Study On Structure Design And Performance Of Fluorescence/Phosphorescence Hybrid White Organic Light Emitting Diodes

With the enhancement of people's quality of life and health awareness,the development of energy-saving,comfortable,environmental protection and healthy green lighting sources has been attached great attention to the academic and industrial fields.Compared with traditional fluorescent lamps and inorganic light-emitting diodes(LEDs),organic light-emitting diodes(OLEDs)are recognized as the latest generation of energy saving and environmental friendly solid-state lighting technology in the 21st century.OLEDs have many unique advantages,such as high efficiency,low energy consumption,no shadow,flat luminescence,soft and non-glare,light and thin,flexible,and no blue light damage.Therefore,OLEDs are considered as the most promising lighting source,and will be widely used in home lighting,automobile lighting,medical lighting and public lighting in the future.Fluorescence/phosphorescence hybrid white OLEDs are prepared by combining high stability blue fluorescent dyes with yellow(or red and green)phosphorescent dyes,which have the advantages of not only the stability of blue fluorescent dyes,but also high efficiency of long wavelength phosphorescent dyes.The hybrid structures are undoubtedly a better choice for commercial white devices.In fluorescence/phosphorescence hybrid white OLEDs,the triplet state energy level of the conventional fluorescent materials used as the blue light emitting layer is lower than that of many phosphorescent materials,so the triplet state excitons will be seriously quenched and the device efficiency will be reduced.How to reduce the quenching is an important issue that needs to be urgently solved for fluorescence/phosphorescence hybrid WOLEDs.The comprehensive performances of the devices,such as efficiency,color rendering index,lifetime and spectral stability,need to be further improved,and the working mechanism also needs to be further studied.Based on the above consideration,this thesis has carried out the studies on the structures and performances in hybrid white OLEDs through the selection of luminescent materials and the design of device structures,the high efficiency and low roll-off hybrid white OLEDs were successfully constructed,and the working mechanism has been deeply discussed.Detail works in this thesis are as follows:In the first part,the high efficiency fluorescence/phosphorescence hybrid white OLEDs based on 4P-NPD:Bepp₂ bulk heterojunction as blue light emitting layer structure were designed and fabricated.The luminescent layer consists of the yellow luminescent layer of the hole-transport material doped with yellow phosphorescent dye,blue light emitting layer mixed by hole-and electron-transport materials,and an ultra-thin green phosphorescent emitter inserted into an electron-transport material.It can be seen that the ultra-thin green phosphorescent layer plays a good role in the regulation of excitons,so that the excitons can be efficiently used.The resulting hybrid WOLEDs exhibit the maximum forward viewing external quantum efficiency(EQE)and power efficiency(PE)of 18.2%and 72.9 lm W^-(16),respectively,and as high as 16.7%and 61.7 lm W^-(16)at 100 cd m^-(17)luminance,and 12.5%and37.7 lm W^-(16)at 1000 cd m^-(17)luminance,showing good properties of high efficiency and low efficiency roll-off.In the second part,the high efficiency fluorescence/phosphorescence hybrid white OLEDs based on 4P-NPD/Bepp₂ double-layer heterojunction as blue light emitting layer structure were designed and fabricated.The light-emitting layer of the two-color white light devices is composed of a ultrathin yellow phosphorescent layer inserted in the hole-transport material,a 4P-NPD/Bepp₂ heterojunction blue light-emitting layer and an assistant layer with triplet-triplet annihilation(TTA)up-conversion characteristics inserted in the electron transport material.It can be seen that the TTA up-conversion layer can make good use of the triplet excitons at high luminance,which greatly reduces the exciton quenching at high luminance and improves the efficiency roll-off.The resulting hybrid WOLEDs exhibited the maximum forward-viewing EQE and PE of 23.6%and 68.8 lm W^-(16),and they only dropped to18.3%and 38.1 lm W^-(16)at 1000 cd m^-(17)luminance and 17.1%and 25.9 lm W^-(16)at 5000 cd m^-2luminance,which are significantly higher than 10.5%and 17.9 lm W^-(16)of WOLEDs without TTA assistant layer at 5000 cd m^-(17)luminance.Furthermore,the operational half-lifetime of the resulting hybrid WOLEDs also reached 600 hours at the luminance of 1000 cd m^-2,nearly two-fold longer than that of the reference WOLEDs,which fully demonstrates the important role of the TTA assistant layer.In addition,the 4P-NPD/Bepp₂bilayer heterojunction was used as blue light emitting layer,and combined with red,yellow and green phosphorescent ultrathin layer method,the high efficiency fluorescence/phosphorescence hybrid white OLEDs were prepared.The resulting hybrid WOLEDs exhibited the maximum forward-viewing EQE and PE of 23.4%and 50.4 lm W^-(16),and they slightly dropped to 20.3%and 31.2 lm W^-(16)at 1000 cd m^-(17),showing high efficiency and low roll-off.Furthermore,the device also shows extremely stable electroluminescent spectra in the whole luminance range with a high CRI of 93 and the Commission International de L'Eclairage(CIE)coordinates of(0.49,0.43)at 1000 cd m^-(17)luminance,which is perfect warm white for indoor lighting.In the third part,the high efficiency fluorescence/phosphorescence hybrid white OLEDs were designed and fabricated based on the bipolar blue light fluorescent material PBI-PPI-TPA with high triplet energy levels as blue light emitting layer.It can be seen that PBI-PPI-TPA is not only an efficient blue fluorescence luminescent material,but also a good phosphorescent host material.As a luminescent layer,PBI-PPI-TPA can well realize the barrier-free injection and transport of charge carriers and improve the utilization of excitons.The two-color(BO)WOLEDs exhibited the maximum forward-viewing EQE/PE up to23.6%/77.8 lm W^-1,and they remained 20.0%/49.5 lm W^-1 at 1000 cd m^-2luminance.The maximum EQE/PE values of the prepared three-color white OLEDs were up to 21.5%/53.2lm W^-1,remained 17.2%/26.8 lm W^-1 at 1000 cd m^-2 luminance,and CRI also reached about70,which fully shows the potential application value of PBI-PPI-TPA for the preparation of high-efficiency fluorescence/phosphorescence hybrid white light OLEDs...