Effect of Al doping on structural, morphological, optical, and electrical properties of Cu 2 ZnSnS 4 thin films prepared by sol-gel spin coating

Preparation of aluminum (Al) -free and Al- doped copper zinc tin sulfide (CZTS) thin films have been reported using low-cost, low-temperature (280 °C) sol-gel spin coating method without sulfurization. Effect of Al doping on structural, morphological, optical and electrical properties of the films have been presented. It has been found that both Al-free CZTS thin film sample (S0) and Al-doped CZTS thin film samples with 1% (S1), 3% (S2) and 4% (S3) show kestrite, polycrystalline tetragonal structure using X-ray diffraction and confirmed with Raman spectroscopy measurements. Scanning electron microscope (SEM) images show that the undoped sample has 3D architecture structure with porous flake-like surface, while the doped samples are smooth and with less porous structure. Energy dispersive spectroscopy results have been presented for reporting chemical composition of the constituting elements. Effect of annealing at two different temperatures of 350 °C and 400 °C for further enhancement of film crystallinity has also been presented. Interference fringes have been observed in the reflectance spectra, which indicate the good optical quality of the films. Optical properties of undoped and Al- doped samples have been reported. It has been found that decrease in the optical bandgap energy from 1.89 eV (undoped sample) to 1.73 eV at 1% Al doping, 1.69 eV at 3% Al doping, and 1.68 eV at 4% Al doping occurs, which is desirable for solar cell applications. At 350 °C, the resistivity of 3% Al doped film (S2) has the lowest value, 9 × 10⁻² Ω cm, compared to the other samples. On the other hand, at 280 °C, the resistivity of the film increases from 1.14 × 10⁻¹ Ω cm for the undoped film, to 1.45 Ω cm for 1% Al doping, 0.877 for 2% Al doping, 0.29 Ω cm for 3% Al, and 2 Ω cm for 4% Al doped sample.