Transparent conductive layers on flexible substrates are important components of today’s optoelectronic technology. They are used in filters for plasma displays, low-e windows, solar cells, etc. At present, in-doped indium oxide (ITO) layers on PET substrate is the predominant transparent conducting oxide film in diverse practical applications. However, ITO is a relatively expensive material because indium is not abundant, but aluminum-doped zinc oxide (AZO) film is emerging as an alternative potential candidate to ITO thin film due to its abundance as a raw material, nontoxic nature, cost-effectiveness, easy fabrication, and good stability in plasma. They have, however, several drawbacks: they exhibit relatively high electrical resistance (sheet resistance, 20-200Ω), considerable emissivity, and significant absorption in the spectral region 1-2μm, in which transition from high transmittance to high reflectance takes place. Furthermore, these films do not block solar thermal radiation (0.7-3μm), which may cause overheating problems to devices such as electro-chromic and photovoltaic devices. On the other hand, ITO/Ag/ITO multilayer films are used to achieve high transparent conducting properties. A thin silver layer of about 10nm thickness is embedded between two ITO layers. The ITO/Ag/ITO film has very low sheet resistance, high optical transparency in the visible range, relatively lower thickness than single-layered ITO film, and better durability than single-layered silver film. In terms of ZnO, which is a wide direct band-gap semiconductor, ZnO has a band-gap energy of 3.37 eV with a binding energy as high as 60 meV at room temperature. ZnO has been applied to various domains for excellent physical and chemical properties, such as piezoelectric sensors, rheostats , gas sensors, semiconductor lasers, and transparent conductive films.
Study of Sputtered ITO Films on Flexible Invar Metal Foils for Curved Perovskite Solar Cells