Molecules And Organic Light-emitting Diodes Based On Thermally Activated Sensitized Emission Mechanism

Materials with thermally activated delayed fluorescence?TADF?have been widely taken as the third generation emitting dyes of organic light-emitting diodes?OLEDs?.To achieve effective emission,a small singlet?S1?-triplet?T1?splitting??EST?and a high S1 radiative decay rate?S rk?should be fulfilled simultaneously,which conflicts with each other.To break this trade-off,we propose a new strategy,named TADF-sensitized emission,that is using TADF materials as the hosts for conventional fluorophors and phosphors.In this way,the TADF materials are mainly responsible for the T1 up-convesion while the guest for the emission,achieving high device performances.1.TADF-sensitized conventional fluorophors with TADF materials as the host for fluorophors: When used as the host for fluorophors,the T1 of the TADF materials can be up-converted into their S1,from where the energy can be transferred to the guest through the F?rster energy transfer?FET?,increasing the ratio of the excitons utilized in fluorescent OLEDs.Fluorescent OLEDs fabricated based on such concept achieve maximum external quantum efficiency(EQE_max)as high as 12.2% and maximum power efficiency(PE_max)of 44 lm/W.By comparing the device performances with different hosts,the conclusion can be made that high device performance require efficient up-conversion efficiency of the hosts T1.In the mean time,direct trapping of the charges on the guest should be reduced.2.TADF-sensitized phosphors with TADF materials as the host for phosphors: As bipolar materials,TADF materials possess small ?EST,which facilates charges transportation and injection from the adjacent layers,rendering TADF materials to be ideal hosts for phosphors.In this manuscript,a series of 1,3,5-triazine/indocarbazole derivations with decreased ?EST have been designed and synthesized.By tuning the energy levels as well as the transporting abilities of the hosts,balanced transporting and injection abilities are achieved.Orange phosphorescent OLEDs?PHOLEDs?based on those hosts achieve EQE_max of 24.5%,with EQE of 23.8% remained at luminance of10000 cd/m2.Further study shows that when TADF materials are used as the host for phosphors,the hosts T1 can be up-converted into S1,from where the energy will betransferred to the phosphors through FET.Since FET is long-range interaction,the energy transfer is more efficient,which will greatly reduce the dopant concentration of PHOLEDs while keeping high efficiency with low efficiency roll-off and long lifetime.3.TADF-sensiztized hybrid white OLEDs based on blue TADF emitters: When blue TADF materials are used in hybrid white OLEDs?WOLEDs?,the T1 of the TADF molecules can be up-converted into their S1 and then directly transferred to the dopant through the long-range FET,eliminating the loss of the excitons transferred through the short-range Dexter mechnisim in hybrid WOLEDs with conventional blue fluorophors.Besides,it is found that the heavy atoms of the phosphors will induce external heavey atom effect?EHA?to the fluorphors,which will greatly reduce the photoluminance efficiency of the conventional fluorophors while only show limited influence on TADF materails.Hybrid WOLEDs based such concept achieved EQE_max as high as 20.8% and PE_max of 51.2 lm/W.Beisdes,the efficeingy roll-off is small with EQE of 19.6%remained at the luminance of 1000 cd/m2.THe high performance of the device can be attributed to the unique energy transfer process which reduces the loss of the excitons.Besides,highly efficient and stable blue TADF emitters based on benzonitrile as the acceptor unit,achieving EQE_max of 21.2% with a T50 of 770 h at an initial luminance of500cd/m2.