*2.3.4 Process and apparatus related parameters*

*Printhead model.* Different inkjet printing systems demand different printhead models. The model of printhead also varies depending on the final application. Printheads for industrial IJP applications are more expensive, display a larger number of nozzles and allow for higher printing resolution. On the other hand, printheads intended for research and laboratory use are cheaper, have fewer nozzles, and print in lower resolution, as displayed in **Table 4** [50]. Regarding laboratory printing setups the most frequently used thorough the literature is the Dimatix Materials Printer, DMP series [50, 92, 108–112].

The general inkjet printer setup can be seen in **Figure 7a**. A 3-degree of freedom (DOF) system is the most frequently adopted and allows for efficient printing. Usually, the printhead only prints in one of the directions x-y directions (left-to-right).

In an inkjet printer the following parameters can be adjusted:

*Resolution.* The resolution should be chosen in accordance with the desired printed pattern, selected ink, and substrate material [109]. In multi-nozzle inkjet printers, the resolution is often controlled by the rotation of the printhead in predefined angles


### **Table 4.**

*Available inkjet printing systems [34, 50].*

### **Figure 7.**

*(a) Illustration of the XYZ cartesian inkjet printing system with mounted printhead. (b) Depiction of the Printhead in its native position and rotated at a 30-degree angle [113].*

(saber angles). To increase resolution, the printhead is rotated towards the movement direction axis. While the printhead is perpendicular to the printing direction, the resolution equals the native distance of the nozzles (lowest resolution). When the angular displacement is decreased, the distance between nozzles is diminished and the printed patterns achieve higher resolution (**Figure 7b**) [113]. In cases where the printhead is manually rotated, operator errors may occur, especially when dealing with small patterns with high resolution, which are more prone to displacement errors. For example, as seen in **Figure 7b**, many patterns demand more than one pass of the printhead be concluded and, if the direction is not perfectly aligned, the pattern will experience displacement between the two passes.

In practice, higher resolution is associated with a higher density of drops per area, which in turn, provides higher conductivity when printing conductive inks. Nonetheless, some issues might arise when the drops are too distanced (low resolution) or too overlapped (high resolution) and printing errors such as flooding, lack of superficial homogeneity, and loss of conductivity can occur, as illustrated in **Figure 8**. Moreover, if several passes are demanded additional attention must be given to assure correct angle alignment [109]. Some designs can also be prone to variability and errors, particularly if the image has sharp right-angle corners or lines whose width does not obey critical spacing rules [108].

Since inkjet printers usually print exclusively when moving in the left–right direction, different orientations of the same patterns may experience differences in the quality output, especially when higher resolutions are involved (> native resolution). This is particularly noticeable if the manual setting of the saber angle is not perfectly aligned. Considering this, to print the correct resolution in the precise position, the inkjet printer software is programmed to compensate for the angular displacement of the head. When this happens, even a slight imprecision during the printhead assembly can propagate printing errors across several printing passes (In **Table 5**, the major figures of merit that allow to identify the quality of printing are summarized, along with their description and observational examples).

*Printhead Height*. The height of the printhead, i.e., the distance between the nozzle and the substrate (the stand-off distance) can influence the splatter pattern

### **Figure 8.**

*Illustration evidencing the relationship between dot spacing and morphology of printed lines. (a) Large dot spacing caused drops to be isolated from each other. (b) Less drop spacing merges drops but their round edges are still visible. (c) Ideally merged drops forming a homogeneously printed line. (d and e) low drop spacing causing localized bleeding and coffee ring effect at the edges of the printed line, respectively. Reprinted with permission from [97]. Copyright (2022) ACS.*

of the drops and their displacing accuracy. Because of this, higher printhead height is correlated with lower printing accuracy (higher prevalence of ink spraying and splatter). An optimum height should be found taking into consideration the type of ink used and the morphology the drops develop during the ejection phase. Some inks, especially metallic nanoparticle-based ones also demand heating of the printhead, as a way of helping the flow of the ink, as this promotes nanoparticle distancing and decreased viscosity, which avoids nozzle clogging [108, 109]. During printing, it is also essential to make sure the distance between the printhead and the substrate remains stable which in some printer models is assured through a vacuum-assisted bed that prevents substrate vibration and displacement [50].

*Voltage Waveform*. The waveform is the parameter that causes the piezoelectric crystal to deform, generating negative pressure at the printhead nozzles, and causing the ink drops to be jetted. To complete the drop ejection process, a bipolar voltage waveform is responsible for the fill/fire pulses (**Figure 9a**). The higher the voltage, the higher the speed of the generated droplet. This occurs because the pressure


### **Table 5.**

*Summary of figures of merit used to classify the quality of the inkjet printing process.*

created inside the chamber is consequently superior [116]. To recreate the desired image, the inkjet printer is connected to adequate computer software that supports the upload of the image and allows for the printing settings to be defined. In the case of the piezoelectric DoD method, a voltage is applied across a piezoelectric crystal under a pre-defined time and amplitude pattern, which generates the voltage

### **Figure 9.**

*(a) Simple depiction of a waveform. (I) Negative pulse that eliminates residual oscillations after each drop ejection; (II) and (III) are the pressurization and ejection phase, respectively; (b) nozzle pressure chamber as the piezoelectric crystal (darker blue) deforms due to step (III) [114, 115]; (c) jet straightness images obtained from a high-speed camera. Reprinted from [50].*

waveform [114]. As depicted in **Figure 9a**, the waveform has four main phases – the damping and relaxation period that prepares the chamber to start the cycle (phase I, or *t*echo), then, the pressurization (phase II) causes the ink chamber and nozzles to fill and defines the volume that will be ejected. During the next phase (phase III, or dwell time) there is a slight pause for stabilization of the ink inside the chamber. The cycle ends in phase IV, with the ejection of the ink droplets due to the pushing pressure caused by the piezoelectric crystal. Depending on the characteristics of the ink, the waveform can be optimized, and the best jetting performance is associated with fast, round, and stable droplets without tails [114]. To achieve this, the total waveform duration, number of phases, and the individual phase amplitude, durations, and slew rates (slopes), can be manually adjusted [114]. Recent inkjet printers are already equipped with waveform tuning ability, and in some cases encompass real-time drop-watcher high-speed cameras that capture images such as the ones in **Figure 9c** [108, 109]. This allows for simultaneous tracking and optimization of the profile of the jetted ink drops.

*Jetting Frequency and Printhead Speed.* The frequency in which the piezoelectric crystal is actuated is intimately related to the waveform and the printhead speed, which can be varied, affecting the rate at which droplets are jetted. If the printhead moves at a low speed the printhead nozzles will be actuated at a lower frequency. By generating waveforms with longer echo and dwell times, the jetting frequency will also be stalled, and vice-versa. As a result, this parameter can be simultaneously studied with the voltage and printhead speed to obtain higher quality printing. As seen in **Figure 10**, the shape of the jetted drops is dependent upon the jetting frequency and varies with the ink characteristic (viscosity, rheology, surface tension).

*Number of printed layers.* It is also a frequently studied parameter, particularly in what concerns its influence on electrical conductivity. To illustrate this, Rihen et al. studied the effects of the number of printed layers (1–5 layers), drop spacing (1016 DPI – 1693 DPI), and curing temperature (75–120°C) in the final conductivity of an Ag ink [89]. In this study, they found that the best conductivity was obtained for 3 printed layers, 1270 DPI, and a curing temperature of 120°C. It was also found that the number of printed layers strongly affected the final conductivity and that when too many layers were printed, excessive ink ejection ended up causing bleeding of the ink during printing and cracks after the heat-treatment was conducted. Hence, when possible, the number of printed layers should be limited to an "optimum minimum"

**Figure 10.** *Jetting behavior with increasing frequency. Reprinted from [117] with permission of AIP publishing.*

as it can end up affecting conductivity and printing quality in a negative manner. Moreover, by finding a compromise between resolution and number of layers the printing process can become more economical in terms of ink used.

*Design-related variables.* The layout of the circuit designs to be printed needs to obey specific rules that depend upon the type of circuit, inkjet printer, printhead, and ink. The process of developing a certain circuit to be printed starts by defining the schematic circuit design in CAD, and after making sure it obeys the rules it is converted to bitmap for printing (**Figure 11a**) [118]. Design-related variables include the minimum resolution, horizontal/vertical line drop spacing, horizontal/ vertical line width, horizontal/vertical line thickness, orientation of the design, and the angles of the line connections and design borders [118]. As depicted in **Figure 11c**, different line spacing between two consecutive lines can cause the lines to overlap. Hence, by managing the way the design is created, different results can be obtained depending on the final objective. Moreover, by anticipating the probability of flooding or coffee ring effect of the ink in specific areas, the design can be manipulated to prevent them through pattern compensation methods, as advanced by Vila et al. [119].

### *2.3.5 Inkjet printing quality indicators*

Fabrication of PE devices using IJP faces a series of challenges for enhancing the technology merit indicators, which are application dependent. These relate mainly to printing quality indicators (printed pattern homogeneity, resolution, consistency), electrical conductivity, mechanical durability, and device flexibility/stretchability. In IJP, the printing quality involves complicated interactions between many factors including the printer, the printhead, the substrate, and the ink [49].

**Figure 11.**

*(a) IJP metal track generation along the x-axis (mask-less): From a CAD layout drawing (a vector file) to a binary plane (a bmp file) and from the binary plane to a physical substrate surface. (b) a unidirectional IJP system. (c) Microscopic images of the two printed lines with varying vector spacings from 190 to 450 μm (scale bar is 500 μm).*

Printed pattern resolution and uniformity are determined by droplet-substrate interactions, ink solvent evaporation rate, and capillary flow inside the ink droplet. The printed lines' dimensions, namely their width, depend upon the drop spacing and coalescing time. There is a relationship between drop spacing, line width, and electrical resistance. The electrical resistance is directly proportional to the drop spacing and inversely proportional to the line width.
