CsPbI₃ and DMA-Incorporated CsPbI₃: How Stable Are They?

The thermal stability of inorganic cesium lead triiodide (CsPbI₃) 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 CsPbI₃-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 CsPbI₃-based solar cell. Next, we show that the DMAI-doped CsPbI₃ 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 CsPbI₃ 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.