Toward Highly Efficient Kesterite Solar Cell and Module

The performance of kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) thin film solar cells is seriously limited by large open circuit voltage deficit (V_OC,def), mainly due to high concentration of defects within the absorber and at the interfaces. To tackle the defect issues, we first mitigated the absorber intrinsic defects by controlling the reaction path/grain growth mechanism of solution processed CZTSSe absorbers. We found the combination of Cu⁺-Sn⁴⁺ precursors produce kesterite structured precursor film that takes direction phase transformation grain growth without involvement of secondary phases, leading to greatly reduced defect concentration and V_OC,def. Second, we mitigated the absorber defects by extrinsic Ag and Cd alloying through the unique phase transformation grain growth, which further reduced banding tailing and V_OC,def. Thirdly, the uniform absorber surface enabled us to investigate the heterojunction interface property of CZTSSe/CdS. We found that the CZTSSe/CdS heterojunction is constructed on a Zn-poor surface due to the dissolution of Zn²⁺ during chemical bath deposition. The occupation of Cd²⁺ on Zn site and re-deposition of Zn²⁺ into CdS creates a defective and lattice-mismatched interface. Low-temperature annealing of the CZTSSe/CdS junction drives elemental di-mixing and thus reconstructed an epitaxial interface, which greatly reduces interface recombination and achieved record efficiency of 13%. The above strategies enabled fabrication of kesterite absorber films with high homogeneity and reproducibility, which encouraged us to make kesterite solar cell modules by series integration through laser (P1, P2) and mechanical (P3) scribing. Characterizations show that the three patterns are well established without observation of metallization. Importantly, benefited from solution process, a thicker absorber is deposited in P1 groove than other area, which reduces lateral charge transport resistance and decreases cell to module loss. By optimization of module structure, kesterite solar cell module with certified efficiency of 10.1% has been achieved with an area of 10 cm². Our results push one step forward for this low-cost and environmentally benign thin film solar cell to practical application.