**5. Conclusions**

564 Recent Advances in Nanofabrication Techniques and Applications

Moreover, the SEM images of a typical printed line as shown in figure 22 also illustrates that nanoparticles are three-dimensionally interconnected with each other, which favorably

Fig. 22. SEM image of continuous silver pattern deposited by multi-nozzle

The main benefit of the electrohydrodynamic atomization in cone-jet mode also known as electrospray deposition can be used for thin film deposition for functional materials on different substrates. The same experiment setup for printing is applicable for the thin film deposition through electrospray. In electrospray mode, the nozzle to substrate distance is larger than the patterning setup; this allows the jet to disintegrate into small monodisperse droplets of equal size due to the repulsive forces in the droplets carrying the charges. These mondisperse droplets landed on the substrate and generate the uniform thin layer of that material. The thickness and area of the layer depends on the flow-rate, distance between

For thin film deposition of CIS (Copper-Indium di-Selenide) through electrospray deposition is performed by using 430µm internal diameter metallic nozzle (Muhammad et al., 2010). Initially the operating envelop for stable cone-jet is investigated for the ink containing CIS nanoparticles. The operating envelop for CIS ink along with different electrohydrodynamic modes is shown in figure 23. For spray purpose higher value of applied voltage is selected. Different experiments are performed by changing the flow rate and also the distance between nozzle and substrate distance. The quality and thickness of deposition of thin layers using ESD depends upon three main factors i.e. distance from nozzle to substrate, spraying time or substrate speed and the flow rate. Therefore as less is the distance between the nozzle and the substrate, higher will be the layer quality and also the layer thickness will be on the higher side and vice versa. Similarly as much is the spraying time, or as less is the substrate speed in this case, the layer quality will be on the higher side, however concerns will be on the thickness of the layer which can negatively affect the device efficiency. Figures 24 present the layer morphology obtained by SEM at a flow rate of 150µl/h and varying nozzle to substrate distances with substrate speed of

electrohydrodynamic inkjet printing (Khan et al 2011)

nozzle and substrate, and also the time of the spray.

affect the electrical conductivity.

**4.4 Thin film deposition** 

Electrohydrodynamic inkjet printing is relatively new but very power tool and process for the direct patterning of the functional materials on substrate. Electrohydrodynamic inkjet

Electrohydrodynamic Inkjet – Micro Pattern Fabrication for Printed Electronics Applications 567

due to accumulation of the nanoparticles on the nozzle opening and cause blocking, which affects the performance of the printing. For micron size nozzle special inks are required

In general, electrohydrodynamic inkjet printing is powerful tools for direct patterning of the functional materials and can be used for high resolution printing. Developing this technology will allow exploring the potential application of electrohydrodynamic printing in high resolution fabrication of electronic devices and appear to be promising direction for

This work was supported by the National Research Foundation of Korea (NRF) grant

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from which printing can be performed.

the future.

**7. Acknowledgement** 

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**8. References** 

printing can be used in continuous mode as well as drop-on-demand mode. The main benefit of the electrohydrodynamic inkjet printing is generation of pattern smaller than the nozzle size, because the printing is performed by pulling the liquid rather than pushing of the liquid, which is limitation in other inkjet technology such as thermal and piezoelectric printing. The advantage of electrohydrodynamic printing over conventional printing is high resolution can be obtained. The other advantage of this direct patterning method is the flexibility in the process in terms of material to be used as well as patterns can be made on different kinds of substrate. In continuous mode the patterning is performed by achieving through thing continuous jet in the stable cone-jet mode, however the thin is difficult to stabilize due to electric field. The drop-on-demand mode can be used as alternative to continuous electrohydrodynamic printing. In drop-on-demand the stable cone-jet is achieved for shorter period of time by applying the pulse voltage. Single nozzle electrohydrodynamic printing has low throughput, in order to increase the efficiency multinozzle printing can be performed as reported. The other advantage of electrohydrodynamic printing is thin film deposition in electrospray mode. Thin films can be deposited on the surface of different substrate without changing the experimental setup. This combination of both the technologies (patterning and thin film deposition) can help in fabrication of the electronic devices such as TFT, OLED or Solar Cells etc. through single technology.

#### **6. Challenges and future trends**

There is lot of research is being performed in the field of the electrohydrodynamic printing by patterning the functional materials even in submicron level (Schirmer et al. 2010). However the functional materials meeting these requirements are a challenge, which can be used for the patterning purpose. The inks containing these functional materials impact the morphology, adhesion, chemical and environmental stability, these factors affect the performance. In addition to this the material used in direct patterning technologies has drawbacks in functional performance such as ion mobility, on-off voltage, threshold voltage and off current. Many researchers are working in the field of chemical and material technologies to overcome these limitations, by introducing the different materials that can be used for fabrication of devices through direct fabrication technology. Inorganic material such as carbon nanotube (single-wall and multi-wall) and metal nanoparticles (silver, gold and copper) based inks and pastes are commercially available for the direct patterning applications. Recently there researches are also working on the organic materials that can be used as the insulation material for electronic devices and also conductive organic material such as PEDOT, which can used for the fabrication of the conductive tracks. Recently composite polymers have received interests in the field of direct patterning material due to improvement in mechanical, thermal, optical and conductive properties. But the major drawback is still the performance of devices fabricated through the direct patterning technology as compared to conventional electronic.

In electrohydrodynamic inkjet printing, for large area manufacturing the design of multinozzle printing head is another bottle neck due to interfering of electric field between nozzles. The miniaturization of the nozzles dimensions cause problems in obtaining the symmetric and stable cone-jet for printing. Other limitation is the fabrication of such small size nozzles. To optimize the performance of small nozzles, the physics and chemistry of the nozzle has to optimize to ensure the printability of different functional materials, with damaging the nozzles. When using the micron size nozzle, the issue of the clogging arises due to accumulation of the nanoparticles on the nozzle opening and cause blocking, which affects the performance of the printing. For micron size nozzle special inks are required from which printing can be performed.

In general, electrohydrodynamic inkjet printing is powerful tools for direct patterning of the functional materials and can be used for high resolution printing. Developing this technology will allow exploring the potential application of electrohydrodynamic printing in high resolution fabrication of electronic devices and appear to be promising direction for the future.
