Formamidinium lead triiodide (FAPbI3) has recently been considered as the most promising candidate to achieve highly efficient perovskite solar cells (PSCs). Excitingly, the state-of-the-art highest efficiency of FAPbI3 based PSCs have reached over 25%. However, their device stability still lags behind other compositions of mixed-cation and mixed-halide perovskites. Interfacial engineering is a very powerful method to address this issue and passivation agents have been intensively developed, however there is a lack of in-depth understanding regarding the solvent selection during post-treatment. Here, we employed cyclohexylmethylammonium iodide (CMAI) as passivation agent, which is investigated using either isopropanol (IPA) or chloroform (CF) as carrier mediator to study the solvent influence on the stabilization of FAPbI3. We observed a suppressed-defect perovskite surface toward distinguished composition with 2D CMA2PbI4 domain and CMAI domain induced by IPA and CF, respectively. Remarkably, post-treatment with solution of CMAI in CF creates a strain-free environment on the perovskite surface, leading to an improved efficiency of approaching 24% and concurrently an extraordinarily stable alpha-phase FAPbI3 PSCs under operation condition, retaining 95% of its initial efficiency after 1050-hour aging. Our resulting device stability is one of the most stable FAPbI3 based PSCs reported in literature.