Phase evolution during annealing of low-temperature co-evaporated precursors for CZTSe solar cell absorbers

Systematic investigations into the phase evolution during reactive annealing of copper–zinc–tin–selenide (CZTSe) precursors for the fabrication of kesterite solar cell absorber layers have been paramount in understanding and suppressing the formation of secondary phases that deteriorate device performance. In this study, the phase evolution during annealing of low-temperature co-evaporated CZTSe precursors is investigated. A detailed analysis of films selenized at different temperatures is used to reveal the possible reaction pathway of CZTSe formation. Utilizing a combination of x-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and energy-dispersive x-ray spectroscopy, it is shown that CZTSe formation starts by Cu out-diffusion to the surface and Cu–Se phase formation at a temperature of 350 °C. An intimate mixing of binaries and ternaries during low-temperature selenization is observed. On the contrary, only binaries are observed at high-temperature selenization. This suggests that the CZTSe formation pathway involves reaction schemes where (i) a competition between binary and ternary phases dominates at low-temperature and (ii) binary reactions dominate the process at high temperatures. However, the number of binary phases decreases with increasing selenization temperature until they become undetectable by XRD and Raman spectroscopy at a temperature of 540 °C (selenization time 10 min). Utilizing the presented selenization conditions, prototype solar cells with an efficiency of up to 7.5%, an open-circuit voltage of 407 mV, and a fill factor of 59%, could be demonstrated. The temperature-dependent current density–voltage characteristics indicate that the performance of the prototype devices is limited by bulk Schottky–Read–Hall recombination.