Parameters Considered During Spray Coating

The characteristics of the films deposited by the spray coating technique depend on a number of factors [166] that include but are not limited to solvent properties, distance between the nozzle and the substrate, the carrier gas pressure, substrate temperature and the solution concentration. These parameters are controlled by the user to optimize the performance of the resulting films and more detailed discussion is given in sections 4.3.2.1 to 4.3.2.7. It is also important to note that it is a trial procedure: to optimise the parameters the user has to play around with coating solution concentration, substrate to nozzle distance etc.

4.3.2.1 Distance between the Substrate and the Airbrush Nozzle (h)

This is a parameter that determines the droplet drying time during flight from the airbrush nozzle to the substrate and was studied in detail by others [163, 166]. They found three regions that exist between the substrate and the airbrush nozzle that affect the properties of the deposited film. The guiding principle in determining this distance is the boiling point of the solvent used to process the coating solution and is worked out from experience / trial rather than calculation. The region close to the nozzle from the substrate is referred to as the wet region, and the coating solution is deposited as a

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solution not droplets. This formed a rough film with many pin-holes and low performance. However, when the substrate is too far from the nozzle, the fine solution droplets dry up before being deposited on the substrate, only “dust” reaches the substrate. They found that a good film was form in the intermediate region with a direct correlation between the film thickness and the spray time. Quantitative examples from previous work demonstrating the concept of substrate to nozzle distance are given in table 2.

4.3.2.2 Concentration of the Coating Solutions

Different concentrations (c) of the coating solutions produced varied film thickness when other parameters are kept constant. An increase in solution concentration results in an increase in film thickness and vice versa. However, as far as sensitizers are concerned, this depends largely on the individual sensitizers. For instance, we found that 50 µM solution of PAN [69], 50 µM solution of Zn(P-CO2H-TPP) [68] and that of

free base EHO (914 µM), all in chloroform solvent, sprayed at a substrate distance of ≈ 7 cm yields a film thickness that is not larger than maximum penetration depth of the evanescent wave. We used 100 µL of each of this solution in a feed-cup of the airbrush.

4.3.2.3 The Carrier Gas Pressure and the Resulting Droplet Velocity

The carrier gas pressure p controls the deposition rate [166] and directs the coating solutions from the feed cup through a needle into the nozzle and a typical value is 20 psi [167]. The velocity of the droplets determine the volume of the solution deposited on the substrate at a given “puff” and hence the film thickness. Quantitative examples from previous work demonstrating the concept of carrier gas pressure are given in table 2. Higher droplet velocities lead to more energetic droplets (kinetic energy = ½ mv2) where v is the droplet velocity and according to Weber number in Eq 4.2, the droplet will overcome the surface tension easily and spread into a film.

4.3.2.4 Spraying Time (t)

At optimized substrate to nozzle distance and other parameters namely (volatility of solvent, ambient temperature, concentration, carrier gas pressure, nozzle diameter) kept constant, the spray time is correlated to the effective film thickness. Thicker films are

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deposited with longer spray time than those deposited with a short spray time [166]. The number of ‘puffs’ from the airbrush determines the film thickness. Many ‘puffs’, resulting in more droplets and hence more solution volume deposited on the substrate.

Table 2. Practical examples of deposition parameters in spray coating process

4.3.2.5 Nozzle Diameter (r) and the Resulting Droplet Velocity

The droplet velocities increase with decreasing nozzle diameter, thus a suitable nozzle diameter is chosen that result in suitable exit droplet velocities, which reduces the sticking of coating solution at the nozzle. On the other hand, too low exit velocities create discontinuity in the coating solution jet, leading to uneven film thickness. Too high exit velocities, results in an increased spray density, leading to an over-wetting of the substrate.

4.3.2.6 Deposition Temperature (T)

The substrate temperature and that of the environment near the spraying area affect the quality of the film formed. In hot substrates, solvent evaporates very fast leading to short drying time of a given volume of droplet, which sometimes may affect the formation of uniform film. This is due to the fact that droplets do not have enough time to merge on the surface of a substrate to form a uniform film. For a substrate at room temperature ie 25 0C, the evaporation of the solvent from the film will depend on the solvent boiling point and this may allow a film enough time to form.

4.3.2.7 Solvent, Surface Tension and Volatility

In spray coating, the basic concept is to choose a solvent with low boiling point at room temperature (ie a fast drying solvent [163]) to avoid solution droplets from re-

Solvent Ingredients Spray

distance (cm) Carrier gas pressure ref Dichlorobenzene/chl orobenzene mixture 16 mg of P3HT/11 mg of PCBM per mL 17 12 psi 162 Dichlorobenzene 2.5 mg of P3HT/5 mg of PCBM per mL 15 - 163 Chlorobenzene 15 mg of P3HT/7.5 mg of PCBM per mL 20 0.1 MPa 166 Dichlorobenzene 15 mg of P3HT/15 mg of PCBM per mL 10 20 psi 167 Dichlorobenzene and p-xylene 2 mg of P3HT/2 mg of PCBM per mL (1:1 ratio) 5 - 170

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dissolving the first deposited layers. Nevertheless, it should also not dry too fast to allow good quality films free of pin-holes to form [163]. The surface tension of the coating solution depends largely on the solvent used. Solvents with low surface tensions and viscosities are preferred to allow easy break up of spraying solution jet into droplets as described quantitatively with Renold, Weber and Ohnesorge constants discussed in section 4.3.3. The lower the surface tension, the larger the droplet spread when it lands on the substrate and the better wet the surface. The properties of the coating solution which include viscosity, solvent boiling point and solvent mixture [163] have an effect on the film morphology.

4.3.3 Renold (Re), Weber (We) and Ohnesorge (Oh) Numbers