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Fabrication And Measurements Of Solution Processed Thermally Activated Delayed Fluorescence OLED Based On Hyper-structured Molecules

Posted on:2019-04-11Degree:MasterType:Thesis
Country:ChinaCandidate:C G HanFull Text:PDF
GTID:2428330542494524Subject:Materials science
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Thermally activated delayed fluorescence?TADF?emitters are capable of efficient reverse intersystem crossing?RISC?from the triplet excited state?T1?to the singlet excited state?S1?to use triplet excitons for photoluminescence with theoretically 100%internal quantum efficiency?IQE?without any noble metals,which has become one of the research hotspots in organic light-emitting diodes?OLEDs?.Usually,these highly efficient TADF-OLEDs are fabricated through high-vacuum deposition,which entails a complex and expensive fabrication process.However,high-vacuum deposition method is difficult to apply to large area OLEDs fabrication because it is incompatible with roll-to-roll process.In fact,most of the solution-processed TADF-based OLEDs are lack of an interlayer between poly?ethylene dioxythiophene?:poly?styrene sulfonate??PEDOT:PSS?and the emitting layer?EML?.As we know,PEDOT:PSS is now commonly used as the hole-injection layer?HIL?to reduce the injection barrier between the indium tin oxide?ITO?anode and EML.However,the electroluminescence?EL?quenching resulting from the natural acidity and ionic state of PEDOT:PSS always exists at the interface between PEDOT:PSS and EML.And PEDOT:PSS is also not very effective at blocking excess electrons passed through the EML to prevent leakage of electrons.Therefore,we explored the effect of hole-transporting layers?HTLs?on the preparation of OLED devices by solution method at first.We found that the performance of poly-TPD/PVK,poly-TPD:poly[bis?4-phenyl??4-butylphenyl?amine],PVK:poly?9-vinylcarbazole?,bilayer device is best of OLEDs.Therefore,poly-TPD/PVK was chosed as the HTL,and the newly synthesized hyper-structures molecular TADF emitting materials were used as light-emitting materials.The OLEDs were fabricated by the solution method and their performances were tested.In the first part,we explored the influence of HTL on the performance of OLED devices fabricated threough solution method and found poly-TPD/PVK bilayer device's performance was the best one.The classical TADF molecule2,4,5,6-tetrakis?9-carbazolyl?-isophthalonitrile?4Cz IPN?wasdopedwith4,4-bis?9-carbazole?biphenyl?CBP?as an EML and TPD,poly-TPD,PVK,poly-TPD/PVK were used as HTLs,respectively.The device structures were ITO/PEDOT:PSS/HTL/10.7wt%-4Cz IPN:CBP/ETL/LiF/Al?ETL:electron-transporting layer,LiF:Lithium fluoride?.The devices with the poly-TPD/PVK bilayerd structure showed the best performance.The turn-on voltage was 3.7 V,the maximum luminance was 19670 cd/m2.and the maximum external quantum efficiency?EQE?was 10.5%.The bilayerd structure of poly-TPD/PVK can combine the advantages of poly-TPD and PVK,i.e.the high hole-transporting ability and HOMO matching with PEDOT:PSS of poly-TPD,and the higher LUMO and triplet energy levels of PVK.As the PVK was very thick?5 nm?,it almost did not affect the hole-transporting ability of HTL.In the second part,using poly-TPD/PVK as HTL and newly synthesized hyper-structured molecules TADF yellow emitting materials,CRA-TXO-PhCz100,CRA-TXO-PhCz50-mCP50,CRA-TXO-PhCz12.5-mCP87.5,the red emitting material CRA-TXO-TPA100 and the green emitting material CRA-PXZ-Trz doped into1,3-bis?N-carbazolyl?benzene?mCP?as EML,the TADF-OLEDs were fabricated by solution methodFirst,the characteristics of their delayed fluorescence of as-prepared light-emitting materials were confirmed.Then the performance of OLED devices were tested.The maximum brightness of the devices were 679 cd/m2,598 cd/m2,966cd/m2,845 cd/m2,and 2424 cd/m2,respectively.Their maximum emission wavelenth were at 544 nm,544 nm,524 nm,600 nm and 533 nm,respectively.The electroluminescenct spectra were stable under different voltages,indicating that the device structure were reasonble.In the third part,a newly synthesized hyper-structured molecule CRA-mCP was used to replace the host material,electron blocking material and hole transport materialintheformerdevice?ITO/PEDOT:PSS/HTL/10.7wt%-4Cz IPN:CBP/TmPyPB/LiF/Al?,respectively.When CRA-mCP was used as HTL to replace poly-TPD/PVK,the device showed the best compresive performance.The device haed a maximum light emission luminance of 6688 cd/m2,an turn-on voltage of 4.9 V,and a current efficiency?CE?of 20.9 cd/A.
Keywords/Search Tags:solution-process, thermally activated delayed fluorescence (TADF), organic light-emitting diodes (OLEDs), hole-transporting layer, hyper-structured molecular emitting material
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