Synthesis And Study On Electroluminescent Properties Of Triphenylmethyl Type Neutral Radicals-a New Way To Break Through The 25% Upper Limit Of Internal Quantum Efficiency Of OLED

Applications of organic light emitting device(OLED)in flat panel display and lighting source are one of the pioneering work owing to some excellent features of OLED,such as self-illumination,wide-viewing angle,flexibility and low-power consumption,which constantly make the OLED a potential leader in display markt.However,there are key issues for OLED materials and devices research,such as how to reduce the materials cost and how it can efficiently utilize the triplet exciton for emission.Although,phosphorescent materials can efficiently solve the problem of the difficulty of using triplet excitons and realize 100% internal quantum efficiency(IQE),the price of the noble metal-based phosphorescent materals is higher than that of the conventional fluorescent materials,meanwhile it is a challenging work to obtain the deep blue phosphorescent materials which satisfy the european broadcasting union(EBU)standard.Resently,thermally activated florescent(TADF)materials effectively solve the problem of high cost of phosphorescent materials and harvest the triplet exciton simutaneously.Triplet-triplet annihilation(TTA)and hybridized local and charge-transfer(HLCT)state mechanisms also show other effective ways to harvest the triplet excitons.All above mentioned mechanisms have their adventages and disadventages,thus it is worth findingout new OLED mechanisms and new types of luminescent materials which are cheap andwith high efficiency.In this thesis,we propose a new kind of OLED,in which organic open shell molecules are used as the emitters to circumvent the transition problem of the triplet excitons and 100% upper limit of IQE can be achieved.For organic radials,because the spin configuration of one unpaired electron is doublet,the transition of radical excitons back to the ground state is spin-allowed.We performed the following studies based on doublet emission from organic neutral radical:1.An emissive and stable radical tris(2,4,6-trichlorophenyl)-methyl(TTM)and two carbazole linked TTM series radical,TTM-1Cz and TTM-2Cz,were synthesized.TTM-1Cz and TTM-2Cz show excellent thermostability and have high luminescent efficiency.Photoluminescent emission spectra of TTM-1Cz and TTM-2Cz in hexane peak at 628 nm and 640 nm,and their quantum efficiencies reach to 53% and 54% respectivly.The photopysical propertis of these radicals indicate that their emission comes from the decay of doublet.However,the emission manners of TTM-1Cz and TTM-2Cz are different from that of TTM.The emissions of TTM-1Cz and TTM-2Cz show the charge chansfer(CT)characteristic,whereas the emission of TTM has the localized property.The aging lifetimes of TTM,TTM-1Cz and TTM-2Cz are 1.6×102 s,9.9×103 s and 4.7×105 s respectivly.The results suggest that the photostabilty of TTM-series radicals increases with the increase of carbazole(donor)unit.Using CBP as a host,we fabricated neutral radical(TTM-1Cz and TTM-2Cz)-based OLEDs with 5% doping concentration ratio.The maximum external quantum efficiency(EQE)of TTM-1Cz and TTM-2Cz-based devices reach 2.4% and 0.8%,and their EL spectra peak at 693 nm and 690 nm,respectively.Magneto-electroluminescence(MEL)results demonstrate that the device emission comes from the doublet exciton.2.Through optimizing the device structre and using the mixture of radical TTM-1Cz and its precursor,nearly 100% doublet exciton formation ratio(?D)in radical-based OLED was obtained.In order to further understand the properties of doping compounds(radical and its precursor),we synthesized the radical precursor(?HTTM-1Cz and ?HTTM-2Cz).Photopysical investigation shows that both of the radical precursors are large band-gap closed-shell molecules with low quantum yield.Study on the different concentration ratio between radical and its precursor shows that mixing radical with its precursor not only resolves the problem of aggregation caused quenching(ACQ)of the pure radical,but although enhances the devices performances.OLED device with 2.5 wt.% doping concentration ratio of radical reaches the maximum EQE of 10.5%,up to date,which is the highest value among the non-phosphrescent deep red fluorescent OLEDs.Mixing radical with its precursor is proved to be a new way to solve the ACQ problem of pure radicals.3.To find out which factors impact the stability and photophysical propertis of TTM series radical,we designed and synthesized two different types of radicals,TTM-1CzBz and TTM-1CztB,in which elctron donor group(tert-butyl)and electron acceptor group(benzoyl)were attached to the carbazole unit.Both new radicals TTM-1Cz Bz and TTM-1CztB show high thermal stability(over than 330?),and strong emission in hexane,emission peaks are 614 nm and 640 nm,and quantum efficiency reach 54% and 55%,respectively.Comparing the emission peaks to radical TTM-1Cz,14 nm blue shift and 12 nm red shift were found for TTM-1CzBz and TTM-1CztB,respectively.The donor and accepter groups connected to the carbazole have great influence on radical stability and photophysical properties.The photostability of benzoyl substituted-radical,TTM-1CzBz,is 15 times as low as that of TTM-1Cz,wheras its electron transporting ability was increased compared to that of TTM-1Cz.Interestingly,for TTM-1CztB,photostability was increased by 4.5 times compared to that of TTM-1Cz,but the electron and hole transporting abilities were much decreased.The maximum EQE of the doping devices of TTM-1CzBz and TTM-1CztB with 2.5 wt.% in CBP host reach 5.2% and 2.8%,respectively.