The Excited State Natures Of 1,2,4-triazole Bipolar Hosts And Performances In Low Roll-off Phosphorescent Devices

Organic light-emitting devices?OLEDs?have attracted considerable attentions in the next-generation lighting and displaying because of the advantages of fast response,high resolution,thin and flexible displays.Phosphorescent organic light-emitting diodes?PhOLEDs?have become a hot topic owing to the fact that they can harvest both singlet and triplet excitons to achieve 100%internal quantum efficiency.However,most PhOLEDs generally suffer from drastic efficiency roll-offs at high current density and high brightness,which has become one of the main bottleneck,which has limited its apllications.One of the main reasons is serious triplet-triplet annihilation?TTA?at high current density and luminances.Herein,based on the previous researches in our group,using 1,2,4-triazole as an acceptor,a series of hosts with special excited states were designed and synthesized by changing the donor's structures,the number of donors and the linkages between donor and acceptor.By adjusting the transition path of triplet excitons,the density of triplet excitons is reduced in the host material,the utilization of triplet excitons is enhanced.Consequently,the stability and luminous efficiency of PhOLEDs has been improved.The paper is mainly based on the following three parts:Firstly,a series of D-?-A type bipolar materials with thermally activated delayed fluorescence?TADF?features using 1,2,4-triazole as the acceptor and phenothiazine?PTZ?,phenoxazine?PXZ?and 9,9-dimethylacridane?DMAC?as the donors are were designed and synthesized by changing the electron-donatingabilityofthedonors,namely10-?4-?1-phenyl-5-?pyridine?-2-yl?-1H-1,2,4-triazol-3-yl?phenyl)-10H-phenothiazine?TAZ-PTZ?,10-?4-?1-phenyl-5-??pyridin-2-yl?-1H-1,2,4-triazol-3-yl?phenyl)-10H-phenoxazine?TAZ-PXZ?and9,9-dimethyl-10-?4-?1-Phenyl-5?pyridin-2-yl?-1H-1,2,4-triazol-3-yl?phenyl?-9,10-acridine?TAZ-DMAC?.The host materials showed excellent thermal properties with the glass transition temperature?T_g?of 79 and 83°C of TAZ-PTZ and TAZ-PXZ which were obviously higher than that of the current host material 4,4'-bis?9H-carbazol-9-yl?biphenyl?CBP,T_g=62°C?.The E_T of TAZ-PTZ,TAZ-PXZ and TAZ-DMAC were 2.51,2.75 and 2.82 eV,respectively.The highest occupied molecular orbital?HOMO?/lowest unoccupied molecular orbital?LUMO?levels of TAZ-PTZ,TAZ-PXZ and TAZ-DMAC were-5.53/-2.68,-5.43/-2.57 and-5.67/-2.42 eV.The red and green PhOLEDs utilizing these bipolar TADF materials as the host matrix and bis?1-phenyl-isoquinoline??acetylacetonato?iridium?III??Ir?piq?₂acac?and fac-tris?2-phenylpyridine?iridium?Ir?ppy?₃?the emitter were fabricated.The green PhOLED with TAZ-PXZ as the host has a lower turn-on voltage(V_on)of2.9 V,the maximum luminance(L_max),maximum current efficiency(?_c,max),maximum power efficiency(?_p,max)and the maximum external quantum efficiency(EQE_max)was 57590 cd/m²,41.6 cd/A,37.8 lm/W and 12.1%,respectively.The efficiency roll-off values at 1000 cd/m² and 10000 cd/m² were1.4%and 14.1%,respectively,lower than the recently reported efficiency roll-off value of PhOLEDs with Ir?ppy?₃ as a guest.Secondly,in order to further improve the carrier transport properties of TADF host materials,a series of symmetrical D-?-A-?-D type bipolar materials with TADF features using 1,2,4-triazole as the acceptor and phenothiazine?PTZ?,phenoxazine?PXZ?and 9,9-dimethylacridane?DMAC?asthedonorsweredesignedandsynthesized,namely10,10'-??5-?pyridin-2-yl?-1H-1,2,4-triazole-1,3-diyl?bis?4,1-phenylene??bis?10H-phenothiazine??TAZ-2PTZ?,10,10'-??5-?pyridin-2-yl?-1H-1,2,4-triazole-1,3-diyl?bis?4,1-phenylene??bis?10H-phenoxazine??TAZ-2PXZ?,10,10'-??5-?pyridin-2-yl?-1H-1,2,4-triazole-1,3-diyl?bis?4,1-phenylene??bis?9,9-dimethyl-9,10-dihydroacridine??TAZ-2DMAC?.The E_T of TAZ-2PTZ,TAZ-2PXZ and TAZ-2DMAC were 2.33,2.35 and 2.62 eV,respectively.Green and red PhOLEDs were prepared by using them as the host and Ir?ppy?₃and Ir?piq?₂acac as the guest.The green PhOLED doped with TAZ-2PXZ has better device performance and?_c,max,?_p,max,max and EQE_max are 40.3 cd/A,37.6lm/W and 11.6%respectively.The roll-off values of efficiency at 1000 cd/m²and 10000 cd/m² are 0.86%and 23.27%,respectively.The maximum EQE of red PhOLED based on TAZ-2PTZ is 13.0%,and it remains 11.9%at 1000cd/m²,the corresponding efficiency roll-off value is 8.2%.Compared with green and blue PhOLEDs,the roll-off of red PhOLEDs is more serious due to its high molecular polarity,narrow optical bandgap and long exciton lifetime.Based on this,we designed and synthesized two bipolar host materials with hybridized locally-electron and charge-transfer?HLCT?properties using 1,2,4-triazole as acceptor and carbazole as donor,termed3,3'-??5-?pyridin-2-yl?-1H-1,2,4-triazole-1,3-diyl?bis?4,1-phenylene??bis?9-phen yl-9H-carbazole??3Cz-TAZ-3Cz?and2,2'-??5-?pyridin-2-yl?-1H-1,2,4-triazole-1,3-diyl?bis?4,1-phenylene??bis?9-phenyl-9H-carbazole??2Cz-TAZ-2Cz?in which acceptor 1,2,4-triazole was introduced at the C-3 and C-2 positions of donor carbazole,respectively.2Cz-TAZ-2Cz and3Cz-TAZ-3Cz have excellent thermal stability,and their thermal decomposition temperatures?T_d?are 457 and 432°C,respectively.2Cz-TAZ-2Cz has better bipolar carrier transport characteristics than the typical host material CBP.The E_T of 2Cz-TAZ-2Cz and 3Cz-TAZ-3Cz were2.66 and 2.65 eV,respectively.The fabricated red devices using 2Cz-TAZ-2Cz or 3Cz-TAZ-3Cz host and Ir?piq?₂?acac?guest emitter realize high device performance.The optimized device based on 2Cz-TAZ-2Cz simultaneously achieved high efficiency and very small efficiency roll-off.The maximum current efficiency and EQE reach 12.4 cd/A and 16.6%,respectively,and EQE shows only a 12.0%and 29.4%drop at practical application luminances of1,000 cd/m² and 10,000 cd/m²,respectively,which is better than that of devices based on CBP and recently reported red PhOLEDs with Ir?piq?₂acac as the guest.