Study Of Fluorescent/Phosphorescent Hybrid White Organic Light-emitting Diodes

White organic light emitting diodes(WOLEDs)have attracted great attention in the fields of solid lighting and flat display due to the advantages of light weight,low power consumption,large area and easy flexibility.Since the stability of the blue phosphorescent emitters is still a big problem which severely restricts the application of all-phosphorescent WOLEDs to the commercial market,hybrid WOLEDs com-bined stable blue fluorescencent emitters and highly efficient green/orange/red phos-phorescent emitters have been considered as the most promising candidate.However,the performances of hybrid WOLEDs still lags behind that of all-phosphorescent WOLEDs to some extend,for triplet excitons will inevitably be captured by tradition-al fluorescent blue emitters,resulting in non-radiative energy loss.In recent years,the development of fluorescent molecules with thermally activated delayed fluorescence(TADF)characteristics has further improved the performance of hybrid WOLEDs.Although the triplet excitons of TADF materials can be up-converted to singlet through the reversed intersystem crossing process due to their small energy gap be-tween the singlet and triplet,and then be radiated to achieve 100%exciton utilization,this also leads to the lifetime of singlet exciton usually in the order of microsecond or even nanosecond,resulting in serious efficiency roll-off in the corresponding WOLEDs.Therefore,it is still necessary to further study on how to achieve WOLEDs with high efficiency and low roll-off of at the same time.In this paper,to make the most use of excitons,we chose high efficient blue TADF emitters and long wavelength phosphorescent emitters.Through reasonable structure design and sufficient exciton utilization,hybrid WOLEDs with high effi-ciency,low roll-off as well as high color rendering index are achieved.Through the systematic study of exciton distribution and exciton utilization process,the reasons for the improved efficiency roll-off are given.Detail works are as follows:1.The optimized blue devices with external quantum efficiency as high as 19.7%were fabricated by using pure layers of TADF material DMAC-DPS as blue emitting layers.The ultrathin layer of orange phosphorescent emitter was employed to study the exciton distribution in the blue device.It is found that the exciton distribution tends to be more uniform within the whole blue emitting layer with increasing the driving voltage.Based on this,efficiency roll-off at high luminance has been im-proveded by increasing the number of ultrathin layer of phosphorescent emitter.Re-placing the orange phosphorescent ultrathin layers with red and green phosphorescent ultrathin layers,WOLEDs with undoped emitting layer structure were fabricated.The turn-on voltage is 2.8 V.The maximum external quantum efficiency,current efficien-cy and power efficiency are 19.1%,41.5 cd/A,and 42.4 lm/W,respectively.When at the luminance of 1000 cd/m2,they still remain 17.3%,39.2 cd/A,and 30.9 lm/W,showing lower efficiency roll-off.The color rendering index is 83.2.B-G-R type WOLEDs based on exciplex of CDBP:PO-T2T as both blue emit-ter and hosts of phosphorescent materials were fabricated.By investigating the influ-ence of thickness of different emitting layers and doping concentration of phospho-rescent materials on the performance of WOLEDs,it is found that the efficienty roll-off turns more seriously with increasing the thickness of blue emitting layer.Based on this,BR-G-R type WOLEDs with improved efficiency roll-off were fabricated by doping very low concentration of high efficient red phosphorescent materials into the blue emitting layer.The turn-on voltage is 2.4 V.The maximum external quantum ef-ficiency,current efficiency and power efficiency are 23.6%,47.0 cd/A,and 58.2 lm/W,which still remain 20.2%,42.7 cd/A,and 35.3 lm/W at the luminance of 1000 cd/m2.The color rendering index is 86.3.Sky-blue OLEDs with external quantum efficiency of 22.7%and emission peak of 488 nm were achieved based on exciplex constructed by TADF emitter Spi-roAC-TRZ as donor and PO-T2T as acceptor.The turn-on voltage is as low as 2.3 V.By investigating the photoluminance chatacteristics of the corresponding films,it is found that the small singet energy gap between TADF donor and exciplex is benefi-cial to the exciton recycling,and improve the utilization of exciton.Combined with orange phosphorescent materials,high efficient single-emitting layer complementary WOLEDs were achieved.The maximum external quantum efficiency,current effi-ciency and power efficiency are 22.8%,73.6 cd/A,and 102.2 lm/W.At the lumi-nance of 1000 cd/m2,they remain 21.0%,65.7 cd/A,and 66.0 lm/W.Furthermore,three-color WOLEDs were also fabricated by adding the red emitting layer.Its maxi-mum external quantum efficiency,current efficiency and power efficiency are 22.1%,46.1 cd/A,and 57.8 lm/W.At the luminance of 1000 cd/m2,the corresponding values remain 19.0%,42.8 cd/A,and 39.5 lm/W.The color rendering index is 86.