Synthesis And Fluorescent Properties Of Thianthrene-9, 9′,10,10′-Tetraoxide-based Thermally Activated Delayed Fluorescence Emitters And Their Host Materials

In rescent years, thermally activated delayed fluorescence(TADF) materials have been new research focus in organic light-emitting diodes(OLEDs). Compared with traditional fluorescent and phosphorescent emitters, TADF emitters can not only realize 100% internal quantum efficiency, but also have the advantages of low cost and environmental friendliness, which are considered as the next generation organic flat panel display and lighting materials. Therefore, in this thesis, I focus on the study of exploiting novel high-efficiency TADF emitters as well as their fluorescent properties and device performance in different surroundings.In chapter 2, two novel evaporation- and solution-process-feasible thianthrene-9,9’,10,10’-tetraoxide-based TADF emitters of ACRDSO2 and PXZDSO2 were designed and synthesized. Maximum EQE values of 19.2% and 16.7% were achieved for the vacuum-deposited OLEDs based on green light emission ACRDSO2 and yellow light emission PXZDSO2, respectively. Notably, the solution-processed device based on ACRDSO2 without any hole-transport layer exhibits obviously lower driving voltage and higher EQE(17.5%) in comparison with the well-known green TADF emitter 4CzIPN((4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile)(13.9%) in the same device structure. In addition, the PXZDSO2 based solution-processed device exhibits a maximal EQE of 15.2%, which is the first reported solution-processed TADF-based OLED that exhibits competitive performance compared with the corresponding vacuum-deposited devices other than green TADF OLEDs. Moreover, all the devices exhibit slight efficiency roll-off with EQE values exceeding 13% at the luminance of 1000 cd/m2, which can be mainly ascribed to the quite short lifetimes(several microseconds) of the delayed components. The current findings validate a practical strategy to develop evaporation and solution process feasible metal-free highly efficient fluorescent emitters for OLEDs applications.In chapter 3, polarities tunable host materials were developed based on oligocarbazoles and diphenylphosphine oxide. Density Funtional theory(DFT) calculations and photoluminescence(PL) spectra in different polar solvents illustrate these host materials have different polarities. We employed PXZDSO2 as guest material and studyed its luminescent properties in designed host materials. Since PXZDSO2 has extremely stong charge transfer(CT), guest-guest interactions are far stronger than host-guest interactions in doped films, thus PL spectra of doped films are almost the same. In addition, their prompt decay lifetimes(around 30 ns) and delayed decay lifetimes(around 2 us) are quite the same respectively. As CZPO has relatively stronger polarity, the EL spectrum of solution-processed device based on 7 wt% PXZDSO2: CZPO is 8 nm red-shifted than other devices. The devices based on DMAC-DPS with weaker CT, achieves a wide range spectra variation, with EL peaks blue-shifting from 496 nm for CZPO to 470 nm for FCZBn.