Synthesis and Characterization of Red-Emitting Thermally Activated Delayed Fuorescence Polymers
Supervisor:ding jun qiao
Thermally activated delayed fluorescence(TADF)polymers show great potential in the pringting organic light-emitting diode(OLED)since they possess both the low cost of fluorescent polymers and the high efficiency of phosphorescent polymers.Nowadays most of the developed TADF polymers emit sky-blue to orange light,whose wavelength maxima are in the range of 470-600 nm.And there is no report about red TADF polymers with up to 600 nm emissive peak.Therefore,in this dissertation,we focus on the construction of red-emitting TADF polymers,in which a small-molecular red TADF dopant is covalently incorporated into the side chain or main chain of a polymeric host.By tuning the triplet energy,highest occupied molecular orbital(HOMO)level,excited lifetime and color,efficient red electroluminescence(EL)is achieved together with low driving voltage,small efficiency roll-off at high luminance,and good color purity.The main research contents are as follows:(1)Polyfluorene(PF),we note,is not suitable to be used as the host for red TADF dopant because of its low triplet energy.To solve this problem,poly(fluorene-co-3,3’-dimethyldiphenyl ether)(PFDMPE)has been newly developed by inserting 3,3’-dimethyl diphenyl ether into the main chain of PF.And the triplet energy is improved from 2.16 eV of PF to 2.58 eV of PFDMPE.Based on PFDMPE as the backbone,a series of side-chain red TADF polymers PFDMPE01～PFDMPE10 are prepared.Their films give soly red delayed fluorescence from the TADF dopant,and the excited lifetimes are in the range of 126-191 us.Among them,PFDMPF-R05 achieves the best device performance,revealing a bright red EL peaked at 606 nm and Commission Internationale de L’Eclairage(CIE)coordinates of(0.57,0.42).At a brightness of 100 cd/m2,the corresponding current efficiency,power efficiency and external quantum efficiency(EQE)are 3.6 cd/A,0.61m/W and 1.95%,respectively.(2)By replacing fluorene by carbazole,the side-chain red TADF polymers PCzDMPF-R03～PCzDMPF-R10 have been synthesized based on poly(carbazole-co-3,3’-dimethyldiphenyl ether)as the backbone.They all show obvious delayed fluorescence with excited lifetimes of 145-161 us.Among them,PCzDMPF-R07 achieves red EL peaked at 607 nm as well as CIE coordinates of(0.56,0.42).At a brightness of 100 cd/m2,the corresponding current efficiency,power efficiency and EQE are 5.6 cd/A,2.4 lm/W and 2.87%,respectively.Meanwhile,thanks to the variation from fluorene to carbazole,the HOMO level of the backbone is up from-6.03 eV to-5.92 eV,which favors the hole injection.As a result,the turn-on voltage is decreased from 10 V to about 5 V,and the power efficiency is improved by about 3 times.(3)To avoid the severe efficiency roll-off at high luminance and poor color purity in the former side-chain counterparts,two types of main-chain red TADF polymers have been designed by sharing the same triphenylamine unit between the red TADF dopant and the hole-transporting TFB.Interestingly,their excited lifetimes are found to be reduced by about 3 orders of magnitude associated with the red-shifted PL.Consequently,TFB-TPAAQIO-PFO obtained a good red color purity with a maximum emission peak of 626 nm and CIE coordinates of(0.61,0.38).Moreover,the corresponding device shows a small efficiency roll-off,and the current efficiency,power efficiency and EQE at 100 cd/m2 are increased to 5.4 cd/A,4.0 lm/W and 4.05%,respectively.