###### Abstract  
In this study, we investigated the ink formulation of poly(3_A_-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) ethanol 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.  
nozzle printing; PEDOT:PSS; ethanol; dilution rate; OLED; grey scale method 2018  
In relation to this, [PERSON] et al. investigated a method to prevent coffee ring and pinhole defects by using a co-solvent system with a different de-wetting velocity mechanism [10]. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is currently the most popular and important hole injection layer (HIL) for the fabrication of solution process based OLED device structures [3]. Generally, it is difficult to fabricate a pristine PEDOT film in a neutral state because pristine PEDOT cannot be dissolved in most conventional organic solvents. Therefore, to apply a solution process to printing, a stable PSS blend system that can approach to process easily was developed [11; 12]. The development of PEDOT:PSS, HIL materials has been studied extensively in the solution based OLED industry. [PERSON] et al. successfully fabricated an all solution tandem white OLED by using various types of PEDOT:PSS [13].  
In this study, we suggested a deposition methodology for PEDOT:PSS with various concentrations of EtOH dilution solvent by using a nozzle stream jet printer. In addition, 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, specifically, the ink formulation, viscosity, surface tension, evaporation rate, and residual containing solvent volume in the PEDOT:PSS solution. Finally, to analyze the film uniformity of the PEDOT:PSS/EtOH solution, we used imaging software and a scanning electron microscope (SEM) measurement system. We then 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.