Mixed tin-lead halide perovskites, with their tunable bandgaps around 1.2 eV, show  great promise for the development of highly efficient all-perovskite tandem solar cells. However, achieving commercial viability and long-term stability for Sn-Pb perovskite solar cells (PSCs) presents numerous challenges. Among various optimization strategies, incorporating additives has proven critical in material engineering. Despite the widespread use of additives to improve performance, their detailed photophysical  mechanisms remain unclear. In this work, we elucidate the mechanistic role of  guanidinium thiocyanate (GaSCN) as an additive in the crystallization of Sn-Pb  perovskites, employing advanced steady-state and time-resolved photoluminescence spectroscopy, hyperspectral imaging, and in-situ photoluminescence techniques. Our findings reveal that GaSCN slows the crystal growth rate during perovskite crystallization, resulting in more homogeneous films with reduced non-radiative recombination. The modified film exhibits a photoluminescence quantum yield of 7.28% and a charge carrier lifetime exceeding 17 µs, leading to a device efficiency of 22.34% and fill factor of over 80%.