Structural Characterization and Room Temperature Low-Frequency Raman Scattering from MAPbI₃ Halide Perovskite Films Rigidized by Cesium Incorporation

The structural instability of organometal halide perovskites (OHP) is one of the major issues concerning commercialization of perovskite solar cells. Probing this intrinsic instability is one of the major milestones and challenging tasks toward enhancing the lifespan of the material. Here we have incorporated Cs ions into methylammonium lead iodide (MAPbI₃) films and studied the thin film structural and optical properties. Incorporation of Cs into MAPbI₃ leads to formation of both α-CsPbI₃ and ∂-CsPbI₃ phases, black and yellow, respectively, as indicated by the evolution of the optical band edge and X-ray diffraction (XRD) spectrum. At a concentration of 20% Cs ions, we observe the existence of a stable α-CsPbI₃ phase. Incorporating 59% or more Cs ions yields the yellow phase of CsPbI₃, due to alloying of Cs with the MAPbI₃ matrix. The structural transformations observed in absorption spectra and XRD are confirmed by low-frequency Raman spectroscopy. The thermally induced structural fluctuations in pure MAPbI₃ films are damped upon Cs incorporation, thus bringing long-range stabilized order to the perovskite structure and enabling for the first time observation of low-frequency Raman scattering at room temperature for OHPs. In addition to this, Cs incorporation rigidizes the perovskite film and sharpens all low-frequency vibrational peaks. This rigidizing effect can explain the importance of incorporating and alloying heavy elements into OHPs to bring both chemical stability and photostability.