Ceiling of Barium Substitution for B-Site Cation in Organometal Halide Perovskite Solar Cells

Significant advancements have been made in the development of perovskite solar cells (PSCs) in recent years, resulting in increased efficiency rates. Nonetheless, the presence of lead within these cells has become a pressing concern. Our study investigates the correlation between tolerance of barium substitution and A-site cation composition through systematic analysis of morphology, crystal structure, optical property, and carrier dynamics. The results show that a high barium substitution ratio of 15 mol.% can be achieved by inducing 60 mol.% of a large A-site cation, formamidinium, into the methylammonium-based perovskite. The induction of large A-site cations in barium-substituted perovskite overcomes the imbalance between hole and electron diffusion lengths, resulting in an electron and a hole diffusion length of 395 nm and 440 nm, respectively. The champion device for 15 mol.% barium-substituted perovskite, FA0.6MA0.4Pb0.85Ba0.15I3-yCly, achieves a PCE of 11.13%, which is the highest record for 15 mol.% of lead reduction. This study provides insights into the limitations of lead substitution in perovskite solar cells and highlights the ceiling for barium substitution ratio in mixed cation perovskite.