Structural stability of transparent conductive oxide tantalum doped tin oxide during high-temperature treatment

The transparent conductive tantalum doped tin oxide is a potential candidate for applications in concentrated solar power technology, dye-sensitized solar cells and dynamic random access memories [1], [2], [3]. In all these fields, high-temperature stability in air is mandatory to preserve its functionality. In this work we demonstrate the compositional and structural in-air-stability of SnO2:Ta thin films at 650 °C and 800 °C for 12 hours. While the element composition and optical spectra were unchanged and the X-ray diffractograms revealed the conservation of a single-phase rutile-type crystal structure, some strong Raman lines of SnO2:Ta underwent substantial changes upon tempering. Quantum ab initio calculations of pristine and Ta-doped SnO2 with systematically varied point defects indicated that preferentially Sn vacancies and excess O atoms are responsible for these strong and unexpected Raman signatures. These defects are partially healed during high-temperature exposure, but that does not affect the functionality of SnO2:Ta as transparent conductor under these harsh conditions. This study provides a comprehensive understanding of crystal and defect structure of Ta-doped SnO2 prior to and after high temperature treatment in air for the first time and encourages its application in different fields where high-T stability, transparency and conductivity are required. [1] F. Lungwitz et al., Transparent conductive tantalum doped tin oxide as selectively solar-transmitting coating for high temperature solar thermal applications, Solar Energy Mater. Solar Cells 199, 84 (2019), doi: 10.1016/j.solmat.2019.03.012 [2] R. Ramarajan, et al. Large-area spray deposited Ta-doped SnO2 thin film electrode for DSSC application, Solar Energy 211, 547-559 (2020), doi:10.1016/j.solener.2020.09.042. [3] C. J. Cho, et al. Ta-Doped SnO2 as a reduction-resistant oxide electrode for DRAM capacitors, Journal of Materials Chemistry C 5, 9405-9411 (2017), doi:10.1039/c7tc03467a Financial support by the EU, grant No. 645725, project FRIENDS2, is gratefully acknowledged.