Main solvent selection criteria and small molecule and polymer binary-solvent inks for inkjet printing

Abstract

In inkjet printing, ink formulation is the foundation to get stable droplets and films with high uniformity. Typically, inks for printing are composed of binary solvents consisting of the main solvent and a co-solvent, where the main solvent provides solubility for the luminescent materials and the co-solvent adjusts the physical parameters of the ink to optimize the ink's printability and film-uniformity properties. However, the principles of main solvent selection are not the same for small molecules and polymers. In this work, we propose the selection criterion of main solvents for small molecules and polymers from three aspects: solvent solubility, film-forming ability, and droplet behavior. The dendrimer Ir(Ph-BM-P2D2)₃ (G2P2) and the polymer poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) were used to compare the differences in solvent selection between small-molecule and polymer inks. Polymer F8BT is poorly soluble but has good film-forming ability. Toluene (TOL), with low viscosity and surface tension, meets dissolution requirements and film-forming integrity. Small molecule G2P2 has many good solvents; however, cyclohexanone (CYC) as the main solvent has not only good solubility but also a high boiling point (155 °C), viscosity (2.2 mPa s) and low surface tension (32.5 mN m⁻¹), ensuring that the film dries slowly without de-wetting. The role of co-solvents for polymers and dendrimers is the same: to eliminate satellite droplets and coffee-ring effects. Cyclohexylbenzene (CHB), with a high viscosity (2.7 mPa s) and a high boiling point (237 °C), was used as a co-solvent. The parameter ranges for printhead suitability were 23.20 < Z < 68.05 for F8BT and 14.87 < Z < 21.17 for G2P2. Finally, the F8BT binary solvent is TOL/CHB = 5/5 and the G2P2 binary solvent is CYC/CHB = 5/5, which inhibits the generation of long trailing and satellite droplets in ink printing and the de-wetting and coffee-ring effects in the film-forming process.