Formamidinium lead triiodide (FAPbI) with an ideal bandgap and good thermal stability has received wide attention and achieved a record efficiency of 26% in n–i–p (regular) perovskite solar cells (PSCs). However, imperfect FAPbI formation on the typical hole transport layer (HTL), high interfacial trap-state density, and unfavorable energy alignment between the HTL and FAPbI result in the inferior photovoltaic performance of p–i–n (inverted) PSCs with FAPbI absorber. Herein, the α-phase FAPbI is stabilized by constructing a buffer interface region between the NiO HTL and FAPbI, which not only diminishes NiO/FAPbI interfacial reactions and defects but also facilitates carrier transport. Upon the construction of a buffer interface region, FAPbI inverted PSC exhibits a high-power conversion efficiency of 23.56% (certified 22.58%) and excellent stability, retaining 90.7% of its initial efficiency after heating at 80 °C for 1000 h and 84.6% of the initial efficiency after operating at the maximum power point under continuous illumination for 1100 h. Besides, as a light-emitting diode device, the FAPbI inverted PSC can be directly lit with an external quantum efficiency of 1.36%. This study provides a unique and efficient strategy to advance the application of α-phase FAPbI in inverted PSCs.