Fabrication and characterization of Si doped ZnO and Cu2P2O7 thin films

Transparent conductive oxides (TCOs) are a group of materials that have good electrical conductivity, high visible and low near infrared (NIR, 790–2500 nm) transparencies. They are widely used in flat-panel displays, thin film solar cells and low-emissivity windows. The current material that dominates over 90% of the TCO market is tin doped indium oxide (ITO), which has superior electrical and optical properties. However, the two issues of ITO are the high cost of indium as well as its incomplete coverage of blockage in the NIR range, especially in the 790–1100 nm range that contains nearly 60% of the NIR solar irradiance. For decades, ZnO has been considered as a potentially low-cost replacement for ITO. But even after years of empirical explorations, the conductivity of the doped ZnO films are still not as good as ITO in many cases. The lack of fundamental studies such as the understandings on the relationships between processing conditions and the microstructure and the chemistries of the films is believed to be an important factor that limits further optimization. In this study, a variety of advanced microscopy techniques such as texture X-ray diffraction, transmission electron microscopy and transmission Kikuchi diffraction were used to study the Si doped ZnO films deposited and processed under different conditions. The results suggest several possible reasons for the large improvement of conductivity after the vacuum annealing and provide a few ideas on avoiding the defects observed in the thin films for future research. In terms of the NIR properties, a more complete coverage of blockage was achieved by the production of Cu2P2O7 thin films, which shows an absorbance of up to 66% of solar irradiance in the 790–1100 nm range while maintaining a good visible transparency (85% of transmittance at 550 nm). Given the little research reported on Cu2P2O7, here for the first time a complete route from the analysis of powder, the production of sputtering targets to the deposition of thin films as well as further analysis on the structure and chemistry of the films is presented. These films are suitable for making low-cost and highly efficient inorganic NIR coating, which can be used in many applications such as energy efficient windows and electromagnetic shielding devices.