The effect of film microstructures on cracking of transparent conductive oxide (TCO) coatings on polymer substrates

We investigate the effects of film microstructures such as crystallinity and surface roughness on cracking of TCO coated on polymer substrates. Indium tin oxide (ITO), which is the most extensively used TCO for electrodes in flexible electronics, is sputter-deposited on polyethylene terephthalate (PET). Varying thickness (t_f) and deposition condition, amorphous ITO (a-ITO) with different surface roughness (R_rms) and a partially crystallized ITO with similar R_rms to a-ITO of the same t_f are coated on PET. Cracking in ITO under tension is analyzed by uniaxial tension test. The mode of cracking and the fracture strength are related to the crystallinity and R_rms of ITO analyzed by high resolution transmission electron microscopy (HR TEM) and atomic force microscopy (AFM), respectively. Our experimental results reveal that the strength of ITO is significantly improved with crystallinity. The important effect of R_rms on cracking is also clearly revealed. In a-ITO, fracture strength is decreased almost linearly with R_rms that increases with t_f. In addition, cracks are always initiated and propagated along the valleys between protrusions on the surface regardless of crystallinity. Based on our experimental results, a low temperature deposition condition to obtain ITO with a high level of functional and mechanical properties is proposed.