Optimization Of Carrier Transport On Solution-processed Blue TADF OLEDs And Its Influence On Device Performance

Organic light-emitting diodes（OLEDs）are developing rapidly as a new generation of display technology.The solution method is the cheap and commercialized preparation method at present.However,the efficiency and stability of small-molecule OLEDs processed by the solution method are still difficult to meet the application requirements.Due to the interaction between small molecules,solution-processed OLEDs（s-OLEDs）will lead to molecular aggregation and disorderly accumulation in film formation.At high brightness,exciton density is concentrated in thermally activated delayed fluorescence（TADF）OLEDs,exciton quenching is severe,and non-radiative recombination increases.In addition,the charge transport in organic films is complicated,and the injection and transport of electrons and holes are not easy to reach balance.All these factors affect the device’s performance.This thesis mainly seeks ways to improve the performance of solution-processed blue TADF OLEDs by improving charge injection,optimizing film morphology and balancing carriers,and realizing more efficient charge transfer and energy transmission in OLEDs.By suppressing exciton quenching,the efficiency of TADF OLEDs can be improved.By testing the capacitance-voltage characteristics,transient electroluminescence,electrochemical impedance spectra,and brightness under AC voltage,we deeply investigated the physical mechanism of carrier transportion in OLEDs to provide data support for the study of blue TADF OLEDs.The specific results are as follows.Firstly,we utilize the structural differences between small molecules to reduce molecular aggregation in organic films.We add a small amount of spiral small molecule TCTA as a dopant into the emitting layer film 4Cz FCN:m CP.The interaction force between TCTA molecules is weak,which reduces the aggregation caused by the movement of small molecules during emitting film formation.The molecular accumulation of the emitting layer is obviously weakened,and the molecular distribution is more uniform.The efficiency roll-off of the device is reduced from 33.3%to 20.5%after doping with 3 wt%TCTA.The doped device exhibits the maximum current efficiency(CE_max=29.6 cd/A),about 60%higher than the undoped device(CE_max=18.5cd/A).Secondly,we use the polar solvent vapor annealing（PSVA）method to change the molecular orientational motion of solution-processed small-molecule films.We verify the feasibility of this method by applying it to the hole injection layer（PEDOT:PSS）and emitting layer（DMAC-DPS:m CP）.The effect of polar solvents on the polar orientation of small molecules in the film formation process is explored using the polarity difference of small-molecule structures.The idea is proved by the measurement of the surface morphology,surface potential,and work function of the emitting layer.The Fermi level of the emitting layer surface increases,the work function of emitting layer decreases by0.36 e V,and the electron injection barrier reduces.Eventually,the two solution-processed films are treated with the PSVA method resulting in improved hole and electron transport.The charge accumulation at the interface is reduced by enhancing the charge transport and optimizing the balance of the two carriers.The ordered arrangement of molecules in the emitting layer increases the electron transmission,reduces the accumulated charge in the device,and reduces the trap density.The efficiency roll-off of the device at high luminance is improved from 33.3%to 26.6%,and the current efficiency is increased.Finally,small-sized gold nanoparticles with a diameter of 3.5 nm～4 nm are synthesized and used to improve the charge transfer capability of PEDOT:PSS film.The small-sized gold nanoparticles are covered in PEDOT:PSS and the quenching of luminescence by metal nanoparticles is reduced.We use the doping of gold nanoparticles in PEDOT:PSS layer to improve the hole injection of OLEDs of three colors without a thick blocking layer.By testing the PL spectrum of the emitting layers,the J-V of the hole-only devices,and the capacitive characteristics of the devices,it is demonstrated that the principle of small-sized AuNPs can be used as dopants for the hole injection layer to exploit their electrical injection capability rather than the localized surface plasmon resonance.PEDOT:PSS doped with gold nanoparticles can reduce the driving voltage and improve the current efficiency.The current efficiency of blue OLED increases from 22.4cd/A to 27 cd/A;the current efficiency of red OLED increases from 5.96 cd/A to 15.6cd/A;the efficiency of green OLED increases from 16.7 cd/A to 29 cd/A.