Hybrid Excited State And Aggregation State Regulation To Achieve High Performance Non-Doped Blue-Emissive Device

The upgrading of emitting materials is the core technology driving the marketization of organic light emitting diode（OLED）,especially blue-emissive materials with high technical barriers.At present,our OLED panel production technology is developing well,and a number of companies have been supplying large quantities to domestic and foreign markets.However,the emitting materials are still in the development period,and heavily dependent on import.Therefore,developing novel emitting materials with independent intellectual property right is of great significance for reducing import dependence,reducing the comprehensive cost of panels and promoting the rapid development of our OLED industry.Hybridized local and charge-transfer（HLCT）state and"hot exciton"material are new luminescent material systems which can breakthrough the upper limit of utilization of electro-exciton raised by scientists in our country.HLCT state is formed by a combination of Locally-emissive（LE）state and charge-transfer（CT）state.Many experiments have showed that HLCT state can take into account both the fluorescence quantum yield（LE component dependent）and electro-exciton utilization（CT component dependent）.In addition,HLCT materials are often used to prepare non-doped device due to its fast utilization of triplet excitons,which can greatly simplify the device preparation process and reduce the production cost.More importantly,the energy of HLCT state is generally higher than that of pure CT state,endowing it a unique advantage in the construction of blue-emissive materials compared to the thermally activated delayed fluorescence（TADF）system proposed at the same time.However,the reported device efficiencies of HLCT materials are difficult to compete with that of TADF materials.On the one hand,the imperfection of the construction of single molecule excited state may lead to the reduction of luminescent efficiency.On the other hand,the irregular state of material aggregation may lead an unreasonable carrier transmission and limited optical coupling output efficiency.In view of the above problems,this paper focus on the development of high-efficiency blue light electro-fluorescent materials as the main line.Based on the successful construction of efficient HLCT excited states,we further study the relationship between the aggregate properties and photoelectric properties of the HLCT materials by combining the theory with experiment methods,and optimize the molecular design through the feedback of device performance.Finally,through new material structure and new aggregation mode,high efficiency and stable performance of pure blue emission and deep blue emission OLEDs are realized.The main research work of this paper is as follows:（1）A large distortion of phenimidazole-donor skeleton structure is designed to realize the dual regulation of blue color purity and the electro-exciton utilization.Firstly,two blue fluorescent materials DP and DPCN based on 9,9-dimethylacridine donor have been designed and synthesized to achieve a better blue color purity compared with the classical HLCT materials TPA-PPI or TBPMCN.The non-doped OLED based on DP achieves decent blue-emissive OLED performance with the color coordinate of（0.154,0.113）and maximum external quantum efficiency（EQE）of 7.45%.In addition,a rare and fast delayed fluorescence phenomenon is firstly observed in the crystalline powder of DPCN.The delayed fluorescence component has a lifetime around 90 nanoseconds,which is only one-tenth to one-hundredth of that of conventional delayed fluorescence materials.Theory combined with experimental analysis,we believe that the delayed component comes from the high energy RISC process,and a reasonable exciton utilization model is proposed,which provides a certain experimental basis and theoretical support for HLCT high energy RISC process.（2）On the basis of maintaining the original excited state properties of DP,the aggregation state structure is further optimized by structural improvement,and the high efficiency non-doped blue fluorescence OLED has been realized.Two blue-emissive materials SP and SPCN have been synthesized by using spirofluorene acridine,which has a better accumulation mode than dimethyl acridine.High efficiency non-doped blue fluorescent OLEDs are realized by optimizing the excited state and aggregation state structure of blue fluorescent materials DP and DPCN.Two blue fluorescent materials SP and SPCN were synthesized by using spirofluorene acridine as the donor.Theory combined with experimental characterization show that the introduction of spirofluorene acridine does not change the original distorted skeleton of DP and DPCN,as well as the properties of their excited states,and both of SP and SPCN can maintain the original blue color purity in OLED.However,the aggregate structure and excited state properties of the SP and SPCN are significantly different form that of DP and DPCN due to their different donor groups.Not only did the fluorene group have a better autoaccumulation mode,but also the C-H···πeffect makes SP exhibit high PLQY and horizontal dipole ratio(Θ_//=89.5%)in relatively amorphous film.Compared with the dimethyl acridine-replaced blue-emissive material DP,SP achives greatly optimized electroluminescence performance:the color coordinate of（0.158,0.068）with the maximum EQE of 11.30%in non-doped OLED;and near-UV doped OLED with the color coordinate of（0.157,0.046）and maximum EQE of 7.45%.Among all the reported HLCT materials with similar optical color,these results all rank in the high level of OLED performance.（3）Furthermore,the molecular design of increasing the proportion of local components is adopted to improve the carrier transport capacity through optimizedπstacking on the premise of preserving color purity,and to improve the electroluminescence efficiency and optimize the efficiency stability of the device.We transfer the position of fluorene group from donor toπbridge to construct long conjugated and separated D-π-A structure.Under the premise of maintaining color purity and improving fluorescence quantum yield,such structure can maximize its contribution to the regularity of molecular packing,further improve the optical coupling output efficiency,and optimize the packing structure to form a carrier transport channel with holes and electrons independent of each other.Specifically,we first synthesized D-π-A HLCT blue light material TDFP using trianiline as donor and tribiphenyl as acceptor with high horizontal orientation factorΘ_//over 90%,which realized a deep blue emission with maximum EQE over 10%and color coordinate of（0.157,0.047）.Furthermore,two pure blue-emissive materials,TDFM and TDFBI,were synthesized by introducing phenylpyridine and benzimidazole groups with decent electron transport properties as acceptor to balance the carrier transport.The maximum EQE of their non-doped OLEDs are both increased to nearly 15%while maintaining highΘ_//（color coordinates are（0.153,0.097）and（0.153,0.109）,respectively）,and the EQE is still close to 12%even at the brightness of 1000 cd m^-2,which is the highest level of non-doped blue fluorescent OLED at present.Among them,the aggregate state of TDFM has its own independent hole and electron transport channel,which greatly improves its electron mobility to 2.51×10^-4 cm² V^-1 s^-1（measured by single carrier device method）compared with the other two materials,which is also the highest level at present.