Production of Al-doped ZnO functional coatings via ultrasonic spray pyrolysis for transparent heater applications

A study was carried out to tailor the properties of ultrasonic spray pyrolysis deposited Al-doped ZnO (AZO) thin films suitable for transparent heater and heat reflecting mirror applications. For this purpose, the influences of precursor and solvent type, spraying solution molarity, critical-doping concentration, nozzle to substrate distance, solution flow rate, carrier gas pressure, deposition temperature, and post-deposition annealing on the structure, morphology, electrical, optical properties, and heating/deicing behaviors of the films were investigated. The optimized samples were produced on 50x75 mm2 soda-lime and Fluorine-doped SnO2 (FTO)-coated glass substrates at 400 °C in open atmosphere. Post-deposition annealing was only preferred for single layer AZO heaters and these samples were annealed under forming gas (Ar+4% H2) flow at 400 °C for 90 min. Single layer AZO (750 nm) transparent heater revealed good electrical properties (sheet resistance value of 38.7 Ω/□ and resistivity of 2.9x10-3 Ω cm) and 83% transmittance in the visible region. However, the reflectance was very low in near-infrared wavelengths. Heating tests showed that the maximum achievable surface temperature was 76 °C under an applied potential of 12 V. The response time, surface temperature uniformity, areal power density, and thermal resistance values were found to be 88 s, 17.7 %, 0.0989 W/cm2, and 409 °C cm2 W-1 for a sample with an active area of 32.5 cm2. Deicing test carried out under 12 V showed that all ice/water can be removed from the surface completely within 250 s starting from a surface temperature of -25 °C. Double layer FTO (550 nm)/AZO (655 nm) heater showed high average transmittance (87%, 400-700 nm) in the visible and reflectance (55%, 2500 nm) in the near-infrared regions. In addition, the sheet resistance and resistivity of the film were measured as 14.85 Ω/□ and 1.78x10-3 Ω cm, respectively. The saturation temperature, response time, surface temperature uniformity, areal power density, and thermal resistance values were found to be 111 °C, 174 s, 11.42 %, 0.299 W/cm2, and 282.8 °C cm2 W-1 for a sample with an active area of 31.5 cm2 and input voltage of 9 V. In addition, dry-ice cooled samples (-25 °C) showed impressive de-icing performance depending on the input power. In case of 12 V, all ice was defrosted, and water droplets were evaporated within 130 s. During this process, a heating rate of ̴ 43 °C/min was achieved. Finally, the study has shown that if properly optimized, solution based-AZO thin films can be produced using the ultrasonic spray pyrolysis method suitable for a variety of smart windows applications including transparent heaters and heat reflecting mirrors.