HUMIDITY-DEPENDENT IMPEDANCE AND DFT ANALYSIS OF PURE AND Cu-DOPED SnO2 THIN FILMS

Tin Oxide has been explored for gas sensing and humidity sensing. In the present work, pure and copper-doped SnO2 is synthesized by controlled spray pyrolysis technique. The films are homogeneous throughout. Rietveld analysis confirms the absence of other phases due to doping. The optical property is studied using UV-Visible spectroscopy which shows a change in the band gap with the introduction of the dopant. An elaborate impedance analysis is carried which showed the effect of doping. Cu-doped thin film showed a faster drop in the impedance when exposed to humidity. Significant change in the cole–cole plot is observed indicating better sensitivity with the doped sample in comparison to undoped. Higher humidity level from 80 to 92 RH is studied as it is important to detect the same in air conditioning systems, electronic devices, aviation systems and food processing systems. The equivalent circuit also reveals that the adsorption of water molecules on the surface of the thin films changes the impedance characteristics. Levenberg–Marquardt algorithm is employed for the theoretical calculations and identifying the equivalent circuit. The charge transfer in the doped sample is subjected to lesser grain resistance according to theoretical calculation and experimental results. A density functional approach is employed to study the band structure and explain the influence of Cu doping on pure SnO2. The calculated result supports the use of Cu as a dopant for better humidity sensing device.