Abstract
In this study, we investigated the ink formulation of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the hole injection layer (HIL) in an organic light emitting diode (OLED) structure. Generally, in a PEDOT:PSS solution, water is incorporated in the solution for the solution process. However, the fabrication of thin film which contained the water, main solvent, could not easily form by using printing technology except spin-coating process because of the high surface tension of water. On the other hand, mixing PEDOT:PSS solution and ethanol (EtOH), a dilution solvent, could restrain the non-uniform layer that forms by the high surface tension and low volatility of water. Therefore, we printed a PEDOT:PSS solution with various concentrations of EtOH by using a nozzle printer and obtained a uniform pattern. The line width of PEDOT:PSS diluted with 90% (volume ratio) ehtanol was measured as about 4 mm with good uniformity with a 0.1 mm nozzle. Also, imaging software and a scanning electron microscope (SEM) were used to measure the uniformity of PEDOT:PSS coated on a substrate. Finally, we fabricated a green phosphorescent OLED device with printed-PEDOT:PSS with specific concentrations of EtOH and we achieved a current efficiency of 27 cd/A with uniform quality of luminance in the case of device containing 90% EtOH.
Highlights
Printed electronics are an important new technology for emerging markets
A printing process can be applied for various low-cost organic electronic device applications such as organic thin film transistors (OTFTs) [1], organic solar cells (OSCs) [2], radio frequency identification (RFID) [3], organic sensors [4], and organic light emitting diodes (OLEDs) [5]
We considered the pattern quality about defect free printing. To prevent defects such as a de-wetting zone and pinholes, we considered the properties of the solution, the ink formulation, viscosity, surface tension, evaporation rate, and residual containing solvent volume in the PEDOT:PSS
Summary
A printing process can be applied for various low-cost organic electronic device applications such as organic thin film transistors (OTFTs) [1], organic solar cells (OSCs) [2], radio frequency identification (RFID) [3], organic sensors [4], and organic light emitting diodes (OLEDs) [5]. Printing technology such as inkjet and nozzle printing process-based OLEDs have received huge interest from many companies and research groups. We fabricated a green phosphorescent OLED device with nozzle printed-PEDOT:PSS containing various amounts of EtOH, and obtained high I-V-L efficiency of an OLED owing to enhanced film uniformity of the PEDOT:PSS layer
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