CsPbI ₃ and DMA‐Incorporated CsPbI ₃ : How Stable Are They?

The thermal stability of inorganic cesium lead triiodide (CsPbI 3 ) is often considered superior to the organic–inorganic variants. However, few reports have demonstrated device thermal stability. The incorporation of dimethylammonium iodide (DMAI) appears to improve the phase stability of CsPbI 3 ‐based devices, but little information on its thermal stability is known. Herein, we propose the need to go beyond chemical stability and consider the high‐temperature phase stability of these inorganic perovskites, where high temperatures induce the conversion of the perovskite to nonperovskite phase. We also put forth the viewpoint that for devices, this high‐temperature phase instability essentially equates to thermal instability of the entire CsPbI 3 ‐based solar cell. Next, we show that the DMAI‐doped CsPbI 3 films also undergo a slower but similar conversion, attributable to the loss of the DMA + cation and the reduced iodide vacancies. We then show that encapsulation helps to slow down DMA + release, which slows down the high‐temperature phase conversion of DMAI‐doped CsPbI 3 films, unlike the case for control films where the effect is negligible. Finally, we show that even with encapsulation, the DMA + cation induces photodegradation that worsens the high‐temperature phase instability, therefore underscoring the need for additional additives alongside DMAI, or a replacement of DMAI additive.