You can use many polymers, preferably ones which do not out-gas (in case of vacuum deposition). PET is one of the most popular and can display particularly good adhesion for films such as ITO etc...
some people use Kapton (a polyamide film from DuPont). It can be nice for vacuum deposition techniques, as it is quite temperature stable for a flexible material.
You can use PET or Kapton they are both suitable as a substrate for thin film deposition. However Kapton polymer substrate has some captivating physical properties among which we can itemize:High thermal stability up to 650K (compare to PET ), Low electrical conductivity, chemical inertness