Effect of the thiourea incorporation velocity and RTA post-deposit treatments, on the properties of CdS films deposited by CBD

Abstract

In this work the influence of the thiourea incorporation velocity, or drip time (td), on the properties of the CdS films deposited by chemical bath deposition technique (CBD) was studied. The thiourea was employed as the sulfur source in the bath solution. The films were deposited on glass substrates keeping the bath solution temperature (Tb) at 90 °C and varying the td from 10 to 60 min. The other reagents used were cadmium acetate, ammonium acetate, ammonium hydroxide and deionized water as an aqueous medium, whose concentrations were constant. The dependence of the thickness with td showed three regions associated with three growth stages: i) induction period, ii) linear growth and iii) saturation stage. The best structural, optical and electrical properties were obtained in the linear growth region (20 min ≤ td ≤ 40 min). These films exhibited a crystallite size around 35 nm and a resistivity value of ∼100 Ω-cm. With respect to their optical properties, an average transmittance value of 70% for λ ≥ 500 nm and Eg of 2.35 eV were obtained. The presence of organic compounds was determined by FTIR spectra. The main organic compound identified was cyanamide, whose concentration was greater and constant at linear growth region. Afterwards, the effect of rapid thermal annealing (RTA) treatments on the properties of the CdS films, was evaluated for three films of the linear growth region. Such treatments did not modify the thickness, crystalline structure and crystallite size of the films. However, there was a change of color from light orange to dark orange, and a shift of Eg to 2.27 eV without the detriment of the transmittance. The CdS optical constants (n, k, ε) obtained from an optical model, confirmed the shift. In addition, photoluminescence spectra after RTA treatment showed a decrease of the defects associated to the radiative bands: green (2.21 eV), red (1.88 eV) and nanoparticles (2.94 eV), this without a change in the orange band (2.04 eV). The cyanamide, main organic compound present in the films, decreased significantly with thermal treatment, producing a resistivity drop from ∼100 to 4 Ω-cm. The drop of resistivity was attributed to the decrease of both, cyanamide and radiative defects concentration. The RTA treatments lead to CdS films with low resistivity and good transmittance, making them appropriate to be used as window material in solar cells.