Sulphurisation of gallium-containing thin-film precursors analysed in-situ

It has been demonstrated that rapid thermal sulphurisation of sputtered Cu/In precursor layers is suitable for industrial production of thin-film photovoltaic modules. The process is relatively straightforward and the underlying fundamental aspects, such as phase formation sequence and reaction rates, have been studied intensively. Using lab-scale preparation technology, incorporation of gallium is known to improve transport properties of the absorber and to enable the fabrication of wide-gap cells. In this work we have used energy dispersive in-situ X-ray diffraction to study the sulphurisation of sputtered Cu:Ga/In precursors. It is the basis for the future development of an industrially feasible production of Cu(In,Ga)S₂ films. Precursor stacking sequence and sulphur partial pressure in relation to precursor temperature have been varied. In many cases, in particular when establishing sulphur partial pressure already at low precursor temperature, we observe a severe reduction of reaction rates after going from pure Cu to Cu:Ga in the precursor. In consequence, single phase films cannot be prepared within the feasible ranges of time and temperature. Adhesion failure and at least intermediate formation of CuIn₅S₈ are other problems frequently encountered. In spite of these problems, promising pathways to single phase Cu(In,Ga)S₂ films prepared from sputtered Cu:Ga/In precursors have now been identified.