Structure investigations of several in-rich (Cu{sub}2Se){sub}x(In{sub}2Se{sub}3){sub}(1-x) compositions: from local structure to long range order

摘要

The observed junction between α-CuInSe{sub}2 and the In-rich compositions in the β-phase domain (e.g. CuIn{sub}3Se{sub}5) appears to play an important role in the photovoltaic process [1]. 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 [2], 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 (Cu{sub}2Se){sub}x(In{sub}2Se{sub}3){sub}(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-CuInSe{sub}2 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 CuInSe{sub}2, CuIn{sub}3Se{sub}5 and CuIn{sub}5Se{sub}8. 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 CuIn{sub}3Se{sub}5 and CuIn{sub}5Se{sub}8.