Precursor Stack Ordering Effects in Cu₂ZnSnSe₄ Thin Films Prepared by Rapid Thermal Processing

Solar cells based on Cu2ZnSnSe4 thin film absorber layers have shown promise as an alternative to more mature thin film technologies because they are composed of more earth abundant elements. To increase device efficiencies there is still much to be investigated about its properties, and film and device processing. Rapid thermal processing is a more industrially viable method of forming thin film absorber layers than time intensive conventional thermal processing. However, optimized conditions for conventional processing are not readily transferable to rapid thermal processing. Thermal processing of this material is complicated through loss of volatile components such as Zn and Sn–Se, in addition to decomposition at elevated temperatures. In this study the effect of stack order has been investigated for Cu–Zn(O)–Sn precursor stacks selenized by rapid thermal processing to form Cu2ZnSnSe4 thin films. Precursor stack ordering is shown to have significant effects on the film properties, including precursor alloy formation, composition and elemental loss, morphology, and secondary phase formation and distribution. Optoelectronic properties of devices prepared from these films also show a dependence on precursor stack order. Most notable is the poor performance of devices with Zn as a bottom layer, due to excessive ZnSe formation at the back contact region. The viability of ZnO as a precursor layer—in place of volatile Zn—is also investigated, and shown to not completely react in the Se atmosphere, leaving residual oxygen in the Cu2ZnSnSe4 films. The best performing device has a conversion efficiency of 4.3%, and uses a stack order of glass/Mo/Sn/Zn/Cu.