Synthesis And Highly Efficient OLEDs Of The Bipolar Host And TADF Materials

Organic light emitting devices(OLEDs)have been widely used in flat panel display and lighting,and have being a hot research direction in the field of organic optoelectronics.Phosphorescent emitters can utilize both single and triplet excitons to achieve 100%theoretical internal quantum efficiency(IQE).Due to the relatively small singlet-triplet splitting energy,thermally activated delayed fluorescence(TADF)materials can achieve reverse intersystem crossing(RISC)from the lowest triplet to the lowest singlet,and can also realizing 100%IQE.In order to achieved the balanced charge transfer and improve the efficiency of phosphorescent OLEDs(PhOLEDs),phosphorescent emitters need to be doped in a bipolar host material.In order to reduce the manufacture cost,TADF material that does not use precious metals needs to be developed.In this paper,novel bipolar host materials and TADF materials are synthesized,and high-efficiency,low-cost OLEDs are fabricated.The details of research works are as follows:(1)Three novel 1,2,4-triazole containing molecules with triazole to carbazole molar ratios of 2:1,1:1 and 1:2,named DTzSCz,DTzDCz and STzDCz,are developed.STzDCz hosted blue and green PhOLEDs exhibit the excellent performances with the efficiencies of 25.0%and 20.3%,respectively.A 1,2,4-triazole/carbazole ratio of 1:2 in the host is the optimal n-type to p-type ratio favorable for the most balanced charge transportation and highest EL efficiency.(2)Dual n-type unit cyanopyridine is selected to design bipolar host by the means of cyanopyridine directly linked onto the carbazole ring(CN-1)or p-and n-type units are separately linked onto a phenylene bridge(CN-2).CN-2 hosted sky-blue PhOLEDs exhibit extremely high efficiency of 34.6%(61.7 cd A-1),which is the highest efficiency among all types of FIrpic based PhOLEDs reported so far.Additionally,CN-2 hosted green device shows the efficiency of 28.2%(94.8 cd A-1),which is close to the best performance of Ir(ppy)3 based PhOLEDs.This result is attributed to the fact that CN-2 can increase the horizontal orientation of emitting molecules,which is conducive to the improve out-coupling efficiency and the device performance.p-type and n-type units are separately linked onto a phenylene bridge is an efficient way to improve the efficiency of PhOLEDs.(3)Compared with o-CzTrz,meta-linked 1,3,5-triazine/carbazole hybrid TADF emitter,m-CzTrz,possesses dramatically higher hole and electron mobilities,which can used as the bipolar host and blue emitter in OLED.As the blue emitter,m-CzTrz exhibits high efficiencies of 19.2%and 16.7%in doped and non-doped devices,respectively.As the bipolar host,m-CzTrz can sensitize traditional green fluorophor and iridium phosphor,with the efficiency of 11.5%and 20.8%,respectively.More importantly,m-CzTrz acting as a host emitter in single-emitting layer hybrid WOLEDs shows the high efficiency of 19.8%.The appropriate steric conformation is the key for the multifunctional applications of m-CzTrz.(4)By introducing double sulfonyl groups and double carbazole groups into phenyl core,green and blue TADF emitters,25DPS-Cz,45DPS-Cz and 46DPS-Cz,are developed.The photophysical properties of these molecules are tuned by changing the sulfonyl groups embedded in donors or on either side of the central benzene ring.The emitter whose donor and acceptor are located close to each other on either side of a central benzene ring exhibits through-space charge transfer,which greatly decreases the delayed lifetime(?DF),enhances RISC process and photoluminescence quantum yield(PLQY).The ?OF and kRISC for 46DPS-Cz are 1.9 ?s and 3.0 × 106 s-1,respectively,and the PLQY is 99.9%.Sky-blue TADF device based on 46DPS-Cz shows the efficiency of 25.9%(53.3 cd A-1).It is shown that the molecule whose donor and acceptor are located close to each other on either side of a central benzene ring is the preferable structure to improve the performance of TADF OLEDs.(5)Blue and green TADF emitters,DBF-SO-DMAC,DBF-SO-PXZ and DBF-SO-PTZ are developed by using 9,9-dimethylacridan,phenoxazine and phenothiazine as donor units and 2-(phenylsulfonyl)dibenzo[b,d]furan(DBF-SO)as acceptor.The efficiencies of doped devices are 22.9%(43.6 cd A-1),21.7%(64.6 cd A-1)and 20.3%(60.5 cd A-1)and the efficiencies of non-doped devices are 18.1%(37.6 cd A-1),18.8%(60.3 cd A-1)and 15.6%(46.6 cd A-1)for DBF-SO-DMAC,DBF-SO-PXZ and DBF-SO-PTZ based devices,respectively.Compared with doped devices,the EL emission peak of non-doped devices are red-shifted by 10,18 and 8 nm,respectively.