The Design And Synthesis Of Organic Small Molecule Electron-transport Materials For Organic Light-emitting Device

Organic light-emitting diodes possess multiple advantages such as self-illuminating,low working voltage,ultrathin,light weight and flexibility,indicating a bright future in the fields of flat panel display and lighting markets.Electron transport/ hole-blocking materials(HBM/ETMs)which are placed between the active layer and the cathode can help balance the hole and electron transport in the OLED,and thus lead to the enhanced efficiency and durability.Organic small-molecule based CIMs show a number of potentially attractive characteristics,including a well-defined chemical structure,ease of synthesis with high purity and low-temperature processability(via thermal evaporation or solution processing).Based on our recent advance,phosphine oxide organic materials are a class of potentially high stability and high performance optoelectronic materials.So,the thesis continues to study(new)aryl phosphine oxide materials,and regulate its electron transport properties,and promote their application.(1)We herein design and synthesize a simple hole-blocking material(biphenyl-3,3'-diyl)bis(diphenylphosphine oxide)(BiPh-m-BiDPO)based on our recent work.Its phosphorescent spectrum in solid film featured two major emission bands peaking at 2.69 and2.40 eV,corresponding respectively to 0-0 and 0-1 vibronic transition(T0-0 = 2.69 eV,T0-1 =2.40 eV).This compound also showed a HOMO/LUMO level of –6.71 eV/2.51 eV,which has been utilized as electron-transport and hole-blocking layer in pin OLEDs.As a result,BiPh-m-BiDPO exhibited comparable luminous efficiencies in the sky blue FLOLED and red PHOLED in relative to Phen-m-PhDPO.For instance,the luminous efficiency of the pin sky blue FLOLEDs reached 11.3 cd A–1@1000 cd m–2.However,the power efficiencies and stability of the devices are beyond satisfactory,which may be mainly attributed to the inefficient electron injection of BiPh-m-Bi DPO due to the higher LUMO level and lower electron mobility.Moreover,the phosphorescent peak of BiPh-m-Bi DPO at 517 nm may contribute to the energy loss of the green phosphorescent OLED.(2)The derivative of 1,10-phenanthroline,for instance,4,7-diphenyl-1,10-phenanthroline(BPhen)has been widely used as electron-transport and interfacial layer in OLED and OPV for the high electron mobility of ~10–4cm2 V-1 s-1,respectively.However,there may also exist certain problems in long term operating devices due to the relatively low glass transition temperature(Tg = 66 oC).Recently,our group reported an versatile electron-transport material(6-(1,10-phenanthrolin-3-yl)naphthalen-2-yl)diphenylphosphine oxide(Phen-NaDPO)with simple synthesis and purification procedure,Phen-NaDPO showed attractive electron-mobility of ~3.9 × 10–4 cm2 V–1 s–1 @ E = 8 × 105 V cm-1)and a high Tg of 116 oC.In this section,we further elaborated the electron conductivity and mobility to 3.3 × 10–4 S m–1 and 10–3 cm2 V-1 s-1 by the doping of 50% Cs2CO3 respectively.The sky blue fluorescent and green phosphorescent OLEDs employing Phen-NaDPO:50% Cs2CO3 as the electron transport layer show improved power efficiency and stability,For instance,LE of ca.14.72 cd A–1 and PE of ca.12.9 lm W–1 @ 1000 cd m–2were obtained,respectively,what's more,the device showed superior stability with t95 of ca.167 h at an initial luminescence of 1000 cd m–2,driven at a constant current density.In addition,we also synthesize two alcohol-processable ETMs for inverted OLED,which can resist dissolution of weakly polar solvents(such as toluene and chlorobenzene).