When you use the PET plastic substrates to coat PEDOT:PSS layer, you need to add some surfactants into the solution of PEDOT:PSS. There are many references you could find. The surfactants could be some SAMs. When you use the PET substrates, there is a trick to put the PET on the glass with certain thickness e.g. 1.1mm thick. You need to drop tiny amount water on the glass, then put the PET plastic on the glass substrate. Normally it stays well during the spin-coating procedure, so you can get similar film quality to your glass substrates. We did a lot of work on such plastic electronic devices. It works well.

Hope it helps.

I have used flexible substrates like PEN not PET, but not bare one, coated with conducting oxide...As Ms Wang suggested to use surfanctant, which can reduce the contact angle and spread well as a film...try to clean the PET surface with ethanol/MEK under sonication and then drop cast PEDOT:PSS, if that spreads well (less contact angle) would get a better coating...if still showing wettability issue, use some other solvents to clean the surface...

PET has a much lower surface free energy (SFE) than glass. This has a number of consequences. Gases and vapours from the atmosphere adsorb much less on PET. While glass usually requires cleaning to remove the layer of adsorbed substances PET usually does not. However, PET films get charged readily and pick up dust. In such a case cleaning is advisable.

Another consequence of the low SFE is that it is only able to rather weak interactions with other substances like your coating solution and your coating. Depending on the solvent of your solution you can have a high contact angle (low wetting) which prevents the film formation. In the case you can obtain a wetting with your solvent the final film does not adhere very well as you observed. In order to improve the adhesion (and in addition the wetting with the solution) you need to “activate” the surface. Activation is a technical term for an oxidation of the surface. It can be done by electrical discharge plasmas (low pressure, corona) or by using chemicals (e.g. sulfuric acid + hydrogen peroxide). You can also apply an UV-ozone treatment. But the latter requires rather long treatment times which might damage the PET film.