Study Of Exciplexe Based High Performance Fluorescent Organic Light-Emitting Diodes

The development of organic light emitting diode(OLED)is gradually shifting from the research stage in the laboratory to the application stage in the market.However,most of high efficiency OLEDs were fabricated by using phosphorescent materials.Phosphorescent materials are expensive,and high efficiency and long operational lifetimes blue phosphorescent materials are scarce,which limited the development of phosphorescent OLEDs.Recently,thermal activation delay Fluorescence(TADF)materials have attractted great interest because they are metal-free,low cost,and a 100% utilization rate of excitons.Most of exciplexs show TADF character.Replacing intramolecular TADF materials,exciplex TADF materials have received much attention because of the advantage of plentiful materials for structuring exciplexs and broad emission spectra for exciplexs.Except for as the emitter,exciplex as the host also has widely applied to OLEDs,and developed rapidly.But the working mechanisms of exciplex based OLEDs is still unclear,and the device performance for monochromatic and white exciplex based OLEDs also need to be further improved.Based on above issues,our main research contents include the following four aspects:1.Using TCTA as donor and 3p-T2 T as acceptor,respectively,an TADF exciplex(TCTA/3p-T2T)was obtained by a 1:1 evaporation deposition of TCTA and 3p-T2 T.Further,the orange OLED was fabricated by doping rubrene into 3p-T2 T layer of exciplex,and the energy transfer from bilayer interface TADF exciplex to dopant was optimize by changing the doping concentration of rubrene in 3p-T2 T.It is found that under the optimized concentration of 1.5%,the device achieved maximum current efficiency,power efficiency and EQE of 25.3 cd/A,22.6 lm/W and 8.1%,respectively.The obtainment of so high efficiency could be attributed to highly efficient RISC efficiency of triplet excitons and energy transfer efficiency from TADF exciplex to dopant.Meanwhile,the optimized concentration of 1.5% also suppress efficiently the Dexter energy transfer between excited triplet state of TADF exciplex and dopant.2.Using mCP as donor and PO-T2 T as acceptor,respectively,an blue TADF exciplex(TCTA: PO-T2T)was obtained by a 1:1 evaporation deposition of TCTA and 3p-T2 T.Further,by doping traditional fluorescent material rubrene into above intramolecular TADF exciplex,a series of fluorescent white OLEDs were fabricated.The device realized the incomplete energy transfer from exciplex to dopant by the control of the doping concentration of rubrene in exciplex host.The optimized white OLED exhibit high device efficiency and color stability.For example,under the optimized concentration of 1.6%,the white device achieved maximum current efficiency,power efficiency and EQE of 14.2 cd/A,12.8 lm/W and 4.9 %,respectively.And as the voltage increases from 4 V,6 V,to 9 V,CIE coordinates of the white device is(0.384,0.439),(0.384,0.440),and(0.383,0.441),showing a slight change.The high efficiency could be attributed to efficient RISC efficiency of triplet excitons and energy transfer efficiency from TADF exciplex to dopant.The improvement of color stability is due to follows fact: 1)there is no energy barrier for carriers injection;2)hole and electron carriers can balanced transport and recombine in emitting layer;3)there is a broadened carrier recombination zone.3.We have introduced broad spectrum yellow-green exciplexs m-MTDATA:TPBi and m-MTDATA/Bepp2,and combined a blue emission of the electron transport material Bepp2,to develop a highly simplified two-color white OLED.In order to improve CRI of white OLED,a thin TCTA film as electrons and excitons blocking layer was incorporated into the electron transport layer of two exciplexs based white OLEDs.By optimizing the thickness of TCTA film,two exciplexs based white OLEDs all show high CRI white emission.For example,m-MTDATA:TPBi exciplex based white device achieved maximum CRI of 90 and maximum current efficiency and power efficiency of 9.3 cd/A,8.9 lm/W,respectively;m-MTDATA/Bepp2 exciplex based white device shows more simple device structure,and also realized maximum CRI of 88.This new design concept opens a new avenue for achieving high performance white OLEDs with simple structures.