The formation energies of neutral Cu, Zn, Ge, and Se vacancies in kesterite-type Cu 2 ZnGeSe 4 were evaluated by first-principles pseudopotential calculations using plane-wave basis functions. The calculations were performed at typical points in Cu-(Zn 1/2 Ge 1/2 )-Se and Cu 3 Se 2 -ZnSe-GeSe 2 pseudoternary phase diagrams for Cu 2 ZnGeSe 4 . The results were compared with those for Cu 2 ZnSnSe 4 , Cu 2 ZnGeS 4 , and Cu 2 ZnSnS 4 calculated using the same version of the CASTEP program code. The

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... results indicate that Cu vacancies are easily formed in Cu 2 ZnGeSe 4 under the Cu-poor condition as in the above compounds and CuInSe 2 , suggesting that Cu 2 ZnGeSe 4 is also a preferable p-type absorber material for thin-film solar cells. The formation energies of possible antisite defects, such as Cu Zn and Cu Ge , and of possible complex defects, such as Cu Zn +Zn Cu , were also calculated and compared within the above materials. The antisite defect of Cu Zn , which has the smallest formation energy within the possible defects, is concluded to be the most hardly formed in Cu 2 ZnGeSe 4 among the compounds.