Symmetric and uniform coalescence of ink-jetting printed polyfluorene ink drops by controlling the droplet spacing distance and ink surface tension/viscosity ratio

- An appropriate drop distance rage forming linear line coalescence morphology was proposed.
- Decreasing the surface tension/viscosity ratio can promote a symmetric and homogeneous coalescence effect.
- The detail mechanisms of the appropriate matching of the spreading and coalescence speed were given.

The film quality is the core to produce long-life and efficient organic light emitting devices by ink-jetting printing technology which is deeply related to the ink droplet coalescence process, especially the coalescence bead morphology and thickness. However, the fluid flow from impacted drop to the previously deposited drops (drop to bead flow) usually leads to an asymmetry and heterogeneity of coalescence beads during droplets coalescence process in the substrate. Here we reduce the spreading speed of impacted drops to achieve a symmetric and homogeneous coalescence effect, by controlling the drops to deposit firstly and then coalescence. A uniform linear line morphology with a smooth straight edge and symmetric ends was obtained at drop distance 1.35 ≤ y ≤ 1.60. Via adjusting the chloride benzene (CB)/cyclohexylbenzene (ChB) proportion of ink formula, when the γ/η value decreases to 4.73 m/s, the h1/h2 value is nearly 1.04, homogeneous bead obtained. Uniform and homogeneous bead was formed for 80/20 CB/ChB sample with a 4.73 m/s surface tension/viscosity ratio. The appropriate matching of the relationships of the spreading and coalescence speed of impacted drops plays a critical role for the final bead morphology during the droplet coalescence. The spreading and coalescence speed have a finite matching for the 80/20 CB/ChB sample with a 4.73 γ/η value, forming uniform beads.

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