Leveling and Drying Dynamics of Printed Liquid Films of Organic Semiconductor Solutions in OLED/OPV Applications

Abstract

Printing process development for fabrication of organic electronic devices is described, with focus on semiconductor layers for organic light‐emitting diodes and photovoltaic cells. This development is considerably more complex than for a graphical printing process. Key aspects are an adequate dosing and transfer of highly volatile inks, the reliable coalescence of the droplets deposited on the substrate to a closed liquid film, and the successful relaxation and leveling of the liquid–air interface in the solvent evaporation phase in the presence of Marangoni stresses and pattern formation instabilities. The conditions for successful implementation of a gravure or inkjet process, using steadily developing, new generations of polymer as well as small molecule semiconductors are, to a large extent, but not exclusively, originating from the molecular features of organic semiconductors and their printable solutions. In addition, recent developments in surface technology, and in the physics of thin‐film dynamics and spontaneous pattern formation contribute to a proper understanding of liquid layer dynamics in printed electronics. The role of ink formulation, Marangoni stresses related to concentration and temperature gradients, the effect of the disjoining pressure, and solvent evaporation are discussed.

Eliminating the Perovskite Solar Cell Manufacturing Bottleneck via High‐Speed Flexography

“…In this work, high density anilox rollers (25 µm cell size) are utilized to achieve ultrathin films while minimizing the wavelength of modulations, allowing for faster leveling. [ 41 ] Based on measured film thicknesses and modulation wavelengths, the inks used here are estimated to level in ≈0.1–0.4s, significantly shorter than the observed drying times of ≈2 s. We note that it is possible to further decrease the ink viscosity to enhance leveling by changing the solvent ratio. For NiO x solutions mixed with ethanol (EtOH) and 2‐ME, increasing the amount of EtOH used decreases the viscosity of the solution (Figure S3, Supporting Information), which is to be expected since the viscosity of pure EtOH (1.1 mPa s) is smaller than the viscosity of pure 2‐ME (1.7 mPa s).…”

Rapid 2D Patterning of High‐Performance Perovskites Using Large Area Flexography

“…Therefore the deposited films show higher RMS variability at lower concentration since they are unable to level and eliminate viscous fingering effects resulting from ink transfer during printing before the film is "frozen" in its final state by drying. [45,49] This shows that both the drying process and the ink viscosity must be considered when optimizing the film uniformity. Thicker wet films printed from higher viscosity inks dry slower, allowing more time for the film to level, which could result in more uniform films.…”

Mild stratification in drying films of colloidal mixtures

“…The distribution of colloidal particles significantly affects their target-oriented properties in functional films used as electrodes in lithium-ion secondary batteries and fuel cells 1,2 as well as in applications such as paints, 3 drug delivery, 4 and optical films. 5,6 It is desirable to control the particle stratification by adjusting process conditions ( e.g. , drying rate and solid concentration) in industrial processes, rather than modifying the material properties of colloidal particles.…”