Sol-gel synthesis of transparent conductive oxide film for thin film based solar cells

Abstract

The present work is based on the deposition of thin films of transparent conductive oxide (TCO) films of zinc oxide (ZnO) and aluminum doped zinc oxide (Al:ZnO) using sol gel spin coating method for the application in thin films based solar cells such as Cu(In,Ga)Se2. The precursor zinc acetate dihydrate (Zn(CH3COO)2.2H2O) was dissolved in isopropanol ((CH3)2CHOH) with monoethanolamine ((HOCH2CH2)NH2) added as stabilizer. Various chemical and processing parameters such as coating thickness, additive ratio, sol concentration, effect of annealing and dopant concentrations were investigated in order to find best combination of films regarding their electrical resistivity and optical transmittance. X-ray diffractrometer (XRD) and scanning electron microscopy (SEM) analysis were used to investigate the crystallinity and surface morphology of the films. Hall Effect Measurement System was used to find resistivity, carrier mobility and carrier density of deposited films. Optical transmittance was investigated using spectrophotometer. In case of intrinsic ZnO films, lowest resistivity of 1.97×102 Ω.cm was shown by films grown using 0.2M sol, annealed at 500 ᵒC for one hour which further decreased to 4.06×10-3 Ω.cm when the films were doped with 1 at. % aluminum. Films with dense granular morphology and preferential texturing in (002) direction were deposited. The optical studies reveal absorption edges at ∼370 nm. The optical band gap energy was calculated to be in range of 3.2-3.5 eV. Transmittance was greater than 80% in the visible region for all deposited films.