Bidentate Pb ²⁺ Chelation Enables Unified PbI ₂ Crystallization Control for Process‐Agnostic Perovskite Fabrication and High‐Performance Tandem Sola

ABSTRACT

The performance of perovskite solar cells depends on how the absorber crystallizes. PbI ₂ nucleates unevenly and grows without order. The resulting films carry iodide‐rich aggregates, nonuniform grains, and a high density of grain‐boundary defects that degrade device performance. Existing strategies for controlling crystallization are typically designed for a single deposition route and rarely work in another. We report that bathocuproine (BCP), a phenanthroline derivative, can be added to the precursor to control how the perovskite crystallizes. Its two pyridyl N sites chelate Pb ²⁺ through several coordination motifs, and the five‐membered chelate is the most stable. Chelation changes the nucleation and growth kinetics of PbI ₂ . We tested BCP in three deposition routes: two‐step spin coating, solvent‐free co‐evaporation of PbI ₂ and BCP, and solvent‐assisted vacuum flash evaporation. In all three, the films came out more uniform. Residual PbI ₂ dropped, grain‐boundary traps were passivated, and stability improved. The higher crystallinity and reduced nonradiative recombination translate into better single‐junction devices. On pyramid‐textured silicon, BCP forms conformal, dense perovskite layers with few vertical boundaries or voids. The best perovskite/silicon tandem reached a TÜV NORD‐certified steady‐state efficiency of 32.22% with negligible hysteresis, and held its performance under MPPT tracking in the ISOS‐L‐2 protocol.