n-Type Doping of CuInSe2 and CuGaSe2

The efficiency of CuInSe2 based solar cell devices could improve significantly if CuGaSe2, a wider band gap chalcopyrite semiconductor, could be added to the CuInSe2 absorber layer. This is, however, limited by the difficulty of doping n-type CuGaSe2 and, hence, in its alloys with CuInSe2. Indeed, wider-gap members of semiconductor series are often more difficult to dope than lower-gap members of the same series. We find that in chalcopyrites, there are three critical values of the Fermi energy EF that control n-type doping: (i) EFn,pin is the value of EF where the energy to form Cu vacancies is zero. At this point, the spontaneously formed vacancies (=acceptors) kill all electrons. (ii) EFn,comp is the value of EF where the energy to form a Cu vacancy equals the energy to form an n-type dopant, e.g., CdCu. (iii) EFn,site is the value of EF where the formation of Cd-on-In is equal to the formation of Cd-on-Cu. For good n-type doping, EFn,pin, EFn,comp, and EFn,site need to be as high as possible in the gap. We find that these quantities are higher in the gap in CuInSe2 than in CuGaSe2, so the latter is difficult to dope n-type. In this work, we calculate all three critical Fermi energies and study theoretically the best growth condition for n-type CuInSe2 and CuGaSe2 with possible cation and anion doping. We find that the intrinsic defects such as VCu and InCu or GaCu play significant roles in doping in both chalcopyrites. For group-II cation (Cd, Zn, or Mg) doping, the best n-type growth condition is In Ga-rich, and maximal Se-poor, which is also the optimal condition for stabilizing the intrinsic InCu GaCu donors. Bulk CuInSe2 can be doped at equilibrium n-type, but bulk CuGaSe2 cannot be due to the low formation energy of intrinsic Cu-vacancy. For halogen anion doping, the best n-type materials growth is still under In Ga-rich, and maximal Se-poor conditions. These conditions are not best for halogen substitutional defects, but are optimal for intrinsic InCu GaCu donors. Again, CuGaSe2 cannot be doped n-type by halogen doping, while CuInSe2 can.