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

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 (CdCl2) activation while its toxic character demands investigation of alternative eco-friendly compounds. Hence in the present study, Magnesium fluoride (MgF2) activation at 100 degrees C, 200 degrees C and 300 degrees 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 MgF2 activated CdS films are polycrystalline in nature having mixed hexagonal and cubic crystalline phases with (22 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 MgF2 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 x 101-1.3 x 102 omega cm. The intensity of photoluminescence (PL) emission peaks is found to be enhanced and the surface morphology demonstrated grain growth with maximum for 200 degrees 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 degrees C as optical window to photovoltaic (PV) devices.

Automated summary

Cadmium sulfide (CdS) thin films were activated using magnesium fluoride (MgF2) at different temperatures, and the effects of activation temperature on the film properties were studied. The results showed that MgF2 activation improved the crystallinity, optical transparency, and optical band gap of the films, making them suitable for applications in photovoltaic devices.