Functional Off-stoichiometry in Cu(In,Ga)Se2. Part I: Topotactic Continuum of Ordered Defect Compounds

Abstract

There is hardly a more bewildering aspect of the chalcopyrite material Cu(In,Ga)Se₂ (CIGSe), used as an absorber in thin-film solar cells, than its relation with off-stoichiometry, which is often manifested in the formation of ordered defect compounds (ODCs). Despite huge efforts devoted to determining the structure and properties of the ODCs, no comprehensive model has been provided. The convention is that the off-stoichiometry is mediated by the charge-neutral [2V_Cu + III_Cu] complexes forming in high concentrations. In this work, we refute this simplistic picture by treating the off-stoichiometry as an integral part of the crystal structure. In Part I, herein, we report a discovery of the topotactic transformation series of stable ODC-like structures with different compositions that enable large Cu deficiency in the material. Crucially, the underlying structures are shown practically invisible to X-ray and neutron diffraction, as the parent lattice is preserved and barely deformed within the relevant compositional range. The model we deduce reconciles several lasting conflicts between experimental findings and first-principles calculations. In Part II, this model is further validated by computing the electronic properties for the structures in the series and comparing the results with prior experimental findings. The developed structural series is uniquely self-consistent and is thus well-positioned to bolster computational analysis of chalcopyrite compounds and alloys.