Research On The Properties Of Solution Processible Organic Light-emitting Small Molecular Thin Films

Recently, organic light emitting diodes (OLEDs) have gained tremendous attentiondue to their significant potential application in flat-panel displays and solid statelighting. However, the research on the properties of solution processible organiclight-emitting small molecular thin films is seldom reported. In this dissertation, thesolution properties of organic light emitting small molecules were investigated, and acomparison study of the organic small molecular thin films prepared by solution processand vacuum deposition was carried out. The charge transport properites of the solutionprepared thin films were investigated. The main contents of this dissertation are asfollows:The viscosity properties of the organic light emitting small molecules (such asTPD、m-MTDATA、TPBI、OXD-7) were systematically investigated. The factors whichcan affect the viscosity properties, such as the solvent, the solution concentration andthe temperature have been studied. It was found at the same solution concentration andtemperature, the solution viscosity of organic small molecules depends largely on thesolvent viscosity. With the solution concentration increasing, the activation energyincreased and the solution viscosity only slightly increased. Raising temperatureresulted in decreasing the solution viscosity.The properties of the solution-processed and vacumm deposited organic smallmolecular thin films were investigated. It was found the solution-processed films ofN,N’-di(3-methylphenyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4-diamine (TPD),4,4’,4’’-tris(3-methylphenylphenyl amino) triphenylamine (m-MTDATA) and2,2’,2’’-(1,3,5-benzenetriyl)-tris[1-phenyl-1H-benzimidazole](TPBI) exhibitedsmoother surface morphology and more hydrophilic compared to the vacumm depositedfilms, respectively. The density of the films of TPD、m-MTDATA、TPBI was measuredby both absorbtion and XRR methods, and an increase in density was observed insolution processed films in comparsion with the vacuum deposited films. The enhancedaggregation is responsible for the red-shift in the PL spectra of the films. Dewettingmachnism was used to explain the why the film surface of the film which was fabricated by the chlorobenzene solution is smoother than that fabricated by the choloformsolution.Taking TPD as an example, the hole-only devices were fabricated and the holemobilities of solution-processed TPD thin films were measured. The hole mobility ofthe solution-processed TPD film is a little higher than that of the vacumm-depositedTPD film. The solution-processed TPD film showed improved hole injection, and theOLED devices with the solution-processed TPD exhibited lower turn-on voltages andenhanced luminous efficiency.