In this work, we present a high-performance, stable formamidinium lead iodide (FAPI) perovskite solar cell (PSC) achieved through the use of 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) bulk passivation and rapid photonic annealing. Utilizing flash infrared annealing (FIRA), we fabricated TEMPO-FAPI PSCs with a power conversion efficiency (PCE) exceeding 20%, exceeding the prior state of the art for this process. The TEMPO additive promotes enhanced crystallization dynamics, yielding films with improved homogeneity and reduced defect densities, as confirmed by photoluminescence (PL), profilometry, and positron annihilation lifetime spectroscopy (PALS). Stability testing under ISOS protocols demonstrated that the TEMPO-FAPI devices retained over 90% of their initial PCE after 4,296 h of operational and thermal stress, showing unprecedented longevity for a rapid processing technique. TEMPO's primary effect on passivating grain boundaries and surface defects is evidenced by a significantly reduced non-radiative recombination rate and low defect density, establishing this molecule as a promising additive for scalable, durable FAPI PSC manufacturing.