Synthesis and characterization of cadmium selenide (Cdse) and lead sulphur selenide (Pbs1-Xsex) thin films by chemical bath deposition method

Cadmium selenide (CdSe) and lead sulphur selenide (PbS1-xSex) thin films have been successfully deposited by chemical bath deposition technique (CBD). The films were characterized by powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and optical absorption spectrophotometer. Cadmium selenide thin films were deposited from chemical bath containing cadmium acetate, tartaric acid, ammonia and sodium selenosulphate, at a bath temperature of 85 0C. The films were annealed in air for 1 hour at 350 0C. The as-deposited films were redishe while the annealed films were dark grey in color. The films were smooth, well adherent and specularly reflective. For the as-deposited films, the XRD analysis revealed a uniform cubic phase of CdSe nanocrystalline thin film with preferred orientation along the (111) plane. The SEM micrographs showed that the film surface was composed of spherically shaped grains. The elemental analyses of the films confirmed that the atomic percentage of Cd:Se is very close to one, indicating that the synthesized thin films were in the desired stoichiometric ratio. The effects of annealing on structural, morphological and optical properties of CdSe thin films were studied. From the XRD analysis it was observed that the annealed CdSe thin film consisted mainly of the cubic CdSe phase. After annealing a new peak appeared at 67.10 and a slight shift in peak positions towards smaller 2θ angles was observed. The average crystallite sizes were found to increase from 40 Å to 45 Å whereas dislocation density and microstrain decreased after annealing. Texture coefficent result revealed that the crystallites were oriented along the (111) plane for both as-deposited and annealed CdSe thin films. Optical studies revealed that the as-deposited sample have direct band transition whose value decreased from 1.86 eV to 1.74 eV after annealing. The variation of optical band gap was due to the change in the crystallite size. PbS1-xSex nanocrystalline thin films were vi synthesized by CBD technique for the entire compositional range of (0 ≤ x ≤ 1). The thin films were deposited from baths containing lead acetate, thiourea, sodium hydroxide, ammonia and sodium selenosulphate, at a bath temperature of 90 0C. The XRD analyses revealed that all the PbS1-xSex thin films were polycrystalline in nature with the diffraction peaks indexed to the face centered cubic structure. The XRD analyses showed that addition of selenium (Se) in PbS shifted the peak positions slightly towards smaller 2θ angles and modified the peaks intensity. The XRD studies also showed that varying the concentration of Se in PbS1-xSex thin films strongly influenced the preferred orientations of the crystallites as well as structural parameters such as average crystallite size, d-spacing and crystal lattice parameter. The average crystallite size was found to increase with Se concentration. The lattice parameter a (Å) and inter planer distance d (Å) increased linearly with the gradual addition of Se in PbS1-x Sex thin films, this result is consistent with Vegard‘s law and supports the formation of a continuous series of solid solution. The EDX analyses confirmed that the films were consistent with the formation of binary and ternary compounds of PbS1-xSex on silica glass slide. In addition, it was observed that the molar percentages of selenium in the as-deposited PbS1-xSex thin films had a good correlation with the calculated molar percentages of Se in PbS1-xSex chemical baths. The SEM micrographs of all the films showed uniform surface morphology over the entire glass substrate without defects like cracks, peeling or pinholes. The films showed a gradual increase in absorbance as Se concentration increases. The optical band gap, calculated from optical absorption of ternary PbS1-xSex samples decreased almost linearly from 1.32 eV to 1.08 eV as Se concentration increased. This linear variation with Se composition revealed that the optical band gap of the PbS1-xSex thin films can be controlled in an optimal region for photovoltaic absorbance as well as for other optoelectronic devices.

Source: KNUST