Construction Of Wide-Bandgap Organic Light-Emitting Molecules And Devices Based On Benzoxazole As Acceptor

Organic light-emitting diodes（OLEDs）have attracted widespread interest from academia and industry for their wide range of applications,including high-resolution full-color displays and energy-efficient solid-state lighting.The main materials used in the emission layer（EML）of OLED products currently on the market are blue fluorescent materials and phosphorescent materials of other light colors.On the one hand,blue fluorescent materials still have the following problems due to their molecular properties:First,the wide band gap causes the carrier injection difficulties and transmission imbalance.Second,color purity and luminous efficiency are difficult to balance.Third,the efficiency roll-off at high luminance is serious.On the other hand,phosphorescent material as EML often require to be doped by the host material,so the development of host material also determines the efficiency of phosphorescent OLEDs（Ph OLEDs）.However,it is difficult for host materials to achieve balanced carrier mobility and high triplet energy level（E_T）at the same time.Since,it is barely able to obtain high luminance,high efficiency and low efficiency roll-off Ph OLED.Therefore,to address the above key scientific issues,we designed and synthesized a series of wide-bandgap organic light-emitting molecules with benzoxazole as the acceptor.And according to the different chemical and physical properties of molecules,they were studied as the host material and blue fluorescent material respectively.The specific content is divided into the following two parts:1.Two wide-band gap and bipolar transmission Ph OLED host materials（PPI33PPPBO and PPI22PPPBO）were designed and synthesized by changing the attachment sites（ortho-or meta-position）between donor phenanthroimidazole and acceptor benzoxazole.Combining photophysical experiments,theoretical calculations,and single-crystal resolution,it is shown that the meta-linked PPI33PPPBO has not only a smallerπ-conjugation but also a more ordered face-to-faceπ-stacking pattern,thus attending simultaneously to the high E_T and balanced carrier mobility.Ultimately,the green,yellow and red Ph OLEDs with PPI33PPPBO as the host show excellent device performance:the turn-on voltage is only 2.8 V,the maximum external quantum efficiency(EQE_max)is 22.8%,26.7%and 17.6%,respectively,and the maximum power efficiency(PE_max)is 94.2,97.0 and 25.7 lm W^-1,and maximum luminance(L_max)is up to 200278,219260,and 105147 cd m^-2,respectively.These far outperform the efficiency of commercial materials CBP and m CP as the hosts under the same conditions.Particularly,they can still achieve EQEs as high as 21.9%,25.5%and 16.4%when L is1000 cd m^-2,corresponding to efficiency roll-offs of only 3.9%,4.5%and 6.8%.This is not only one of the best internationally reported performances of doped phosphorescent devices,but also the first time that benzoxazole groups are efficiently applied to Ph OLEDs in distorted linkage manner.2.Two rigid and efficient pure-/deep-blue emitters（PIPMe PBO and PIPDMe PBO）were designed and synthesized by linearly linking the acceptor benzoxazole and the donor phenanthroimidazole to form a conjugated system,and adding different numbers of steric hindrance groups as the conjugated bridge.The rigid structures give them excellent thermal and photophysical properties,and both materials have a regularπ-stacking pattern and good carrier mobility,resulting in L_max of 12212 and 38928 cd m^-2and EQE_max of 8.1%and 8.0%for PIPMe PBO-and PIPDMe PBO-based nondoped devices,respectively.Simultaneously,at a high luminance of 10000 cd m^-2,they still maintain an EQE of 7.0%and above,showing a low efficiency roll-off.In particular,the nondoped device of PIPDMe PBO shows a deep-blue electrofluorescence of 427 nm with the Commission Internationale de L’Eclairage（CIE）coordinates of（0.156,0.048）,which meets the requirement of deep-blue light for high-definition television（HDTV）.The PIPDMe PBO-based device is one of the best efficiencies among the nondoped deep-blue OLEDs with similar light color.