Formation mechanism of Cu2ZnSnSe4 absorber layers during selenization of solution deposited metal precursors

Phase-pure Cu2ZnSnSe4 (CZTSe) layers are necessary for achieving efficient thin film solar cells. This requires the knowledge of intermediate phases and their existence regions during the evolution of the CZTSe phase within its homogeneity range. Here we investigate the growth mechanism of different phases when solution deposited metal salt precursors are selenized into CZTSe layers. A combination of in situ and ex situ X-ray diffraction, Raman spectroscopy, energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy at successively increasing substrate temperatures is used to track evolving crystal phases. The growth starts with the fast formation of binary Cu-Se phases that are present between 190 °C and 320 °C. Overlapping diffraction patterns of CZTSe/Cu2SnSe3/ZnSe phases evolve from 280 °C onwards and remain until a final temperature of 550 °C. The ternary Cu2SnSe3 phase co-existing with CZTSe between 340 °C and 370 °C is confirmed by Raman spectroscopy and point EDX measurements. No individual zinc or tin binary phases can be detected. We propose the kinetically driven formation mechanism, which starts with the selenization of Cu requiring the lowest activation energy for reaction, and then proceeds via the gradual incorporation of Sn and Zn to yield the final CZTSe phase.