Carrier Management To Improve The Performance Of Solution-processed Blue TADF OLEDs

With the increasing maturity of applications in the field of flat panel displays and solid-state light sources,organic light-emitting devices(OLEDs)have attracted more attention,especially OLEDs based on thermally activated delayed fluorescence(TADF)emitters which exclude heavy metals.TADF materials can utilize triplet excitons through the effective reverse intersystem crossing(RISC)from the lowest triplet state(T₁)to the lowest singlet state(S₁),and theoretically can achieve 100%internal quantum efficiency(IQE).A unique advantage of OLEDs is that they can be prepared by low-cost and large-scale solution methods.However,devices with complex structures cannot be prepared by the solution method due to the problem of mutual solubility between layers,resulting in limited carrier injection and transport.The performance of the solution-processed devices are not as good as vapor deposition OLEDs,especially blue OLEDs.Therefore,in order to improve the performance of blue TADF OLEDs,we propose a collaborative method to manage the density and distribution of carriers in the emission layer(EML),based on2,3,4,6-tetra(9H-carbazol-9-yl)-5-fluorobenzeonitrile(4Cz FCN)emitter,and obtain high-efficiency blue TADF OLEDs.The main contents are as follows:(1)We optimized the EML from the aspects of the EML structure,the doping concentration of guest material,the EML thickness and the host material,etc.,and explored the influence of these factors on the device performance and the optimization mechanisms.We obtained the best doping concentration of 15 wt%,and the OLED with the highest current efficiency of 16.3 cd/A and the highest luminance of 5187.7 cd/m².Compared with other hosts,m CP enables the device achieving the shortest emission wavelength,the lowest turn-on voltage,the highest current efficiency,and the highest luminance.This is mainly due to the strong hole transport capability of m CP,which is conducive to the generation of excitons and carrier balance at low voltage.(2)We regulated the EML interface to manage the distribution of carriers and excitons in the EML,thereby optimizing device performance,and obtaining high-efficiency blue TADF OLEDs.First of all,we introduce TSPO1 as a hole and exciton blocking layer adjacent to the EML.TSPO1 can effectively prevent hole leakage and exciton diffusion to the electron transport layer,and improve the radiative recombination of carriers and the utilization of excitons in the EML,thereby improves the device efficiency.However,because TSPO1 hinders the transport of electrons,the device luminance is reduced.Hence,we screened two materials with deep highest occupied molecular orbits(HOMO)level,deeper lowest unoccupied molecular orbits(LUMO)level and high electron mobility as the electron transport layer.In particular,B4Py MPM can effectively block holes and enhance the injection and transport of electrons.Thus,the corresponding device achieves a high current efficiency of 19.6 cd/A and a high luminance of 5368.2 cd/m².Finally,we choose PVK,a polymer with low hole mobility,as the hole transport layer between PEDOT:PSS and EML.The PVK layer can effectively prevent electron leakage and reduce the hole current in the device,thereby leading to more balanced carrier distribution in the EML and further improving the device performance.In short,a maximum luminance of 7625.0 cd/m² and a high external quantum efficiency of 24.0%have been achieved through the above optimization.The efficiency is the highest value reported on 4Cz FCN and a higher value among the reported blue TADF OLEDs prepared by solution method in this wavelength range by now.