How are conductive inks designed?

The advantages of print used in printing books and newspapers – such as scalability, lower cost, simplicity and speed – can also be used today in production of advanced microelectronics. The inks employed in printed electronics are to some extent similar to the inks used in graphics printing. The contribution of the first-mentioned to the overall performance of the end-use product is however much greater. First and foremost, materials for printed electronics are designed to bring a functionality of electrical conductivity. Therefore, in addition to the basic ink constituents such as solvents and additives, conductive inks must contain components which introduce movement of electric charges to the printable fluids. Secondly, the choice of conductive constituents can augment the properties of printed structures with various additional features, which result from the intrinsic characteristics of the employed functional materials. As components imparting conductivity, a variety of inorganic and organic nanomaterials are applied, such as metal nanoparticles, carbon nanomaterials, conductive polymers, or quantum dots. The choice of a conductive material is, however, only a part of the complex process of conductive ink design.

To successfully deposit conductive patterns of desired performance, it is necessary to understand the relationships between physicochemical parameters of ink and specificity of the applied printed technology. The ink properties must fit the requirements of the chosen printing method resulting from its working principle. Further, the ink formulation should be compatible with the desired substrate, meaning appropriate wettability and good adhesion. The  list of other important factors contributing to the end-use device performance, includes also adjustment of process parameters and sintering conditions, prior substrate treatment, yet the very first challenge is the proper ink design.

The ink must be processable and should demonstrate good printability, which strongly depends on both its rheological characteristic and the size of employed functional particles. Rheology of fluids refers to their behavior in response to an applied force. An ink is subjected to different types of stress, during the printing process and when impacting a substrate. Accordingly, to achieve printouts of good quality, it is crucial to control such fluid parameters as its viscosity, surface tension, evaporation of liquid components, wetting of a surface, and others.

Viscosity is a measure of the resistance of a fluid to flow, which is related to the internal friction of a moving fluid. This factor is highly sensitive to temperature, and the relationship is reciprocal: with increasing temperature, viscosity decreases. Each printing technique requires a specific range of ink viscosity, which the formulation should be tailored to. Eventually, the viscosity optimization often comes down to a trade-off between processability versus achievable printing resolution and precision. Too low viscosity may give rise to satellite droplet formation and overspray reflecting on the resolution and quality of the printed structures. Too high viscosity may result in difficult deposition and may lead to clogging of the printing nozzle [link to the other article]. Nevertheless, in the light of the recent developments and demand for printing high-performance conductive features, higher ink viscosity is currently recommended. The additional reasons are attributed to more solid contents remaining after solvent evaporation, and because the deposited patterns are less affected by the substrate properties when using a more viscous ink [].

The fluid behavior is also governed by surface tension. This is the effect of cohesive forces of liquid molecules that cause the molecules on the surface of a liquid to be pushed together, minimizing the occupied area. A liquid with a low surface tension spreads over a substrate surface more than a liquid with a higher surface tension. Generally, high surface tension of the printable fluid is desirable, since it results in higher printing resolution and fewer satellite droplets []. Nonetheless, to give good adhesion of the printouts to the substrate, the surface tension of the ink must be lower than the surface energy of the substrate. The adhesion can be further facilitated by addition of surfactants.

Another crucial factor is the evaporation rate of the ink. Rapid drying could be favorable enabling higher printing speed, but it also promotes clogging of the printing nozzle, which constitutes a primary problem with printing. The evaporation rate also affects the morphology of the deposited structures, as too low drying rate may cause uneven distribution of the printed material. This is an unfavorable phenomenon as the non-uniform morphology of  printed features may disturb the device performance. Faster drying helps conserve homogenous distribution due to the faster evaporation of the solvent []. The drying characteristic can be optimized by proper selection of the solvent system or optional adding of a humectant in case of aqueous medium [].

Further, wetting the surface by ink constitutes a major factor affecting morphology of the printed film [],its adhesion to the substrate, as well as the printing resolution []. To achieve proper ink behavior on the substrate surface, wetting conditions must be appropriate, which means surface tension of printing material must match the surface energy of the substrate []. For drops spreading while keeping them together creating uniform layers, the surface energy of the substrate must be superior to that exhibited by the ink []. This allows achieving homogeneity of the lines, the minimum possible line width, as well as the resolution between adjacent lines []. The wetting compatibility can be fixed by a surface treatment, for example by corona or plasma treatment.

Achieving high-quality with printed conductive patterns is an effect of a complex optimization of several factors intertwined by network-dependencies, The crucial role in this process is played by a proper ink design. Importantly, it can be infeasible to attain the greatest values of all the parameters at the same time (in terms of e.g., electrical conductivity of a printed layer, its uniformity, resolution, flexibility, stretchability, stability, etc). Eventually,  the properties of conductive inks should be tailored towards the specific criteria of a particular printing technology or a chosen application, leading to the possibly highest overall end-use performance of created conductive structures.

If you have printed electronics requirements and are looking for fine-tuned high-performance conductive inks, feel free to contact our team for details. Discover what the line of unique XTPL nanoinks can offer you to help you realise your ambitious goals.

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