In the present work, sputtered In2S3 buffer layers are deposited on Cu(In,Ga)Se-2 absorbers with no or minimal sputter damage. Buffer deposition at slower sputter rates (0.22 angstrom/s) with H2S as a reactive gas improved the interface quality and uniformity. We obtained crystalline In2S3 films at room temperature with the deposition parameters used in this work. Elemental intermixing effect at the In2S3/CIGSe heterointerface at different annealing temperatures was mapped in 3-dimensions using atom probe tomography (APT). APT results revealed the induced sputter damage during buffer layer deposition, and the effect of diffusion and segregation of elements at the heterointerface. Knowledge of elemental redistribution at the buffer-absorber heterointerface can help better understand the relation of the chemical intermixing with electrical performance of the cell. X-ray photoelectron spectroscopy (XPS) reveal accelerated Cu+ doping of the In2S3 buffer at 225 degrees C. Cu-depletion on CIGSe surface up to a few nanometers improves the cell performance. However, at higher annealing temperature of 275 degrees C, CuIn5S8 and Cu(InxGa1-x)(5)Se-8 phases are formed which is detrimental for cell performance. Na diffusing from the CIGSe absorber segregates at the In2S3/CIGSe heterojunction. This actively suppresses the formation of charged antisites defects, lowering the recombination in the space charged region. Cu-depletion at CIGSe surface, Cu and Na diffusion in In2S3 and passivation of interfacial defect states by Na, giving an efficiency of 13.84% (14.83% with CdS-reference) for sputtered In2S3 buffer layers. Consequently, this work elucidates the chemistry of buried hetero-interfaces and their significance to improve the electrical performance of solar cells.