Synergistic effect of 3-mercaptopropionic acid and MOE: DMF solvent engineering on crystallization and charge transport in sol-gel derived CZTSSe thin films

This work systematically investigates the synergistic effect of 3-mercaptopropionic acid (MPA) coordination and solvent engineering on the structural, morphological, vibrational, and electrical properties of sol-gel-derived Cu 2 ZnSn(S,Se) 4 (CZTSSe) thin films. CZTSSe absorbers were fabricated using precursor solutions with and without MPA under different solvent environments (MOE: DMF solvent ratios of 50:50 and 25:75), followed by selenization. GIXRD and Raman analyses confirmed the dominant formation of kesterite CZTSSe absorbers, while the MPA-treated samples exhibited larger crystallite sizes, reduced microstrain, and improved local structural ordering. FESEM and AFM observations revealed that MPA incorporation promoted grain coalescence, reduced surface roughness, and produced more compact absorber layers. Conductive AFM further revealed that the MPA-treated films exhibited a more spatially connected current distribution, indicating improved local electrical connectivity and reduced defect-assisted charge transport. Hall effect measurements further showed an increase in carrier concentration from ~10 14 to ~10 19 cm⁻³ and a corresponding reduction in resistivity, confirming improved charge transport after MPA incorporation. However, DMF-rich systems exhibited smaller crystallite sizes and greater electrical inhomogeneity than films prepared using balanced solvent compositions, suggesting that an excessive DMF fraction limits long-range crystallite development through changes in precursor coordination and film formation. Among all the samples, M5, prepared with MPA under balanced solvent conditions (MOE: DMF = 50:50), exhibited the most favorable combination of crystallinity, compact morphology, and electrical performance. This study highlights the importance of balancing ligand coordination and solvent chemistry to optimize solution-processed CZTSSe absorbers.