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Inkjet

Markus Hösel

Inkjet printing is a well recognized technology from the daily (home-) office use and gained relevance in other areas outside of conventional printing in the late 1990s.DOI:10.1146/annurev-matsci-070909-104502 It can be divided into the main categories: continuous inkjet (CIJ), and drop on demand (DOD). Here, the focus is mainly on piezo-based DOD because of its current relevance and increased application in the field of print functionalities.

Simplified principle of inkjet printing
Figure 1. Simplified principle of inkjet printing for R2R processing.

The majority of todays inkjet printheads employed in functional printing use piezoelectrically actuated transducers to eject droplets on demand out of the nozzle. A simplified schematic is shown in figure 1. An applied voltage waveform (firing pulse) to the piezo induces a mechanical actuation and propagates a pressure pulse through the ink held in the chamber behind the printing nozzle. The droplets get ejected once the pressure exceeds the threshold at the nozzle. Ink is held inside the chamber due to surface tension and static pressure that stabilizes the meniscus at the nozzle. A similar actuation method is achieved using microelectromechanical systems (MEMS) and electrostatic forces.

State of the art industrial printheads have e.g. a native resolution of 300 dpi but can go beyond 1000 dpi of effective resolution due to variable drop size volumes. Firing frequencies over 40 kHz are commercially available with typical drop volumes between 5-80 picoliter depending on type and manufacturer. Lab-scale based systems such as the often-used Fujifilm Dimatix DMP printer can print with small drops down to 1 pl. This system is perfect for research studies due to its cartridge system (1.5 ml ink volume). For R2R systems inkjet heads with large print swathe widths are preferred. They often have >1000 nozzles and can print >70 mm wide at speeds beyond 50 m/min$^{1}$. These setups often rely on high-volume ink circulation systems that prevent frequent ink changes and are preferably used just for production runs.

The interplay between ink fluid rheology (viscosity $\ll$50 mPa$\cdot$s), inkjet head, surface energies of the substrate, surface tension of the ink, and print parameters such as print speed and drop distance is very critical and has to be adjusted carefully for a satisfying drop generation, layer homogeneity and line definition.DOI:10.1002/adma.200300385DOI:10.1146/annurev-matsci-070909-104502Magdassi, The Chemistry of Inkjet Inks In general, the pattern resolution is limited by the drop spreading on the surface, the drop overlap, and the coalescence with adjacent drops. The drop footprint (diameter) is approximately 3 times the drop diameter in flight. Feature sizes down to 30 µm without additional surface modifications can be achieved. Different line formations and the coffee ring effect are characteristics of the droplet interaction with the surface and the environment.DOI:10.1021/la7026847DOI:10.1016/j.tsf.2009.10.126DOI:10.1038/39827 Print parameter settings and the choice of solvents mixtures (low and high boiling point) have a crucial impact on the layer print results. Inkjet printing is a very complex technology with a huge parameter space that needs to be taken into account for the fabrication of functional structures. The big advantage is virtually waste-free printing using additive processes and digital printing forms, which are literally free and can be changed on-the-fly.

Inkjet printing for OPV

Inkjet printing for the fabrication of OPV devices has seen an increasing attention within the last years. The aim for printing active layers and PEDOT:PSS electrodes or intermediate layers are homogenous full layers whereas AgNP ink was preferably used for current collecting grid structures.DOI:10.1016/j.orgel.2010.06.007DOI:10.1166/jnn.2013.7500DOI:10.1002/aenm.201201050DOI:10.1016/j.solmat.2013.06.033 It was found that inkjet printing can produce active layer morphologies compared to spin coating without sacrificing the device performance. Inkjet printing requires optimized ink that must be technically printable but also suitable for the surface that it is printed on. That is why the majority of the studies report on tailored solvent systems and process conditions.DOI:10.1016/j.solmat.2012.10.011. Additives have also a strong influence on the morphology, printability, and optoelectronic properties of the active layer and PEDOT:PSS.DOI:10.1016/j.orgel.2010.06.007DOI:10.1016/j.orgel.2009.01.015 The strength of inkjet printing is the digital patterning of fine line and grid structures that can be used in OPV devices, especially for front grid electrodes,DOI:10.1016/j.solmat.2012.04.039 DOI:10.1016/j.orgel.2013.08.001 but also for back electrodes.DOI:10.1016/j.solmat.2013.06.033DOI:10.1002/aenm.201201050

Although inkjet printing has been used a lot for OPVs the holy grail in real-world upscaling has not yet been shown and virtually no reports on R2R printed OPVs except for conductive grid structures exist.DOI:10.1039/c2nr31508d The complexity of large-scale inkjet setups that typically involves recirculation systems, the required ink volumes and the change of drying conditions that has an impact on the layer formation and finally on the device performance tend to be the main bottleneck.

M. Hösel, Large-scale Roll-to-Roll Fabrication of Organic Solar Cells for Energy Production, PhD thesis

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