Simultaneously improving the morphology and defect properties of CZTSSe film via hybrid grain-growth mechanism

Multi-layer crystallization and uncontrollable defects were the main factors that limiting the quality of Cu2ZnSn(S,Se)4 (CZTSSe) films. Here a hybrid crystallization strategy was proposed to simultaneously improve the morphology and defect properties of CZTSSe film. Firstly, we found the variation of the microstructures (constitution, chemical environment of the elements, etc.) in precursor films can greatly alter the grain-growths of CZTSSe films. Detailed characterizations revealed that the chemical environment of S in the precursor film may influence the S-to-Se substitution of the film during the selenization process. This can further affect the grain-growth mechanism and the properties of CZTSSe film. While the S-to-Se substitution of the film mainly occurred below 500°C, the NaSex on the surface of the film could dominate the grain-growth (Na-mechanism). For this mechanism, the CZTSSe film exhibited bi-layer morphology and relatively high band-tail energy, but shallow-acceptor was detected in the film. The solar cells based on the CZTSSe films reached the efficiency of 10.6%. As a comparison, liquid phase could be generated when the S-to-Se substitution of the film shifted to higher temperature (>500°C). In this condition, the grain-growth would be dominated by the liquid phase (LP-mechanism). Such mechanism can eliminate the multi-layer crystallization and reduce the band-tail energy, but the surface properties of the CZTSSe film was deteriorated and the formation of shallow-acceptor in CZTSSe was inhibited. The fabricated solar cells achieved the efficiency of 10.3%. Based on the results above, a bi-layer precursor film contained sub-layers with different microstructures was proposed to regulate the crystallization of CZTSSe. The co-existence of different microstructures in the precursor film could induce a hybrid grain-growth mechanism for the CZTSSe film (e.g., the Na-mechanism worked on the surface while the LP-mechanism worked in the bottom simultaneously). Such grain-growth mechanism can yield CZTSSe film with uniform morphology and favorable defect properties. The CZTSSe solar cells based on the hybrid grain-growth mechanism achieved the best efficiency of 12.6%. The investigation in this paper would provide new solutions for the crystallization regulation on CZTSSe photovoltaic material.