Gradient band gap CZTSSe prepared via sputtering from quaternary ceramic targets followed with annealing under different atmospheres

CZTSSe is a thin-film solar cell absorber material with long-time application potential, but its development is limited as its severe open-circuit voltage(Voc) deficit. To further improve the Voc, gradient band gap structure of absorber is expected to achieve this aim without large change of its overall band gap. In this work, a stack structure precursor was deposited by sputtering from quaternary ceramic targets. The top gradient band gap CZTSSe was achieved by annealing the precursor under pure Se or S/Se mix atmosphere. The effects of the atmosphere and the layers-stack precursor structure on the film properties and device performance were studied. Compared with selenization process, sulfo-selenization could effectively suppressed the severe reaction at the back contact interface. It would reduce Mo(S,Se)2 and secondary phases formation and improve solar cell efficiency. The Voc of the double-layers precursor solar cell successfully improved under both atmospheres as expected, representing this method could be a feasible direction to solve the large Voc,def problem. However, for higher efficiency sulfo-selenization cells, the Voc improvement in stack precursor samples did not bring obvious increase of efficiency due to the deterioration of fill factor, indicating that further optimization of device electrical properties should be applied in future study.