The Drying of Inkjet Printed Drops on Patterned Substrates

Abstract

Inkjet printing provides a promising method for the fabrication of OLED displays but currently, inkjet printed displays are not commercially viable. This thesis focuses on understanding the drying processes that occur once drops have been delivered to the patterned substrates necessary for OLED devices. To this end, internal flows in drops evaporating within wells were
investigated and the changing drop profiles during drying were imaged. A method was suggested for successful fabrication of OLED devices.

Particle tracking was carried out on both pure solvents and binary solvent mixtures within square wells. Due to the large particle size in comparison to the depth of fluid these experiments were not very informative, though they did confirm evaporation was faster at the
contact line than in the centre of the drops. Evaporation was also slightly faster in the corners of the wells relative to the straight edges.

Studies on pure solvents identified the influence of evaporation rate on profile development in drying drops. Two main drying regimes were identified and the main influence on drop profile development was found to be the evaporation rate of the solvent. Slow drying drops gave U-shaped profiles and fast drying drops gave W-shaped profiles.

The influence of thermal effects on drop profiles was also considered. Thermal Marangoni flows were found to have a profound influence on profile development, with drops giving M-shaped profiles. Thermal effects could not always be reliably reproduced and it was concluded that further experimentation in this area was necessary. The lack of repeatability in the results was assumed to be due to the sensitivity of the drop profile to its initial behaviour.

Binary solvent mixtures were also found to have an impact on profile progression during drying. Solutal Marangoni flows gave M-shaped profiles in the case where the more volatile solvent had a lower surface tension and enhanced drainage from the corners of the wells towards the centre in the case where the more volatile solvent had a higher surface tension.

The thesis then moved on to investigate the effect of active materials on drop profiles. The active materials used were found to increase the surface tension of the solvents, giving M-shaped profiles when dissolved in single solvents. In some slow drying solvents, diffusion of the material evened out concentration gradients during drying and U-shaped profiles were seen. When solvent mixtures which had shown flows in opposition to those caused by active materials were used to print the actives, the profile development showed enhanced drainage
from the corners of the wells suggesting solvent driven Marangoni flows were dominant over active material driven Marangoni flows. Crystallisation of the active material in this case showed re-circulatory flows were present with the active materials following the flows. This
suggested particle tracking should be possible in these systems.

A proposed method for obtaining flat deposits from printed drops was then presented, along with some initial results towards that goal. The initial results were promising but more investigation is needed in this area.