In this work, we focus on the front and back interfaces of absorber, which, along with the crystalline quality of the absorber, are definitely important in obtaining high efficiency for Cu2ZnSnS4 (CZTS) thin-film solar cells. For the back interface, the ZnS segregation is a well-known problem for formation of CZTS absorbers. The segregation has been believed to cause high series resistant. We found that while the ZnS segregation is hardly dependent on the absorber composition, the sulfurization condition and the precursor stack structure have an effect on it. Furthermore, we found that ZnS segregation at the backside of the absorber could have beneficial effects. For the front interface, the compositions of Cd-, Zn-, and In-based buffer layers were studied by X-ray photoelectron spectroscopy (XPS). The buffer layers consist of mixed chemical states, ie, sulfides, oxides, and hydroxides, however, little is known about how these complex states distribute within the buffer layers. By using Ar+ sputtering in conjunction with XPS, the depth profiles for each chemical state were observed. These results are informative in concerning deposition process. Assisted by these studies, recently we have achieved an efficiency of 9.2% on 5× 5 cm2-sized CZTS submodule with antireflection coating.