be a useful group in novelE_TMs.