Structure investigations of several In-rich (Cu₂Se)_x(In₂Se₃)_1-x compositions: From local structure to long range order

The observed junction between α-CuInSe₂ and the In-rich compositions in the β-phase domain (e.g. CuIn₃Se₅) appears to play an important role in the photovoltaic process. There remain, however, inconsistencies and uncertainties about the boundary and structure of this phase. In general the structure of this phase belongs to defect tetrahedral family of structures, which can be described as normal tetrahedral structures with a certain fixed number of unoccupied structure sites. In this work, the local structures of various (Cu₂Se)_x(In₂Se₃)_1-x semiconductor alloys in the β-phase domain were studied by Extended X-ray Absorption Fine Structure (EXAFS) and the results were compared to those for the α-CuInSe₂ phase. The long-range order of these compositions was studied by X-Ray powder Diffraction (XRD) and electron diffraction. It was found the local structures of these compounds are well defined. These compounds, however, could not be well described by any long-range order structure model, especially the selenium position. First-principles band structure calculations were performed to assist in assigning crystal structures to CuInSe₂, CuIn₃Se₅ and CuIn₅Se₈. The calculations indicated that the local environments of these compounds are well defined. Their long-range order might depend sensitively on growth history and the configurational entropies as suggested by the similar formation energies of several possible crystal structures for CuIn₃Se₅ and CuIn₅Se₈.