Tin perovskites suffer from poor stability and a self-doping effect. To solve this problem, we synthesized novel tin perovskites based on superhalide with varied ratios of tetrafluoroborate to iodide and implemented them into solar cells based on a mesoscopic carbon-electrode architecture because film formation was an issue in applying this material for a planar heterojunction device structure. We undertook quantum-chemical calculations based on plane-wave density functional theory (DFT) methods and explored the structural and electronic properties of tin perovskites FASnI(3-x)(BF4)(x) in the series x = 0, 1, 2, and 3. We found that only the x = 2 case, FASnI(BF4)(2), was successfully produced, beyond the standard FASnI(3). The electrochemical impedance and X-ray photoelectron spectra indicate that the addition of tin tetrafluoroborate instead of SnI2 suppressed trap-assisted recombination by decreasing the Sn4+ content. The power conversion efficiency of the FASnI(BF4)(2) device with FAI and Sn(BF4)(2) in an equimolar ratio improved 72% relative to that of a standard FASnI(3) solar cell, with satisfactory photostability under ambient air conditions.