Performance of CZTSSe thin film solar cells fabricated using a sulfo-selenization process: Influence of the Cu composition

•CZTSSe thin film solar cells were fabricated by sulfo-selenization of sputtered metallic Mo/Zn/Cu/Sn/Cu precursors.•The surface composition of the absorbers was Cu depleted and Zn rich compared to the bulk of the absorbers.•The ideality factors ‘A’ over 2 suggest that increased interface recombination in the devices which reduced the open circuit voltage of the devices.•By tuning the Cu content in the absorbers solar cells with conversion efficiency about 8.03% was obtained.And the best efficiency of 8.17% was achieved without an antireflection coating.