The effect of tin doping on the structural optical and electrical properties of copper oxide thin films

In this research, copper oxide (CuO) thin films doped with various elements were prepared on glass substrates using the spin coating technique, followed by annealing at 500 °C in an air environment. The primary aim was to explore how the introduction of dopant materials influences the structural characteristics of CuO films, with a particular emphasis on parameters such as crystallite size, preferred orientation, microstrain, and dislocation density, assessed via X-ray diffraction (XRD) analysis. The XRD results confirmed that all samples, whether doped or undoped, exhibited a polycrystalline structure consistent with the monoclinic phase of CuO. The incorporation of dopants led to subtle shifts in both the positions and intensities of the diffraction peaks, suggesting alterations in lattice parameters, the development of internal stresses, and modifications in the dominant crystal orientation. Furthermore, variations in peak sharpness and intensity pointed to significant effects of doping on crystal growth behavior and the overall degree of crystallinity within the films. Optical absorption measurements demonstrated that the undoped CuO thin film featured a prominent absorption peak around 374 nm. With the introduction of 1 mol% samarium (Sm) as a dopant, a slight redshift of the absorption edge to approximately 377 nm was detected, suggesting a reduction in crystallite size—likely attributed to quantum confinement effects. These absorption features imply an increased level of structural order within the nanostructures, which can be linked to donor–acceptor interactions among metal ions. Such interactions are known to significantly influence the electronic configuration, resulting in size-dependent quantum effects associated with interband transitions. In the ultraviolet region, a decrease in overall absorption was observed for both undoped and Sn/Sm-doped CuO films, along with the appearance of a weak absorption peak. The diminished absorption intensity observed in the undoped CuO sample may be attributed to structural defects arising from newly formed intermolecular bonds between cations and anions, which can interfere with electronic transitions. In all samples, a pronounced decline in absorption with increasing wavelength was evident, reinforcing the conclusion that dopant incorporation significantly alters both the structural characteristics and optical absorption behavior of CuO thin films. The optical, structural, and morphological characteristics of undoped, Sm-doped, Sn-doped, and Sm/Sn co-doped CuO thin films were systematically examined. The films were fabricated on glass substrates via the spin coating method. The introduction of Sm, Sn, and combined Sm/Sn dopants was found to notably influence the optical absorbance of the CuO films, enhancing their potential for solar cell applications. Key optical parameters—including the refractive index, high-frequency dielectric constant, and static dielectric constant—were calculated to further understand these effects. These values were derived using established theoretical models, specifically the Moss relation, the Herve and Vandamme approach, and the Ravindra model. Notably, the calculated values obtained from the various theoretical models showed strong agreement, confirming the consistency and reliability of the optical analysis. Among all the compositions investigated, the undoped CuO thin film exhibited the highest absorption coefficient within the visible spectrum—an important result, as a higher absorption coefficient enhances light harvesting, which is critical for the performance of solar cells. Additionally, the undoped film demonstrated a greater skin depth at higher band gap energies, indicating favorable behavior for specific optoelectronic applications. Moreover, it showed the highest optical conductivity in the wide band gap region, a property that is particularly advantageous for devices such as solar cells and photodetectors. The superior optical performance of the undoped CuO thin film highlights its potential as an effective active (absorber) layer in solar cells and photodetectors. Its enhanced absorption capability and high optical conductivity suggest improved solar energy conversion efficiency, aligning well with the increasing demand for sustainable energy technologies. Overall, the findings of this study indicate that doping CuO thin films with Sm, Sn, and their combination is a promising approach to tuning and enhancing the material's optical properties. Such modifications could significantly expand the applicability of CuO in next-generation solar cell devices and other optoelectronic applications. The influence of samarium and tin doping at varying concentrations on the surface morphology of CuO thin films was examined through scanning electron microscopy (SEM). SEM analyses were conducted on undoped CuO, 1% Sm doped CuO, 1% Sn doped CuO, 1% Sn+1% Sm doped CuO, and 1%Sm+2% Sn doped CuOfilms, all fabricated via the spin coating technique. The results revealed that the undoped CuO, 1% Sm doped CuO, 1% Sn doped CuO, and 1%Sm+2% Sn doped CuOfilms exhibited smooth, uniform surfaces with strong adhesion to the glass substrates. Notably, the CuO-3 film, containing 1% Sn dopant, displayed a densely packed surface morphology with an enhanced nanoparticle coverage, indicating improved film compactness. These morphological features were found to significantly influence key electrical properties, including resistivity, carrier concentration, and mobility. Furthermore, slight agglomeration and the coexistence of polygonal and quasi-spherical nanostructures were observed in the CuO and 1% Sm+2% Sn doped CuOfilms, suggesting a degree of heterogeneity in nanoparticle shape and distribution.