Constructing High Performance Exciplex Emitting Materials Based On Benzimidazole-Triazine Derivatives Accepotrs

Organic luminescent materials are more and more popular in the applications of flat pannel display,lighting technology,medical and so on.There are many commercial products in the market so far,especially the organic light emitting diode?OLED?screens for mobile phone,television,vehicle display and so on.Cost,efficiency and lifetime of the orginc luminescent materiasls are the most significant factors to consider facing challenge of industrialization.Highly efficient full luminescent materials have been achieved since 2012,when new luminous mechanism called thermally actived delayed fluorescence?TADF?was proposed by Adachi.Exciplex system is one type of TADF materials based on intermolecular charge transfer.Compared with intramolecular charge transfer characterized unimolecule TADF material based devices,exciplex-based devices show many advantages,such as lower driving voltages,higher power efficiencies,lower efficiency roll-offs,showing enormous potential in commercialized productions.In this dissertation,benzimidazole and triazine units are combined for a series of exciplex electron acceptor materials,the thermostability,photophysical property,electrochemistry property and electron transport property of them are measured and discussed systematacially.These electron acceptor materials are applied as the emitting layers and cohosts in OLED devices by mixing applicable electron donor materials,the designing strategy of exciplex materials and preponderance of exciplex cohosts are discussed.1.In chapter II,four electron electron acceptor materials with strong electron transporting property composed of benzimidazole and triazine hybrids were synthesized by simple coupling and cyclization reactons with high yields.They possess high glass-transition temperature?over 95??and thermal decomposition temperature?over 418??.Photophysical property measurements indicated that the four compounds showed characteristic of intramolecular charge transfer owing to the bipolar character of benzimidazole units and stronger electron-withdrawing ability of triazine units,which was also demonstrated by theoretical calculations.The deep highest occupied molecular orbital?HOMO?energy level of-5.90 eV and the lowest unoccupied molecular orbital?LUMO?energy level of-2.90 eV matched well with that of the frequently-used cathode material were calculated by electrochemistry measurements.Electron-only devices proved that these electron acceptor materials possess comparative electron transporting ability with TmPyTz.All the results suggest this series of electron acceptor materials have the potential of fabricating highly efficient exciplex devices.2.In chapter III,the compounds synthesized in chapter II were fabricated exciplex devices as the emitting layers by mixing aromatic amine electron donor materials TAPC,TCTA,and Tris-PCz.Firstly,devices with TAPC:A1,TCTA:A1,and Tris-PCz:A1exciplex blends as the emitting layers were fabricated in order to explore the influence on exciplex devices performance caused by different electron donor materials.The results indicate that compound TAPC possesses shallowest HOMO energy level and highest triplet energy level compared with those of TCTA and Tris-PCz,which are beneficial for charge accumulation and inhibiting energy pass back.As a consequence,the TAPC:A1 blend film showed highest photoluminescence quantum yield?PLQY?of93.6%.All the exciplex blends showed TADF characters and the TAPC:A1 film showed the highest reverse intersystem crossing?RISC?efficiency.Devices with TAPC:A1 as the emitting layer showed the best performance with the highest exernal quantum efficienvy?EQE?of 21.7%and the maximum power efficiency of 97.3 lm/W,the EQE could maintain 19.8%at 1000 cd/m²,which is the best performance among the reported exciplex devices.In addition,TAPC:A2,TAPC:A3,and TAPC:A4 were all employed as the emitting layers to fabricate exciplex devices to explore the influence on exciplex devices performance caused by different electron acceptor materials.Although compounds A2^A4 possess similar LUMO energy levels and triplet energy levels,the respective blend films with TAPC exhibited different PLQYs and proportions of delayed fluorescence,which are related to the interaction between the two compounds under film state.3.In chapter IV,we fabricated full-fluorescence OLED devices by employing TAPC:A1,TCTA:A1,and Tris-PCz:A1 as exciplex cohost and traditional green fluorescent material C545T,red material DCJTB as the dopants.Devices with TAPC:A1 as the cohost showed the best performances because of the highest RISC efficiency of TAPC:A1.For green device,the device showed the maximum EQE of20.2%and power efficiency of 86.6 lm/W when the doping concentration of C545T was 0.6wt%.The red device showed the maximum EQE of 10.3%.Both the devices displayed lower 10%efficiency roll-offs at 1000 cd/m².These results show that exciplex cohosts with high RISC efficiency are applicative for traditional fluorescent materials and indicate the excellence of the exciplex cohost we designed.4.In chapter V,TAPC:A1,TAPC:A2,TAPC:A3 and TAPC:A4 were all applied as cohosts for green phosphor materials Ir?ppy?₃.The electroluminescence spectra of the devices with TAPC:A1 and TAPC:A3 as the emitting layer is hypochromatic shifted compared with those of TAPC:A2 and TAPC:A4 based ones,indicating TAPC:A2 and TPAC:A4 possess more lower triplet energy levels.As a consequence,devices with TAPC:A2 and TAPC:A4 as cohosts showed incomplete energy transfer from cohosts to Ir?ppy?₃ and inferior device results.Devices with TAPC:A1 and TAPC:A3 as cohosts showed more complete energy transfer and better results,they displayed the highest EQE of around 30%and the highest power efficiency of 120.8¹28.0 lm/W.As for red phosphor material dopants,we synthesized two pure red iridium complexes via introducing two electron-withdrawing trifluoromethyl groups on auxiliary ligands.In this way,the electron donating ability of the auxiliary ligands was weakened and therefore the HOMO energy levels were lowered but the LUMO energy levels were maitined simultaneously.A slightly increase of energy gaps contribute to pure red emission.For comparing,devices with exciplex cohost and traditional host were all fabricated.Devices with exciplex cohost showed increased efficiency and reduced efficiency roll-offs.Hole-only and electron-only devices of the emitting layers showed that exciplex cohost-based films possessed more balanced and faster hole-electron migration,which is beneficial for enlarging the exciton recombionation zone and therefor the efficiency is improved and exciton annihilation is restricted at high luminance.In summary,we have designed and synthesized a series of efficient exciplex acceptor materials composed of benzimidazole and triazine hybirds.Exciplex-based OLED devices were fabricated employing these compounds and well-matched electron donor materials mixed films as the emitting layers,which all exhibite low driving voltages,high efficiencies and low efficiency roll-offs.Especially,device with TAPC:A1 as the emitting layer showed the record efficiency values.Exciplex cohosts can show the high RISC efficiency and bipolar characters adequately.Green and red traditional fluorescent and phosphor materials were all used as dopants for exciplex cohosts devices,which show significant efficiencies suggesting the advancement of the electron acceptor materials.This work provides referential experience for developing highly efficient exciplex electron acceptor materials and highly efficient exciplex-baed devices.