Optimization Of Device Structure And Performance For Highly Efficient White Organic Light-emitting Diodes

As an emerging lighting and display technology,organic light-emitting diodes(OLED)has came into our daily life since it was firstly discovered in 1987.OLED has many incomparable merits compared with other existing lighting and display technologies,such as ultra-thin structure,low power consumption,flexible,wide color gamut,large viewing-angle,harmlessness to human eyes and environmentally friendly.However,there are still many key issues urgently needed to be addressed,for example,the stability of blue emitting materials,light out-coupling efficiency,the cost of the fabrication,etc.From the point of view of organic emitters,we can divide the development history of OLED into three main stages.The earliest organic emitters are the fluorescent materials.Due to their intrinsic properties,only singlet excitons can be-harvested for radiative decay,therefore,the maximum internal quantum efficiency(IQE)is limited to 25%.After that,Forrest et al.invented the phosphorescent materials in 1998.The heavy metal atom in this kind of materials turn room-temperature radiative decay from triplet excitons into possible.The IQE of the devices based on phosphorescent materials can reach nearly 100%.Over the past few years,Adachi et al.made a breakthrough to achieve IQEs of over 25%for conventional fluorescent OLEDs based on thermally activated delayed fluorescence(TADF)materials.Through TADF channel in this kind of material,triplet excitons can up-convert to singlet excitons via reverse inter-system crossing,hence the IQE of TADF based OLEDs can also be close to 100%.Nevertheless,instead of quantum efficiency,the power efficiency is the mostly used parameter to measure the power consumption of a light source in the practical use.The key point to improve the power efficiency is to reduce the driving voltage of the devices.According to the previously reported works,the most common methods to optimize the driving voltage is to reduce the charge injection and transporting barriers and improve the carrier mobility of organic materials,which could be accomplished by looking for suitable charge injection and transporting materials,modifimg the interfaces between organic materials and electrodes,optimizing the device structure and so forth.Up to now,the power efficiency of green OLED is over 100 1m W-1 while the power efficiency of xwhite OLED is still between 80-90 lm W-1(without light out-coupling techniques).To remain competitive for OLED products in lighting and display market,it is necessary to have new ways to improve the power efficiency.Meanwhile,reduce the manufacturing cost of OLED by simplifying the device structures,etc.In considering of the aforementioned challenges,we present two approaches in this thesis to simplify the device structures and improve the power efficiency of white OLEDs.First,we proposed an interlayer-free structure for OLEDs based on ultra-thin emitting layers.Generally,there are interlayers between the conventional ultra-thin emitting layers,which is used to modulate the emissive spectrum of the devices and optimize the device performance.However,the additional interlayers not only create more interfaces and enhance the complexity of device,but also cause an increase in materials cost and device processing time.To overcoma these problems,we first designed an ultra-thin emitting layer structure without interlayers.The maximum current efficiency,power efficiency and external quantum efficiency of two-color white OLED based on this novel structure are 56 cd A-1,55.5 lm W-1 and-9.3%respectively with a turn-on voltage of only 3.2 V.Besides,the three-color white OLED based on this structure realizes the maximum current efficiency of 45.5 cd A-1,power efficiency of 42 lm W-1 and external quantum efficiency of 17.6%.This novel device structure not only simplifies the OLED structure,but also saves the materials cost and device processing time.Therefore,we suppose that this novel emitting layer has great potential in the practical use in the near future.Second,in order to further improve the power efficiency of white OLEDs,we chose two organic materials to construct a new blue exciplex host.The devices based on this host can perfectly confine the exictons and significantly reduce the carrier injection/transporting energy barrier.Accordingly,the blue OLED based on this host achives power efficiency of 48.0 lm W-1,current efficiency of 41.3 cd A-1 and external quantum efficiency of 17.3%.Furthermore,the warm white OLED based on this host exhibited an unusually high power efficiency of 105.0 lm W-1.It should be pointed out that the efficiency is the forward-viewing efficiency without any light out-couling techniques.To the best of our knowledge,this power efficiency is the highest value among the published white OLED up to now.Also,it is the first white OLED with a power efficiency of exceeding 100 lm W-1.