Physical and optical properties of In₂S₃ thin films deposited by thermal evaporation technique for CIGS solar cells

Indium sulphide is an promising buffer material with higher bandgap for CIGS solar cells. Promising results on the optical properties and morphology of deposited In_xS_y were found. In₂S₃ thin films were deposited onto glass substrates using thermal evaporation technique. Indium (In) and sulphur (S) powder were evaporated with different In/S ratio, where the stoichiometry and non-stoichiometry composition of InxSy influenced the optical bandgap and surface morphology. The films were structurally and optically characterized by X-ray diffraction, atomic force microscopy and UV measurements. It has been found that, the optical direct bandgap varied from 2.3 eV to 2.5 eV with the different composition ratio of In_xS_y. The X-ray diffraction data shows that the films have cubic β-In₂S₃ structure onto the glass substrates. AFM images illustrate the surfaces quite smooth and uniform with a low surface roughness. These results can be explained in the practical work as non-stoichiometric composition of indium sulphide may result in different band gaps. Hence, a specific stoichiometric composition which results in the highest band gap is desirable to achieve high efficiency In_xS _y-CIGS solar cell. This is due to the lesser photon loss in the buffer layer as the light passes into the absorber layer. From the fabrication results, numerous influences of In₂S₃ buffer layer are investigated that can be implemented to the fabrication of high efficiency CIGS solar cells.