We investigated defect formation energetics at the grain boundary (GB) in CuInSe 2 (CIS) using first-principles calculations. We focused on the (112) ) were investigated in the bulk and GB. We found that CIS shows characteristic atomic relaxation after vacancy formation, and V Cu 0 is the most energetically favorable defect in both the bulk and the GB. Furthermore, we found that (112)[1 10] â3 twin GB does not promote the formation of the Cu vacancy under all conditions, whereas it relatively promotes the formation of the Se vacancy under metal-rich conditions.