The poor intrinsic perovskite absorber stability is arguably a central limitation challenging the prospect of commercialization for photovoltaic (PV) applications. Understanding the nanoscopic structural features that trigger instabilities in perovskite materials is essential to mitigate device degradation. Using nanostructure characterization techniques, we observe the local degradation to be initiated by material loss at stacking faults, forming inherently in the (011)-faceted perovskite domains in different formamidinium lead triiodide perovskite compositions. We introduce Ethylene Thiourea (ETU) as an additive into the perovskite precursor, which manipulates the perovskite crystal growth and results in dominantly in-and out-of-plane (001) oriented perovskite domains. Combining in-depth experimental analysis and density functional theory calculations, we find that ETU lowered the perovskite formation energy, readily enabling crystallization of the perovskite phase at room temperature without the need for an antisolvent quenching step. This facilitated the fabrication of high-quality large area 5 cm by 5 cm blade-coated perovskite films and devices. Encapsulated and unmasked ETU-treated devices, with an active area of 0.2 cm(2), retained > 93 % of their initial power conversion efficiency (PCE) for > 2100 hours at room temperature, and additionally, 1 cm(2) ETU-treated devices maintained T80 (the duration for the PCE to decay to 80 % of the initial value) for > 600 hours at 65 degrees C, under continuous 1-sun illumination at the maximum power point in ambient conditions. Our demonstration of scalable and stable perovskite solar cells represents a promising step towards achieving a reliable perovskite PV technology.