A promising successive ionic layer adsorption and reaction (SILAR) method with a modified sequence has been designed and developed to deposit CZTS thin films with high absorbing characteristics. The influence of copper concentration in the precursor solution on the structural, compositional, morphological and optical properties and on the photoelectrochemical performance of the CZTS thin films has been investigated. The copper concentration in the precursor solution was varied from 0.02 to 0.008 M in 0.004 M intervals with constant Zn/Sn ratio. The films deposited using the optimized copper concentration of 0.012 M exhibit a prominent CZTS phase with a Cu-poor and Zn-rich composition, a dense microstructure and optical band gap energy of 1.43 eV. The device based on a Cu-poor (0.012 M) CZTS absorber layer exhibits the highest current density of 15.23 mA/cm² with a power conversion efficiency of 3.81%.