Working Stability Development For Organic Light-emitting Diodes With Small Singlet-triplet Energy Split Materials

After the development of organic light-emitting diodes（OLEDs）for over 30 years,it is of the great interest to regard it as the candidate for new generation for full-color display and lighting technology.Up to date,the first-generation of conventional fluorescent materials and the second-generation of phosphorescent materials have already been developed,while the red and green-color OLEDs have met the commercial demand for their efficiency and working lifespan.However,their low-efficiency or high-cost issues further restrict the progress of OLED technology.In the past few years,a novel type of material with small singlet-triplet energy split has attracted great hopes as the advantages of potentially low-cost and high efficiency.Hence,our current thesis focuses on such small singlet-triplet energy split materials to disscuss the working stability,including novel thermally activated delayed fluorescence（TADF）materials,exciplex cohosts and the device configuration for purely organic white organic light-emitting diodes（WOLEDs）.In chapter 2,a series of thioxanthone derivatives consisting 9H-thioxanthen-9-one-S,S-dioxide（SOXO）or thioxanthone（TXO）as an electron acceptor were developed as TADF emitters to fabricate highly efficient fluorescent OLEDs.Their emission color was successfully tuned from blue to orange-yellow by changing the sulfur atom valence state to tune intramolecular charge transfer effect and dominating the molecular conjugation.Amogst these emitters,maximum external quantum efficiency of 13.6%with Commission Internationale de L’Eclairage（CIE）coordinates of（0.37,0.57）was achieved in green light emission molecule CzSOXO which contain SOXO acceptor and carbazole donor in a D-A structure.The efficiency roll-off were well controlled and maintained 8.8%of EQE at 1000 cd m^-2,as the introduction of sulfonyl moiety would further promotes reverse intersystem crossing（RISC）process and restrain the exciton-exciton annihilations.Besides,the blue molecules consisting of TXO acceptor could also give EQE values exceeding 11%,but their efficiency roll-off was considerably severe with increasing current density.After efficiency simulations by adopting TTA model,the results indicate that TTA annihilations are taken the major response for the efficiency trade-off,while long delayed fluorescent lifetime and slow RISC process would suffer more from the luminous decline.In chapter 3,a simplified but effective approach was demonstrated to give molecular stability prediction and analyze their possible degradation mechanisms by considering molecular dissociated activation energy（ΔE_A）description and internal exciton dynamics correlations.Degradation of molecules in charged states,excited states,charged-excited states and highly excited states provide the possible dissociation mechanisms for exciplex cohost candidates.As the result,hole transporting moieties in cohost molecules possess distinct weak bonds under the molecules in excited states（or highly excited states）and charged-excited states,in consequence of homolytic cleavage reactions at C-N or C-C weak bonds and generating dissociated products in aromatic amine.Fortunately,rapid reactive exciton relaxation from highly excited state to stable state has the intrinsic talent to access potentially more reliable exciplex cohosts,achieving extended T₅₀(time to 50%of initial luminance of 1000 cd m^-2)lifetime of 10169 h and 9051 h for the predicted long-lived exciplex cohosts BCzPh:SF2-TRZ and NPB:T2T.Degradation behaviors further confirm that the deteriorated source is attributed to the formation of exciton quenchers and hole traps from excited states（or highly excited states）and charged-excited states,respectively.In chapter 4,we demonstrate a feasible configuration of exciton-adjustable interlayers Bepp2:mCBP/Bepp2,to achieve high efficiency and operational stable purely organic fluorescent WOLEDs consisting of a TADF assistant host 4CzPN and a strategic exciton management by multi interlayers.After investigating the exciton density distribution and exciton dynamic behaviors,the main recombination region can be properly controlled to located at yellow emission layer（Y-EML）and interlayer interface,inhibiting the generally quench of long-range triplet excitons by blue fluorophore.As the result,outstanding maximum EQEs of 15.1%(48.9lm W^-1)and 14.7%(38.4 lm W^-1)were performed in two-color and three-color WOLEDs,the efficiency trades off could be favorably depressed.The current findings also indicate that restraining exciplex quenching at carrier blocking layer and emitting layer interface make significant roles on enhancing operational lifetime.Therefore,the conventional blocking layer TCTA was replaced by a wide bandgap material to achieve impressive T₅₀ lifetime of²000 h.To our knowledge,this is the first pioneering work for realizing high efficiency,low efficiency roll-off and long working lifetime purely organic WOLEDs based on TADF materials.