Synthesis Of Vinyl-Based Thermally Crosslinked Hole Transporting Materials And Application In Solution Processed OLEDs

Compared with the evaporation process,the printing process can be prepared in a large area,does not require a higher vacuum degree and has a high material utilization rate,and thus has attracted widespread attention.However,because OLEDs require a multilayer functional layer structure to achieve their high performance,the hole transport layer and the light-emitting layer will have inter-layer miscibility problems.Therefore,the formation of an insoluble crosslinkable functional layer is very important for solution-processed OLEDs.Cross-linking is considered to be the best way to obtain solvent resistance.Among them,vinyl has become the most studied crosslinking group due to its simple synthesis,no initiator and harmless small molecules.Cross-linkable hole transporting materials（HTMs）play a vital role in solution-processed OLEDs with the purpose of reducing costs and improving productivity.In this paper,two cross-linked hole transporting materials with styryl group as cross-linking groups have been designed and developed:3,3’-（1,3,4-Oxadiazole-2,5-diyl）bis（N-phenyl-N-（4-vinylphenyl）aniline）（OXZ-VPAN）and 3,3’-（4-phenyl-4H-1,2,4-triazole-3,5-diyl）bis（N-phenyl-N-（4-vinylphenyl）aniline）（TRZ-VPAN）.The two target compounds and their intermediate products were identified and characterized by hydrogen NMR spectroscopy and MOLDI-TOF mass spectrometry.The performance of the two materials was systematically studied.First,through the photophysical performance test,the HOMO energy levels were-5.64 and-5.52 e V,and the band gaps were 3.33 and 3.24 e V,respectively;the thermal properties of the two materials were characterized by thermosgravimetry analysis and differential scanning calorimetry.The thermal decomposition temperatures T_d were as high as 453and 473°C,respectively,indicating that the two hole transporting materials have excellent thermal stability after cross-linking;due to the difference of the mother nucleus,the material exhibits exothermic peaks at different temperatures during the heating process,which are respectively 90～170 and 200°C;the absorption spectra of the film before and after crosslinking before and after being immersed in toluene at different temperatures were tested by ultraviolet-visible absorption spectroscopy,and the actual crosslinking temperature of OXZ-VPAN and TRZ-VPAN was determined to be 160℃;single-carrier devices of two materials were prepared,and the hole transporting performance of the cross-linked hole transporting layer film was tested by the space charge limited current method.The hole mobility was 2.27×10^-4 and 6.28×10^-5 cm² V^-1 s^-1 respectively.The crosslinking agent tetrakis（3-mercaptopropionic acid）pentaerythritol ester（PETMP）was doped in the vinyl-based thermally crosslinked hole transporting material to lower the crosslinking temperature.The crosslinking temperature of the doped OXZ-VPAN and TRZ-VPAN films is significantly reduced.After irradiating under 365 nm ultraviolet light for 5 min,the film is heated at a low temperature of 120℃for 20 min,and the solvent resistance reached 100%,indicates that it can be completely cross-linked.However,since the ester group does not have transporting properties,the hole mobility has a certain degree of decline,respectively 9.98×10^-5 and 2.82×10^-5 cm² V^-1s^-1.The application of a crosslinked hole transporting layer with a low crosslinking temperature to a green phosphorescent OLED device achieves an external quantum efficiency of up to 16.0%.