Effect of pre-annealing on the phase formation and efficiency of CZTS solar cell prepared by sulfurization of Zn/(Cu,Sn) precursor with H₂S gas

The effect of pre-annealing on the phase formation behavior and efficiency of CZTS thin film solar cell prepared by sulfurization of sputtered Zn/(Cu,Sn) metal precursor with H₂S gas was investigated. Precursor with stacking structure of Zn/(Cu,Sn) was deposited by sputtering of Cu, Zn, and Sn metal targets. The depth profile of metal elements and cell efficiency of the sulfurized CZTS films with H₂S were observed to be critically dependent on the pre-annealing conditions. For the CZTS film prepared by sulfurization in N₂-5 vol.% H₂S at 550 °C after pre-annealing at 350 °C in Ar, segregation of SnS phase at the surface region was observed to be pronounced. When the pre-annealing was performed at 350 °C in N₂-5 vol.% H₂S, however, uniform depth profile of metal elements with a small amount of CuS phase was observed. The CuS phase was disappeared with increase in the pre-annealing temperature in N₂-5 vol.% H₂S. The phase formation behavior influenced by pre-annealing condition was observed to affect solar cell performance of the CZTS thin film synthesized at 550 °C in N₂-5 vol.% H₂S. In contrast to the CZTS thin film prepared with pre-annealing at 350 °C in Ar showing bad efficiency (∼0.93%), the CZTS solar cells fabricated with pre-annealing at 450 °C in H₂S shows higher efficiency of 3.04%. By the optimization of Zn layer thickness, solar cell efficiency of 4.40% was obtained in the CZTS thin film prepared with pre-annealing at 450 °C in N₂-5 vol.% H₂S. This phenomenon was due to the change in the secondary phase formation behavior during sulfurization of the Zn/(Cu,Sn) metal precursor with various pre-annealing conditions.