Definitely a particulate contamination problem, either in the substrate or in the solution to be deposited. I suggest to use an adequate surface cleaning immediately before deposition and possibly filtering your solution, you can use a syringe filters, for example.

It is either contamination on the surface of FTO or the organic of pvk is too much,

for the 1st thing, UV-Ozone 10-15 min, or Plasma treatment. for the 2nd try to minimize the organic concentration by 10% then compare the results.