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

Abstract

Currently, the efficiency of Cu 2 ZnSnS 4 (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 Br 2 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 SnS 2 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% Br 2 –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. • Optimal conditions for combinational post-treatment of CZTS crystals were found. • The content of Cu ↑ and Zn ↓ in the bulk of CZTS crystals by increasing annealing temperature. • The formation of SnS 2 flakes on crystals surfaces in the annealing process was determined. • The PCE of CZTS solar cells enhanced from 3.8% to 9.44% by optimizing post-treatment conditions.