Investigation of Se supply for the growth of Cu ₂ ZnSn(S _x Se _1-x ) ₄ (x ≈ 0.02-0.05) thin films for photovoltaics

In this work, the selenization growth of electrochemically deposited Cu ₂ ZnSn(S _x Se _1-x ) ₄ (CZTSSe) films with x ≈ 0.02-0.05 was optimized by two groups of experiments in a graphite box. The selenization of CZTSSe is strongly dependent on the Se supply in the graphite box. Insufficient Se supply left the selenization incomplete. Higher Se supply to CZTSSe either by increasing the Se powder usage or by increasing the external pressure of nitrogen resulted in the degradation of CZTSSe films with lower degree of crystallinity. Our experimental results show that the refined usage amount of Se powder is 0.3 g and the optimized external pressure of flowing nitrogen in the furnace is from 0.5 Torr to 2 Torr. The characterization of XRD, Raman and SEM confirmed the films obtained under the best conditions were well-developed CZTSSe films with compact faceted grains and good crystallinity. Additionally, the CZTSSe film grown using 2 Torr of nitrogen pressure showed more orientation along (2 2 0/2 0 4).