In Situ Dual‐Interface Modulation for Homogeneous Sn─Pb Perovskites and Efficient Tandem Solar Cells

All-perovskite tandem solar cells (TSCs) show great promise as the next-generation photovoltaic technology with high theoretical efficiency and low fabrication cost. However, further progress in the TSCs is critically hampered by the subpar performance of mixed tin-lead narrow-bandgap bottom subcells, which arises from the uncontrolled crystallization, unbalanced Sn²⁺ oxidation, and undesirable band alignment. Here, we develop an in situ dual-interface modulation strategy for tin-lead (Sn─Pb) perovskite solar cells (PSCs) by incorporating planar rigid tetrathiafulvalene (TTF) into the precursor solution. The interactions between electron donor TTF and Sn─Pb perovskite precursor constituents, coupled with the in situ self-assembled dual-interface enrichment of TTF, collectively regulate the crystallization dynamics, homogenize the Sn oxidation states, facilitate the carrier extraction and transport in the perovskite bulk and dual interfaces, and stabilize the perovskite structure. Such improvements enable homogeneous single-junction Sn─Pb PSCs to achieve a champion power conversion efficiency (PCE) of 24.30%, together with a record-high fill factor of 83.59% and excellent stability. Furthermore, we obtained a high PCE of 29.14% (certified 29.07%) in all-perovskite TSCs. Encapsulated tandem retains 80% of its initial efficiency following 964 h of maximum power point tracking under simulated 1-sun illumination in ambient air.