The inverted formamidinium (FA)-rich perovskite solar cells possess great potential in realizing high power conversion efficiency (PCE) and excellent stability. However, the uncontrollable crystallization and poor film quality of FA-rich perovskites are the main obstacles to further advancing photovoltaic performance. Here, we first propose a novel intermediate phase for assisting the crystallization strategy to fabricate high-quality perovskite films. After incorporating phenformin hydrochloride (PFCl) into the precursor solution, the intermediate phase PFCl·FAI decreases the generation of the δ yellow phase and promotes the orientational growth of the α-phase perovskite during crystallization. Combining multi-active-site S-methylisothiosemicarbazide hydroiodide (SMI) post-treatment, the bulk and interfacial trap-assisted nonradiative recombination losses are minimized, which is ascribed to much improved crystallization, reduced defects and released residual stress. As a result, the devices with PFCl@SMI demonstrate maximum PCEs of 24.67% (0.09 cm²) and 22.48% (1 cm²). The unencapsulated target devices exhibit promising thermal and light stabilities, retaining 84% of their initial PCE after 1008 h of continuous light illumination and 90% of their original PCE after 864 h of continuous heating at 85 °C. This work provides valuable guidelines for minimizing bulk and interfacial nonradiative recombination losses by rational organic salt engineering.