<p>Methylammonium lead iodide (MAPbI₃)–based perovskites are often regarded as unstable under light and heat; however, previous studies have not fully clarified the influence of residual lead iodide (PbI₂) in thin films, which may have led to misinterpretations of their intrinsic stability. In this work, we propose a precursor-engineering strategy in which pre-synthesized MAPbI₃ single crystals are re-dissolved to prepare coating solutions, thereby producing MAPbI₃ films free of residual PbI₂. Devices fabricated using this method achieved a power conversion efficiency (PCE) of 21.55%, significantly higher than the 18.61% obtained from control devices prepared using conventional precursor solutions. Moreover, the single crystal-derived devices exhibited remarkable long-term stability, retaining 98% of their initial efficiency after 1000 h of storage, whereas the control devices degraded to below 50% of their initial performance. Extensive characterizations of the single crystal-derived films revealed the degradation pathways induced by residual PbI₂ in MAPbI₃, confirming the superiority of this precursor design strategy. Importantly, these single crystal-derived films were further applied in mini-modules, achieving a PCE of 19.82% under 1-sun illumination and 39.66% under 6000 lux indoor light, underscoring their great potential for both outdoor and indoor photovoltaic applications.</p><p></p>