Blue Fluorescent Materials With High Solid-state Luminescence Efficiencies And Their Application In White Organic Light-emitting Diodes

White organic light-emitting diodes（WOLEDs）have been regarded as the most promising technology in next-generation solid-state lighting because of the advantages of high efficiency,low power consumption,self-illumination and flexibility.Fluorescent-phosphorescent hybrid WOLEDs,combining the merits of low cost and good stability of fluorescent materials and high efficiency of phosphorescent dyes,are considered to be the most ideal device structure for WOLEDs.In order to realize the large-scale commercial applications of hybrid WOLEDs,it is necessary to further simplify device structure,improve device efficiency,reduce efficiency roll-off and improve color quality via materials innovation and device structure optimization.The key factor in hybrid WOLEDs is the blue fluorescent materials.The properties of blue materials such as triplet state energy level and solid-state photoluminescent quantum efficiency（PLQY）have a crucial influence on the design of device structure and resulting performance.In this paper,we carried out the structure design and performance research of fluorescence-phosphorescence hybrid WOLEDs by adopting blue fluorescent materials with high solid luminescence efficiencies,aiming at achieving WOLEDs with simple device structure,high efficiency and low roll-off.The main works include the following four parts:1.Blue fluorescent emitters usually undergo fluorescence quenching and spectral red-shift in aggregate state and thus require doping technologies to fabricate OLEDs,which commonly results in complicated device structure of OLEDs and relatively large efficiency roll-offs.To solve these problems,tetraphenylethene（TPE）group with characteristic of aggregation-induced emission（AIE）and triphenylamine unit possessing twisted conformation are introduced on the central core of anthracene to obtain an AIE molecule TPAATPE.Its PLQY in thin film can reach 83%.The non-doped device using TPAATPE as the emitter achieves pure blue emission with Commission International de L’Eclairage（CIE）coordinates of（0.15,0.16）and the maximum external quantum efficiency（EQE）of 6.97%.Furthermore,by utilizing TPAATPE as the non-doped blue-emitting layer（EML）,and PTZ-B,a bipolar thermal activation delayed fluorescence（TADF）molecule,as the phosphor host,a series of hybrid WOLEDs with relatively simple device structures are successfully prepared.When PTZ-B acts as the host of the orange phosphor PO-01,two-color WOLED achieves high-efficiency warm white emission with CIE coordinates of（0.44,0.44）.The maximum forward-viewing power efficiency（PE）and EQE are 69.5 lm W^-1 and25.2%,respectively.In order to improve the color rendering index（CRI）,three-color（blue-green-red）WOLED is fabricated by utilizing PTZ-B as the green component as well as the host of red phosphor Ir（piq）₃,and achieves pure white emission with CIE coordinates of（0.34,0.38）,high CRI of 92,maximum EQE and PE value of 25.3%and47.3 lm W^-1,respectively.This work provides a new idea for the preparation of hybrid WOLEDs with high CRI value using blue fluorescent materials possessing low triplet energy levels.2.When blue fluorescent materials with high triplet energy levels are used in hybrid WOLEDs,they not only provide blue emitting component,but also act as the host of phosphorescent dyes,thus simplifying the device structure of WOLEDs.In this type of WOLEDs,except for high triplet energy levels,blue fluorescent materials with high PLQY,bipolar transporting properties and fine device performance as host materials for phosphorescent dyes are also required.In this chapter,two phenanthroimidazole derivatives with high solid-state luminescence efficiency,PPPIS and PPIDPS,are selected as the blue-emitting hosts in WOLEDs.PPPIS and PPIDPS have similar molecular structures but with the different oxidation state of sulfur atom,which provides them with various photophysical properties.PPPIS shows weak charge transfer excited state property and exhibits deep-blue emission.However,because of the oxidation of sulfur atom,the charge transfer excited state property of PPIDPS is significantly enhanced,and the spectrum is red-shifted to sky-blue region.The two colors WOLED using PPPIS as the host of phosphor PO-01 shows better device performance with the maximum PE and EQE of 80.1 lm W^-1 and 27.2%,respectively.At the luminance of 1000 cd m^-2,the EQE can still maintain 22.1%.3.In this chapter,the device performance of phenanthroimidazole-biphenyl（PPIM）molecule,which also possesses the weak charge transfer excited state property,is systematically investigated.The non-doped device achieves a maximum EQE of 7.8%with the CIE coordinates of（0.153,0.068）.When PPIM serves as the phosphor host,the maximum EQE of green,orange and red phosphorescent devices can reach 23.5%,30.8%and 22.4%,respectively.On this basis,two-color WOLED achieves the maximum EQE of 28.5%and maximum PE of 82.8 lm W^-1,and the EQE and PE can still maintain 27.9%and 70.3 lm W^-1 at the luminance of 1000 cd m^-2 with negligible efficiency roll-offs.In addition,by doping red phosphor Ir（piq）₃in PPIM and PTZ-B layer at low concentration,three-color WOLED achieves a high CRI value of 93 and the maximum EQE of 24.2%.Finally,compared with the model compound,the introduction of benzene ring at the axis of phenanthroimidazole-phenyl（PPI）entitles the resulting molecule with weak charge transfer state property,significantly improves the luminescence efficiency and device performance.4.In prior studies,pyrenoimidazole derivatives can be applied in non-doped OLEDs with the properties of high brightness,high efficiency and low roll-off.However,due to the largeπ-conjugated plane,pyrenoimidazole derivatives usually show sky-blue electroluminescence.According to the work in chapter 4,we also add a benzene ring at the horizontal axis of pyrenoimidazole-phenyl to obtain pyrenoimidazole-biphenyl（PPy IM）molecule,which has a high luminescence efficiency of 73.3%in thin film.The non-doped device achieves pure blue emission with the CIE coordinates of（0.160,0.128）and the maximum EQE is 7.6%.Under the high luminance of 5000 cd m^-2,the EQE can still reach 7.2%,showing a very low efficiency roll-off.By using PPy IM as the non-doped blue EML and PPIM as the host of phosphor PO-01,hybrid WOLED with high efficiency and low efficiency roll-off is realized.The maximum EQE and PE can reach 23.5%and 51.5 lm W^-1,respectively.Even at the high luminance of 5000 cd m^-2,the EQE can still reach 21.2%,and the EQE roll-off was only 8%.Moreover,the device exhibits very stable warm white emission with CIE coordinates of（0.454,0.439）at the luminance of 1000 cd m^-2,and the CIE coordinates variation is only（0.004,0.003）in the luminance range of 400 cd m^-2 to10000 cd m^-2.