The Sol-gel process is an attractive method for thin film deposition because of the easy control of the composition and fabrication of large-area films with low cost. Several paper deal with doping and codoping (you may see "J. Alloy. Compd. 512 (2012) 171-178").

The bandgap is similar to the gap of ZnO films produced by other techniques and can be adapted. The optical transmission normally is >80%. The values for specific resistivity do vary from around 10^-2 to 10^8 Ohm.cm. So depending on the sols used, the temperatures and also the dopands, these films are comparable to ZnO films prepared by other established methods.

To my opinion the main challenge(s) is(are) the specific resistivty in combination with a suitable crystallite size and electron mobility.

Thank you, Ivo. I see you are from Germany, so congrats for winning the World Cup! I ask this questions because I would like to know how sol-gel AZO films are compared with sputtered or CVD-deposited AZO films, especially in the application of transparent electrodes in solar cells.

I see that you have a couple of great publications on CVD ZnO. I agree with you that the resistivity and electron mobility are definitely the main issue. Except for the influence of crystallite size on the electron mobility, does crystallite size itself affect the performance of AZO films as transparent electrodes for solar cells? For example, in the light of stability, etching characteristics, etc? In general, is larger or smaller crystallite size better? How is the optical transmission of the best sol-gel AZO films compared with sputtered or CVD AZO? 

Any input to any of the above questions is appreciated.

There is always trace amount of organic residues in sol-gel deposited AZO. Therefore, although the process is cost effective and more Al can be effectively doped in ZnO lattice with sol-gel method, the resistivity is one to two order of magnitude higher than sputtered ZnO. Also, the required relatively high calcination temperature in sol-gel deposited films can potentially deteriorate pre-deposited layers depend on solar cell type. In terms of transparency, the thickness should be raised to obtain lower resistivity, which can strongly affect the transparency of the film. However, in our previous work, the transparency was not affected even in 500 nm thick film.

Thank you, Dr. Golobostanfard. Do you know whether sol-gel AZO have already been used as electrodes in solar cells? If not, are they used as a conductive coating in other industrial applications (oven window, display, electrochromic window, etc) at all? What are the other non-piezoelectric industrial applications of sol-gel zinc oxide films in general? I'm just trying to determine if sol-gel ZnO films are used in industry at all and their feasibility if not yet.

In addition to other answers, ZnO reacts with both Acids and bases, so its lithography and etching process are sensitive and challenging. Its adhesion to substrate is low.