The Design And Synthesis Of Organic Molecular Electron Transport Materials Based On 1,3,5-Trizaine For Organic Light-Emitting Diodes

Organic light-emitting diodes?OLEDs?have attracted intensive attention from academia and industry for realizing high-contrast,ultrathin and light-weight flat panel displays and next generation eco-friendly solid state lighting.Organic electron transport materials?ETMs?,the basic component of OLEDs,assist in the injection of electrons from cathode into emitting layer,and avoid luminescence quenching caused by the direct contact of the cathode and emitting layer,which play a vital role in determining device performances and stability.High-performance electron transport materials usually require suitable LUMO?lowest unoccupied molecular orbital?energy levels,high thermal and film morphology stability,as well as good electron transport property.Nevertheless,improving the electron transport mobility?m_e?requires increased molecule conjugation,which often results in poor solubility and hence difficulty to purify,especially for removing halogen impurities.To compound the situation,even a trace of residual halogen impurities in the ETMs resulting from the reactants and/or its reaction intermediates would exert a fatal effect on device lifetime.In addition,there exists a trade-off between increasing glass transition temperature and electron mobility.Although a large number of organic electron transport materials have been reported,there is few ETM applicable to practical OLEDs.Consequently,this thesis focuses on developing high-performing and stable electron-transport materials based on the 1,3,5-triazine unit that possesses the advantages of high electron affinity,electrochemical reduction stability and convenient structural modification.The main contents are as follows:1?We designed and synthesized a series of electron transport materials TRZ-m-TPO,TRZ-p-TPO,TRZ-Py-TPO,and BPTRZ-Py-TPO,by combining triarylphosphine oxide moiety and triazine unit.The compounds have good solubility in weak polar solvents such as dichloromethane,which allows for easy purification mainly through column chromatography.It is worthy to note that the halogen impurities could be facilely removed,thus avoiding the fatal influence on the device stability.Introducing of bulk,rigid and steric triarylphosphine oxide moiety is beneficial for forming the amorphous ETMs mentioned above with a glass transition temperature between 80-123 ^oC.The LUMO levels of TRZ-m-TPO and TRZ-p-TPO are-2.76 and-2.87 e V,respectively.When the electron-deficient pyridine unit introduced,the LUMO levels of TRZ-Py-TPO and BPTRZ-Py-TPO are effectively reduced to-2.98 and-3.06 e V.In addition,the compounds possess deep HOMO?highest occupied molecular orbital?and high triplet energy levels,for specially,the E_Tof BPTRZ-Py-TPO is as high as 2.88 e V.Upon n-doping with 8-hydroxyquinolatolithium?Liq?,TRZ-p-TPO:Liq produces low electron mobility of only 4.62×10^-7-6.45×10^-6cm²V^-1s^-1@E=?2-5?×10⁵V cm^-1,while the?_eof TRZ-m-TPO:Liq and TRZ-Py-TPO:Liq is improved by about an BPTRZ-Py-TPO:Liq achieves the highest?_e,of ca.4.66×10^-5-3.21×10^-4@E=?2-5?×10⁵V cm^-1.In bottom-emission red phosphorescent OLEDs,the luminous efficiency?LE?and power efficiency?PE?of TRZ-m-TPO:Liq and TRZ-p-TPO:Liq devices are 11.0 cd A^–1/7.8lm W^–1and 8.6 cd A^–1/5.6 lm W^–1@1000 cd m^-2,respectively,inferior to those of TRZ-Py-TPO:Liq(14.7 cd A^–1/10.5 lm W^–1)and BPTRZ-Py-TPO:Liq(13.9 cd A^–1/10.4 lm W^–1)devices,due to the lower electron mobility.However,TRZ-m-TPO:Liq and TRZ-p-TPO:Liq devices have achieved higher stability,t₉₅are 372 and 486 h@1000 cd m^–2,which may be attributed to the lower hole blocking property of the electron transport layer resulting in reduced polaron-triplet exciton annihilation.In view of the high electron mobility,the top-emission green phosphorescent OLEDs based on TRZ-Py-TPO:Liq and BPTRZ-Py-TPO:Liq reveal outstanding performances and long term stability.For instance,at a luminance of ca.1000 cd m^–2,LE=72.1 cd A^–1and PE BPTRZ-Py-TPO:Liq device.Compared to the bottom-emission red phosphorescent OLEDs,the injection of electron and hole carriers in the emitting layer of the top-emission green phosphorescent OLEDs appeared more balanced,and thus polaron-triplet exciton annihilation was effectively suppressed.Consequently,driven by a constant current for ca.640 h,the initial luminance of 1000 cd m^–2appears almost no decay,and the operating voltage remained nearly stable.2?We further present the triarylphosphine oxide moiety modified triazine derivatives BPTRZ-25Py-TPO and TRZ-Phen-TPO.The compounds possess high thermal stability with T_gof 127 and 150 ^oC,deep HOMO/LUMO levels of–6.48/–3.11 and–6.62/–3.27 e V,and high triplet energy levels ca.2.92 and 2.66 e V.Upon Liq-doping?1:1,wt/wt?,at an electric field of?2-5?×10⁵V cm^-1,the?_eof BPTRZ-25Py-TPO:Liq is 2.21×10^-5–1.94×10^-4cm² BPTRZ-Py-TPO:Liq film.The LE and PE of the BPTRZ-25Py-TPO:Liq top-emission green phosphorescent OLED are 49.1 cd A^–1/51.4 lm W^–1vs.49.7 cd A^–1/52.1 lm W^–1@1000 cd m^–2for TRZ-Phen-TPO:Liq device.Upon continuous operation under a constant current,t₉₇of BPTRZ-25Py-TPO:Liq device is ca.206 h,while that of TRZ-Phen-TPO:Liq device is over 270 h.3?By introducing naphthylanthranyl to 1,3,5-triazine unit,we report electron-transporting materials Di Na AN-m-TRZ and Na AN-m-TRZ.Both compounds are easy to synthesize and purify,and exhibit high T_gover 157 ^oC,and the HOMO/LUMO levels are–5.62/–2.70 and–5.76/–2.84 e V,respectively.When n-doping with 50 wt%Liq,the electron mobility of Di Na AN-m-TRZ and Na AN-m-TRZ is 1.06×10^-5-2.18×10^-4and 6.23 phosphorescent OLEDs,the LE/PE are 13.3 cd A^-1/9.5 lm W^-1for Di Na AN-m-TRZ vs.12.7cd A^-1/8.7 lm W^-1@1000 cd m^-2for Na AN-m-TRZ.The LE and PE of the Di Na AN-m-TRZ:Liq top-emission green phosphorescent OLED are 78.5 cd A^-1/88.0 lm W^-1vs.72.2 cd A^-1/81.0 lm W^-1@1000 cd m^-2for Na AN-m-TRZ:Liq device.Upon continuous operation under a constant current,both devices exhibited an extraordinory lifetime with t₉₇?300-400 h@1000 cd m^-2.4?Through modifying the 1,3,5-triazine moiety with a large planar and rigid electron-deficient 1,10-phenanthroline unit,we present new electron-transporting materials,BPTRZ-m-Phen and TRZ-Ph-Phen.Both the compounds have a T_gof over 120 ^oC,BPTRZ-m-Phen and TRZ-Ph-Phen show deep HOMO/LUMO energy levels of–6.40/–3.24and–6.49/–3.11 e V,respectively.The electron mobility values of BPTRZ-m-Phen:Liq and ?2-5?×10⁵V cm^-1.The LE/PE of the top-emission green phosphorescent OLEDs based on BPTRZ-m-Phen:Liq and TRZ-Ph-Phen:Liq electron-transport layer are 62.1 cd A^-1/69.7 lm W^-1and 72.0 cd A^-1/80.7 lm W^-1@1000 cd m^-2,respectively.In summary,a series of triazine-based organic small molecule ETMs was designed and synthesized.In particular,BPTRZ-Py-TPO exhibits very promising properties,including a high T_gof 123 ^oC,triplet energy level of 2.88 e V,and low LUMO level of-3.06 e V.n-Doping with Liq?1:1 wt/wt?leads to attractive electron mobility with 4.66×10^-5-3.21×10^-4cm²V^-1s^-1@E=?2-5?×10⁵V cm^-1.Besides,the top-emission green phosphorescent OLED based on BPTRZ-Py-TPO:Liq exhibits high efficiency and ultra-high stability.Among others,the present results shall promote organic phosphinyl-functionalized photoelectric materials for OLED applications.