Synthesis And Characterization Of Organic Electron-transport Materials Based On Triarylphosphine Oxide-triazine Conjugates For OLEDs

Organic light-emitting diodes are increasingly used in the fields like flat-panel displays and solid-state lighting for the advantages including self-illumination,flexibilityle,tunable emission and light weight.Because of the important role the electron-transport layer(ETL)plays in OLEDs,electron-transport materials(ETMs)have been intensely pursued.High mobility,deep LUMO level,enhanced thermal stability and glass transition temperature are usually needed for ETMs.Triazine,a kind of electron-deficient heterocycle with reversible electrochemical reduction,is conducive to enhance electron injection and transport,while triarylphosphine oxide moieties with a rigid and steric structure are beneficial to construct amorphous organic functional materials.In this thesis,a series of electron-transport materials involving both triarylphosphine oxide and triazine moieties are proposed and characterized.It is worth pointing out that,2-(3-(9-(3-(diphenylphosphinyl)phenyl)anthracen-10-yl)phenyl)-4,6-diphenyl-1,3,5-triazine(TPOAN-m-TRZ)has a low refractive index(n)below 1.8 within the wavelength range at 467–800 nm.Upon 50%Liq-doping,the refractive index is futher decreased and less than 1.8 in a broader visible region(423–800 nm).Owing to the impoved light output given by TPOAN-m-TRZ:Liq with low n as single Liq-doped electron-transport layer,a top-emission green phosphorescent OLED produces appealing performances.The main findings are as follows:(1)Compounds TPOAN-m-TRZ and TPOAN-m-Ph TRZ are obtained through coupling the triphenylphosphine oxide with 2,4,6-triphenyl-1,3,5-triazine and 2-(3-phenylbiphenyl)-4,6-diphenyl-1,3,5-triazine moieties via a 9,10-anthracene linker,respectively.They are easy to be purified and have a high T_g of 151 ^oC/152 ^oC and low refractive index(n_{(TPOAN-m-TRZ)}<1.8 within 423–800 nm,n_{(TPOAN-m-Ph TRZ)}<1.8 within 435–800 nm in the visible).Upon 50wt%Liq doping,the electron mobility is 3.50×10^–5–2.78×10^–4 cm² V^–1 s^–1 for TPOAN-m-TRZ and 1.05×10^–5–9.67×10^–5 cm² V^–1 s^–1@(2–5)×10⁵ V cm^–1 for TPOAN-m-Ph TRZ.Utilizing TPOAN-m-TRZ:Liq and TPOAN-m-Ph TRZ:Liq as a single ETL in bottom-emission red PHOLEDs,the latter achieved better device efficiency.Both red PHOLEDs have promising stability,such as t_{95(TPOAN-m-TRZ)}?495 h,better than a previous analogue ETM reported earlier with t_{95(BPTRZ-Py-TPO)}?304 h under similar conditions.In top-emission green PHOLEDs,TPOAN-m-TRZ:Liq device exhibits better characterisitics:LE=87.7 cd A^–1,PE=98.4 lm W^–1and EQE=22.3%@1000 cd m^–2,which may be due to its lower refractive index and improving the light output.Under diven at a constant current,the TPOAN-m-TRZ:Liq top-emission green PHOLED showed a lifetime t₉₅?325 h@3000cd m^–2.The extrapolated t₉₅?2103 h@1000 cd m^–2.Further optimizing the OLED structure including introducing an electron-injection layer such as Yb would improve the device performance and operational stability.(2)The coupling of diphenylphosphine oxide with 4,6-diphenyl-1,3,5-triazine moiety via2,2?-naphthyl-6,6?-diyl or with 4,6-di(4-phenylphenyl)-1,3,5-triazine moiety via 2,6-naphthyl affords DPO-2Na-TRZ and DPO-Na-BPTRZ,respectively.Both compounds possess facile synthesis and purifying,a high T_gof over 130 ^oC and deep HOMO/LUMO levels of–6.03/–3.08 e V and–6.57/–3.31 e V,respectively.Upon 50 wt%Liq doping,the?_e of DPO-2Na-TRZ and DPO-Na-BPTRZ is 6.71×10^–6–8.11×10^–5 cm² V^–1s^–1 and6.30×10^–5–4.22×10^–4 cm² V^–1s^–1@(2–5)×10⁵ V cm^–1.Because of the low E_T1 of DPO-2Na-TRZ and DPO-Na-BPTRZ(<2.3 e V),a 5 nm exciton-blocking/electron-transport layer is introduced between the green phosphorescent emitting layer and the doped electron-transport layer DPO-2Na-TRZ:Liq or DPO-Na-BPTRZ:Liq to suppress exciton energy transfer and enhance electroluminescence efficiency.As a result,DPO-2Na-TRZ:Liq green PHOLED produces high performance:LE=91.6 cd A^–1?PE=102.8 lm W^–1and EQE=22.1%.In addition,both OLEDs feature an impressive stability.For instance,the decay of the initial brightness of 1000 cd m^–2 is less than 2%after a continuous operation for 600 h.