Preparation And Performance Study Of High-efficiency All-fluorescent White Light Organic Light-emitting Diode

White organic light-emitting diodes（WOLEDs）have significant application prospects in the fields of flexible flat panel displays and large-area solid-state lighting due to their merits of ultrathin form factor,low cost,and large-scale production.All fluorescence WOLEDs based on conventional fluorescent（CF）materials are limited by the 25%exciton utilization,which greatly limits efficiencies.The stability of WOLEDs based on all phosphorescent materials is a bottleneck in commercial applications due to the lack of stable blue phosphorescent materials.Thermally activated delayed fluorescence（TADF）materials have 100%exciton utilization and relatively stable molecular structures,which is a new generation material system for preparing high efficiency,stable and high-quality all fluorescence WOLEDs.Especially,the use of TADF molecules as sensitizers for CF materials to prepare superfluorescence OLEDs enables CF emitters to achieve～100%internal quantum efficiency.This provides a new strategy to prepare all fluorescence WOLEDs with high external quantum efficiency（EQE）,low efficiency roll-off,high color rendering index（CRI）,and long life.However,due to the complex exciton energy transfer pathways in all fluorescence WOLEDs,the efficient utilization of triplet excitons remains a formidable challenge.This paper mainly focuses on the high efficiency all fluorescence WOLEDs.The TADF sensitizing system is adopted to optimize the exciton utilization pathways,and a co-host is used to balance charge carrier transport and broaden the exciton recombination region.Introducing a cascaded exciton transfer layer to control the distribution of excitons,which suppresses the quenching of excitons and improves the utilization of excitons.As a result,all fluorescence WOLEDs with high EQE（>20%）,low efficiency roll-off,and high CRI（>90）are achieved.The main work is summarized as follows:1.Preparation and performance study of high efficiency TADF yellow OLEDs.A series of yellow OLEDs are prepared using different host materials doped with the yellow TADF material t Bu Cz DBA.Compared to single-host yellow OLED,the co-host-based yellow OLEDs show significantly lower driving voltage and efficiency roll-off due to the excellent carrier balance of the co-host system.The yellow device based on CBP:PPF system has a low turn-on voltage of 2.8 V,with maximum current efficiency（CE）,power efficiency（PE）,and EQE of 93.3 cd A^-1,104.6 lm W^-1,and 31.7%,respectively.At a luminance of 1000 cd m^-2,the efficiencies remain 73.9 cd A^-1,59.5 lm W^-1,and 25.2%,respectively.2.Preparation and performance study of high efficiency two-color complementary all fluorescence WOLEDs.Further,m CBP:PPF is used as the co-host for DMAC-DPS,combining with CBP:PPF:t Bu Cz DBA yellow light-emitting layer（EML）,to prepare two-color complementary all fluorescence WOLEDs.The maximum CE,PE,and EQE of the two-color complementary all fluorescence WOLEDs are 71.3 cd A^-1,76.8 lm W^-1,and 26.1%,respectively,while remaining 60.4 cd A^-1,54.5 lm W^-1,and 22.1%at a luminance of 1000cd m^-2.The corresponding CIE and CRI are（0.31,0.41）and 64,respectively.It is noteworthy that the device displays excellent color stability in the luminance range of 1000-5000 cd m^-2,with a chromaticity coordinate（CIE）drift（ΔCIE）of only（0.001,0.003）.Subsequently,the exciton recombination zone in the two-color complementary all fluorescence WOLED is investigated by inserting an ultrathin phosphorescence layer and designing the comparative experiments with different hosts for EMLs.These results show that the co-host system optimizes the balance of carrier transport and widens the exciton recombination zone,which suppresses the aggregation and quenching of excitons and improves the device performance.3.Preparation and performance study of high efficiency three-color all fluorescence WOLEDs.Based on the co-host strategy,the CF material DBP is chosen to fabricate three-color all fluorescence WOLEDs.The DMAC-DPS-t Bu Cz DBA-DBP dual-sensitization pathway is constructed to regulate the diffusion and distribution of excitons by introducing the yellow cascade exciton transfer layer.In addition,multiple F?rster resonance energy transfer（FRET）processes are formed,effectively improving the exciton utilization.By modulating the thickness of cascade exciton transfer layer,the maximum CRI of 92 is attained for three-color all fluorescence WOLEDs.Moreover,the maximum CE,PE,and EQE are 49.5 cd A^-1,55.5 lm W^-1,and 22.4%,which maintain 35.7 cd A^-1,28.3 lm W^-1,and16.2%at a luminance of 1000 cd m^-2,with a CIE and CRI of（0.37,0.40）and 90.The transient photoluminescence（PL）decay experiments and the t Bu Cz DBA sensitizing DBP device reveal the efficient FRET process,and the working mechanism of the high efficiency three-color all fluorescence WOLED is illustrated.Finally,the maximum output coupling efficiency calculated by optical simulation is compared with the experimental test values.The result confirms that the three-color all fluorescence WOLED achieves nearly 100%exciton utilization,which further validates the effectiveness of the proposed design strategy for high efficiency all fluorescence WOLEDs.