Electrical and optical properties of ultrasonically sprayed Al-doped zinc oxide thin films

Aluminium-doped ZnO (AZO) films were deposited by ultrasonic spray pyrolysis (USP) technique to investigate its potential application as antireflection coating and top contact layer for copper indium gallium diselenide (CIGS) based photovoltaic cells. The solution used to prepare AZO thin films contained 0.2 M of zinc acetate and 0.2 M of aluminium pentanedionate solutions in the order of 2, 3 and 4 at.% of Al/Zn. AZO films were deposited onto glass substrates at different substrate temperatures starting from 450 °C to 500 °C. XRD and FESEM analysis revealed the structural properties of the films and almost all the films possessed crystalline structure with a preferred (0 0 2) orientation except for the 4 at.% of Al. Grain size of AZO films varied from 29.7 to 37 nm for different substrate temperatures and atomic percentage of aluminium. The average optical transmittance of all films with the variation of doping concentration and substrate temperature was 75–90% in the visible range of wavelength 600–700 nm. Optical direct band gap value of 2, 3 and 4 at.% Al-doped films sprayed at different temperatures varied from 3.32 to 3.46 eV. Hall studies were carried out to analyze resistivity, mobility and carrier concentration of the films. AZO films deposited at different substrate temperatures and at various Al/Zn ratios showed resistivity ranging from 0.12 to 1.0 × 10−2 Ω cm. Mobility value was ∼5 cm2/V s and carrier concentration value was ∼7.7 × 1019 cm−3. Minimum electrical resistivity was obtained for the 3 at.% Al-doped film sprayed at 475 °C and its value was 1.0 × 10−2 Ω cm with film thickness of 602 nm. The electrical conductivity of ZnO films was improved by aluminium doping.