Effects of thermal treatment and layers' number on SnO2 thin films properties prepared by sol-gel technique

Tin oxide layers are very important in photovoltaic applications. This paper presents an in-depth study of the SnO2 thin films deposited by the gel dip coating technique on glass substrates. The heat treatment and the number of layers effect on structural, morphological, optical and electrical properties were studied. Differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) showed that SnO2 crystallized at 402.5 degrees C. The diffraction spectra x-rays revealed that a formation of tetragonal cassiterite structure with a preferred growth orientation according to (101) plane. Exceptional film prepared at 200 degrees C preheated temperature is according to the (110) plane growth orientation. The size of the crystallites is strongly influenced by the preheated temperature, annealing temperature and a number of layers. The AFM images show homogeneous surfaces for all samples except the film prepared at a preheated temperature of 200 degrees C. However, it becomes smoother by increasing the number of layers. UV-Visible spectroscopy has shown that greater transparency is found in the infrared and visible domains (75%-96%). For all films, the optical band gap energy varies randomly with the preheated temperature; conversely, it increases from 3.83 eV to 3.88 eV for three-layer and six-layer films respectively. The energy gap decreases for samples with more than 6 layers. The minimum resistivity values are 8.1 10(-1) Omega cm determined by the four probe method for the sample preheated to 400 degrees C without annealing temperature.