9.4 % efficient kesterite solar cells on bare FTO enabled by synergistic Ag/Na doping and annealing optimization

The fabrication of kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells based on fluorine-doped tin oxide (FTO) substrates, as an alternative to conventional molybdenum, is highly attractive for applications in semitransparent, bifacial and tandem photovoltaic devices. Although FTO exhibits phase stability during the high-temperature annealing process required for CZTSSe grain growth, its electrical conductivity decreases significantly, impairing carrier transport and limiting device performance. Addressing this issue necessitates lowering the annealing temperature without compromising the crystallinity of the CZTSSe absorber layer. In this study, we demonstrate the growth of high-crystallinity CZTSSe absorbers on FTO substrates at a reduced temperature of 500 °C, facilitated by silver (Ag) and sodium (Na) doping in the precursor film. The absorbers were fabricated from a 2-methoxyethanol-based molecular ink, and the annealing duration was systematically optimized. We found that the crystallinity and morphology of the CZTSSe layer are strongly influenced by annealing time. An optimal film morphology was achieved with a 15-min anneal, whereas shorter annealing (≤10 min) resulted in incomplete crystallization, and extended annealing (>20 min) promoted void formation within the absorber due to Ostwald ripening. Furthermore, the optimized annealing process reduces the interface defect density and broadens the depletion width of the device, leading to a record power conversion efficiency of 9.4 % for CZTSSe solar cells on bare FTO substrates. These results provide important insights for developing efficient bifacial CZTSSe solar cells.