Local polar fluctuations in lead halide perovskite crystals (1604.08107v3)

Abstract: Hybrid lead-halide perovskites have emerged as an excellent class of photovoltaic materials. Recent reports suggest that the organic molecular cation is responsible for local polar fluctuations that inhibit carrier recombination. We combine low frequency Raman scattering with first-principles molecular dynamics (MD) to study the fundamental nature of these local polar fluctuations. Our observations of a strong central peak in both hybrid (CH$_3$NH$_3$PbBr$_3$) and all-inorganic (CsPbBr$_3$) lead-halide perovskites show that anharmonic, local polar fluctuations are intrinsic to the general lead-halide perovskite structure, and not unique to the dipolar organic cation. MD simulations show that head-to-head Cs motion coupled to Br face expansion, on a few hundred femtosecond time scale, drives the local polar fluctuations in CsPbBr$_3$.

• The paper reveals that both hybrid and inorganic lead-halide perovskites exhibit intrinsic local polar fluctuations.
• It employs low-frequency Raman spectroscopy coupled with DFT-based MD simulations to correlate vibrational modes with dynamic structural disorder.
• Findings challenge conventional views by showing that polar fluctuations arise even without polar organic cations, influencing photovoltaic performance.

Overview of Local Polar Fluctuations in Lead Halide Perovskite Crystals

The paper presented in the paper titled "Local Polar Fluctuations in Lead Halide Perovskite Crystals" investigates the inherent polar fluctuations in both hybrid and inorganic lead-halide perovskite materials using a combination of low-frequency Raman scattering and first-principles molecular dynamics (MD) simulations. This investigation is crucial for understanding the dynamic processes responsible for the unique electronic and structural properties of these materials, which are highly relevant for photovoltaic applications.

The research focuses on two perovskite compounds, CH₃NH₃PbBr₃ (a hybrid variant) and CsPbBr₃ (an inorganic variant), both known for their promising photovoltaic characteristics. The essential finding is that significant local polar fluctuations, previously thought to be associated predominantly with the presence of polar organic cations, are also present in all-inorganic perovskites. This challenges the conventional understanding of structural behaviors in these materials.

Experimental and Computational Approaches

The paper employs low-frequency Raman spectroscopy to analyze vibrational modes and local polar fluctuations within these perovskite crystals as a function of temperature, exploring phases from orthorhombic to cubic. Raman scattering spectra reveal a central peak—an indicator of dynamic polar phenomena—which becomes increasingly pronounced in higher temperature phases. These results suggest a substantial role of dynamic disorder intrinsic to both hybrid and inorganic perovskites.

In parallel, MD simulations based on density functional theory (DFT) are used to provide insight into atomic-level processes leading to the observed Raman features. The simulations confirm that dynamic local polar fluctuations are driven by head-to-head cesium motion coupled with bromide ligand expansion within the CsPbBr₃ structure, offering a detailed picture of the disorder mechanisms.

Key Findings and Implications

1. Local Polar Fluctuations: The research demonstrates that anharmonic polar fluctuations are intrinsic to the lead-halide perovskite structure itself, not exclusively tied to the presence of polar organic cations like CH₃NH₃⁺. This finding underscores structural dynamics as a pervasive trait among perovskites.
2. Raman Spectroscopy and MD Correlation: Consistent peaks identified in the Raman spectra and corresponding theoretical predictions from MD simulations provide robust evidence of the structural dynamics, revealing anharmonic motions of Cs and coupled Br in the CsPbBr₃ lattice.
3. Anisotropic Motion: The anisotropic nature of the polar disorder is highlighted by the modulation of Raman intensity with varying polarization angles. This suggests that despite differences in ionic constituents, the polar fluctuations share common traits facilitated by the lead-bromide network in both CsPbBr₃ and CH₃NH₃PbBr₃.
4. Theoretical and Practical Implications: The paper refines the understanding of perovskite stability and electronic properties, potentially impacting the design of solar cell materials. It suggests alternative pathways for enhancing carrier mobility and reducing recombination by managing local structural disorders.

Future Directions

This research opens doors for future explorations into materials with similar dynamic behaviors and paves the way for engineered perovskite systems with tuned properties. Continued investigations into the coupling between electronic excitations and local lattice dynamics could yield insights valuable for enhancing photovoltaic efficiency and stability. Additionally, exploring other inorganic substitutes and their effects on structural dynamics may further diversify the applicability of perovskite materials in various optoelectronic devices.

In summary, this paper significantly contributes to the understanding of dynamic polar phenomena in perovskites, providing a strong foundation for theoretical models and experimental designs in material science and photovoltaic research.