Copper aluminum disulfide (CuAlS2) is one of the promising wide-gap (Eg of 3.6 eV) chalcopyrite-like compounds that can be used in thin film solar cells. The band gap energy of the CuAlS2-based absorber must be significantly reduced. An obvious approach for such reduction of the CuAlS2 band gap consists in combining CuAlS2 in solid solutions with other ternary compounds that have a lower band gap, for instance, CuFeS2. The goal of the present paper is to study the phase relations and to determine the limits of solubility in the CuAlS2 - CuFeS2 system by X-ray powder diffraction (XRPD) and Scanning Electron Microscopy (SEM).
The thermobaric treatment (techniques of high pressure and temperature) was applied to prepare samples of the alloys from previously prepared ternary compounds CuAlS2 and CuFeS2. Synthesis of the initial ternary compounds CuAlS2 and CuFeS2 was performed in quartz ampules by melting the elements at a temperature that exceeds the melting point of the compound by 20 K. After preparation of the initial compounds, their homogeneous mixtures with molar part of CuFeS2 (x) equaling to 0.025, 0.05, 0.10, 0.125, 0.20, and 0.30 were prepared and treated at the high pressure of 5.5 GPa and temperatures ranging from 1170 to 1270 K.
The X-ray studies of the CuAlS2 - CuFeS2 alloys were carried out using monochromatic Cu Ka-radiation (1.5406 Å, step size 0.01° or 0.04°, counting time 10 s). The Rietveld analysis of the X-ray powder diffraction data was done using the FullProf software.
Phase formation in the (CuAlS2)1-x-(CuFeS2)x system was investigated and the unit-cell parameters (the lattice constants and the unit-cell volume) were computed as a function of composition. It was found the absence of complete solubility in the (CuAlS2)1-x-(CuFeS2)x system. The formation of solid solutions with the tetragonal structure of chalcopyrite was detected for the compositions with molar part of CuFeS2 x<0.20. The system with the compositions with higher content of CuFeS2 contains two phases with the chalcopyrite-like structure – CuAlS2-based and CuFeS2-based phase. The CuFeS2-based phase is not stable and may decompose into two phases. Both of these phases are Fe-substituted phases. One of the phases is a phase with the structure of chalcopyrite, for which the ratio ([Cu]+[Fe])/[S] is ~1. The second phase is a phase with the structure of talnakhite, for which the ratio ([Cu]+[Fe])/[S] is >1. The phase assemblages of these lamellar intergrowths are different depending on the proportion of Cu and Fe in the high-temperature solid solution. The solid solutions with Cu>Fe form an intergrowth of chalcopyrite (with a composition close to CuFeS2) and a phase similar to talnakhite (Cu9Fe8S16). The solid solutions with Cu<Fe form an intergrowth of chalcopyrite with a phase having Cu<<Fe considered to be the high-temperature cubanite.
Acknowledgment. B. Korzun would like to thank PSC-CUNY for financial support of the studies under project TRADA-48-527.