Study Report on the Formation of Exciton Polarons and High Carrier Relaxation in Rigid Dion–Jacobson Type Two-Dimensional Perovskite

Two-dimensional organic-inorganic hybrid perovskites (HOIPs) have garnered widespread attention due to their strongly confined exciton states and reduced dielectric screening effects resulting from their two-dimensional layered structure. These properties render them promising for applications in light-emitting devices, photodetectors, photovoltaics, and quantum emitters. However, the complex interplay between electronic and lattice dynamics plays a crucial role in the efficiency of these materials. The functional role of exciton-phonon interactions remains particularly uncertain. This paper aims to uncover the strong polaronic nature of excitons in these materials and their relationship with high carrier cooling behavior through a combination of ultrafast spectroscopy and electronic structure calculations.

Source of the Paper

The paper was published in the top-tier journal “Nature Materials,” with the DOI: https://doi.org/10.1038/s41563-024-01895-z. The primary authors include Somnath Biswas, Ruyan Zhao, Fatimah Alowa, and others from prestigious institutions such as Princeton University, University of Toronto, and Boston University. The article was received on April 9, 2024, with the exact online publication date pending.

Research Background and Objectives

Two-dimensional Dion–Jacobson (DJ) type perovskites are more stable compared to another type, Ruddlesden–Popper (RP) perovskites. Although studies have shown that the performance of these materials highly depends on the complex interactions between electronic and structural dynamics, the role of exciton-phonon interactions in DJ-type materials and their influence on material performance have not been fully understood. This study aims to fill this gap by investigating the formation of exciton polarons, the regulation of exciton-phonon interactions, and their impact on carrier cooling behavior in DJ-type two-dimensional perovskites.

Research Methods

The study described in the paper was conducted through a series of precise experiments and calculations following the specific steps outlined below:

Workflow

1. Material Preparation and Characterization:

   • DJ-type perovskite thin films were prepared using organic diamine and lead iodide in a solvent mixture. The desired two-dimensional perovskite phase was obtained through spin coating and appropriate thermal treatment.
   • The crystallinity and phase purity of the samples were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM).

2. Ultrafast Spectroscopy Measurements:

   • Femtosecond transient absorption (TA) spectroscopy was employed to measure the TA signals of simple components (fpp)PbI4 and complex components (fpt)PbI4 at different probe energies and time delays.

3. Electronic Structure Calculations:

   • Density functional theory (DFT) was used for calculations, employing a special displacement method to estimate the coupling strengths of different modes to electrons and holes.

Experimental Design

• Vibrational Dynamics of Exciton Polaron Formation: Direct evidence of exciton polaron formation was provided by observing molecular-like phonon wave packet motion, using coherent phonon generation from the resonance Raman scattering mechanism.

• Electronic Structure Calculations of Low-Frequency Phonon Modes: The calculated bandgap changes were used to estimate the reorganization energies of different phonon modes, thus evaluating exciton-phonon coupling.

• Carrier Relaxation Experimental Measurements: By comparing carrier relaxation decay curves at different excitation densities (high (>10^18 cm^-3) and low excitation density), the effect of exciton-phonon interactions on carrier cooling was investigated.

Research Results

Exciton Polaron Formation

The study confirmed the polaronic nature of excitons in DJ-type two-dimensional perovskites through electronic structure calculations and ultrafast spectroscopy measurements. Specifically, the electronic structure calculations of (fpt)PbI4 showed that octahedral twisting mode at ~40 cm^-1 and octahedral rocking mode at ~33 cm^-1 play important roles in modulating exciton transition energy. The TA data highlighted the significance of these low-frequency phonon modes in exciton polaron formation. The results indicated that the polar interactions between excitons and the lattice stabilize the exciton, forming a polaron.

Carrier Cooling

At high excitation density, strong exciton-phonon coupling extended the lifetime of hot carriers, creating a hot phonon bottleneck effect. In these DJ-type perovskites, (fpt)PbI4 and (fpp)PbI4 exhibited different hot carrier relaxation behaviors. At high excitation density, the carrier cooling time in (fpt)PbI4 samples was dependent on exciton-phonon coupling strength, demonstrating that the hot phonon bottleneck effect becomes significant due to the non-equilibrium phonon population generated at high excitation levels.

Conclusion

The study, through experimental observations and calculations, demonstrated that the exciton polaron properties in DJ-type two-dimensional perovskites and their relationship with carrier cooling can be controlled by adjusting the structure of organic ligands. Notably, the molecular-like coherent phonon wave packet node was observed at room temperature for the first time, offering new avenues for modulating carrier cooling behavior through ligand modification. These findings provide insights for developing hybrid semiconductor materials with tailored properties, which are significant for future applications such as hot carrier solar cells.

Research Highlights

• Observed the molecular-like coherent phonon wave packet node in DJ-type perovskites at room temperature for the first time, indicating strong exciton-phonon coupling in these materials.
• Experimental and computational results demonstrated that exciton-phonon coupling strength and its effect on hot carrier cooling could be significantly influenced by manipulating organic ligands.
• Discovered the mechanism of hot phonon bottleneck effect formation at high excitation density, providing new perspectives for understanding and controlling the performance of these materials in optoelectronic applications.

This study provides a theoretical foundation and experimental evidence for the application of DJ-type two-dimensional perovskites in optoelectronic devices, showcasing their potential for future uses in fields like solar cells.