The upsurge of absorption coefficient in CuInS₂ thin film with Ru doping: an energetic absorber layer in a superstrate solar cell

In this work, a clear increase in electrical mobility and absorption coefficient has been demonstrated in spray pyrolysis-deposited CuInS₂ (CIS) thin films by ruthenium (Ru) doping. This film was then employed as an efficient absorber layer in superstrate photovoltaic cells for the first time. Ru doping into the CIS tetragonal crystal structure increases the tensile strain causing structural modifications including average crystallite size and dislocation density. Further, the Ru causes an adjustment in the Brillouin zone boundary of the CIS semiconductor leading to alternative morphologies on the surface of the substrate such as particles (2 wt%), rods (4 wt%), and leaf-like mixed nanoparticles (6 wt%). The absorbance and emission properties of this CIS absorber layer were studied for possible enhancement in optical properties due to the surface modification. A higher absorption coefficient (1.20 × 10⁶ cm⁻¹ compared to pristine CIS 0.84 × 10⁶ cm⁻¹) was observed in the Ru-doped CIS film. The possibility for Ru-4d to S-3p transition explains the electronic origin of an enhanced absorption coefficient in Ru-doped CIS thin films. The atomic elements such as Cu, In, S, and Ru and their corresponding 1⁺, 3⁺, 2⁻, and 4⁺ oxidation states remained unchanged following doping illustrating an intact lattice. The Ru-doped CIS films, optimized at 6 wt%, exhibited both higher electrical and photo responses as compared to a pristine CIS film. Solar cells were fabricated using the ITO/AZO/CdS/CuInS₂ superstrate configuration with and without the Ru doping (0 and 6 wt% Ru-doped CIS)/Au showing an increase from 2.84% to 4.46% PCE a 1.62% increase overall. Following this, similar durability over 500 h was observed in both device types.