Insights into the Formation Pathways of Cu₂ZnSnSe₄ Using Rapid Thermal Processes

Recent advances in Cu2ZnSn(S,Se)4 (CZTSe) thin film photovoltaics open the possibility for the future industrialization of this technology. Nevertheless, major progresses in CZTSe have been achieved using conventional thermal processing annealing routes (CTP), which rely on time-consuming processes with tubular furnaces, incompatible with the requisites of fast methodologies for the Industry. Changing from conventional to rapid thermal processes (RTP) using halogen lamps as heating method is not at all obvious, since the system becomes kinetically controlled, and the CZTSe formation mechanisms as well as crystallization pathways can drastically change. In this work we present the transfer of our kesterite production baseline (Cu2ZnSnSe4:Ge) from a conventional thermal process using a tubular furnace toward a rapid thermal process using an adapted system, by comparing them and analyzing the differences between both processes in terms of formation mechanisms as well as photovoltaic absorber properties. For this purpose, the rapid annealing process is stopped at different steps, analyzing the compositional, structural, and morphological properties of the CZTSe absorber at these different stages. Using a combination of XRF, SEM, Raman spectroscopy, and XRD characterization techniques, it is demonstrated that, in contrast to CTP routes, when RTP is used, kesterite is being formed in large amounts in the very early stages. This suggests a fast formation of CZTSe promoted by the higher Se vapor pressure that can be quickly achieved with this methodology. The formation of kesterite seems to proceed via two competitive reactions (binaries vs ternary compound). Additionally, the fast reaction observed in the system avoids the possible Sn loss in an efficient way. Through the optimization of this RTP treatment a device with 8.3% efficiency has been obtained (the total time of the thermal process is 12 min), comparable with the efficiencies obtained so far with CTP routes. Finally, the consequences of all these changes for the future interpretation of the formation reaction mechanisms of kesterites are discussed.