Effect of annealing on chemically deposited Cu2Se thin films

Abstract

Cu2Se is a promising candidate in the field of photovoltaic and thermoelectric devices. The pristine Cu2Se thin films are observed to be unstable under ambient conditions. One of the instabilities reported in this material is the formation of dendritic structures on the surface. The present work discusses the effect of post-deposition annealing on the dendritic growth formation. The pristine Cu2Se films were vacuum annealed at 60 °C, 80 °C, 100 °C, 150 °C, and 180 °C and the properties of the samples were investigated. The XRD peak profile analysis was done to evaluate the microstructural parameters like crystallite size, microstrain, stress and energy density. The various techniques used for this study include Scherrer method, Modified Scherrer Method, Williamson-Hall Plots, and Size Strain Plot. The modified variants of W–H Plots were employed which included Uniform Deformation Model, Uniform Stress Deformation Model and Uniform Deformation Energy Density Model. In all these cases, the crystallite size was found to increase on annealing. This is attributed to the reduction in defects and dislocations due to annealing of the sample. Microstrain is also observed to increase with the increase in annealing temperature. The calculation of crystallite size and lattice strain using UDM model deviates more from other models. This may be due to the incompatibility of its basic assumption of isotropic strain in UDM model. The W–H plots also suggest the presence of tensile strain in the lattice. The Cu ions in pristine Cu2Se thin films are in a disordered state due to their high mobility and diffusion coefficient. These mobile Cu ions form the centres of anisotropy and participate in dendritic growth formation with suitable stimulants. With an increase in annealing temperature, these ions get ordered due to strain hardening which in turn stabilizes the Cu2Se structure. The minimum temperature required to resist dendritic growth formation was found to be 150 °C. Thus annealing of pristine sample is suggested as a simple but effective method towards stabilization of the matrix. During the annealing process in the temperature range 60 °C–180 °C, it is observed that the optical band gap decreased from 2.2 eV to 1.9 eV.