Towards halide treatment on CdS thin films for solar cell applications: An evolution to ion size impact on segregation and grain boundaries passivation

Abstract

In this study, an impact of halide ion size on the physical properties of 200 nm thin thermally evaporated CdS films is explored. The ex-situ halide treatment is performed using 0.2 M solutions of halide compounds viz. CdCl2, CdI2, MgF2, MgCl2 and MgI2 separately and subsequently air annealing at 200 °C. A detailed grain growth mechanism in CdS films based upon nature and size of cations and anions to the treatment species is deeply discussed. The recrystallization responsible for modulation in structural, optical, topographical, photoluminescence, electrical, morphological and compositional properties of CdS films is explored via formation of different complexes. The XRD patterns reveal formation of polycrystalline CdS thin films having mixture of cubic (with space group F4̅ 3 m) and hexagonal phases with (220) cubic preferential orientation. Different crystallographic constants are calculated and discussed where the crystallite size is varied from 34 nm to 48 nm with treatment. Surface topographical study indicates that surface roughness is altered with the halide treatment and observed maximum for MgCl2 treated CdS thin films. The pristine and halide treated CdS thin films show Ohmic nature owing to linear current-voltage behavior. Optical study demonstrates that MgI2 treated films have higher transmittance and lower absorbance and energy band gap is measured out in 2.04–2.48 eV range. PL spectra of CdS films reveal a strong red emission band appeared at ∼685 nm corresponding to Sulfur vacancies (VS) and possible A centers (XS+VCd2−)−. FESEM exploration reveals grain growth with probable activation where maximum grain size is obtained for MgF2 treated films. The EDS patterns confirm deposition of CdS films and also integration of F, Cl, I and Mg in the associated treated CdS films. The investigated findings reveal that size of halide ions plays a decisive role in modification of the physical properties of CdS thin films which could be implicated as optical window to CdTe and CIGS based photovoltaic devices.