Characterization of Solution Processes for Small Molecule Organic Light-Emitting Diode

다국어 초록 (Multilingual Abstract)

Solution-processed organic light emitting diodes (OLED) have been demonstrated with small molecule by using three solution processes. Spin-coating, electrospray deposition process(ESD) and electrohydrodynamic (EHD) jetting are employed as the way for deposition method. Mostly, polymer host has been used for solution process with good film formation property. However, polymer host has some problem such as difficulty of purifying and low electroluminescence property. Therefore, 4,4'-N,N'-dicarbazolebiphenyl (CBP) which is typical small molecule based ink is used for spin-coating and electrospray method. For three solution processes, various process variables and materials properties were investigated including solvent effect, annealing conditions, ETL molecular structures and so on.
In spin-coating process, electron transporting material, annealing conditions and solvent effect was investigated to get highly efficient OLED device. Since 2,2',2''-(1,3,5- Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)(TPBi) had low highest occupied molecular orbital and high thermal stability, efficiency dramatically increased with TPBi. And annealing conditions for both removing solvent and avoiding crystallization were studied. Proper annealing condition(60℃ for 1hour) could raise the current efficiency almost twice. In case of solvents, dichloroethane(DCE) showed good film formation and device performance because of high viscosity and appropriate boiling point. Consequently 46.22cd/A of efficiency was obtained in use of TPBi as an ETL, annealing in 60℃ for 1hour with DCE as a solvent.
ESD process has potential of bulk production in comparison CBP. However, it was hard to form uniform thin film with CBP based ink because of aggolomerate phenomenon. This was from molecular structure of CBP which didn't have branch structure. To make uniform thin-film, derivatives synthesis of CBP structure was fulfilled by removing one carbazole group in CBP. Due to elimination of carbazole group which had high hole mobility, device performance was lower than original CBP based device. 5 devices were fabricated with varying the ratio of CBP and its derivatives. Finally, 23.45cd/A of efficiency was obtained when the ratio between derivates and CBP was 1:4. The result showed possibility of ESD process which can be an alternative of spin-coating
With EHD process, patterning fine lines for high resolution OLEDs was fulfilled for large area display. EHD jet printing is strongly dependent on ink properties and process conditions such as applied voltage, pressure, distance nozzle and substrate, speed of the moving stage and surface properties. For characterization those dependences, instead of polymer, small molecule ink was used because polymer could not fill the lighting area and make stable jetting. Considering vaporization problem and nozzle clogging problem, dichloroethane and dichlorobenzene were selected as solvent. Fine line patterning trends were investigated by changing applied voltage, line motor velocity and distance between nozzle and substrate. As a result, up to 30um pattern was achieved and it could be a promising technology for solution process.
It is expected that these solution processes can be a good alternative of vacuum deposition technology.

목차 (Table of Contents)

• List of Figure
• List of Table
• Abstract
• 1. Introduction
• 1.1 Small molecule OLEDs with solution
• process
• 1.2 Literature review
• 1.3 Scope of this work
• 2. Experiment
• 2.1 Solution process of CBP based small molecule OLED
• 2.2 Electrospray deposition process for small molecule organic light-emitting layers
• 2.3 High Resolution OLED Printing Using Electrohydrodynamic jetting
• 3. Result & Discussion
• 3.1 Solution Process of CBP based Small Molecule OLED
• 3.1.1 Effect of vacuum-deposited electron transporting layer
• 3.1.2 Effect of electron transporting material
• 3.1.3 Effect of annealing condition
• 3.1.4 Effect of solvents
• 3.2 Electrospray Deposition Process for Small Molecule Organic Light-Emitting Layers
• 3.3 High Resolution OLED Printing Using Electrohydrodynamic jetting
• 4. Conclusion
• Reference