Study On Organic Light-Emitting Diodes Based On Rough-Vacuum Thermal Evaporation

Recently,organic light-emitting diodes(OLEDs)have shown great potential in the area of display and lighting with their rapid developments.Flexible,cambered,transparent,light and thin display or lighting are coming true thanks to the technology of OLEDs.However,the cost of production still remains at a high level.To a large degree,the high cost is mainly on account of the expensive vacuum system.For avoiding this issue,the deposition technology of solution processing has been developed.Solution-processing method not only reduces the production cost of OLEDs,but also provides an effective route to realize the device fabrication with large area.In this thesis we studied the crystallization issue of solution-processed organic small molecular films and put forward an effective method to suppress the film crystallization.We found that severe crystallization was observed in solution-processed dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile(HAT-CN)films,whereas not in vacuum thermal evaporated ones.As a result,the deteriorated film would result in short circuit of the device when operation.The film crystallization could be suppressed after mixing 2,3,5,6-Tetrafluoro-7,7,8,8,-tetracyano quinodimethane(F4-TCNQ)with HAT-CN with different ratios.In addition we also explored the intersolubility issue between the solution-processed hole-injecting layer(HIL)and light-emitting layer(EML).We proposed a simple method by spin-coating an additional layer of aqueous MoO3:TS-CuPc was spin-coated onto the HAT-CN:F4-TCNQ,to separate the two oily layers.As a result,the efficiency of hole injection bilayer based solution-processable blue phosphorescent OLED was improved largely.Combined with vacuum thermal evaporation and solution process,we also proposed a new method,named as flash evaporation,to fabricate OLEDs.The new method does not need the expensive experimental apparatus.What's more,it avoids the intersolubility issue between two adjacent layers could be easily avoided.In addition,we also analyzed and contrasted the surface morphology,molecular structure,film density and molecular orientation in vacuum-evaporated,flash-evaporated and solution-processed films.We found that there was nearly no difference in surface morphology and molecular structure among these three films.However,the film density of flash-evaporated film was much smaller than the other two films.Also,the molecular orientation,which determines the film properties,was very different from each other.