Understanding thermo-mechanical durability of nanostructured silver and metal oxide composite on PET

In flexible electronics, the substrate's durability during device processing and operation is critical to ensure good performance and long-term stability. In this article, we aim to understand the difference in thermo-mechanical durability of a transparent conducting electrode (TCE) made on polyethylene terephthalate (PET). The TCEs are composed of layers of silver (Ag) nanowires and indium zinc oxide on a PET substrate containing Ag metal bus lines. TCEs made on all substrates show similar optical, electrical, chemical, and physical properties; however, some TCEs are durable, i.e., maintain conductivity and structural integrity, during device fabrication, while others exhibit catastrophic cohesive failure and become non-conductive. To investigate whether these observations are linked to the surface properties of the PET substrates, surface-sensitive characterizations, including contact angle, ellipsometry, Fourier transform infrared spectroscopy in total attenuated reflection mode, and atomic force microscopy, are employed. Our results show that TCEs made on the bare PET side of the substrate are thermo-mechanically durable, while those fabricated on the primer side always fail when heated during device fabrication. Adhesion of TCE materials to bare PET is found to be stronger than to the primer side. Thus, realizing functional flexible electronics on plastic substrates requires a holistic design of substrate and inorganic thin film stacks, so that the thermal requirements for device fabrication and operation are met.