Understanding grain growth and grain boundary inversion in CdS thin films by CdI2 activation

Abstract

The post deposition halide treatment to Cadmium (Cd) based solar cell devices is recognized as vital step for passivation of grain boundaries and influences the performance parameters drastically. In view of these facts, herein, a concise investigation on the impact of post-deposition Cadmium iodide (CdI₂) treatment on the physical properties of thermally evaporated Cadmium sulfide (CdS) films is carried out at 100 °C, 200 °C and 300 °C temperature. The structural, electrical, optical and topographical properties are explored by using characterization tools concerned. The crystallographic results reveal that the films are polycrystalline in nature having mixed hexagonal (wurtzite) and cubic (zinc blende) phases and preferentially oriented along (220) cubic plane. Augmentation in crystallite size of CdS films from 34 nm to 41 nm is observed with CdI₂ treatment where optical energy band gap is tuned in 2.33–2.39 eV range with treatment temperature. The broad photoluminescence emission peak is achieved for all the CdS films in the spectral range of 660–710 nm which is associated to red emission band where intensity of peak is continuously enhanced with temperature. The electrical study shows Ohmic nature of the as grown and CdI₂ treated CdS films where conductivity is lessened for treated films as compared to the as grown films. Surface topographical analysis reveals to formation of almost nanospherical grains having variable size at different activation temperature. The transmittance of >75 % beyond 600 nm wavelength is observed for CdS films activated at 300 °C. The findings revealed that CdS films treated at 300 °C are suitable to use as window layer in fabrication of Cd based thin film solar cells.