To minimize carrier recombination at interfaces and maximize the open circuit voltage and resultant power conversion efficiency, it is imperative to characterize and optimize the heterointerfaces of Cu(In,Ga)Se₂ (CIGS) thin film solar cells. In this study, we investigated the surface chemical and electronic structure of CIGS with In-final and Ga-final co-evaporation processes and observed that the In-final CIGS contained larger grains and showed a slightly larger bandgap than the Ga-final CIGS. We also analyzed the band alignment of the heterointerface consisting of the Zn(O,S) buffer layer/CIGS deposited by atomic layer deposition using X-ray photoelectron spectroscopy. We found that a S stoichiometric concentration of less than 20% in Zn(O,S) is essential to achieve a power conversion efficiency (PCE) of ∼16.7% with conduction band offsets at 0.53 eV with 14% of S content. However, when the S content exceeded 20%, no PCE was obtained with conduction band offsets at >1 eV. Therefore, this study suggests that an appropriate S content in the Zn(O,S) buffer layer is a key factor for achieving improved PCE in Zn(O,S)/CGIS thin film solar cells because the carrier injection at this interface is very strongly affected by the interfacial conduction band offset.