The approaches to stabilize the perovskite structure of formamidinium lead iodide (FAPI) commonly result in a blue shift of the band gap, which limits the maximum photoconversion efficiency. Here, we report the use of PbS colloidal quantum dots (QDs) as a stabilizing agent, preserving the original low band gap of 1.5 eV. The surface chemistry of PbS plays a pivotal role by developing strong bonds with the black phase but weak ones with the yellow phase. As a result, a stable perovskite FAPI black phase can be formed at temperatures as low as 85 °C in just 10 min, setting a record of concomitantly fast and low-temperature formation for FAPI, with important consequences for industrialization. FAPI thin films obtained through this procedure reach an open-circuit potential (Voc) of 1.105 V, 91% of the maximum theoretical Voc, and preserve the efficiency for more than 700 h. These findings reveal the potential of strategies exploiting the chemi-structural properties of external additives to relax the tolerance factor and optimize the optoelectronic performance of perovskite materials.