OLEDs With Low Efficiency Roll-off Based On Several Novel Iridium(?) Complexes

The organic light-emitting diodes?OLED?based on organic materials with the characteristics of self-illumination is featured with outstanding performances in screen lightness,response speed,imaging efficiency and flexible display,and their market penetration rate is increasing year by year.Emiting materials are the key for OLED,and among them the iridium?III?complexes are prominent due to their high efficiency,variable ligand structure and tunable emission color.However,due to some influence of factors such as imbalanced charge carrier and high local exciton densities,OLED devices generally face serious efficiency roll-off.To solve this problem,we focused on the design of the ancillary ligand structures,and several iridium complexes?III?with good electron mobility were synthesized.Their photoelectric properties were studied and discussed in detail,and the OLED exhibit good electroluminescence performances with extremely low efficiency roll-off.1.A novel ancillary ligand of pyridine sulfonic acid?Py SO₃?was developed for two green-emitting iridium?III?compounds,Ir1(?_max=496 nm)and Ir2(?_max=504nm),with trifluoromethyl substituted 2-phenylpyridine derivatives as main ligands.Due to the strong electron-withdrawing ability and the special structure of Py SO₃,both complexes have relatively low LUMO energy levels and good electron mobility,which enhance their electron injection/transport?EI/ET?abilities,benefit the charge balance,alleviate the exciton annihilation and finally slow down the efficiency roll-off of the deivces.Respectively,the device D1 using Ir1 as emitter obtains a maximum luminance(L_max)of 44885 cd m^-2,a maximum external quantum efficiency(EQE_max)of 21.7%,and the EQE still remains 20.5%at a high luminance of 10000 cd m^-2 with an efficiency roll-off ratio of 5.5%.Device D2 using Ir2 as emitter obtains better performances with a L_max of 92297 cd m^-2 and an EQE_max of 25.5%.At the high luminance of 10000 cd m^-2,the EQE remains 24.9%with an efficiency roll-off ratio of 3.2%.Even at the high luminance of 20000 cd m^-2,the efficiency fells by only9.5%.It can be seen that Py SO₃ not only has the advantages of simple structure and easy synthesis,but also has unique advantages in enhancing EI/ET capacity of complexes when used as ancillary ligands,providing a new method for the development of high-performance OLEDs with low efficiency roll-off.2.The ancillary ligand bis?diphenylphorothioyl?amide?Stpip?was utilized in two sky-blue iridium?III?complexes?dfppy?₂Ir?Stpip?and?dfppm?₂Ir?Stpip?with 2-?2,4-difluorophenyl?pyridine?dfppy?and 2-?2,4-difluorophenyl?pyrimidine?dfppm?as main ligands,respectively.Compared with the reported ancillary ligand of tetraphenylimidodiphosphinate?tpip?,the introduction of S atom enhances the coordination ability between the ancillary ligand and the metal center,moderate synthesis conditions,and improve the synthesis and sublimation yield of the complexes.At the same time,Stpip which has a large steric hindrance and is rich in heteroatoms and unsaturated bonds,can effectively inhibit the accumulation of molecules,improve the electron mobility of complexes and finally enhance the performances of devices.Device D1 using?dfppy?₂Ir?Stpip?emitter exhibits a L_maxof 11182 cd m^-2,a?_c,max of 21.5 cd A^-1 and an EQE_max of 12.7%.At the luminance of1000 cd m^-2,the EQE still remains as 12.6%.As the pyrimidyl has better electron accepting character and capacity in giving the enhanced MLCT transition,?dfppm?₂Ir?Stpip?obtains higher quantum efficiency,and the performances of the device D2 were also greatly improved with a L_max of 16592 cd m^-2,a?_c,max of 34.7 cd A^-1 and an EQE_max of 19.4%.At the brightness of 1000 cd m^-2,the EQE remains as19.2%with an efficiency roll-off ratio of 1.0%,indicating an inappreciable efficiency roll-off.When Stpip serve as the ancillary ligand,the performances of the blue device are improved with the obviously suppressed efficiency roll-off,which provides a feasible strategy for alleviating the efficiency roll-off of the blue OLED device.