Font Size: a A A

Design, Synthesis And Organic Light-emitting Diodes Of Bipolar Phosphorescent Host Materials

Posted on:2016-02-07Degree:DoctorType:Dissertation
Country:ChinaCandidate:B HuangFull Text:PDF
GTID:1108330503476693Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
Organic light emitting diodes (OLEDs) have gained tremendous attention due to their significant potential applications in flat-panel displays and solid state lighting. In contrast to traditional fluorescent OLEDs, the singlet exitions and trplet exitions can be utilized in PhOLEDs, leading to the limited internal quantum efficiency 100%. The choice of host materials for high-efficiency PhOLEDs is very important. Bipolar phosphorescent host materials have attracted major interest because not only they can transport holes and electrons, but also they are potent to improve the efficiency and simplify the structures of PhOLEDs. In principle, when the energy gap between singlet and triplet (△EST) of bipolar materials is small(<0.5 eV), the endothermic reverse intersystem crossing(RISC) can be overcome by the thermal motions of the molecule atoms. These materials are thermally activated delayed fluorescence (TADF) materials. As a result, the triplet excitons are transformed to singlet excitons via RISC in TADF materials, both triplet and singlet excitons can be harvested. On the basis of this concept, TADF materials can be used as substitutes of phosphors based on rare metals for high-efficiency and low-cost OLEDs. It is very important for us to develop novel bipolar phosphorescent host materials and TADF materials.In this thesis, we have designed and synthesized a series of novel bipolar materials based on electron-transporting groups such as 2,4,6-trisphenyl-l,3,5-triazine, diphenyl sulfone, benzonitrile, benzophenone and hole-transporting groups such as triphenylamine, carbazole. The structure-property relationships including their thermal, photophysical, electrochemical and electroluminescent properties have been systematically investigated. The main contents are described as follows:(1) A novel N-phenyl carbazole substituted 2,4,6-trisphenyl-1,3,5-triazine host material (TPCPZ) for solution processed green phosphorescent organic light-emitting devices (PhOLEDs) has been synthesized. The optical, electrochemical and thermal properties of TPCPZ have been characterized. The solution-processed single-layer device using TPCPZ as the host for fac-tris[2-(4-phenylpyridine)]iridium (Ir(ppy)3) exhibits a low turn-on voltage of 3.5 V, a maximum current efficiency of 20.8 cd A"1 and a maximum luminance of 18000 cd m-2.(2) A series of bipolar materials for TADF based on 3,6-di-tertbutyl-carbazole and diphenyl sulfone, have been synthesized. In these materials, the 3,6-di-tertbutyl-carbazole group is linked at the 3-,4-position or 3’-,4’-position of diphenyl sulfone. The effects of the conjugation connectivity on the electronic, photophysical and electrochemical properties of these materials have been systematically investigated. The △EST in these materials is tuned from 0.39 eV to 0.22 eV by manipulation of conjugation of the electron donor units.(3) A series of ambipolar compounds using triphenylamine/N-phenyl carbazole as electron donors and benzonitrile/2,4,6-trisphenyl-1,3,5-triazine as electron acceptors have been synthesized. The electronic, photophysical and electrochemical properties of these materials can be effectively tuned by manipulation the constitution of acceptor and donor units. The △EST in these materials is tuned from 0.75 eV to 0.18 eV by systematically changing the electron donor and acceptor units. Triphenylamine/N-phenyl carbazole substituted 2,4,6-trisphenyl-1,3,5-triazine exhibit small △EST (0.18 eV and 0.37 eV), and are potential for thermally activated delayed fluorescence materials.(4) A series of bipolar host materials (36DDPSPC and 27DDPSPC) based on N-phenyl carbazole and diphenyl sulfone for blue phosphorescent organic light-emitting devices (PhOLEDs) have been synthesized. Their optical, electrochemical and thermal properties have been characterized.36DDPSPC and 27DDPSPC exhibit high triplet energy of 3.00 eV and 2.86 eV, and can be used as blue phosphorescent host materials.(5) A series of benzophenone derivatives based on benzophenone and 3,6-ditert-butyl-carbazole/3,6-bis(3,6-ditert-butyl-carbazol-9-yl)-carbazole (t-BuCz2BP and t-BuTCz2BP) for TADF have been synthesized. The optical, electrochemical and thermal properties of t-BuCz2BP and t-BuTCz2BP have been characterized. In comparison with the reported benzophenone derivatives, t-BuCz2BP and t-BuTCz2BP exhibit better solubility and smaller △EST. t-BuTCz2BP exhibits a high Td(corresponding to 5% weight-loss)value of 489 ℃, a triplet energy of 2.59 eV, and a small △EST of 0.05 eV. The solution-processed non-metal green TADF OLED based on t-BuTCz2BP exhibits a low turn-on voltage of 4.5 V, a maximum current efficiency of 9.2 cd A"1 and a maximum luminance of 4200 cd m-2.
Keywords/Search Tags:organic light emitting diodes, electrophosphorescence, organic small molecules, host materials, bipolar, solution-processed, thermally activated delayed fluorescence
PDF Full Text Request
Related items