Theoretical insights into kesterite and stannite phases of Cu-2(Sn1-xGex)ZnSe4 based alloys: A prospective photovoltaic material

A comparative study of kesterite (KS) and stannite (ST) phases of Cu-2(Sn1-xGex)ZnSe4 (CTGZSe) alloys has been carried out using a hybrid functional within the framework of density functional theory (DFT). Our calculations suggest that KS phase is energetically more stable. We find that the total energy of the KS phase decreases with increasing concentration (x) of Ge. The calculated positive binding energies suggest that the alloy systems are stable. The formation enthalpy clearly indicates that CTGZSe alloys are thermodynamically stable and its growth can be achieved by following the route of an exothermic reaction. The calculated energy band gaps of the alloys agree well with the experimental data for the KS phase. The band offsets of KS and ST phases as a function of Ge concentration (x) can be explained on the basis of the calculated energy band gaps. We find a slight upshift in the conduction band edges while the valence band edges remain almost the same on varying the concentration (x) of Ge. Our results could be useful for the development of CTGZSe alloys based solar cells