The rapid expansion of Big Data and Internet of Things (IoT) has driven significant advancements in indoor photovoltaics (IPVs), which provide power to wireless IoT devices. Tin halide perovskites (THPs) have garnered significant attention for IPVs due to their excellent optoelectronic properties without the environmental risks of lead exposure. However, THPs face challenges in controlling their fast crystallization process. Here, we introduce a novel approach to precisely control the crystallization kinetics of FASnI2Br perovskite via the formation of the 6H-intermediate phase, supported by the mesomeric (+M) interaction effect of 4-aminopyridine hydrochloride (4APCl) in the perovskite precursor. The grazing-incidence wide-angle X-ray scattering measurements indicated the formation of 6H-intermediate phase for the FASnI2Br-4APCl perovskite during the crystallization process. The in-situ ultraviolet-visible absorption spectroscopy during the spin coating and annealing process confirmed the reduction of crystal growth rate after the 6H-intermediate phase formation. Thus, high-quality perovskite films were obtained with reduced defects. The resulting IPVs achieved an efficiency of 21.55% under indoor illumination at 1000 lux, exceeding all types of lead-free perovskite IPVs.