Chemical Composition and Phase Evolution in DMAI-Derived Inorganic Perovskite Solar Cells

Inorganic CsPbI3 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/CsPbI3 layer during thermal treatment has been studied. The results demonstrate that, with the common DMAI/CsI/PbI2 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 Cs4PbI6. Further thermal treatment will transform the mixed perovskite phase directly to gamma-CsPbI3 and then spontaneously convert to delta-CsPbI3. 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 gamma-CsPbI3. 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 gamma-CsPbI3 perovskite.