Cesium-Triiodide-Mediated Crystallization Regulation of Pure-Iodide Wide-Bandgap Perovskites Enables Perovskite-Silicon Tandem Solar Cells Exceeding 30% Efficiency

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Pure-iodide wide-bandgap (WBG) perovskites (∼1.68 eV) featuring eliminated photoinduced halide segregation are compelling top-cell absorbers for perovskite-silicon tandem solar cells. However, their complex crystallization readily induces heterogeneous composition and structural defects, hindering performance improvements in the resulting devices. Here, we develop a cesium triiodide (CsI3)-mediated strategy to regulate intermediate-phase evolution of pure-iodide WBG perovskites. The CsI3-derived precursor chemistry reconstructs Pb-I coordination networks, suppresses premature organic-dominated intermediate nucleation, and promotes homogeneous phase conversion. The modulated crystallization pathway yields highly crystalline films with improved iodine uniformity and reduced non-radiative recombination losses. Consequently, single-junction solar cells achieve a stabilized power conversion efficiency (PCE) of 22.40%, which retains 95% of its initial efficiency after 1500 h of maximum power point tracking. More importantly, the associated perovskite-silicon solar cells deliver a stabilized PCE of 30.74% along with improved thermal stability, highlighting the great potential of pure-iodide WBG perovskites for efficient and durable tandem photovoltaics.