LAUSR

Thermal stability is a critical criterion for assessing the long‐term stability of perovskite solar cells (PSCs). Here, it is shown that un‐encapsulated co‐evaporated MAPbI3 (TE_MAPbI3) PSCs demonstrate remarkable thermal stability even in an n‐i‐p structure that employs Spiro‐OMeTAD as hole transport material (HTM). TE_MAPbI3 PSCs maintain over ≈95% and ≈80% of their initial power conversion efficiency (PCE) after 1000 and 3600 h respectively under continuous thermal aging at 85 °C. TE_MAPbI3 PSCs demonstrate remarkable structural robustness, absence of pinholes, or significant variation in grain sizes, and intact interfaces with the HTM, upon prolonged thermal aging. Here, the main factors driving TE_MAPbI3 stability are assessed. It is demonstrated that the excellent TE_MAPbI3 thermal stability is related to the perovskite growth process leading to a compact and almost strain‐stress‐free film. On the other hand, un‐encapsulated PSCs with the same architecture, but incorporating solution‐processed MAPbI3 or Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3 as active layers, show a complete PCE degradation after 500 h under the same thermal aging condition. These results highlight that the control of the perovskite growth process can substantially enhance the PSCs thermal stability, besides the chemical composition. The TE_MAPbI3 impressive long‐term thermal stability features the potential for field‐operating conditions.