Lead‐Free, Sn‐Based All‐Perovskite Tandem Solar Cells with an Efficiency Over 15%

Recent advances in tin-based perovskite solar cells (TPSCs) have yielded significant gains in power conversion efficiency (PCE), yet progress on wide-bandgap (WBG) tin perovskites remains limited, primarily due to the complexities of halogen composition tuning and the associated phase segregation. Here, a halogen composition-independent strategy is presented for realizing WBG TPSCs by partially substituting formamidinium with dimethylammonium (DMA) in the A-site of the perovskite lattice. This substitution expands the lattice, widening the bandgap from 1.63 to 1.72 eV without requiring additional bromine. Comprehensive structural and optical analyses reveal enhanced crystallinity, reduced strain, and improved film morphology. Furthermore, ultraviolet photoelectron spectroscopy confirms enhanced band alignment with the hole transport layer, enabling more efficient charge extraction. By employing a dielectric/metal/dielectric transparent electrode, semi-transparent TPSCs (ST-TPSCs) are fabricated with a PCE of 10.37% and high near-infrared transmittance, which is well-suited for tandem applications. Stacking this ST-TPSC with a narrow-bandgap TPSC yields the first four-terminal, lead-free perovskite tandem device, achieving a combined PCE of 15.02%. These findings show that DMA incorporation effectively addresses the challenges of WBG TPSCs without relying on halogen adjustments, providing a robust pathway toward high-efficiency, eco-friendly photovoltaics and highlighting the promise of tin-based perovskites for next-generation tandem solar cells.