In this article, we present a systematic investigation of the influence of CdS layer thickness on the structural and compositional properties of CZTSSe films and photovoltaic performance of superstrate solar cell devices. The selenization of the CZTS films results in Cd diffusion into the absorbing layer. This leads to a significant change in the lattice parameters and crystallinity, and also causes the inhomogeneous distribution of S and Se ions along the thickness of the CZTSSe layer and formation of CZCTSSe kesterite phase. Alternatively, an increase in Cd content for a certain amount also leads to the enhancement of the device performance parameters of CZCTSSe films which are attributed to the improved crystallinity, morphology and, possibly, graded band-gap. The achieved power conversion efficiency of 3.3% is among the highest for the spray pyrolysis deposited solid-state CZTSSe solar cells of superstrate architecture.