ZnO:Al is a transparent conductive oxide and a very important component in flexible devices application such as solar cells. Moreover, the surface characteristics of polymer are very important factors to determine the interfacial properties and control the thin film technological performance. This work studied the feasibility of polyethyleneterepthalate (PET) surface modifications using different Argon plasma treatment. This method can be considered as an alternative process to modify the functional properties of ZnO:Al thin films deposited on top. The surface morphology of both PET and ZnO:Al films was evaluated by atomic force microscopy (AFM) using Kelvin Probe tip from NEXT-TIP. The results showed physically stable ZnO:Al films with mechanical stability and good adherence to the substrate using appropriated plasma treatment conditions. Finally, the performance of flexible solar devices were prepared to demonstrate the potential applications of the optimized ZnO:Al thin films with suitable optoelectronic and adhesion properties.