Device Structure Design And Optoelectronic Performances Of High Efficiency Blue Fluorescent Organic Light-emitting Diodes

Organic light-emitting diodes（OLEDs）are a new generation of technology of display and lighting.Compared with green and red OLEDs,blue OLEDs have some shortcomings in efficiency,color purity and stability.Although blue phosphorescent OLEDs have the advantage of high efficiency,the exciton quenching process in phosphorescent devices is serious,which leads to serious efficiency roll-off and short lifetime.Compared with blue phosphorescent materials,organic blue fluorescent materials utilize singlet excitons with short lifetime for radiative decay,so their device efficiency roll-off is negligible and device lifetime is relatively stable.Among,triplet–triplet annihilation（TTA）up-conversion,thermally activated delayed fluorescence（TADF）,hybridized local and charge-transfer（HLCT）and aggregation-induced emission（AIE）materials are the currently widely studied fluorescent materials.Compared to TTA and TADF material systems,AIE and HLCT materials have attracted attention due to their fast and efficient reverse intersystem crossing（h RISC）from the high-lying triplet states（T_n,n≥2）to the singlet states（S_m,m≥1）.Moreover,as the h RISC process of AIE and HLCT materials is a nanosecond fast process,it can effectively suppress exciton loss.Therefore,AIE and HLCT materials can achieve a theoretical internal quantum efficiency（IQE）of 100%without efficiency roll-off.However,the efficiency of blue OLEDs based on AIE and HLCT materials did not meet our expectations and the exciton loss processes resulting in low efficiency is still unclear,which urgently needs further study.In this thesis,a series of in-depth researches on the exciton dynamics and optoelectronic properties of AIE and HLCT materials are deeply studied,clearly revealing the exciton loss processes in the electroluminescence（EL）of AIE and HLCT materials.Further,we fabricated high-efficiency blue fluorescent OLEDs based on AIE and HLCT materials through device structure design.The main work is summarized as follows:1.We studied the exciton dynamics and electroluminescence properties of AIE blue fluorescentmaterial4’’’’-（diphenylamino）-2’’,5’’-diphenyl-[1,1’:4’,1’’:4’’,1’’’:4’’’,1’’’’-quinquephenyl]-4-carbonitrile（TPB-PAPC）and HLCT blue fluorescent material 2-（4-（10-（3-（9H-carbazol-9-yl）phenyl）anthracen-9-yl）phenyl）-1-phenyl-1H-phenanthro[9,10-d]imidazole（PAC）,and successfully fabricated two high-efficiency blue fluorescent OLEDs by doping blue fluorescent material BD into TPB-PAPC and PAC.Among them,the maximum external quantum efficiency(EQE_max)of PAC:2%BD device reached 17.4%with Commission Internationale de I’Eclairage（CIE）coordinates of（0.14,0.22）,achieving a breakthrough in the efficiency of blue fluorescent OLEDs.Through transient EL,magnetic field EL and exciton dynamics analysis,TPB-PAPC exists the h RISC process of T₃→S₁ and the internal conversion（IC）of T₃→T₁ that is inevitable exciton loss process.Similarly,PAC exists inevitable exciton loss process of T₂→T₁.The experiments clearly show that the doping fluorescent material BD in emitting layer can effectively promote the h RISC process of T_n→S_m and inhibit the IC process of T_n→T_n-1,greatly reducing the loss of high-energy triplet excitons and improving device performance.2.We studied the exciton dynamics and electroluminescence properties of two AIE deep-blue materials 4′-（4-（diphenylamino）phenyl）-5′-phenyl-[1,1′:2′,1′′-terphenyl]-4-carbonitrile（TPB-AC）and 4′′′-（diphenylamino）-2′,5′-diphenyl-[1,1′:4′,1′′:4′′,1′′′-quaterphenyl]-4-carbonitrile（TPB-TPA-CN）.The transient EL decay and photophysical measurements clearly prove that TPB-AC and TPB-TPA-CN devices exist serious singlet-triplet annihilation（STA）processes.By introducing the TTA up-conversion material DMPPP into the AIE emitting layers,the loss excitons of IC process are reused,effectively inhibiting the STA exciton losses in TPB-AC and TPB-TPA-CN devices.As a result,the efficiency and efficiency roll-off of the resulting devices are improved.Among them,the EQE_max of TPB-AC:30%DMPPP device is 11.1%with CIE color coordinates of（0.15,0.07）and the EQE_max of TPB-TPA-CN:30%DMPP device is 11.8%with CIE color coordinates of（0.15,0.07）.These devices exhibit negligible efficiency roll-off.3.We studied the exciton dynamics and electroluminescence properties of AIE sky-blue material 2’,5’-bis（4-（diphenylamino）phenyl）-[1,1’:4’,1’’-terphenyl]-4,4’’-dicarbonitrile（i TPB-2AC）.Through the calculation of the excited state energy level and spin orbit coupling（SOC）matrix element values of i TPB-2AC,it is found that i TPB-2AC exist three h RISC processes of T₉→S₃,T₄→S₂ and T₃→S₁.Because the high-lying excited triplet excitons cannot avoid the IC process,the internal exciton loss of the high-lying excited triplet excitons and the serious STA loss process are caused.By introducing TTA up-conversion materials of DMPPP and Cz PA into i TPB-2AC,loss excitons of the IC process are reused and the STA process is suppressed.Thanks to the improvement in Dexter energy transfer（DET）and h RISC process,the EQE_maxof the resulting blue fluorescent OLEDs reached 14.8%with CIE color coordinates of（0.15,0.18）,keeping 14.4%at the luminance of 1000 cd m^-2.