n-type doping principles for doping CuInSe2 and CuGaSe2 with Cl, Br, I, Mg, Zn, and Cd

We develop n-type doping principles for chalcopyrite photovoltaic materials, considering donor doping of CuInSe₂ and CuGaSe₂ by halogen (X = Cl, Br, I), and divalent (M = Mg, Zn, Cd) atoms. We determine equilibrium defect concentrations as a function of growth conditions, using defect formation energies obtained by first-principles supercell calculations. We find: (i) In CuInSe₂ under Se-poor conditions, halogen doping does not yield a higher net n-type doping level than intrinsic doping by In_Cu. (ii) In CuInSe₂, divalent doping leads to a higher net n-doping, but a high compensation level is present (iii) In CuGaSe₂, neither halogen nor divalent doping yields net n-type doping under equilibrium conditions, because formation of Cu vacancies pins the Fermi level low in the gap. This pinning of E_F also limits the amount of band bending that can be obtained inside the Cu(In,Ga)Se₂ absorber of a solar cell.