| New-generation Displays and lightings based on Organic Light Emitting Diodes(OLEDs)have been commercially available,but they are too expensive at present.The emitting species in commercial OLEDs are generally Ir-or Pt based phosphorescent complexes.The scarcity of these noble metal elements would increase production cost.Moreover,Ir-or Pt based phosphorescent materials are unable to produce efficient deep blue emission.Therefore,developing efficient fluorescent substitutes is important in terms of reducing commercial cost as well as enhanching color gamut.However,the triplet excited state is generally non-emissive for fluorescent molecules,leading to poor device performance.Hence,developing fluorescent molecules that can utilize triplet excitons is a pressing issue.There are mainly two types of triplet utilization mechanism,i.e.,thermally activated delayed fluorescence(TADF)and triplet-triplet annihilation(TTA).TADF can fully use both singlet and triplet excitons and achieve high device efficiency that is comparable to the efficiency of phosphorescent complexes.But the devices normally suffer from serious efficiency roll-off at high luminescence partially as a result of the relatively slow reverse intersystem crossing(RISC)of T1?S1.TTA is able to slow down the efficiency roll-off at high luminescence and is suitable for nondoped devices.But most of the reported TTA molecules can only obtain high efficiency in doped devices.The devices normally need to be operated at high luminescence in practical application.Displays should be able to constantly operate at the luminescence of 100 cd m-2.Mobile phones should stably operate at even higher luminescence,such as 500 cd m-2,under strong sunlight so that we can read and watch clearly.Solid lightings should be able to constantly operate at the luminescence of 1000 cd m-2.Only acquiring high efficiency at high luminescence with negligible efficiency roll-off can fluorescent materials be useful in practical applications.Green-emitting fluorescent molecules have realized relatively satisfactory device performance.In contrast,the efficiency of blue-and red-emitting devices needs to be improved.Extending the spectrum into the near infrared(NIR)region is also an attractive subject.Additionally,developing fluorescent materials that can realize high efficiency in nondoped device is also meaningful for practical application since nondoped device can simplify device fabrication and lower production cost.In this dissertation,we focus on designing and synthesizing fluorescent molecules that are capable of acquiring high device efficiency at high luminescence with small efficiency roll-off.We also try to develop fluorescent materials suitable for high performance nondoped device.By using phenanthroimidazole as donor,we report a series of D-A molecules whose emission spectra can cover the entire visible region.The main results of this dissertation are summarized as follows:(1)We investigated the different photophysical properties resulting from two different linking modes between phenanthroimidazole(donor)and acceptor.In the case of attaching the acceptor to the C-position of imidazole ring of phenanthroimidazole,the acceptor is less twisted against phenanthroimidazole because of weaker steric repulsion.In other words,the donor and acceptor share better conjugation.Hence,the overlap between hole and electron distributions of S1 state is large.Correspondingly,the Coulomb attraction of hole-electron pair is strong which leads to the difficulty of the spin flip of the excited electron.This means that the intersystem crossing(ISC)from S1 to triplet excited state is of low possibility.The C-position linking mode is beneficial for achieving higher photoluminescence quantum yield(PLQY)and better device performance.In the case of connecting the acceptor with the N-position of imidazole ring,the acceptor is more twisted against phenanthroimidazole because of the steric repulsion of neighboring H atoms.As a result,the hole and electron orbitals of S1 state barely overlap.Therefore,the Coulomb attraction between hole and electron is weak and as a result of which,the spin flip of the excited electron can readily take place.In other words,the ISC from S1 to triplet excited state is efficient.The N-position linking mode also results in much lower PLQY and more inferior device efficiency.Since the triplet energy of phenanthroimidazole is relatively low and the donating ability is weak,even though the twisting angle between acceptor and phenanthroimidazole is large,TADF is not feasible for blue-emitting molecules constructed via N-position linking mode.(2)We have synthesized a series of full-color emission phenanthroimidazole derivatives and realized high device efficiency at high luminescence with little efficiency roll-off for blue-,green-and red emission.For deep blue emission,a maximum external quantum efficiency(EQE)of 6.80%is attained.The EQE is 6.63%and 5.64%at the luminescence of 100 cd m-2 and 1000 cd m-2.The electroluminescence(EL)peak is 445 nm with a CIE coordinates of(0.15,0.08).This result is based on doped device of phenanthroimidazole-sulfone D-A molecule PMSO.For sky blue emission,a maximum EQE of 9.44%(13.16 cd A-1)is obtained at the luminescence of 1000 cd M-2.The EQE can still remain as high as 8.09%(11.29 cd A-1)at the luminescence of 10000 cd m-2.The EL peak is 470 nm and the CIE coordinates is(0.14,0.19).This result is obtained in nondoped device of phenanthroimidazole-cyano substituted anthracene molecule PIAnCN.For green emission,a maximum EQE of 8.31%(26.85 cd A-1)is achieved.The EQE can still retain at 8.27%at the luminescence of 1000 cd m-2.The EL peak locates at 537 nm with CIE coordinates of(0.37,0.56).The result is carried out in doped device of phenanthroimidazole-benzo[c][1,2,5]thiadiazole D-A molecule PIBzP.For orange-yellow emission,a maximum EQE of 9.50%(26.14 cd A-1)is realized.The EQE is 9.15%at 1000 cd m-2.The EL shows a CIE coordinates of(0.48,0.51)with an emission peak of 575 nm.This results is acquired in doped device of phenanthroimidazole-cyano substituted benzo[c][1,2,5]thiadiazole D-A molecule PIBzPCN.For orange-red emission,the maximum EQE is 9.00%(23.35 cd A-1)and decreases to 8.64%at 1000 cd m-2.The EL shows a CIE coordinates of(0.52,0.47)with a emission peak of 582 nm.This result is derived from doped device of phenanthroimidazole-benzo[c][1,2,5]thiadiazole-triphenylamine non-symmetry D-A-D' molecule PIBzTPA.For red emission,the maximum EQE is 6.66%(10.81 cd A-1).The EQE is 6.51%and 5.11%at 100 cd m-2 and 1000 cd m-2,respectively.The EL peak is 612 run,and the CIE coordinates is(0.60,0.40).This result is from nondoped device of PIBzTPA.Last but not the least,phenanthroimidazole-naphtho[2,3-c][1,2,5]thiadiazole-triphenylamine non-symmetry D-A-D'molecule PINzTPA exhibits efficient NIR emission with the PLQY of 33.65%in neat film.The nondoped device of PINzTPA displays NIR emission with EL peak of 688 nm and CIE coordinates of(0.69,0.30).The maximum EQE is 2.76%which is high among nondoped fluorescent NIR OLEDs.The EQE is increased to 4.96%in doped device showing NIR EL with emission peak of 680 nm and CIE coordinates of(0.69,0.30). |
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