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1. Direct Observation of Bandgap Oscillations Induced by Optical Phonons in Hybrid Lead Iodide Perovskites

   https://doi.org/10.1002/adfm.201907982  

   Guo, Peijun; Xia, Yi; Gong, Jue; Cao, Duyen H.; Li, Xiaotong; Li, Xun; Zhang, Qi; Stoumpos, Constantinos C.; Kirschner, Matthew S.; Wen, Haidan; et al (April 2020, Advanced Functional Materials)

   Abstract Hybrid organic–inorganic perovskites such as methylammonium lead iodide have emerged as promising semiconductors for energy‐relevant applications. The interactions between charge carriers and lattice vibrations, giving rise to polarons, have been invoked to explain some of their extraordinary optoelectronic properties. Here, time‐resolved optical spectroscopy is performed, with off‐resonant pumping and electronic probing, to examine several representative lead iodide perovskites. The temporal oscillations of electronic bandgaps induced by coherent lattice vibrations are reported, which is attributed to antiphase octahedral rotations that dominate in the examined 3D and 2D hybrid perovskites. The off‐resonant pumping scheme permits a simplified observation of changes in the bandgap owing to theA_gvibrational mode, which is qualitatively different from vibrational modes of other symmetries and without increased complexity of photogenerated electronic charges. The work demonstrates a strong correlation between the lead–iodide octahedral framework and electronic transitions, and provides further insights into the manipulation of coherent optical phonons and related properties in hybrid perovskites on ultrafast timescales. 

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2. Observation of the polaronic character of excitons in a two-dimensional semiconducting magnet CrI3

   https://doi.org/10.1038/s41467-020-18627-x  

   Jin, Wencan; Kim, Hyun Ho; Ye, Zhipeng; Ye, Gaihua; Rojas, Laura; Luo, Xiangpeng; Yang, Bowen; Yin, Fangzhou; Horng, Jason Shih An; Tian, Shangjie; et al (September 2020, Nature Communications)

   Abstract Exciton dynamics can be strongly affected by lattice vibrations through electron-phonon coupling. This is rarely explored in two-dimensional magnetic semiconductors. Focusing on bilayer CrI₃, we first show the presence of strong electron-phonon coupling through temperature-dependent photoluminescence and absorption spectroscopy. We then report the observation of periodic broad modes up to the 8th order in Raman spectra, attributed to the polaronic character of excitons. We establish that this polaronic character is dominated by the coupling between the charge-transfer exciton at 1.96 eV and a longitudinal optical phonon at 120.6 cm⁻¹. We further show that the emergence of long-range magnetic order enhances the electron-phonon coupling strength by ~50% and that the transition from layered antiferromagnetic to ferromagnetic order tunes the spectral intensity of the periodic broad modes, suggesting a strong coupling among the lattice, charge and spin in two-dimensional CrI₃. Our study opens opportunities for tailoring light-matter interactions in two-dimensional magnetic semiconductors. 

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3. Frenkel defects promote polaronic exciton dissociation in methylammonium lead iodide perovskites

   https://doi.org/10.1039/d1cp00222h  

   Guan, Yuhan; Zhang, Xu; Nan, Guangjun (March 2021, Physical Chemistry Chemical Physics)

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   Hybrid organic–inorganic perovskite materials, such as CH 3 NH 3 PbI 3 , exhibit substantial potential in a variety of optoelectronic applications. Nevertheless, the interplay between the photoinduced excitations and iodine Frenkel defects which are abundant in CH 3 NH 3 PbI 3 films remains poorly understood. Here we study the light-triggered electronic and excitonic properties in the presence of iodine Frenkel defects in CH 3 NH 3 PbI 3 by using a combination of density functional theory (DFT) and time-dependent DFT approaches, the latter of which treats electron–hole and electron–nucleus interactions on the same footing. For isolated Frenkel defects, electrons are trapped close to the iodine vacancies and the electron–hole correlation brings the holes in close vicinity to the electrons, yielding tightly bound polaronic excitons. However, in the presence of multiple interactive Frenkel defects, the holes are pulled out from an electron–hole Coulomb well by the iodine interstitials, leading to spatially separated electron–hole pairs. The X-ray photoelectron spectra are then simulated, unravelling the light-triggered charge transfer induced by Frenkel defects at the atomistic level. We also find that the energy and spatial distributions of polaronic excitons at the Frenkel defects can be controlled by the dynamical rotation of organic cations. 

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4. Benchmark thermodynamic analysis of methylammonium lead iodide decomposition from first principles

   https://doi.org/10.1088/2515-7655/ad139d  

   Heine, Douglas; Yu, Hui-Chia; Blum, Volker (December 2023, Journal of Physics: Energy)

   Abstract Hybrid organic–inorganic perovskites (HOIPs) such as methylammonium lead iodide (MAPbI₃) are promising candidates for use in photovoltaic cells and other semiconductor applications, but their limited chemical stability poses obstacles to their widespread use.Ab initiomodeling of finite-temperature and pressure thermodynamic equilibria of HOIPs with their decomposition products can reveal stability limits and help develop mitigation strategies. We here use a previously published experimental temperature-pressure equilibrium to benchmark and demonstrate the applicability of the harmonic and quasiharmonic approximations, combined with a simple entropy correction for the configurational freedom of methylammonium cations in solid MAPbI₃and for several density functional approximations, to the thermodynamics of MAPbI₃decomposition. We find that these approximations, together with the dispersion-corrected hybrid density functional HSE06, yield remarkably good agreement with the experimentally assessed equilibrium betweenT= 326 K andT= 407 K, providing a solid foundation for future broad thermodynamic assessments of HOIP stability. 

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5. Predicting dynamic spin splitting in 2D hybrid organic-inorganic perovskites via machine learning model

   https://doi.org/10.1063/5.0261959  

   Bhattacharya, Sampreeti; Thomas, Aidan J; Kanai, Yosuke (August 2025, Chemical Physics Reviews)

   Hybrid organic–inorganic perovskites (HOIPs) have emerged as a promising class of materials for optoelectronic and spintronic applications. Layered two-dimensional (2D) HOIP variants have received considerable attention, primarily due to their unique properties that can be modulated through the tailored selection of both organic and inorganic components. The spin splitting in the band structure due to the strong spin–orbit coupling is one of the most intriguing properties of such 2D HOIPs materials for their potential utility in spintronics. In addition to observing the spin splitting in equilibrium due to the non-centrosymmetric structure, the possibility of having dynamic spin splitting at room temperature of the otherwise centrosymmetric systems has become a topic of great debate. While modern first-principles molecular dynamics (FPMD) simulation is able to address such a question in principle by taking into account the lattice anharmonicity in electronic structure calculation, the finite-size error poses a great challenge in practice. In this work, we employ a machine learning (ML) model to overcome this practical limitation to investigate the dynamic spin splitting in phenylethyl ammonium lead iodide 2D HOIP. Specifically, we use the deep potential molecular dynamics approach [Zeng et al., J. Chem. Phys. 159(5), 054801 (2023)] for ML FPMD simulation, and we also develop a surrogate model for predicting the spin splitting based on the recent finding that relates the spin splitting to structural descriptors in 2D HOIPs. Our work shows that even in globally centrosymmetric structures, the inclusion of lattice anharmonicity can induce dynamic spin splitting at room temperature. 

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