Optical Phonons in Methylammonium Lead Halide Perovskites and Implications for Charge Transport (1607.08541v2)

Abstract: Lead-halide perovskites are promising materials for opto-electronic applications. Recent reports indicated that their mechanical and electronic properties are strongly affected by the lattice vibrations. Herein we report far-infrared spectroscopy measurements of CH${3}$NH${3}$Pb(I/Br/Cl)${3}$ thin films and single crystals at room temperature and a detailed quantitative analysis of the spectra. We find strong broadening and anharmonicity of the lattice vibrations for all three halide perovskites, which indicates dynamic disorder of the lead-halide cage at room temperature. We determine the frequencies of the transversal and longitudinal optical phonons, and use them to calculate, via appropriate models, the static dielectric constants, polaron masses, electron-phonon coupling constants, and upper limits for the phonon-scattering limited charge carrier mobilities. Within the limitations of the model used, we can place an upper limit of 200$\,$cm$^{2}$V$^{-1}$s$^{-1}$ for the room temperature charge carrier mobility in MAPbI${3}$ single crystals. Our findings are important for the basic understanding of charge transport processes and mechanical properties in metal halide perovskites.

• The paper identifies significant anharmonic optical phonon broadening, revealing dynamic lattice disorder in perovskites.
• It determines both TO and LO phonon frequencies using far‐infrared spectroscopy, confirming strong LO-TO splitting in lead-halide cages.
• The study estimates an upper mobility limit (around 200 cm²/Vs in MAPbI₃), emphasizing the impact of phonon scattering on charge transport.

Optical Phonons in Methylammonium Lead Halide Perovskites and Implications for Charge Transport

The paper "Optical Phonons in Methylammonium Lead Halide Perovskites and Implications for Charge Transport" offers a comprehensive investigation of the lattice dynamics in methylammonium lead halide perovskites (MAPbX$_3$, X = I, Br, Cl) at room temperature. Through a series of far-infrared spectroscopy measurements and subsequent analysis, the authors provide insight into the inherent lattice vibrations and their implications on charge transport phenomena in these materials.

Key Findings and Results

1. Lattice Vibrations: The paper identifies strong broadening and anharmonicity in the optical phonon modes of these perovskites, indicating significant dynamic disorder within the lead-halide frameworks even at room temperature. This dynamic state is notable given the material's crystallinity and highlights a critical aspect of its electronic properties.
2. Optical Phonon Spectra: Through careful measurement and modeling, the authors determine both transverse optical (TO) and longitudinal optical (LO) phonon frequencies. The observed LO-TO splitting across all materials substantiates the assignments of these modes to vibrations predominantly within the lead-halide cages.
3. Dielectric Constants and Electron-Phonon Coupling: By employing the Cochran-Cowley relation, the static dielectric constants are calculated, providing a basis for further estimation of electron-phonon interaction parameters. The paper evaluates the Fröhlich interaction parameter, establishing a framework to discuss the role of strong ionicity and polaron effects.
4. Charge Carrier Mobility: Using a combination of derived physical parameters and theoretical modeling, an upper limit for charge carrier mobility, defined predominantly by optical phonon scattering, is estimated. For instance, the mobility in MAPbI$_3$ is calculated to have a ceiling around 200 cm²/Vs, a value aligned with experimental observations albeit at the higher spectrum.

Implications and Future Directions

The findings underscore the fundamental role of lattice dynamics on the optoelectronic properties of hybrid perovskites. The demonstrated broadening and anharmonicity emphasize potential challenges in achieving high mobilities, thereby influencing device performance. Specifically, the observed dynamic disorder which extends across several unit cells, suggests that traditional models based on harmonic vibrations may not be adequately descriptive for these compounds.

From a practical standpoint, this detailed understanding of lattice vibrations could guide efforts to mitigate phonon scattering, which is crucial for optimizing charge transport. The experimentally determined LO-TO splittings and subsequent derived couplings provide necessary parameters for further computational modeling aimed at predicting and enhancing material performance.

Future research might explore strategies to reduce the anharmonic contributions through compositional engineering or explore alternative perovskite structures that might present less scattering. Additionally, expanding the theoretical frameworks to accommodate the pronounced non-harmonic behavior observed could lead to better predictive models for charge dynamics.

In conclusion, the paper adds substantial depth to our understanding of methylammonium lead halide perovskites, providing both a detailed empirical foundation and a theoretical perspective that can drive more effective optimization strategies for perovskite-based optoelectronic devices.