High Performances Organic Light-emitting Diodes Based On Novel Spirofluorene Derivatives As Hosts

In recent years,organic light emitting diodes(OLEDs)have received widespread attention because of advantages of fast response speed,backlight free and fine mechanical flexibility.With a further researchement,display screen and solid-state lighting based on OLEDs have been gradually applied to our daily life.However,due to the high triplet energy level of blue phosphors,the lack of host materials with high triplet energy levels causes the poor performances of blue OLED,which further affects the whole development of OLED.In order to improve the performances of blue OLED,the commonly used methods are synthesing new host materials and optimizing device structure,of which synthesing new host materials is the most effective and most commonly used approach to improve the performances of blue OLED.In this thesis,we first synthesized a new spirobifluorene derivative 2'-(9-phenylfluoren-9-yl)-9,9'-spirobi[fluorene](PF-SBF)through coupling reaction.And then,we designed several types of OLEDs using PF-SBF as emitting material or host after a series of characteriztions of PF-SBF.The details were as follows:(1)Two kinds of simple-structure OLEDs using PF-SBF as emitting material or single host were fabricated,but the performances of devices were poor;(2)By further employing hole-transporting material di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane(TAPC)and PF-SBF as co-hosts and optimizing structures,the maximum luminance and current efficiency of the blue OLED were as high as 7857 cd/m2 and 16.70 cd/A;(3)By finally employing TAPC and PF-SBF as co-hosts of green phosphor tris(2-phenylpyridine)iridium(III)(Ir(ppy)3),the maximum luminance and current efficiency of green OLED having a similar structure with that of blue OLED were 23390 cd/m2 and 50.50 cd/A.With the optimization of device structures,the performances of OLED based on the PF-SBF host were greatly promoted,however the performances of the most optimized device were barely satisfactory due to a lower triplet energy level of PF-SBF than that of FIrpic.In order to obtain blue OLED with higher electroluminescent performances,we further synthesized a series of spiro[fluorene-9,9'-xanthene] derivatives 2-(9-phenyl-fluoren-9-yl)spiro[fluorene-9,9'-xanthene](PF-SFX),2-(9-(4-(octyloxy)-phenyl)-9H-fluoren-9-yl)spiro[fluorene-9,9'-xanthene](C8OPF-SFX),2',7'-bis(9-(4-(o-ctyloxy)phenyl)-9H-fluoren-9-yl)spiro[fluorene-9,9'-xanthene](C8OPF-SFX-C8OPF)and 2',2',7',7'-tetrakis(9-(4-(octyloxy)phenyl)-9H-fluoren-9-yl)spiro[fluorene-9,9'-xanthen e](4C8OPF-SFX)using similar method with PF-SBF.And then using them as hosts,we obtained several high-performance blue OLEDs with structural optimization.The details were as follows:(1)The turn-on voltage,maximum luminous efficiency and maximum luminance of our blue OLED employing the PF-SFX host were 6.80 V,3247 cd/m2 and 8.74 cd/A;(2)Using similar cohost device structures by only replacing PF-SBF with PF-SFX,C8OPF-SFX,C8OPF-SFX-C8 OPF or 4C8OPF-SFX,the maximum current efficiencies and maximum luminances in these devices reach 22.56,25.93,19.06,19.43 cd/A and 6421,6196,5806,2257 cd/m2,respectively.The research results show that breaking C-C bond and introducing oxygen can increase the triplet energy level of host,and more matching the triplet energy level of commonly used phosphor,which effectively improved the efficiency of blue OLEDs.