Grain growth in thermally evaporated CdS thin films: An approach to MgF2 activation for window layer applications

Abstract

• Grain growth evolution is undertaken in CdS films by MgF 2 activation. • Films are crystallized along cubic phased (2 2 0) plane with increased band gap. • Surface roughness is varied with treatment and films showed Ohmic behavior. • PL peak at ∼685 nm owing to surface states and morphology demonstrated grain growth. • Findings warrant that films activated at 200 °C could be applied as window layer. Cadmium sulfide (CdS) is an attracting window layer material to Cadmium telluride (CdTe) solar cells where pristine CdS thin films consist of colossal amount of native defects and grain boundaries (GBs) which behave like recombination centers and trap states and are major roadblocks in achievement of higher device performances. Typically, the influence of these defects is controlled employing post-deposition Cadmium chloride (CdCl 2 ) activation while its toxic character demands investigation of alternative eco-friendly compounds. Hence in the present study, Magnesium fluoride (MgF 2 ) activation at 100 °C, 200 °C and 300 °C temperature is carried out on thermally evaporated CdS thin films in order to seek viability of other halide composites as an activation agent. The XRD findings show that all the MgF 2 activated CdS films are polycrystalline in nature having mixed hexagonal and cubic crystalline phases with (2 2 0)C preferred reflection where intensity of this orientation is diminished with activation temperature. The crystallite size is observed to be increased from 35 nm to 39 nm, optical transmittance is found to fluctuate and the optical energy band gap of CdS films is enhanced from 2.35 eV to 2.39 eV with MgF 2 activation temperature. AFM images demonstrated hill-like topography wherein grain size is varied from 54 nm to 68 nm with activation temperature. The current voltage characteristics revealed to the development of Ohmic contacts where electrical resistivity is determined in range 4.7 × 10 1 –1.3 × 10 2 Ωcm. The intensity of photoluminescence (PL) emission peaks is found to be enhanced and the surface morphology demonstrated grain growth with maximum for 200 °C activated films. Appearance of Cd, S, Mg and F peaks in EDS patterns validated deposition of CdS films with incorporation of Mg and F into the CdS lattice. Findings demonstrated applicability of activated films at 200 °C as optical window to photovoltaic (PV) devices.