Lattice stabilization and strain homogenization in Sn-Pb bottom subcells enable stable all-perovskite tandems solar cells

All-perovskite tandem solar cells (PTSCs) offer a promising approach to surpass the Shockley-Queisser (SQ) limit, driven by efficiently reducing thermalization and transmission losses. However, the efficiency and stability of the narrow-bandgap (NBG) subcells, which are essential for PTSC performance, remain severely constrained by challenges such as lattice instability, strain accumulation and halide migration under illumination. This study introduces a rigid sulfonate-based molecule, sodium naphthalene-1,3,6-trisulfonate (NTS), into tin-lead (Sn-Pb) perovskites, where it strengthens the Sn-I bond through Sn-trisulfonate coordination and reduces light-induced dynamic lattice distortions via the rigid NTS backbone. These molecular interactions alleviate strain heterogeneity within the lattice and homogenize the Sn-Pb compositional gradient, thereby enhancing the structural integrity and long-term stability of Sn-Pb perovskites under operational conditions. As a result, Sn-Pb single-junction perovskite solar cells (PSCs) achieve a power conversion efficiency (PCE) of 23.2%. When integrated into a tandem configuration, the device attains an impressive PCE of 29.6% (certified PCE of 29.2%, one of the highest certified efficiencies to date), with 93.1% of the initial efficiency retained after 700 h of continuous operation. By stabilizing the lattice structure, this work lays a solid foundation for achieving both high efficiency and long-term durability in next-generation perovskite photovoltaics.