Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal and cubic phases of methylammonium lead iodide

Abstract: The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural behaviour, with successive transitions between orthorhombic, tetragonal and cubic polymorphs at ca. 165 K and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The equilibrium structures compare well to solutions of temperature-dependent powder neutron diffraction. By following the normal modes we calculate infrared and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degradation of the sample. Despite a clear separation in energy between low frequency modes associated with the inorganic PbI3 network and high-frequency modes of the organic CH3NH3+ cation, significant coupling between them is found, which emphasises the interplay between molecular orientation and the corner-sharing octahedral networks in the structural transformations. Soft modes are found at the boundary of the Brillouin zone of the cubic phase, consistent with displacive instabilities and anharmonicity involving tilting of the PbI6 octahedra around room temperature.

• The paper presents a DFT-based analysis revealing detailed phonon spectra for the orthorhombic, tetragonal, and cubic phases.
• The paper shows that low-frequency vibrations are driven by the inorganic framework, while high-frequency modes arise from organic cations.
• The paper uncovers soft modes and anharmonic effects in the cubic phase, highlighting key implications for perovskite solar cell performance.

Lattice Dynamics and Vibrational Spectra of Methylammonium Lead Iodide Phases

This paper presents a comprehensive first-principles investigation into the lattice dynamics and vibrational spectra of the methylammonium lead iodide (\ce{CH3NH3PbI3}) perovskite's orthorhombic, tetragonal, and cubic phases. The research is grounded in the general thermodynamic behavior of these materials, which undergo structural transitions based on temperature variations—transforming from orthorhombic to tetragonal to cubic phases at approximately 165 K and 327 K, respectively.

Lattice Dynamics and Structural Analysis

The study employs density functional theory (DFT) with the PBEsol functional framework to calculate the phonon spectra for each phase. The findings emphasize the distinction and interaction between the inorganic \ce{(PbI3^-)_n} frameworks and organic \ce{CH3NH3+} components, revealing significant vibrational coupling. In particular, the research identifies that low-frequency modes are predominantly driven by the inorganic framework, while high-frequency vibrations are typically associated with the organic cations. However, notable coupling between these components is observed, suggesting complexity in the perovskite's structural dynamics.

Equilibrium structural parameters derived from DFT show satisfactory agreement with experimental data obtained from temperature-dependent powder neutron diffraction. This agreement further extends to infra-red (IR) and Raman spectral intensities for normal modes, which were calculated and compared against experimentally measured single-crystal data.

Soft Modes and Anharmonicity

The research discloses the existence of soft modes at the Brillouin zone boundaries in the cubic phase, indicative of displacive instabilities and anharmonic effects. These soft modes are linked to rotations and tilting of \ce{PbI6} octahedra and are consistent with the material's observed dynamic lattice distortions. The occurrence of these modes highlights the importance of considering anharmonicity, especially when exploring applications in photovoltaic systems.

Implications for Photovoltaics

The implications of these findings are significant for understanding and engineering the performance of hybrid perovskite solar cells. The detailed phonon spectrum, revealing mode coupling and the dynamic behavior of the lattice, provides insights that are crucial for modeling charge transport and recombination processes under operational conditions. The interaction between electron-phonon coupling and dielectric screening plays a vital role in influencing charge carrier dynamics, necessitating advanced phononic consideration in future photovoltaic analyses.

Conclusion

This paper delivers valuable insights into the vibrational properties and structural dynamics of methylammonium lead iodide perovskites. By advancing the understanding of phase-dependent vibrational modes and their coupling, this study aids the theoretical foundation required for the development of enhanced perovskite-based photovoltaic technology. Future research directions may include deploying non-harmonic approaches to better capture the intricate lattice dynamics at elevated temperatures and leveraging this understanding to optimize material properties for specific photovoltaic applications. Such work could potentially lead to more efficient energy conversion systems, fitting within the escalating demands for sustainable and efficient energy solutions.