Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites (1606.01841v1)

Abstract: We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations.1 Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm-1. Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamics effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity. We show that optical rather than acoustic phonon scattering is likely to prevail at room temperature in these materials.

• The paper establishes a novel framework by combining Raman, terahertz spectroscopy, and DFT calculations to assign vibrational modes in MAPbX₃ perovskites.
• It demonstrates that phase transitions induce dynamic disorder, broadening Raman peaks and generating additional spectral features.
• The research reveals that exceptionally short phonon lifetimes and strong anharmonic coupling critically affect charge mobility and thermal conductivity.

Overview of the Vibrational Spectra and Phonon Dynamics in Methylammonium Lead Halide Perovskites

The paper offers a comprehensive analysis of the vibrational spectra and phonon dynamics in methylammonium lead halide perovskites (MAPbX₃, where X = I, Br, Cl) through a combined experimental and theoretical approach. The paper uses Raman and terahertz absorption spectroscopy across a wide temperature range to elucidate the intricate coupling of molecular and lattice dynamics within these materials. The work is supported by density functional theory (DFT) phonon calculations and provides a robust framework for the assignment of vibrational modes, a crucial aspect for the advancement of hybrid perovskite applications in optoelectronics and photovoltaics.

Key Findings

• Phase Transition and Dynamic Disorder: The research illustrates that the orthorhombic-to-tetragonal phase transition is pivotal in unlocking the rotational degrees of freedom of the methylammonium cations. This dynamic disorder contributes to the broadening of Raman peaks and the emergence of additional spectral features, which previous studies may have misattributed to overtones or harmonics.
• Phonon Lifetimes and Thermal Conductivity: The material system demonstrates exceptionally short phonon lifetimes, particularly in MAPbI₃, which correlates with its low thermal conductivity. This characteristic derives from significant anharmonic phonon-phonon coupling, suggesting that optical phonon scattering predominantly governs the phonon relaxation processes at room temperature.
• Implications for Device Performance: The detailed phonon dispersion and lifetime data provide critical insights into the charge carrier mobility and optical losses within these materials. The findings suggest that optical, instead of acoustic, phonon scattering might limit electron mobility at elevated temperatures, a critical consideration for device efficiency.

Implications and Future Directions

The elucidation of vibrational spectra and phonon properties in MAPbX₃ materials presents significant implications for their development and optimization in photovoltaic and other semiconductor applications. Understanding the nature of phonon scattering mechanisms and dynamic disorder offers pathways to engineer material compositions that enhance thermal and charge transport properties. Moreover, the demonstrated coupling between molecular motion and lattice dynamics might be leveraged to design new hybrid perovskites with tailored thermoelectric properties.

Speculatively, future research might extend to similar hybrid organic-inorganic frameworks under varying environmental conditions to explore stability and performance under operational stresses. Advanced computational methods could further refine the understanding of disorder effects, potentially enabling the prediction of emergent properties in novel perovskite-based materials. Additionally, integrating these insights into high-throughput spectroscopic techniques could streamline the quality assessment of perovskite films during the manufacturing process, accelerating their implementation in commercial technologies.

Overall, the paper provides a fundamental baseline for interpreting vibrational spectra in these complex materials and sets the stage for further exploration and technological advancements in hybrid perovskite systems.