3D/1D Heterostructure Perovskite Engineering via 1D TMSPbI ₃ Templated Growth Toward Improved Efficiency and Moisture Stability in Solar Cells

Perovskite solar cells (PSCs) combine high power conversion efficiency (PCE) with low fabrication cost, yet their instability under ambient conditions remains a major barrier to commercialization. The protic nature of conventional A‐site cations renders perovskite films vulnerable to moisture‐induced degradation. Here, we demonstrate a mixed‐dimensional (3D/1D) compositional engineering strategy using an aprotic additive, trimethylsulfonium bromide (TMSBr), to enhance both the moisture stability and photovoltaic performance of PSCs. Incorporation of TMSBr into a ternary‐cation perovskite precursor promotes the in situ formation of one‐dimensional TMSPbI ₃ during preannealing, which organizes preferentially along the grain boundaries to form a 3D/1D heterostructure. The hydrophobic 1D TMSPbI ₃ effectively suppresses water penetration owing to the weak interaction between TMS ⁺ and H ₂ O molecules, resulting in improved film crystallinity and suppressed intergranular degradation. Devices based on this architecture achieve a high PCE of 21.65% and retain over 84% of their initial efficiency after 40 days of ambient exposure. These findings highlight aprotic sulfonium additives as a promising route toward intrinsically moisture‐stable and durable perovskite photovoltaics.