Study On Highly Efficient And Simplified White Organic Light-Emitting Diodes Based On Exciplex

White light organic light-emitting diodes(OLEDs)have a lot of excellent characteristics such as high performance,self-luminous,flexible and transparent display and environmental protection,which capture plenty of attention in the solid-state lighting and full-color display.However,high fabricating cost has greatly hindered the large-scale commercial application of OLEDs.Currently,fabricating OLEDs with simple structures and high performance is still a challenging subject.Recently,due to the discovery of thermally activated delayed fluorescence,the research on exciplex has been greatly developed,which shows great application potential as emitting compoments and doped host in OLEDs.This paper intends to take advantage of the unique properties of exciplex and employ exciplex to design a series of white OLEDs with simplified structure,and aslo their emitting mechanism are investigated in detail.The specific research contents of this article are as follows:(1)By employment of wide emission spectrum of the exciplex and dopant-free technology,fluorescent white OLEDs based on the exciplex emission are prepared.Firstly,TAPC:TPBi based white OLEDs are fabricated by conbining blue exciplex emission and orange electromer emission.By adjusting the doping ratio of TAPC and TPBi,the carrier transmission and the relative emission intensity of blue and orange emission are adjusted,achieving maximum efficiencies of 1.5 cd/A and 1.6 lm/W.Then we introduce m-MTDATA:TPBi exciplex into the two-color white OLED to fabricate three-color white OLED with higher efficiency and wider spectral coverage.By further adjusting the ratio of TAPC and TPBi,the carrier transmission balance and the relative emission intensity of the three colors in the device are optimized.Finally,white OLED with a half-peak width of 190 nm is obtained with maximum efficiencies of 11.3 cd/A and 11.8 lm/W.As a result,two emisson layers and three organic transmission materials are used in the fabrication process,which greatly reduce the difficulty of OLED fabrication.(2)Based on the structure of three-color fluorescent white OLED,the performance of the device is further improved by introducing red and orange phosphorescent materials.We first compare the performance of devices with two different exciplex hosts and find that m-MTDATA:TPBi is more suitable as the host for red and orange phosphorescent materials,achieving a maximum efficiency of 18.2 lm/W for red device and 74.7 lm/W for orange device.Then,we combine TAPC:TPBi blue emission layer and phosphorescent layers to fabricate white OLED based on multiple emission layers.As a result,the two-color white OLED achieves maximum efficiencies of 55.3 cd/A and 46.8 lm/W,and the three-color white OLED achieves maximum efficiencies of34.1 cd/A and 26.8 lm/W.The improved efficiencies of devices could be contributed to multiple energy transfer pathways,which enhance the utilization of triplet excitons efficiently.In addition,the blue exciplex emission is used to instead of expensive blue phosphorescent material in the device,which could achieve excellent performance with reduced manufacturing cost.(3)A novel exciplex host MCP:B4PyPPM is used to fabricate phosphorescent white OLED.Firstly,red,orange,green and blue devices are fabricated based novel exciplex host.Compared with devices using traditional hosts,the performance of the four-color OLEDs based on exciplex host show significantly improvement,achieving lower turn-on voltage with maximum power efficiencies of 35.3 lm/W,106.1 lm/W,121.3 lm/W,and 47.4 lm/W,respectively.Then we used the complementary colors to prepare white OLEDs based on double emission-layer structure,achieving maximum efficiency of 73.6 lm/W with supressed efficiency roll-off.Moreover,the result device also achieves excellent spectral stability,the Commission Inter-nationale de L Eclairage coordinates variation of is only(0.003,0.001)in the brightness range of 500cd/m~2 to 5000 cd/m~2,which is a significant improvement in the reported white OLEDs based on the exciplex host.By investigating the exciton distribution and carrier transmission performance in the device,the high spectral stability could be attributed to balanced carrier transmission and highly efficient energy transfer between host and dopants.Compared with previous reports,emission purity and spectra stability are significantly improved in the devices based on MCP:B4PyPPM exciplex host,which can be widely used in monochromatic and white OLEDs.(4)A novel interfacial exciplex system 26DCzPPy/B4PyPPM is used to build white OLEDs based on single-emission-layer structure.It is found that direct charge recombination of excitons is prevail in 26DCzPPy hosted devices,while F?rster energy transfer is dominated in B4PyPPM hosted device.Based on the abouve results,we optimize the complementary monochromatic devices by adjusting doping concentration based on the interfacial exciplex system,achieving maximum power efficiencies of51.4 lm/W for blue device and 125.9 lm/W for orange device.Furthermore,by doping complementary-emitter into B4PyPPM layer,single-emission-layer warm WOLED is achieved with efficiencies of 102.6 lm/W and 81.0 cd/A along with a low turn-on voltage of 2.43 V,which is one of the best values in WOLEDs based on simplified architecture.The enhanced performance could be attributed to reduced energy loss of triplet excitons in the host through efficient F?rster energy transfer between the exciplex and the dopants and suppressed Dexter energy transfer process between blue and orange dopants due to low doping concentration of orange dye.Additionally,single-emission-layer white OLED based on interfacial exciplex system only requires doping three materials simultaneously with no need of high-performance host materials,which could provide a promising method to design simplified white OLEDs with high performance for commercial solid-states lighting and display.