Tin-based halide perovskite solar cells (PSCs) hold the most promise among lead-free PSCs, but they are plagued with inadequate environmental stability and power-conversion efficiency (PCE). Here we demonstrate that the optimum incorporation of a bulky divalent organic cation, 4-(aminomethyl)- piperidinium (4AMP), in FASnI(3) thin films improves stability, optoelectronic properties, and PSC performance. The optimized PSC yields a maximum PCE of 10.9% and good 500-h operational stability under continuous illumination. This is attributed to the unique thin-film structure, where the strong ionic bonding afforded by divalent 4AMP may provide near-full-coverage functionalization (encapsulation) of FASnI(3) grain surfaces and grain boundaries, retarding O-2/H2O ingression and mitigating Sn-defects for reduced photocarrier nonradiative recombination.