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Device Engineering Of High Performance Cold/Warm White Organic Light-Emitting Diodes

Posted on:2021-02-16Degree:MasterType:Thesis
Country:ChinaCandidate:W LuoFull Text:PDF
GTID:2428330605465245Subject:Physics
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Organic light-emitting diode(OLED)has been widely used in solid-state lighting and full-color flat panel display because of its high efficiency,low power consumption,wide color gamut and flexible in form factor.In order to further enhance the competitiveness of OLED in the lighting and display market,development of high-performance white OLEDs(WOLEDs)is of particular importance.WOLEDs based on phosphorescent emitters which can harvest both the singlet and triplet excitons to achieve a theoretical internal quantum efficiency(IQE)of 100%have been investigated extensively.However,the absence of stable blue phosphors make it easy to lose the color fidelity in a display.In recent years,thermal activated delayed fluorescence(TADF)materials have aroused much interest because they can effectively harvest excitons in devices and obtain 100%in IQE.Especially,the wide emission spectra of TADF materials make it easy to achieve high quality white light emission.Therefore,WOLED based on TADF materials is a promising approach to achieve high efficiency,high color purity and color stability.In addition,conventional WOLEDs usually have rather complicated device structures which may lead to a high driving voltage,which become an obstacle for OLED display and lighting in lowering the power consumption and manufacturing cost.Therefore,device with a simplified structure should be also considered.In my thesis,I present two methods to achieve WOLEDs with high efficiency,highcolor purity,good color stability and simplified device structure.Firstly,we obtained high color purity and spectral stable WOLEDs by constructing a blue-orange-blue sandwich emissive layer(EML)structure consisting of all-TADF emitting materials.We optimize the performance of WOLED and study the exciton dynamics of the recombination zone in this sandwich EML system,and found that this structure can improve the charge balance and broaden the exciton recombination zone in the EML,thus properly utilizing the singlet and triplet excitons to achieve high-performance white emission.Our WOLED shows pure white emission with a very stable Commission International de I'Eclairage(CIE)coordinates of(0.33±0.01,0.33±0.01)in a brightness range between 250 to 5000 cd/m2,corresponding to a color rendering index(CRI)of 81-83.The device yields a maximum external quantum efficiency(EQE)and current efficiency(CE)of 12.0%and 28.0 cd/A.This work suggests that our sandwich EML structure is promising for the fabrication of high-performance pure WOLEDs for display applications.Secondly,we design an interlayer-free structure for hybrid WOLED by introducing a non-doped blue fluorescence emitting layer to the device.This structure acquires a good carrier balance through controlling the dopant concentration and selecting appropriate host materials.In addition,by regulating the thickness ratio between the orange EML and blue EML and optimizing the thickness of electron-transport layer,we can effectively utilize the excitons and hence the exciton quenching could be greatly reduced,resulting in high device efficiency.Our WOLED shows a maximum power efficiency(PE),CE,and EQE of 82.3 lm/W,70.0 cd/A,and 22.2%,respectively.This design provides a new option for promoting simple-structure hybrid WOLEDs with superior performance.
Keywords/Search Tags:organic light-emitting diodes, white emission, thermally activated delayed fluorescence, high color quality, non-doped blue emitting layer, high efficiency
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