Dynamic control of layered MoSe2 crystallographic orientation for optimized CZTSSe/Mo interface contact in CZTSSe solar cells

During high-temperature selenization of CZTSSe solar cells, the Mo back electrode reacts with Se to form a MoSe2 interfacial layer, whose crystallographic orientation critically affects interfacial properties owing to its layered nature. In this work, ZnO was deposited on MoOx/Mo substrates and subjected to pre-selenization for different durations to form the ZnSe/MoSe2/Mo structure. During pre-selenization, the crystallographic registry between ZnSe(111) and MoSe2(002) promotes the growth of basal plane-oriented MoSe2. This configuration facilitates Se retention near the back interface during the early stage of absorber growth. The retained elemental Se is subsequently buried at the bottom of the absorber precursor and serves as an internal Se reservoir during the early stage of high-temperature selenization. Notably, such pre-embedded Se is generally difficult to achieve because elemental Se readily reacts with the Mo electrode; here, the basal plane MoSe2 layer acts as a barrier that prevents this reaction. As selenization proceeds, ZnSe is gradually consumed and incorporated into the CZTSSe absorber, while MoSe2 remains predominantly in the edge-on configuration, and this tendency becomes more pronounced after complete selenization. The optimized device then exhibits a thicker absorber, a more homogeneous elemental distribution, suppressed cation loss toward the back interface, reduced deep-level defect densities, and improved carrier transport. Consequently, the champion efficiency increases from 11.51% to 13.38%. This work provides an effective approach for regulating the MoSe2 orientation and optimizing the back interface of CZTSSe solar cells.