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We studied the defect physics in CuInSe 2, a prototype chalcopyrite semiconductor. We showed that (i) it takes much less energy to form a Cu vacancy in CuInSe 2 than to form cation vacancies in II-VI compounds (ii) defect formation energies vary considerably both with the Fermi energy and with the chemical potential of the atomic species, and (iii) the defect pairs such as (2 V Cu−+ I n Cu 2+) and (2 C u In 2−+ I n Cu 2+) have particularly low formation energies (under certain conditions, even exothermic). Using (i)–(iii), we (a) explain the existence of unusual ordered compounds CuIn 5 Se 8, CuIn 3 Se 5, Cu 2 In 4 Se 7, and Cu 3 In 5 Se 9 as a repeat of a single unit of (2 V Cu−+ I n Cu 2+) pairs for each n= 4, 5, 7, and 9 units, respectively, of CuInSe 2;(b) attribute the very efficient p-type self-doping ability of CuInSe 2 to the exceptionally low formation energy of the shallow defect Cu vacancies;(c) explained in terms …