This paper primarily focuses on the performance enhancement of cadmium sulfide/cadmium telluride (CdS/CdTe) solar cells by replacing CdS window with zinc sulfide (ZnS)/CdS bilayer window. The comparative analysis of thermally evaporated CdS and ZnS/CdS heterostructure-based CdTe solar cells was done by employing structural, morphological, optical, and electrical analysis. Core objective of the work was to reduce the absorption loss at window layer by widening the total transmittance of window system for CdTe absorber. In addition, interface behavior was studied using advanced spectroscopic techniques, which help to analyze the diffusion. For the first time, to the best of our knowledge, ultraviolet photoelectron spectroscopy measurement has been employed on all the layers of the present superstrate structure to estimate the energy band diagram. Further, their suitability in a multi-layer graded bandgap structure was correlated to the final device parameters, concerning economic viability. Along with the fill factor increment (55% to 73%) offered by the ZnS/CdS bilayer window, limitation posed by the single-layer CdS window has been discussed. Replacement of CdS by ZnS/CdS bilayer window has enhanced the conversion efficiency from 3.73% to 7.12%.