Band gap change induced by defect complexes in Cu2ZnSnS4

Understanding the impact on the electronic and optical properties of kesterite Cu2ZnSn(S,Se)(4) due to the presence of native defects is crucial for designing and manufacturing efficient solar cells. In this work, we complement earlier published theoretical studies by investigating the change in the Cu2ZnSnS4 band gap energy induced by the dominating isovalent (Zn (Cu) + Cu-Zn) and (Zn-Cu + V-Cu) defect complexes and the corresponding larger [(Zn-Cu + Cu-Zn)+(Zn-Cu + V-Cu)] complex. The study is based on the generalized gradient approximation with corrections for on-site Coulomb interactions (GGA + U) and the Heyd-Scuseria-Ernzerh hybrid functional method (HSE06) within the first-principles density functional theory. We find that (Zn-Cu + Cu-Zn) and (Zn-Cu + V-Cu) affect the band gap energies in an opposite way: (Zn-Cu + Cu-Zn) decreases the gap whereas (Zn-Cu + V-Cu) increases the gap. However, there is a strong correlation between these two defect complexes. The presence of the Cu-poor (Zn-Cu + V-Cu) defect complex lowers the formation energy of the antisite (Zn-Cu + Cu-Zn) defect complex in close configuration, and vice versa. Thereby, and considering the opposite effect on the band gap for (Zn-Cu + Cu-Zn) and (Zn-Cu + V-Cu), our results indicate that the band gap energy will be compensated and stabilized in Cu-poor Cu2ZnSnS4. We discuss the underlying mechanisms behind the band gap physics of the considered isovalent defect complexes. (C) 2012 Elsevier B.V. All rights reserved.