Ab initio investigation of potential indium and gallium free chalcopyrite compounds for photovoltaic application

The amazing opto-electronic properties of pseudo-ternaries Cu(In,Ga)(S,Se)₂ compounds are, for a great deal, related to their defect chemistry, for which ab initio calculation have already provided new insights. As Indium and Gallium are rare and expensive elements, the search of an alternative may become an important issue. Ab initio calculation of energies and electronic structures of various chalcopyrite type structures replacing In by other elements has been performed exploring over 16 compounds with some 10 different elements of the periodic table. Among the different possibilities, the substitution of In by the isoelectronic couples such as (Zn, Sn) and other cationic couples having an average valency of III have been evaluated in details, as well as the influence of the anion (S or Se). The most important point defects and defect complexes have been explored. Low copper vacancy (V_Cu) and defect complexes (such as V_Cu+Zn_Cu) formation energies have been found in these new compounds: for instance the formation energy of V_Cu in Cu₂SnZnSe₄ is 0.49 eV as compared to 0.48 eV in CuInSe₂ (CIS). In addition to point defect characteristics, different electronic properties (such as band structure, bonding character) are compared to the one of CIS, which is considered as our reference structure. The method used is based on the density functional theory within the framework of pseudo-potentials and plane waves basis. The results are discussed in view of the existing data, models and calculations. Based on these properties as well as the ionic/covalent bonding character some trends are exposed.