Efficient Cu2ZnSn(Se,S)4 solar cells with 79% fill factor using two-step annealing

CZTSeS absorbers were prepared by two-step annealing under Ar carried H2Se gas, namely, annealing holding at 400 °C then at 560 °C. The additional holding results in a more compact morphology, same microstructure of phases and higher emission energy of CZTSeS absorbers. The FF as high as 79% has been achieved by improving Rsh, together with the contributions of Rs as low as 0.1 Ω·cm2 and a ohmic contact. This work highlights great potential for sputtering processing based on quaternary target and demonstrates that fill factor of 79% can be experimentally achieved in CZTSeS solar cells.