4.1.1 Technology Background

The first introduction of the spray pyrolysis technique by Chamberlin and Skarman [29] in 1966 was for the growth of CdS thin films for solar cell applications. Since then, the process has been investigated with various materials, such as SnO$_x$ [107], In$_2$O$_3$ [180], Indium Tim Oxide (ITO) [138], PbO [106], ZnO [168], ZrO$_2$ [150], YSZ [172] and others [153].

The main advantages of spray pyrolysis over other similar techniques are:

- Spray pyrolysis is cost effective and can be easily performed.

- Substrates with complex geometries can be coated.

- Spray pyrolysis deposition leads to relatively uniform and high quality coatings.

- No high temperatures are required during processing (up to $\sim$500$^o$C).

- Films deposited by spray pyrolysis are reproducible, giving it potential for mass production.

The major interest in spray pyrolysis is due to its low cost, while it is increasingly being used for some commercial processes, such as the deposition of a transparent layer on glass [137], the deposition of a SnO$_2$ layer for gas sensor applications [107], the deposition of a YSZ layer for solar cell applications [172], anodes for lithium-ion batteries [161], and optoelectronic devices [19].

The general simplified scheme for spray pyrolysis deposition is shown in Figure 4.2, where three processing steps can be viewed and analyzed.

Figure 4.2: General schematic of a spray pyrolysis deposition process.

The three processing steps for spray pyrolysis deposition are

1. Atomization of the precursor solution.
2. Aerosol transport of the droplet.
3. Droplet evaporation, spreading on the substrate, and drying and decomposition of the precursor salt to initiate film growth.

These three steps are individually addressed in the sections to follow.