**4.4 Thin film deposition**

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

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 0.5mm/s. As clear from the figures, the best layer quality is achieved at a nozzle to substrate distances of 6 mm and 8 mm the dense layers is produced and particles are completely intact and no pores or islands are visible as in the case of nozzle to substrate distances of 13mm the layer is porous and contain voids. These pores will case defect in the functionality of the deposited layer.

Fig. 23. Operating envelope for the CIS ink, showing different modes (Muhammad et al., 2010)

Fig. 24. FE-SEM micrographs of the deposited layer at stand-off distance of a. 6 mm, b. 8 mm, c. 10 mm, d. 13 mm, at substrate speed of 0.5 mm/s and flow rate of 150 ml/h (Muhammad et al., 2010)
