Study On White Organic Light-emitting Diodes With Interface-exciplex Sensitized Phosphorescent Ultrathin Layer

White organic light-emitting diodes(WOLEDs)are considered as a new generation of lighting source because of their advantages of surface light source,flexibility,light weight and thin thickness,soft light color,no blue damage and large-area preparation.In the future indoor lighting,medical lighting,decorative lighting and other fields,it has broad application prospect.Currently,WOLEDs still has some problems,such as complicated device structure and strict preparation process,which restrict their commercialization process.As the host of phosphorescent materials,exciplexes can significantly improve the performance of the device.However,the doping process and the control of low concentration phosphorescent materials with different colors will lead to low yield and high cost as well as the color stability and color quality to be improved.Based on this,we investigated WOLEDs of ultrathin phosphorescent layers(Ph-UEMLs)combined with interfacial exciplex The scheme effectively avoids the complex doping process and saves the use of expensive phosphorescent materials.At the same time,Ph-UEMLs gives WOLEDs more flexibility in device structure design.The specific research contents are as follows:1.The physical mechanism of Ph-UEMLs sensitized by interfacial exciplex and its application in the development of high-performance monochromatic OLEDs were explored.When a blue Ph-UEML is introduced into the interfacial exciplex composed of m CP and POT2 T,the maximum external quantum efficiency of the prepared blue device is 4.1 times that of the reference device(introduced into the interface of m CP and TPBi,no exciplex is formed).It is proved that single interfacial exciplex as the host of Ph-UEML can sensitize Ph-UEML and improve the exciton utilization of the device.In addition,the maximum external quantum efficiency of the device is increased by increasing the number of interfaces of the exciplex,which can achieve the efficiency close to that of the device based on bulk phase exciplex.It is proved that the synergy between the multi-interfacial exciplex can also improve the exciton utilization of the device.The sensitization and synergistic mechanism were combined to design the device structure and realize the preparation of higher efficiency blue devices with PhUEMLs sensitized by interfacial exciplex.The maximum current efficiency,power efficiency and external quantum efficiency of the optimized blue device are 42.1 cd/A,40.1 lm/W and20.0%,respectively.Red,yellow and green devices with similar structures were prepared,which proved the universality of the research scheme.2.WOLEDs with Ph-UEMLs sensitized by a single interfacial exciplex was proposed.On the basis of the above monochromatic devices,we adopt the main structure of "donor layer/PhUEMLs/acceptor layer" to prepare a series of white devices by adjusting the color combination and thickness of Ph-UEMLs.Due to the restriction of charge carriers and excitons by the energy level gap at the donor/acceptor layer interface and sensitization of the exciplex,all the devices achieve low turn-on voltage and stable white light emission.By simply changing the thickness of yellow Ph-UEMLs in the trichromatic white light device,it can be flexibly adjusted from cold white light to warm white light.The optimized trichromatic white device exhibits very high color stability.Under the driving voltage of 5-7 V,the CIE coordinates of the device change only(?0.00,?0.02),the color rendering index is 88,and the correlated color temperature is about 3000 K.However,the limited phosphorescent sites in the "donor layer/phUEMLs/acceptor layer" structure make the efficiency and brightness of the device need to be further improved.3.WOLEDs with Ph-UEMLs sensitized by multi-interfacial exciplexes were developed.On the basis of the above white devices,we design the main structure of "donor layer/PhUEML/acceptor layer/Ph-UEML/donor layer/Ph-UEML/acceptor layer...",and a series of white devices were prepared by changing the number of interfacial exciplexes,the sequence of different color Ph-UEMLs and the thickness of donor/acceptor layer.Due to the sensitization of Ph-UEML by interfacial exciplex and the synergistic effect of multi-interfacial exciplexes,the maximum current efficiency and external quantum efficiency of dichromatic white device reach 51.9 cd/A and 20.1%,respectively.The color rendering index of the optimized redyellow-blue white device is above 80 at different voltages,and the correlated color temperature is about 3500 K.The maximum current efficiency and external quantum efficiency reach 27.9cd/A and 14.9%,respectively.The color rendering index of the tetrachromatic white device is87,and the maximum current efficiency and external quantum efficiency are 30.8 cd/A and13.1%,respectively.Compared with the white devices with similar color quality in the previous chapter,the efficiency of all devices is significantly improved.The carrier recombination region and luminescence mechanism were determined by the probe device.It is proved that the device with Ph-UEMLs sensitized by multi-interface exciplexes,"donor layer/Ph-UEML/acceptor layer/Ph-UEML/donor layer/Ph-UEML/acceptor layer..." has great potential in the development of WOLEDs.