High-Efficiency Exciplex With Spectrum Adjustable And Its Application

For organic electroluminescent(OEL)materials,traditional fluorescent materials have the advantages of high stability and low cost,but their internal quantum efficiency(IQE)is only 25%.Although the IQE of phosphorescent materials based on heavy metal effect has reached 100%in theory,the blue phosphorescent materials are still in the research and development stage due to the poor stability.Now blue fluorescent materials are still be used as luminescent materials in industrial applications.In addition,the efficiency roll-off caused by the long-life time of the triplet exciton in phosphorescent materials is also an urgent problem to be solved.Thermally activated delayed fluorescence(TADF)materials have become a research hotspot in recent years because they can not only use triplet excitons,but also avoid the use of precious metals.TADF materials are divided into single molecule TADF materials based on intramolecular charge transfer and exciplexes with TADF characteristics based on intermolecular charge transfer.Compared with the single molecule TADF material,the exciplex with TADF characteristics with the highest occupied orbital(HOMO)and the lowest unoccupied orbital(LUMO)of the molecule located on the donor and the acceptor molecule respectively,can achieve complete separation in space,have a smaller singlet triplet energy gap(?)compared with the single molecule TADF material,so effective reverse intersystem crossing(RISC)efficiency can be obtained.However,due to its low photoluminescence quantum yield(PLQY)and wide luminescence wavelength,it will show such as low luminescence efficiency and poor color purity in application.However,when the exciplex is used as the host material for phosphorescent or fluorescent guest with narrow wavelength and high PLQY,exciplex transfer energy to the guest,then the guest molecules emit light.The problems of low luminous efficiency and wide spectrum can be solved at the same time.In addition,compared with conventional host materials,exciplex host have great advantages,such as reducing driving voltage,improving exciton utilization and reducing efficiency roll off and so on.In this paper,we studied an exciplex that can adjust the wavelength range from sky blue to green by only adjusting the ratio of donor to acceptor without reducing the performance of the device.Then,the exciplex was used to the host of phosphorescent materials and the exciplex-phosphorescent hybrid white OLED devices were obtained by using the phenomenon of spectral movement;When it was used as the host of traditional fluorescent materials,the device performance of fluorescent devices is improved by using the phenomenon of spectral movement.Details as follows:Different donor acceptor ratios in the exciplex will affect its PLQY and charge transfer balance normally.So,we studied how the different ratios of donor/acceptor(DMAC-DPS/PO-T2T)(ranging from 1:9,3:7,5:5,7:3,9:1,5%PO-T2T and 1%PO-T2T)influence the performance of exciplex OLED devices.The results shows that the EQE of exciplex OLED devices almost close under different donor/acceptor ratios,with the maximum EQE ranges from 10.4%to 12.1%under different donor/acceptor ratios.Experiments show that the hybrid film has high PLQY under different donor receptor ratios,and the better transmission balance of for the bipolar transmission characteristics of DMAC-DPS is the reason for obtaining better device performance under different donor/acceptor ratios.In addition,when the donor/acceptor ratio is 1:9,3:7 And 5:5,the EL spectrum of the device is almost kept at 536 nm,showing standard green light emission.With the decrease of the proportion of receptors,we observed a large spectral blue shift,and when the concentration of PO-T2T was 1%,the peak wavelength of the device moved to the short wavelength by 40 nm to 496 nm,and entered the blue emission region.We believe that the change of film polarity caused by the different ratio of DMAC-DPS to PO-T2T is the reason for the spectral shift.Using the high-efficiency exciplex with adjustable spectrum as the host of phosphorescent material,high-efficiency orange red light and exciplex-phosphorescent hybrid warm/cold white OLED devices are realized respectively.Among them,with a donor/acceptor ratio of 5:5 exciplex host,orange red OLED devices are obtained at 2%guest doping concentration,the maximum current efficiency is 37.0 cd/A,the maximum power efficiency is 37.3 lm/W and the maximum EQE is 18.3%.The device has low roll-off efficiency,the roll-off rate of EQE is only 10.9%compared with the maximum EQE at 1000 cd/m² brightness,and the roll-off is only 23.3%ranges from the brightness of 1000 cd/m² to 10000 cd/m².Using the phenomenon of spectrum adjustable,we use1%acceptor and donor to form exciplex,change the emits light from green to blue through spectral shift,and then take it as the host of red phosphorescent material,reduce the doping concentration of guest at the same time,a simple white OLED device is realized by using the mechanism of incomplete energy transfer.The white OLED devices with CIE coordinates of(0.48,0.41)warm white light and(0.28,0.37)cold white light were realized at the guest doping concentrations of 1.0%and 0.5%,respectively.It also shows high EL efficiency,the maximum current efficiency,power efficiency and EQE of warm/cold white OLEDs are 37.6/38.3 cd/A,41.3/43.7 lm/W and 16.6/17.8%,respectively.The exciplex with TADF characteristics is applied to the traditional fluorescent materials for breaking the limitation of 5%EQE of fluorescence and achieve higher EQE than traditional fluorescence.We firstly studied the sensitization of the conventional exciplex 26DCz PPy:PO-T2T(5:5)host to the fluorescent material C545T and DCJTB for more overlaps of the PL spectrum of the exciplex and the absorption spectrum of the guest.We obtained a green fluorescent device with a maximum external quantum efficiency of 6.0%at the concentration 0.5%.Then 26DCz PPy:PO-T2T(5:5)is also be used as the host of the red fluorescent material DCJTB for the spectral matching of the PL of the exciplex and the absorption of the guest.We obtain a single-layer all fluorescent white OLED with the color rendering index(CRI)of 80 and the maximum EQE of 6.9%at the guest doping concentration of 0.5%.Then,we used DMAC-DPS:PO-T2T exciplex with spectrum adjustable as the host of the fluorescent material C545T,a green fluorescent device with the maximum current efficiency,power efficiency and external quantum efficiency of 35.8 cd/A,46.8 lm/W and 10.4%respectively is obtained by getting the PL spectrum of the exciplex and the absorption spectrum of the guest more overlaps through spectrum adjustment.In conclusion,we have studied the high-efficiency and spectral shift of spectral tunable exciplex,and used the exciplex as the host of phosphorescent and fluorescent materials obtains valuable results.We have reason to believe that the phenomenon that DMAC-DPS:PO-T2T exciplex which can change the spectrum in a wide range by only changing the donor/acceptor ratio without affecting the luminescence efficiency is by no means an example.We will continue to explore similar exciplex systems and further expand their applications for developing the blue and red exciplex and used it as host of blue materials.