Structure Design And Performance Study Of White Organic Light-emitting Diodes Based On Exciplexes

People pay more and more attention to energy-saving and environment-friendly lighting sources due to the improvement of life quality and the enhancement of health awareness.In contrast to conventional lighting technologies,such as fluorescent tubes,light-emitting diodes?LEDs?and organic light-emitting diodes?OLEDs?have been considered as the most promising candidates for healthy solid-state lighting technologies in the 21^st century,especially for OLEDs.Their extraordinary characteristics,such as eye-friendly soft light,being free of UV,IR,glare,lower power consumption,homogenous illumination,flat shadowless emission,low weight,and easy flexibility,which become a more ideal healthy lighting source candidate,promote rapid development of its applications in household lighting,medical lighting,automobile lighting,public lighting and other fields.In recent years,the emergence of exciplex with the characteristic of thermally activated delayed fluorescence prompted the development of high efficiency OLEDs.However,based on exciplex OLEDs,the efficiency roll-off problem at high luminance,the efficiency,color rendering index and spectral stability still need to be improved for meet the demands of green lighting.What's more,the device working mechanism still need to further research and study.On the basis of above considerations,a series of high-performance WOLEDs with rational utilization of exciplex,smart device structure designs and deep studies on the working mechanism have been developed by combining the advantages of bipolar carrier transport and low driving voltage of exciplex and high efficiency of phosphorescent emitter.Detail works in this thesis are as follows:In the firt part,high efficiency,low efficiency roll-off,and low driving voltage complementary white OLEDs were gradually achieved by doping blue phosphor,introducing double ultrathin layers?UTLs?and green phosphorescent UTL in a single exciplex co-host system.The resulting device exhibited the maximum forward-viewing power efficiency?PE?and external quantum efficiency?EQE?of 95.3 lm W^?1 and 22.8%and remained at 55.5 lm W^?1 and 21.9%at 1000 cd m^-2 without using light out-coupling technique.Remarkably,at the luminance of 5000 and 10000 cd m^-2,the EQEs were still as high as 20.4%and 18.9%.The results showed that the introduction of blue dopant can play a nonnegligible role in the uniformity of exciton distribution in emissive layer,and the introduction of the double UTLs and green UTL can increase the energy transfer and radiative transition channels of the excitons,which alleviate the exciton quenching.On this basis,color-tunable white OLEDs with a wide correlated color temperature?CCT?from 2143 to 8448 K have been achieved by combining red and green phosphorescent UTLs.The EQEs still keeped over 17.1%at 5000 cd m^-2,exhibiting very low efficiency roll-off.This is one of the state-of-the-art color-tunable OLEDs reported so far.In the second part,high efficiency,low efficiency roll-off,and high color rendering index?CRI?white OLEDs have been achieved by the optimal selection of the best exciplex for different phosphors and the easonable arrangement of emissive layers.In the double exciplexes co-host system with the same acceptor and different donors,high quality white OLEDs with a CRI over 80 realized the EQE of 21.6%at 1000 cd m^-2,showing a very small efficiency roll-off.In the double exciplexes co-host system with different acceptors and donors,the resulting device showed a warm-white light with a CRI of 82,a CCT of 3223 K,and a Commission International de I'Eclairage?CIE?Coordinates of?0.421,0.396?at 1000 cd m^-2.More importantly,with the luminance increasing from 1000 to 15000 cd m^-2,the device exhibited a stable spectra with very small CIE variation of?0.010,0.004?.The resulting PE and EQE were 43.2 lm W^-1 and 20.1%at 1000 cd m^-2,and the corresponding total PE was predicted to be around 73.4 lm W^-1?without light out-coupling techniques?,which can meet the demands of green and healthy lighting sources.In the third part,we have achieved high efficiency and low efficiency roll-off white OLEDs by rational choice of donor,spacer,and acceptor materials and dopant concentration optimization in the spatially separated interfacial exciplex co-host system.The results showed the electron-hole coupling distance could be over 9 nm.When this system was used as orange phosphorescent host,the resulting device exhibited the maximum forward-viewing PE and EQE of 76.0 lm W^-11 and 24.0%,which is 2.24 times more than that of the interfacial exciplex without spacer?The maximum EQE was 10.7%?.More importantly,the EQE roll-off at 1000cd m^-2 reduced from 49.6%to 7%.Effective manipulation and utilization of excitons are the keys for achieve high-performance.On this basis,the complementary white OLEDs with doping blue and orange phosphors not only realized the stable EL spectra,but also exhibited low efficiency roll-off with the EQE of 21.3%at 1000 cd m^-2.