Structural and mechanical properties of pre-strained transparent conducting oxide films on flexible substrate

Among popular transparent conducting oxide (TCO) materials, indium tin oxide (ITO) and aluminum-doped zinc oxide (AZO) are expected to be the most applicable for the TCO layer of solar cells due to their resistance to a reductive ambient and their transparency to infrared wavelengths. In this study, ITO and AZO are deposited by a sputtering system with various pre-strain forces applied to the substrate. According to the X-ray diffraction results, the intensities of the characteristic peaks for ITO and AZO films decrease at a certain pre-strain ratio. Decreases for ITO and AZO films occur at pre-strain levels of 2% and 4%, respectively. Scanning electron microscopy images show that some fissures and cracks appear for the 2%-ITO film. The mechanical properties and the interface adhesion energy between the films and the substrates are investigated using a nanoindentation system. The results demonstrate that the hardness and elastic modulus of ITO and AZO thin films reach a critical point at pre-strain levels of 2% and 4%, respectively. When the pre-strain level is over 4%, cracks appear in the ITO thin film.