Sol–gel synthesis of intrinsic and aluminum-doped zinc oxide thin films as transparent conducting oxides for thin film solar cells

Sol–gel synthesis of intrinsic (i-ZnO) and aluminum-doped zinc oxide (Al:ZnO) thin films was carried out via spin coating process using 0.2 M Zn2 + precursor salt, monoethanolamine to Zn2 + ratio of 0.75, and 1 and 2 at.% aluminum as a dopant. After annealing at 500 °C for 1 h, the structural, compositional, electrical, and optical properties of the films were investigated. Scanning electron microscope studies revealed smooth, dense film microstructure with granular cross-sectional morphology. The average grain size was in the range of 25 to 53 nm, depending on film composition, and indicated reduction in size with more Al incorporation. The processing conditions favored (002) preferential growth in all the films, with degree of preferred growth as high as 0.45, as determined from X-ray diffraction analysis. Al:ZnO film with 1 at.% Al was found to exhibit minimum resistivity value of 4.06 × 10− 3 Ω cm and carrier concentration as high as 5.52 × 1019 cm− 3. From optical transmittance spectra, the absorption edge of the films was determined to be at ~ 370 nm with ≥ 80% transmittance in visible and near-infrared regions of the spectrum. The calculated values of band gap indicated continuous increase from 3.35 to 3.41 eV upon Al doping of the films.

► Sol–gel technique is used to produce intrinsic and aluminum-doped zinc oxide films.
► Doping with Al promotes 002 preferred orientation and decreases surface roughness.
► Films with 1 at.% Al show electrical resistivity of 4.5 × 10− 3 Ω cm.
► Optical transmittance is up to 85% with band gap in the range of 3.35 to 3.41 eV.
► 2 at.% Al in films causes deterioration of electrical and optical properties.