XRD studies of an electrochemically co-deposited Cu–Zn–Sn precursor and formation of a Cu2ZnSnSe4 absorber for thin-film solar cells

Abstract

A Cu–Zn–Sn (CZT) precursor film on Mo/soda lime glass was electrochemically co-deposited in a stirred citrate solution and further annealed and selenized at elevated temperatures. Modern x-ray diffraction and Raman spectroscopy methods have been used for determination of the phase composition of as-deposited, preliminary annealed and selenized precursor films. It has been determined that the as-deposited CZT film contains up to four metallic phases: the hexagonal η-Cu6.26Sn5, tetragonal Sn, cubic γ-Cu5Zn8, and cubic β′-CuZn phase. During the CZT preliminary annealing at a temperature of 230 °C pure tin only melted and crystallized into the tetragonal Sn phase when the sample was naturally cooling down. When preliminary annealing was conducted at a temperature of 350 °C the whole pure tin was used for the formation of Sn solid solutions both in η-Cu6.26Sn5 and γ-Cu5Zn8 phases. However, a segregation of Sn was found out after ageing of pre-annealed precursor for 2 days. XRD studies of CZT films selenized at temperatures 350–500 °C have shown that formation of Cu2ZnSnSe4 (CZTSe) started at temperatures of about 400 °C, however, the selenized film contained a large quantity of ZnSe phase and somewhat less that of CuSe. The amount of ZnSe decreased with selenization temperature until it became undetectable by XRD at a temperature of 500 °C. The paper also deals with the possible routes of Cu2ZnSnSe4 formation.