Synthesis And Properties Of Adamantane-based Host Materials With Wide Energy Gaps For Phosphorescent OLEDs

Organic light-emitting devices (OLEDs) have attracted considerable research attention because of their potential applications in solid-state lighting and flat-panel displays. The early OLEDs based on fluorescent organic materials could only obtained 25% internal quantum efficiency, since the ratio of singlet and triplet excitons formed under electrical excitation is approximately 1:3. The performance of OLEDs can be greatly improved by incorporating phosphorescent materials as emission layer because both the singlet and triplet excitons can be harvested, which overcome the limitation of 25% efficiency of conventional fluorescent OLEDs. Although phosphorescent organic light-emitting diodes (PhOLEDs) could potentially approach nearly 100% internal quantum efficiency, the relatively long lifetimes of triplet excitons formed from phosphorescent heavy metal complexes commonly induced multiparticle annihilation at high currents, which seriously reduces the efficiencies of electrophosphorescent devices. Therefore, heavy metal complex phosphors are dispersed in some matrixes to restrain the concentration quenching and triplet-triplet annihilation (TTA). In this thesis, through rational design, we developed series of host materials with wide energy gaps based on adamantane for highly efficient PhOLEDs.In chapter 1, a brief introduction of OLEDs including the development of OLEDs, device structures and different functional materials and the emission mechanism are given first. Then the recent progress of host materials in phosphorescent OLEDs is reviewed. Finally, the desigh strategies and main contents of this thesis are outlined.In chapter 2, two hole-transporting type host materials, namely 2Cz-Ad and 2MTPA-Ad, are designed and synthezied by incorporating carbazole or triphenylamine groups with adamantane bridge via a simple C-N coupling reaction. Their thermal, electrochemical, UV-vis absorption spectroscopy and photoluminescent properties are fully investigated. High glass transition temperature (Tg) of more than 120 ? are observed due to the introduction of the rigid adamantane. Sky-blue phosphorescent OLEDs hosted by 2Cz-Ad show a maximum current efficiency of 36.4 cd A-1, a maximum power efficiency of 28.6 lm W-1 and a maximum external efficiency of 14.2%.In chapter 3, three electron-transporting type host materials 2OXD-Ad,2BID-Ad and 2TAZ-Ad are synthesized and fully characterized by 1H NMR,13C NMR, mass spectrometry and elemental analysis. Their thermal, electrochemical, UV-vis absorption spectroscopy and photoluminescent properties are studied. These host materials reveal high glass transition temperature (more than 123 ?) and high triplet enegry levels (more than 2.77 eV). The phosphorescent OLEDs using FIrpic as guest and 2BID-Ad as host show a maximum current efficiency of 13.8 cd A-1, a maximum power efficiency of 12.8 lm W-1 and a maximum external efficiency of 6.4%.In chapter 4, two electron-transporting type host materials 2CN-Ad and 4CN-Ad are facilely synthesized through simple cyano substitution reaction. Their thermal, electrochemical, UV-vis absorption spectroscopy and photoluminescent properties are studied. 2CN-Ad show relatively low Tg due to its small molecular weight and planar molecular structure.4CN-Ad possesses high Tg of 178 ? because of the stretched molecular structure. The triplet energies of 2CN-Ad and 4CN-Ad are ultra high (3.29 and 3.28 eV, respectively). Sky-blue phosphorescent OLEDs hosted by 2CN-Ad reveal a maximum current efficiency of 25.7 cd A-1, a maximum power efficiency of 24.5 lm W-1 and a maximum external efficiency of 12.0%.In chapter 5, three electron-transporting type host materials 2Py-Ad,2PO-Ad and 2SO-Ad are designed and synthesized. Their thermal, electrochemical, UV-vis absorption spectroscopy and photoluminescent properties are studied. The incorporating of non-conjugated adamantane makes the host materials with high triplet energies of more than 2.75 eV. The phosphorescent OLEDs using FIrpic as guest and 2Py-Ad as host show a maximum current efficiency of 22.6 cd A-1, a maximum power efficiency of 20.9 lm W-1 and a maximum external efficiency of 9.6%.In chapter 6, four bipolar host materials CzCN-Ad, CzPy-Ad, CzPO-Ad and CzSO-Ad are synthesized by incorprating hole-transporting group carbazole and electron-transporting groups benzonitrile, pyridine, triphenylphosphine oxide and sulfonyldibenzene via adamantane bridge. Their thermal, electrochemical, UV-vis absorption spectroscopy and photoluminescent properties are fully investigated. These materials show high glass transition temperature (more than 87 ?) and high triplet enegry levels (more than 2.84 eV). The phosphorescent OLEDs using FIrpic as guest and CzCN-Ad as host show a maximum current efficiency of 57.0 cd A-1, a maximum power efficiency of 45.9 lm W-1 and a maximum external efficiency of 24.1%. The blue phosphorescent OLEDs based on FIr6 and CzCN-Ad exhibit a maximum external efficiency of 14.2% and a very high luminance of 34262 cd m-2. To our best knowledge, this is the highest luminance for the FIr6-based blue PhOLEDs reported in references.In chapter 7, three bipolar host materials Cz1F-Ad, Cz2F-Ad and Cz3F-Ad are synthesized by incorprating hole-transporting group carbazole and electron-transporting groups fluorobenzene derivatives via adamantane bridge. Their thermal, electrochemical, UV-vis absorption spectroscopy and photoluminescent properties are fully studied. These host materials exhibit similar physical propoerties due to their similar molecular structures. The high triplet energies of 3.02 eV make them suitable for hosting FIr6 and FIrpic. The phosphorescent OLEDs using FIrpic as guest and Cz3F-Ad as host show a maximum current efficiency of 35.5 cd A-1, a maximum power efficiency of 26.9 lm W"1 and a maximum external efficiency of 16.6%. The devices using FIr6 as guest and Cz1F-Ad as host show a maximum current efficiency of 42.8 cd A-1, a maximum power efficiency of 38.4 lm W"1 and a maximum external efficiency of 18.5%.In chapter 8, four bipolar host materials MTPACN-Ad, MTPAPy-Ad, MTPAPO-Ad and MTPASO-Ad are synthesized by incorprating hole-transporting group triphenylamine and electron-transporting groups benzonitrile, pyridine, triphenylphosphine oxide and sulfonyldibenzene via adamantane bridge. Their thermal, electrochemical, UV-vis absorption spectroscopy and photoluminescent properties are studied. These host materials possess high triplet energies of more than 2.95 eV. The phosphorescent OLEDs using FIr6 as guest and MTPASO-Ad as host show a maximum current efficiency of 47.0 cd A-1, a maximum power efficiency of 49.2 lm W-1 and a maximum external efficiency of 25.1%. To our best knowledge, this is the highest efficiency for the FIr6-based blue PhOLEDs reported in references.In chapter 9, four solution-processed host materials 4TPA-Ad,4MTPA-Ad,4Cz-Ad and 4tBuCz-Ad are designed and synthesized through simple C-N coupling reations. The incorporation of rigid adamantane block greatly improves their thermal and morphological stability. Their thermal, electrochemical, UV-vis absorption spectroscopy and photoluminescent properties are fully studied. These host materials possess high triplet energies of nearly 3.00 eV. The solution-processed phosphorescent OLEDs using FIrpic as guest and 4MTPA-Ad as host show a maximum current efficiency of 16.6 cd A-1, a maximum power efficiency of 7.1 lm W-1. These efficiencies are much better than those of the control device using TAPC as host (a maximum current efficiency of 3.8 cd A-1 and a maximum power efficiency of 0.6 lm W-1).