Selectively Regulating Sn²⁺ Crystallization in Mixed Sn–Pb Perovskite for Efficient and Stable Narrow-Bandgap Solar Cells

In high-performance perovskite tandem solar cells, the bottom cell is responsible for harvesting near-infrared photons in sunlight and its narrow bandgap at 1.2–1.3 eV is achieved by alloying Sn²⁺ and Pb²⁺ in the perovskite structure. Owing to the tendency of Sn²⁺ toward premature crystallization, solution deposition of mixed Sn–Pb perovskite films faces challenges in phase segregation and defect formation. In this work, we employed an organic molecule trimethylthiourea (3T) as a Lewis-base ligand for Sn²⁺ in the precursor solution and on film surfaces of the mixed Sn–Pb perovskite. According to infrared spectroscopy, the 3T ligand preferentially coordinated to Sn²⁺ over Pb²⁺ in the precursor solution. As a result, crystallization process of the mixed Sn–Pb perovskite went through with regulated crystallization kinetics and enhanced composition homogeneity, giving rise to smooth and compact films consisting of micron-sized crystallites showing preferential (100) orientation. Enhanced film quality also yielded charge-carrier lifetimes as long as 0.9 μs and device fill factors over 80%. We subsequently fabricated complete photovoltaic devices having the structure ITO/perovskite/C₆₀/Ag. Thanks to the high-quality perovskite layer and the absence of a hole transport layer (HTL), our mixed Sn–Pb perovskite solar cells exhibited energy conversion efficiencies up to 21% and storage stability of at least 6000 hours, which are excellent performances for HTL-free, narrow-bandgap perovskite solar cells. Our results present an effective method to enhancing film deposition of mixed Sn–Pb perovskite, and lay a solid foundation for high-efficiency tandem perovskite solar cells.

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