Functional Off-stoichiometry in Cu(In,Ga)Se2. Part II: Electronic Properties in a Wide Range of Compositions

Abstract

Off-stoichiometry in Cu(In,Ga)Se₂ (CIGSe) is a critical materials characteristic that has resisted a proper understanding due to incompatible models and conflicting interpretations. In Part I of this study, we reported a discovery of the topotactic transformation series of stable ordered defect compounds that properly explain the extension of single-phase Cu-deficient CIGSe solar absorbers. In Part II, herein, we advance this model by computing the electronic properties of all structures from the series and comparing the results with existing experimental reports. Combining the results obtained with various exchange-correlation functionals, we show monotonic band gap widening caused by the reduced p-d repulsion at the valence band edge and small dispersion of the valence band maximum for the Cu-deficient structures. The small band edge dispersion is interpreted as yielding an anisotropic increase in hole mass with decreasing Cu content. These trends are consistent with prior experimental findings, giving support to the proposed model of off-stoichiometry in chalcopyrite solar energy materials.