Conventional wisdom is that the MAI and FAI are stable in the solution, but actually they are not. We demonstrated that the MAI first deprotonated to form methylamine (MA), and then MA reacted with FAI to form two condensation products N-methyl FAI and N, N 0 -dimethyl FAI. Moreover, triethyl borate was introduced to stabilize the perovskite precursor solution, which significantly reduced the impure phase in the perovskite film and enhanced the device performance and reproducibility.

Inorganic CsPbI 3 is promising to enhance the thermal stability of perovskite solar cells. The dimethylamine iodide (DMAI) derived method is currently the most efficient way to achieve high efficiency, but the effect of DMAI has not been fully explained. Herein, the chemical composition and phase evolution of the mixed DMAI/CsPbI 3 layer during thermal treatment has been studied. The results demonstrate that, with the common DMAI/CsI/PbI 2 recipe in DMSO solvent, a mixed perovskite DMA 0.15 Cs 0.85 PbI 3 is first formed through a solid reaction between DMAPbI 3 and Cs 4 PbI 6 . Further thermal treatment will transform the mixed perovskite phase directly to γ-CsPbI 3 and then spontaneously convert to δ-CsPbI 3 . It has been also demonstrated that the DMA 0.15 Cs 0.85 PbI 3 phase is thermodynamically stable and shows a bandgap of 1.67 eV, which is narrower than 1.73 eV of γ-CsPbI 3 . The device efficiency of the mixed DMA 0.15 Cs 0.85 PbI 3 perovskite is therefore highly improved in comparison with the pure inorganic γ-CsPbI 3 perovskite.