Systematic Development of a Novel Ternary Solvent System for Uniform Inkjet Printing of Organic Light-Emitting Diodes

This study investigates the suitability of three solvents commonly used in the cosmetic, food and pharmaceutical industries for inkjet printing of an organic light emitter. First, the solubility of a representative host material-4,4',4-Tris(carbazoyl-9-yl)triphenylamine (TCTA)-was examined in ethyl-4-methylbenzoate (EMB), 3-ethylbiphenyl (EBP), and 2-ethylhexyl benzoate (EHB) in comparison to some conventional organic solvents. Using a combinatorial design tool (Minitab), six ternary ink formulations based on EMB, EBP, and EHB were then selected. The role of the specific ink properties, such as viscosity and surface tension, on jetting behavior and drying dynamics were systematically analyzed. As a result, we determined a solvent mixing ratio that met three key requirements for the ink: At a ratio of 10:80:10 (EMB/EBP/EHB), the ink showed good host solubility and stability without satellite or tail formation while producing very smooth patterns. This ink was used to fabricate an organic light-emitting diode (OLED) device with an inkjet-printed emissive layer, with TCTA used either as a single host or in combination with 2,2',2″-(1,3,5-Benzinetryl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) to accommodate the emitter Tris[2-(p-tolyl)pyridine]iridium(III) (Ir(mppy)3). While the device in the single-host configuration achieved a rather modest external quantum efficiency (EQE) of 2.22% and a current efficiency (CE) of 5.21 cd/A at 1000 nits, performance increased significantly to an EQE of 5.84% and a CE of 18.5 cd/A in the mixed-host configuration.