Structure Design And Exciton Management Of High Efficiency All Fluorescent White Organic Light-emitting Diodes

White organic light-emitting diodes(WOLEDs)have the advantages of ultrathin,flexible,flat emission,large area production,low driving voltage,low power consumption,soft emission,without the harm of blue emission and so on,which have shown much strong market competiveness in display and lighting fields and aroused wide attention and research.The conventional fluorescent emitters have the advantages of low cost,good stability and low efficiency roll-off,but they only can utilize 25%singlet excitons for radiation,which seriously restricts the efficiencies of the fabricated WOLEDs.However,using exciplexes,TADF or phosphorescent molecules as sensitizer,conventional fluorescent materials as emitter and selecting reasonable materials and designing device structures to fabricate WOLEDs can realize all the excitons formed in emitting layer(EML)to effectively transfer to the fluorescent emitters for radiation.Thus,the fluorescent WOLEDs not only can not only realize 100%exciton utilization but also have the advantages of low roll-off and good stability of conventional fluorescent emitters,which can fabricate high efficiency,low efficiency roll-off and long lifetime all fluorescent WOLEDs.Although the phosphor or TADF sensitized fluorescent monochrome OLED have made good progresses in device performances,the high performance fluorescent WOLEDs are still rarely reported.In this thesis,we used exciplexes as hosts,different types of molecules as sensitizers and conventional fluorescent materials as emitters,the high efficiency,low efficiency roll-off,long lifetime and high quality white light emission all fluorescent WOLEDs were successfully fabricated by optimizing the device structures and regulating the exciton distribution of excitons to increase the utilization of excitons and reduce the exciton quenching.The main works in this thesis are as follow:1.We used the highly efficient conversation process of exciplex sensitized fluorescent emitter to achieve high efficiency,low efficiency roll-off all fluorescent WOLEDs.We first used mCP:PO-T2T exciplex as blue host and the low doping concentration TBRb yellow fluorescent material as guest to fabricate complementary-color all fluorescent WOLEDs by optimizing the doping concentration of guest and regulating the incomplete energy transfer process between host and guest to realize both host and guest radiative emission simultaneously.The turn-on voltage of the resulting WOLEDs was 2.6 V,and the maximum current efficiency(CE),power efficiency(PE)and external quantum efficiency(EQE)were30.2 cd/A,36.5 lm/W and 10.4%,respectively,and remained 12.9 cd/A,6.7 lm/W and 5.1%at the luminance of 1000 cd/m².We further used 26DCzPPy:PO-T2T as blue host and the yellow fluorescent emitter TBRb as guest,and inserted a thin hole-transport layer TCTA into the yellow EML of TBRb doped in 26DCzPPy:PO-T2T host to fabricate high efficiency and low efficiency roll-off all fluorescent WOLEDs.The formation of TCTA/PO-T2T interface exciplex by TCTA and PO-T2T increases the energy transfer process from26DCzPPy:PO-T2T host to TBRb guest.It can be seen that the formation of TCTA/PO-T2T interface exciplex not only increased the conversation process of triplet excitons,but also broadened the exciton recombination zone,which reduces the exciton quenching and improves the efficiency roll-off.The maximum CE,PE and EQE of the resulting WOLEDs were 32.0 cd/A,35.9 lm/W and 10.1%,respectively,and at the luminance of 1000 cd/m²,these values still remained 25.6 cd/A,16.1 lm/W and 8.1%,respectively.The Commission Internationale de L'Eclairage(CIE)coordinates were(0.36,0.53),and the electroluminescence(EL)spectra were stable at the range of 500 cd/m² and 5000 cd/m²luminance,showing the much stable white light emission.2.We used the highly efficient conversation process of phosphor sensitized fluorescent emitter to achieve high efficiency and low efficiency roll-off all fluorescent WOLEDs.The research showed that the effect of hole trapping by phosphor sensitizers plays a very important role in increasing the conversation efficiency of triplet excitons in phosphor sensitized fluorescent emitter.We fabricated phosphor sensitized blue,green,yellow and red fluorescent OLEDs with the maximum EQE of 12.1%,13.1%,17.0%and 18.9%,respectively.On the above basis,we designed the structure of phosphor-sensitized blue and yellow EMLs and regulated the thicknesses and positions of these two EMLs to realize high efficient blue and yellow emission simultaneously to fabricate high efficiency and low efficiency roll-off all fluorescent WOLEDs.The maximum CE,PE and EQE of the resulting WOLEDs were 42.4cd/A,36.4 lm/W and 14.3%,respectively,and at the luminance of 1000 cd/m²,these values remained 34.2 cd/A,19.8 lm/W and 11.6%,respectively,and the CIE coordinates were(0.41,0.47),exhibiting the availability of the phosphor sensitized fluorescent emitter strategy to fabricate high efficiency all fluorescent WOLEDs.Furthermore,we utilized highly efficient conventional blue EML and phosphor sensitized yellow fluorescent EML to fabricate high efficiency,low efficiency roll-off and long lifetime all fluorescent WOLEDs.The maximum CE,PE and EQE of the resulting WOLEDs reached 44.8 cd/A,56.5 lm/W and 15.2%respectively,and at the initial luminance of 1000 cd/m²,the half-lifetime(LT₅₀)reached 1680h,exhibiting excellent comprehensive performance.3.We used the highly efficient conversation process of TADF sensitized fluorescent emitter to achieve high efficiency and low efficiency roll-off all fluorescent WOLEDs.The exciplex was used as blue EML and host,and conventional fluorescent materials were used as guests.We further introduce the sensitized materials with TADF characteristic to form multiple RISC processes in EML,which can effectively convert triplet excitons into singlet excitons,resulting in increasing the exciton utilization and reducing exciton quenching.We used mCBP:PO-T2T exciplex as blue EML,TADF sensitizer AIA-TRZ and conventional fluorescent material TTPA co-doped in mCBP:PO-T2T exciplex host as green EML,and TADF sensitizer Pr-1 and conventional fluorescent material DCJTB co-doped in mCBP:PO-T2T exciplex host as red EML to fabricate three-color fluorescent WOLEDs by optimizing the thicknesses and positions of different EMLs.The maximum CE,PE and EQE of the resulting WOLEDs were 48.4 cd/A,58.4 lm/W and 17.8%,respectively,and at the luminance of 1000 cd/m²,these values still remained 36.4 cd/A,28.5 lm/W and 13.8%,respectively,and the CIE coordinates were(0.48,0.44)and the CIR was 83.Furthermore,we used TADF material spiroAC-TRZ and electron-transport material PO-T2T to form exciplex as blue EML,and the TADF sensitized fluorescent emitters as green and red EMLs to fabricate three-color fluorescent WOLEDs.The maximum CE,PE and EQE of the resulting WOLEDs were 35.9 cd/A,42.7 lm/W and 15.0%,respectively,and at the luminance of 1000cd/m²,these values were 36.0 cd/A,24.0 lm/W and 12.4%,the CIE coordinates were(0.46,0.45)and the CRI reached 84,exhibiting much good comprehensive performances.