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.