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 photo-conversion efficiency.Here, we report the use of PbS colloidal quantum dots (QDs) as 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, stable perovskite FAPI black phase can be formed at temperatures as low as 85°C in just 10 minutes, 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 hours. 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.

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