In2S3 as an alternative Cd-free buffer in Cu(In, Ga)Se-2 (CIGS) solar cells was deposited on CIGS substrate by a chemical bath deposition and characterized after post annealing to optimize film properties for CIGS solar cells. A uniform and pinhole-free In2S3 film was deposited on a CIGS substrate by H2O2 treatment prior to chemical bath deposition. The In2S3 layer was an amorphous state due to the co-existence of In -S, In-O, and In-OH bonds. Annealing at 200 degrees C induced copper diffusion from CIGS into In2S3 layer and lowered the band gap from 3.3 to 1.9 eV, leading to phase change from amorphous state to crystalline state. The conduction band alignment at the In2S3/CIGS interface can be controlled by the post annealing. The shunt current through In2S3 film was prevented down to the thickness of 30 nm and a 1.15 eV shallow defect was eliminated by the annealing. The results indicated that post annealing in air is a critical to fabricate CIGS solar cells with a sub-30 nm CBD-In2S3 buffer layer.