Efficiency enhancement of Cu2ZnSnS4 monograin layer solar cells via absorber post-growth treatments

Currently, the efficiency of Cu2ZnSnS4 (CZTS) monograin layer solar cells is mainly limited by the recombination losses in the bulk and at the surface of absorber/buffer that need to be reduced. One approach to significantly improve the performance of CZTS monograin layer solar cells is to post-treat the surfaces of monograin powder crystals before the buffer layer deposition. This study includes the optimization of two sequential post-treatment steps: 1) chemical etching of as-grown powder crystals with Br2 in methanol followed by KCN; and 2) annealing at different temperatures (550–850 °C) under different sulfur vapor pressures (100–2050 Torr) in a two-temperature zone furnace in closed ampoules. Energy-dispersive X-ray spectroscopy, high-resolution scanning electron microscopy and Raman studies using multiwavelength excitation revealed that SnS2 and ZnS secondary phases were formed on the crystals’ surfaces in the thermal annealing process. At the same time, by increasing the annealing temperature, the ratio of Cu/(Zn + Sn) increased and Zn/Sn decreased in the bulk of annealed CZTS crystals compared to as-grown monograin powder crystals. Optimal conditions for combinational post-treatment to adjust the absorber material composition and to improve monograin layer solar cells' performance were found as follows: the chemical etching of as-grown powders with 1% Br2–MeOH for 5 min + 10% KCN for 5 min followed by annealing in sealed ampoules at 850 °C in a sulfur atmosphere of 2050 Torr for 1 h. The highest power conversion efficiency of 9.44% was obtained under these conditions.