Fabrication And Properties Of Solution-processed Blue Fluorescent Organic Light-Emitting Diodes

Blue light is an essential part of the full-color display,however,the current efficiency of blue organic light-emitting diodes(OLEDs)is lower as compared with that of OLEDs with other colors,especially the blue OLEDs fabricated by solution-processing.In this thesis,a novel blue fluorescent material,(CH3OH)4THE,and its application in OLEDs are investigated.Based on the studies on the basic properties of this material,we optimized the device performance on the basis of the optimizations in the film preparation process by solution method,the emissive layer(EML),the device structure and so on.The main research contents are as follows:(1)The physical properties of(CH3OH)4THE material such as light absorption,luminescence and carrier transport characteristics are characterized,and the factors influencing the solution-processed films are investigated.(CH3OH)4THE is a blue fluorescent small molecule material with an absorption peak at 380 nm and an emission peak at 440 nm.Its hole transport ability is better than that of electrons.The best solvent for dissolving the material was screened by comparing the device performance of the OLEDs based on the EML fabricated from different solvents including chlorobenzene,chloroform,toluene,O-Xylene,ethanol and dimethyl sulfoxide.It is found that chlorobenzene is the best one.Then,a heating process for the(CH3OH)4THE chlorobenzene solution was introduced for further improving the device performance.The luminescence properties of the device such as brightness and current efficiency were significantly improved.The maximum efficiency of the device prepared with the unheated solution was 0.22 cd/A,whereas that prepared with the heated solution was 0.33 cd/A.The maximum brightness of the device prepared with the heated solution also increased from 233 cd/A to 784 cd/A.(2)The electron transport material TPBi was doped in EML to improve the device efficiency.Since the hole mobility of the(CH3OH)4THE thin film is higher than its electron mobility,the electron mobility is improved by doping the electron transport material TPBi.Moreover,TPBi was used as an electron transport layer(ETL)to simultaneously enhance the interface electron injection.Different concentrations of TPBi doped in(CH3OH)4THE were investigated.It was found that the device with 10%TPBi doping has the highest efficiency,which is 0.18cd/A,and the lowest efficiency roll-off.The improved efficiency behaviors can be due to the electron transport ability of the TPBi in EML and the hole blocking effect of TPBi ETL.which leads to a more balanced carrier distribution in EML.(3)The device structure of the OELDs with(CH3OH)4 THE EML was also optimized by adding a cathode modified layer and adjusting the thickness of EML and ETL.After adding a cathode-modified layer,the device current density is reduced,the brightness and the efficiency are increased.The maximum brightness at an applied voltage of 4V is increased from 122 cd/cm2 voltage to 784.8cd/cm2,and the current efficiency is improved from 0.08 cd/A to 0.32 cd/A.By comparing the current densities of single carrier-only devices under the same electric field,the problem of the shift in the emission region and the causes of the unbalanced exciton recombination were analyzed.The thickness of the EML is found to affect the carrier transport and recombination region.The addition of the ETL blocks the holes and reduces the non-radiative recombiantion at the metal electrode interface,which greatly improves the current efficiency of the device to 1.20 cd/A.