ABSTRACT

The deposition of undoped and Y-doped CuSe, CdSe and CoSe with different dopant concentration (0.01 – 0.04mol%) and varying substrate temperature (140^oC – 200^oC) have been successfully carried out using spray pyrolysis technique. The effect of the yttrium doping and substrate temperature variation on the optical, electrical and structural properties of all the deposited samples were reported in this research work. For the undoped and Y-doped CuSe, the optical studies revealed an energy bandgap ranging from 3.0eV to 2.5eV and introduction of Y dopant was observed to narrow the energy bandgap of pure CuSe. The electrical studies result supported the fact that the thin films are semiconductors. The XRD pattern reveals that the deposited films exhibited a polycrystalline cubic structure with preferred orientation along (200) and improved crystallinity with yttrium doping. For variation in substrate temperature, the optical analysis showed that the samples have energy band gap value ranging from 1.44 eV to 1.8 eV. Increase in substrate temperature was found to decrease the conductivity thereby increasing the resistivity and thickness. The XRD pattern revealed a polycrystalline cubic structure with most preferred orientation along (111) plane for all the films irrespective of the substrate temperature. YCuSe deposited at 140^oC was seen to exhibit the best crystallinity. In summary, we conclude that substrate temperature of 140^oC is the optimum substrate temperature to grow YCuSe. For the undoped and Y-doped CdSe, the influence of Y-incorporation and variation (0.01, 0.02, 0.03 and 0.04mol%) were clearly seen. The energy bandgap was observed to decrease from 1.71eV for undoped CdSe to 1.41eV for Y-doped. Increase in Y-concentration from 0 to 0.04m0l%, led to a decrease in the resistivity of CuSe with decreasing film thickness and increasing conductivity value. The XRD patterns reveal that all the deposited films are polycrystalline in nature having hexagonal structure with the preferred orientation and highest intensity along the (100) plane. Introduction of yttrium dopant was observed to improve the crystallinity of CdSe. The general result shows that YCdSe films are good materials for the production of photovoltaic cells.  For variation in substrate temperature, YCdSe deposited at 180^oC exhibited better optical property when compared to other films with an energy band gap value of 1.50eV. The electrical studies, shows that increase in substrate temperature of the thin material decreases the resistivity with increasing thickness and increasing conductivity which reveals that the films are semiconducting. The XRD analysis revealed a polycrystalline hexagonal structure with most preferred orientation along (100) plane for all the films irrespective of the substrate temperature.Y/CdSe deposited at 160^oC gave the best crystallinity. For the undoped and Y-doped CoSe, the optical analysis gave moderate (> 50%) transmittance with energy bandgaps ranging from 1.25eV to 1.81eV. Incorporation of yttrium dopant to the CoSe films was observed to enhance the optical features and the electrical studies reveals that the films are semiconducting materials. The XRD result revealed a polycrystalline cubic structure and introduction of higher concentration of yttrium dopant on the films enhanced the crystallinity. For variation in substrate temperature, the optical properties were found to vary with substrate temperature despite the fact that the variation was not totally linear as the film deposited at substrate temperature of 160^oC deviated from the linearity in all the optical properties. The energy band gap of the deposited samples ranges from 1.25 eV – 1.75eV. The electrical analysis revealed that the substrate temperature and the film thickness varies directly with conductivity and inversely with resistivity which is one of the features of a typical semiconductor. The XRD result shows that the films are cubic polycrystalline in nature andY/CoSe grown at substrate temperature of 180^oC gave the most excellent crystalline quality and a preferential orientation along (111) direction. From our findings we observe that these new materials can be used for photovoltaic purposes.