Study On Structure Design And Performances Of White Organic Light-emitting Diodes Based On Exciplexes And Phosphor Ultra-thin Layers

White organic light emitting diodes(WOLEDs)have exhibited an extensive application prospect owing to outstanding characteristics,such as scalable,flexible,high efficiency,green non-pollution,eye-friendly soft light,which are considered the most promising new solid-state lighting technology in the 21st century.And now,WOLEDs have a variety of mature products in lighting(lamp shape and structure design),display(flexible display)and other fields,which show tremendous potential in the future.In general,since only singlet excitons can be utilized in conventional fluorescent materials,as a result,the fabricated OLEDs show a low electroluminescence(EL)efficiency due to the nonradiative transition.In recent years,the structures taking advantage of exciplex with the characteristic of thermally activated delayed fluorescence as host have been made great progress in the development of high efficiency OLEDs.The internal quantum efficiency(IQE)of the exciplex can reach 100%owing to reverse intersystem crossing(RISC)process,which is greatly beneficial to fabricate high efficiency OLEDs.However,doping multi-EMLs architecture generally used in these devices,which needs to accurately regulate the doping ratios of phosphors to co-hosts and is detrimental to the application.Moreover,the efficiency,color rendering index and spectral stability of OLEDs are still necessary for future research.Based on the above reality,this thesis carries out the structural design and study on the EL performance of white organic light-emitting diodes based on exciplex.By screening different exciplex system and combining with the simple structure of phosphorescent ultra-thin layers,a series of high-performance WOLEDs has been successfully fabricated through the management and effective use of excitons in the light-emitting layers,as well as the optimization of device's structure and charge transfer balance.At the same time,the working mechanism of devices has also been deeply studied.The main work of this paper is summarized as follows:In the first part,blue OLEDs with high efficiency,and low turn-on voltage were prepared by introducing multiple FIrpic blue phosphorescent ultra-thin layers at different positions in m CBP:PO-T2T host.This structure not only expands the width of the exciton formation region,but also effectively increases exciton utilization,which significantly improves the efficiency and the efficiency roll-off of the devices.The resulting blue OLEDs exhibited low turn-on voltage of 2.4 V,of which the maximum forward-viewing power efficiency(PE),current efficiency(CE)and external quantum efficiency(EQE)reached 57.3 lm W^-1,48.3 cd A^-1 and27.4%.Moreover,the EQE is still as high as 24.3%even at 1000 cd m^-2,exhibiting outstanding EL efficiency.In the second part,full-phosphorescent white organic light emitting diodes with high efficiency,high CRI and stable spectrum have been fabricated by strategically adjusting the position,thickness and the arrangement order of red,green,and blue ultra-thin layers in interlayer of m CBP:PO-T2T.Additionally,the adjustment of the position of the phosphorescent ultra-thin layers not only increases the efficiency of the device,but also greatly improves the spectral stability of the WOLEDs.The maximum EQE,PE,and CE of the optimized WOLED are 26.1%,50 lm W^-1,and 40 cd A^-1,respectively.Meanwhile,the CRI has been maintained above 81,indicating good warm white light.In the third part,by using m CP:B3PYMPM doping with FIrpic phosphors as blue emitting layer and inserting a yellow phosphorescent ultra-thin layer in the appropriate position of the electron transport layer,high efficiency,low efficiency roll-off and high spectral stability cool white light WOLEDs have been fabricated.Research suggests that structure achieves the separation of exciton recombination region and light-emitting region,which effectively improved the efficiency roll-off and spectral stability.The maximum EQE,PE,and CE of resulting WOLED are 51.9 lm W^-1,46.2 cd A^-1,18.8%,respectively,and at 1000 cd m^-2,the EQE was reduced by only 6%.Besides,the CIE of the optimized WOLED was barely changed about(±0.002,±0.001)with the lightness increasing from 1000 cd m^-2 to 10000 cd m^-2,which exhibited excellent EL performance.