Stability Issues Of Blue Thermally Activated Delayed Fluorescent Organic Light-emitting Diodes And The Improvements By Host Modification

Thermally activated delayed fluorescence(TADF)materials,as the organic electroluminescence display materials,using simple aromatic compounds without the use of expensive heavy metal complexes for triplet exciton harvesting like in phosphorescent emitters,can achieve 100% utilization of excitons and have ability to achieve high performance OLEDs.However,the poor device stability of the device limits the application of TADF OLED.In particular,the device lifetime of blue TADF OLEDs is a great challenge that needs further research.In this work,we investigate the degradation origins of the blue TADF OLEDs based on 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-spiro[acridine-9,90-fluorene](Spiro AC-TRZ)TADF emitter,and enhance the device performance and stability by incorporate different materials into the emittinglayer(EML).The underlying mechanisms for the improvements are also explored.(1)In this thesis,based on the solution-processed high efficiency sky blue TADF OLED,by comparing the electroluminescence stability of the devices and the photoluminescence stability of the materials films,determined that the degradation of the host material is the main factor for the low device stability.Then we change the hostguest doping ratio to further optimize the device performance and stability.(2)Based on(1),we first doped different ratio of PVK into the emitting layer(EML)to improve the morphology of the solution-processed film,but high concentration doping ratio affects the performance of the device.The experiment shows that the 10% PVK doping ratio can improve the stability of the device and improve the performance of the device.By tested and collected the changes in the electroluminescence(EL)spectrum of devices prepared by using only the host material as the EML and changing the carrier transport layer adjacent to the EML under constant current density,we found that the host material m CBP would interact with the exciton blocking material DPEPO to form a new band,and the doping of PVK into the host material can significantly weaken the band.The solution-processed films of wide band gap carbazole-based host material commonly used in blue light devices are more likely to be degraded by excitons due to their relatively lower molecular packing density and larger free volume which allow for a less-restricted molecular re-organization and mobility.Moreover,due to the poor film quality of the EML fabricated from solution-processes,the host molecules are easier to aggregate and form quenching centers and/or complexes with electron transport layer(ETL)molecules,which severely degrades the device performance.We doped small molecule ETL materials as molecular spacers into the EML to improve the device.The doping of POT2 T both improves device performance and device stability.By tested and collected the changes in the EL spectrum of devices prepared by using only the host material as the EML doping different ratio of PO-T2 T into the EML under constant current density,we found that the doping of PO-T2 T can also weaken the band that creat by the interaction between m CBP and DPEPO.And through the test of the film,it is found that a small amount of PVK and POT2 T mixed with m CBP can significantly improve the morphology and stability of the m CBP film.This thesis contains 26 figures,6 tables,and 75 references.