A Study Of White Organic Light-Emitting Diodes Based On Exciplex Emission

White organic light-emitting diodes(WOLEDs)can be used as lighting resources for daily life and applied to full-color display.WOLEDs have drawn enormous research interests because of their attractive merits such as thin,high luminance efficiency,flat emitting with large area,flexibility combined with flexible substrates,etc.In the study of WOLEDs,high efficiency is always the goal of many researchers.Phosphorescent materials have been widely used in WOLEDs because they can achieve 100% internal quantum efficiency.However phosphorescent emitters generally contain heavy metal atoms,which has disadvantages of high cost,limited resource,and environmental pollution.Thermally activated delayed fluorescence(TADF)materials can achieve a maximum internal quantum efficiency of 100% without heavy atom effect because the energy gap between singlets and triplets is very small,which facilitates the up-conversion of triplets to singlets.Meanwhile,exciplex with TADF characteristic has been one of research focus because of the simplicity of acquiring exciplex.In many studies of exciplex,a relatively complicated doping system is widely used in the structures,which increases the difficulty of the device fabrication.This thesis reports the high efficiency non-doped device of blue exciplex emission and WOLEDs based on blue exciplex emission and ultrathin yellow fluorescent emitter.Based on this topic,the following work was carried out:First,we proved the existence of exciplex in the mixed film of hole transport material CDBP and electron transport material PO-T2 T by photoluminescence(PL)spectra,and also verified the existence of exciplex in bilayer structure.Based on the result of PL spectra,we designed a simple doping-free blue exciplex emission device.Due to the TADF characteristic,this device exhibited a maximum external quantum efficiency of 9.6% and a maximum current efficiency of 18.9 cd/A.In order to obtain white emission,we introduced an ultrathin yellow fluorescent emission layer into a blue bilayer exciplex system.It was found that the location of ultrathin TBRb in the devices played a key role on the performance of WOLEDs.When the ultrathin yellow layer was inserted at the CDBP/PO-T2 T interface,blue emission existed only when the thickness of yellow layer is very small,which made the devices difficult to reproduce.When the TBRb layer is inserted into CDBP,direct trapping of holes by TBRb occurred,which lowered the efficiency of the devices.While TBRb is inserted into PO-T2 T,direct trapping of carriers and Dexter energy transfer from exciplex to TBRb were both suppressed by adjusting the distance between TBRb and CDBP/PO-T2 T interface.This led to a high efficiency WOLED.In addition,it is proved that the excitons are mainly formed as exciplex,and then part of excitons are transferred to TBRb by F?rster energy transfer process while Dexter energy transfer is effectively suppressed when the distance between TBRb and exciplex is larger than 3 nm by theoretical calculations.Finally,The WOLED with a simple dopingfree structure achieved a maximum current efficiency of 27.2 cd/A and a white emission with Commission Internationale de I'Eclairage(CIE)coordinates of(0.35±0.02,0.38±0.02)...