Synthesis Of Thermally Cross-linkable Hole Transporting Materials And Application In Solution-processed OLED

Developing novel insoluble cross-linkable hole transporting layers（HTLs）plays a vital role in solution-processed organic light emitting diodes（OLED）.Here,four hole transport materials（HTMs）with different conjugated structures and molecular flexibility were synthesized,including N⁴,N^4’-di-p-tolyl-N⁴,N^4’-bis（4-vinylphenyl）-[1,1’-biphenyl]-4,4’-diamine（V-p-TPD）,4,4’-bis（3-vinyl-9H-carbazol-9-yl）-1,1’- biphenyl（V-CBP）,N⁴,N^4’-bis（4-（（E）-4-（phenyl（4-vinylphenyl）amino）styryl）phenyl）-N⁴,N^4’-di-p-tolyl-[1,1’-biphenyl]-4,4’-diamine（V-TPAVTPD）and 4,4’-（（1E,1’E）-（[1,1’-biphenyl]-4,4’-diylbis（9H-carbazole-9,3-diyl））bis（ethene-2,1-diyl））bis（N-phenyl-N-（4-vinylphenyl）aniline）（V-TPAVCBP）.HOMO levels of these four materials were-5.45,-5.81,-5.25 and-5.37 e V,respectively.Energy gaps were 3.05,3.31,2.73 and 2.85 e V,respectively.After cross-linking,they showed excellent thermal stability.The thermal decomposition temperatures were 476,480,517 and 537°C,respectively.Due to the difference in flexibility around the cross-linkable groups,the four materials exhibited different cross-linking temperatures,which were 160,270,190,and 190°C, respectively.The cross-linked hole transporting layers possessed good hole transporting properties,and hole mobilities were estimated at 2.5×10^-4、1.8×10^-4、2.9×10^-4and 2.4×10^-4 cm² V^-1 s^-1,respectively.The effect of blended and ordinally-deposited cross-linked hole transport layers on improving hole injection was investigated.The band gaps and HOMO energy levels of the materials in the blended cross-linked film remained unchanged.Cross-linking reaction between the two materials hardly occurred.The properties of blended film were similar to physical mixing.In OLED devices,cross-linked hole transport layers with ordinal HOMO energy levels could improve the hole injection efficiency.However,more ordinally-deposited hole transport layers leaded to more interfaces,which would reduce the film quality and affect the efficiency of devices.The performance of the devices based on the blended film showed no obvious advantages compared with the devices based on ordinally-deposited film.Photoinitiator 4-octyloxydiphenyliodonium hexafluoroantimonate（OPPI）was introduced to vinyl-based thermally cross-linkable HTMs,aiming at lowering cross-linking temperature and enhancing hole mobility.For V-TPAVTPD and V-TPAVCBP,the cross-linking temperatures of doped HTLs were greatly reduced.After the irradiation by 365 nm UV for 2 min,cross-linking could occur at an expressly low temperature of 120°C.In addition,the hole mobilities were dramatically enhanced by two orders to 1.2×10^-2 and 4.9×10^-3 cm² V^-1 s^-1, respectively for V-TPAVTPD and V-TPAVCBP.When applying HTLs with low cross-linking temperatures and high hole mobilities to solution-processed green and red phosphorescent OLED,the highest external quantum efficiency（EQE）of 15.5%for green devices and EQE of 15.0% for red devices were obtained.