The Design And Synthesis Of Pyridazin Based Bipolar Hosts And Corresponding OLEDs Study

Organic Light-Emitting Diode(OLED)is a current-type light-emitting device based on organic materials,which has the advantages of all solid,self emission,fast response,high color contrast,large viewing angle,low operating voltage,light weight,low power consumption and being capable for flexible substrates and so on.Being promising for both applications of display and lighting,OLED has attracted tremendous attension in the past three decades.This work focuses on the developing function materials for OLED,e.g.,the design and synthesis of new pyridazine-based bipolar hosts and the corresponding OLEDs study,which includes the following contents:Firstly,two novel bipolar host materials were designed and synthesized,i.e.,9,9'-(3-(6-(9Hcarbazol-9-yl)pyridazin-3-yl)pyridine-2,6-diyl)bis(9H-carbazole)(3CzPyaPy)and 3,6-bis(2,6-di(9Hcarbazol-9-yl)pyridin-3-yl)pyridazine(4CzPyPyaPy).The compounds were characterized by NMR spectroscopies,high-resolution mass spectrometry,and the molecular structures were further verified by single-crystal X-ray diffraction(XRD)analysis.Based on the studies of thermal analysis,optical spectroscopies,and cyclic voltammetry(CV),as well as quantum chemistry computations,valuable insights into key molecular electronic structure and interesting structure-properties relationship were disclosed.It was also found that pyridazine incorporated bipolar hosts are rare in literature.Sencondly,since thermal stability is one of essential requairement for functional materials on OLED applications,to disclose the thermal revolution of 3Cz PyaPy,films of 3CzPyapy were prepared and thermally annealed at different temperatures.It was found that there were no significant changes in the strength and profile of the absorption and emission spectra upon thermal treatment from 25 °C to 180 °C.The scanning electron microscopy(SEM)and atomic force microscopy(AFM)showed that 3CzPyaPy films had a smooth and uniform morphology.Consequently,we comcluded that 3Cz PyaPy posseses high thermal and morphological stabilities.Thirdly,before utilizing 3CzPyaPy in OLED devices,mono-carrier devices of 3Cz PyaPy have been fabricated and proved the bipolar transporting capability of 3Cz PyaPy.Then we studied the following five types devices using 3CzPyaPy as a host: a)phosphorescent devices based on Ir(ppy)3(green)were prepared and studied to optimize the structure and doping concentration.It was found that at the doping of 2%,device presented the best performance,with very high efficiencies of 54.0 cd A-1(17.7%);b)4CzIPN based TADF device(TADF: thermal activated delayed fluorescence)achieved a maximum efficiencies of 15.6 cd A-1 and 5.9%;c)in the study of blue OLED: since the triplet of 3CzPyaPy is slightly lower than that of blue phosphor FIrpic,a blue phosphorescent of FIrpic could not effectively confine the exciton in the emitting layer,which leads to low efficiencies;d)a fluorescent OLED based on 2% BD-1 yielded a maximum efficiencies of 12.1 cd A-1 and 8.2%,which is the best among blue fluorescent devices;e)finally,the orange phosphorescent OLED device with Ir(BPPya)2acac phosphor revealed high performance,with maximum efficiencies of 28.2 cd A-1 and 10.9%.