In situ (on devices) force–distance curves acquired during the approach-contact-and-detachment of the a-PVA:Li and PVA:Li films with the reference PE film in their respective TENGs (1-year old samples). FPVA:Li and Fa-PVA:Li refer to the adhesion force measured during contact between the PVA:Li or the a-PVA:Li films with the PE film, respectively.