Journal of the American Chemical Society university researchers studied mixed tin–lead (Sn–Pb) halide perovskites with tunable bandgaps of 1.2–1.4 eV for tandem solar cells. Achieving stable high efficiency in Sn–Pb perovskite solar cells was challenging. Additives were found to play a critical role in modulating crystallization. Guanidinium thiocyanate, a chaotropic agent, was examined using hyperspectral imaging and in situ photoluminescence spectroscopy. The agent modulated crystal growth, producing homogeneous films and reducing nonradiative recombination. Crystallization was observed to continue during cooldown, not only after solvent evaporation. Films showed a photoluminescence quantum yield of 7.28% and charge carrier lifetimes exceeding 11 μs. Devices reached an efficiency of 22.34% with a fill factor above 80%. The study provided insights into additive-mediated crystal growth and cooldown dynamics in Sn–Pb perovskites.