Synthesis And Optoelectronic Properties Of Thermally Activated Delayed Fluorescent Polymer

Organic light-emitting diodes（OLEDs）have become one of the most promising flat panel display technologies due to the advantages of wide viewing angle,short response time and flexibility.The development of organic light-emitting diodes（OLEDs）can be classified as three major stages according to the emission mechanism in emitters,i.e.,from the traditional fluorescent materials to phosphorescent materials and then thermally activated delayed fluorescence（TADF）emitters.Under electric field,the singlet excitons（25%）and triplet excitons（75%）are generated with the ratio of 1:3 in OLED device.The luminescence of the fluorescent materials can only originate from the de-excitation of singlet while the triplet excitons are wasted as the form of non-radiation,resulting in the theoretical maximum internal quantum efficiency of only 25%.Therefore,the way to make full use of triplet excitons to participate in the radiative process has become the key to improve the efficiency of organic light emitting materials.As the third generation of OLED light-emitting materials,thermally active delayed fluorescence（TADF）materials can effectively transfer triplet excitons into singlet excitons through reverse intersystem crossing（RISC）process under thermal activation without using precious metals to emit fluorescence,achieving 100%theoretical maximum exciton utilization rate and internal quantum efficiency.Compared with TADF small molecules,TADF polymer based OLED device can be prepared by solution processing technology because of the excellent solubility and film-forming properties.With the advantages of low cost and large area manufacture,TADF polymers have shown great development potential.As the molecular structures and vacuum evaporation device performances of TADF small molecules keep evolving,there are relatively few reports on TADF polymers.New polymeric structures and highly efficient PLED device compatible with TADF small molecules are limited.Therefore,the design of new polymer backbones and the preparation of high-performance TADF polymers are crucial to the development and application of TADF polymers.Based on the background,we successfully prepared a series of highly efficient TADF polymers with new structures and fluorescent polymers based on TADF material sensitization.Then their optoelectronic properties were systematically studied through reasonable molecular design in this paper.Finally,a series of solution-processed OLED devices were finally constructed based on these light emitting polymers.In chapter 1,the development process,device structure,light-emitting mechanism and performance indicators of organic electroluminescent materials were briefly described.Then the structure and performance of the functional materials of each layer in OLED device are introduced.Finally,the light-emitting layer materials of OLED devices,especially TADF polymers,are described in detail.In chapter 2,a novel TADF polymer backbone was constructed through the copolymerization of tetraphenylsilicon units and carbazole units.Based on the polymer backbone,we introduced the TADF luminescent units into the side chain through flexible alkyl chain by side chain engineering strategy and a series of green TADF polymers P3～P12 were finally prepared by adjusting the content of TADF luminescent units.All of these polymers possess obvious TADF properties and high photo-luminescent quantum yield（PLQY）.In addition,a series of spin-coated non-doped OLED devices were successfully constructed using these TADF polymers as the light-emitting layer,and the maximum external quantum efficiency（EQE）reached 7.0%.In chapter 3,the Friedel-Crafts hydroxyalkylation polymerization of isatin and carbazole was used to construct the polymer skeleton with high triplet energy level（2.88 e V）.The reaction and purification are simple because the catalyst that is hard to get rid of is not needed in this type of reaction.By changing the content of TADF units,a series of sky blue TADF polymers PACDPS-5～PACDPS-15 were successfully constructed.The target polymer has a small single-triplet energy level difference(△E_ST)and typical TADF properties.The solution-processed device based on PACDPS-5～PACCDPS-15 as the light-emitting layer material achieved high device efficiency,among which,device D3 achieved a maximum external quantum efficiency of 5.4%.In chapter 4,TADF polymers were constructed through the Friedel-Crafts hydroxyalkylation polymerization reaction of carbazole and isatin,and the TADF luminescent units PXZ-DPS-Me O was connected to the backbone through a flexible alkyl chain.By changing the content of the side chain TADF units,a series of side chain type sky blue TADF polymers PPXDPS-4～PPXDPS-16 were finally synthesized.The target polymer successfully inherited the luminescence characteristics of the side chain TADF units with small△E_STand typical TADF properties.The solution-processed device based on PXDPS-16 as the light-emitting layer material achieved a maximum external quantum efficiency of 4.1%.In chapter 5,based on the Suzuki polymerization of fluorene and benzothiadiazole,the fluorescent polymer PFBT was designed and synthesized.In addition,the TADF materials DMAC-DPS and DMAC-DP-Cz were used as sensitizers,which were connected by covalent bonds and physically doped to fluorescent polymer PFBT respectively.After sensitization of TADF material,the luminous efficiency of PFBT has been greatly improved.The maximum external quantum efficiency of the non-sensitized fluorescent polymer based device D1 was1.9%.After sensitization,the device efficiency was significantly improved,and the maximum external quantum efficiency was 3.6%.In chapter 6,highly efficient TADF molecule DMAC-DP-Cz was used as the sensitizer and connected to the side chain of the fluorescent polymer PFBT through a flexible alkyl chain.Two types of single-molecule sensitized polymers PCz BT-50 and PCz BT-100 were finally constructed.After intra-molecular sensitization,the device efficiency of fluorescent polymers has been significantly improved.The maximum external quantum efficiency of device D2 was 3.8%.Compared with physically doped sensitization,the sensitized polymer achieves intra-molecular sensitization through covalent bonding,which not only improves the efficiency of the device,but also improves device stability as a result of the avoided phase separation of the luminescent material during operation of the device.