Ultrafast physics in 2D halide perovskites for applications in optoelectronic devices

Project Information

ULTRA-2DPK

Grant agreement ID: 101106654

DOI

10.3030/101106654

Project closed

EC signature date 23 March 2023

Start date 1 May 2023

End date 30 April 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 211 754,88

Coordinated by

INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE RENNES
France

Periodic Reporting for period 1 - ULTRA-2DPK (Ultrafast physics in 2D halide perovskites for applications in optoelectronic devices)

Reporting period: 2023-05-01 to 2025-04-30

Summary of the context and overall objectives of the project

In this project, we investigated some of the fundamental mechanisms that determine the power conversion efficiencies of halide perovskites to facilitate the optimization of perovskite-based optoelectronic devices. The key scientific outcomes were: (i) a new computational code and approach to anharmonic electron-phonon coupling with extensions to nonequilibrium dynamics, (ii) the support of experimental discovery of new layered halide perovskites and heterostructures, and the calculation of their optoelectronic and vibraitonal properties, (iii) the generation of a database of locally disordered perovskite structures to support future first-principles machine-learning driven approaches. This project was carried out by Dr. Marios Zacharias (MZ) in the FOTON institute, hosted at INSA Rennes, under the supervision of Prof. Jacky Even (JE). Another key outcome was the professional maturity and enhancement of competences and skills of MZ to proceed to his career’s next step and secure a tenure track position as an Assistant Professor at the Cyprus Institute, an EU research organization.

The scientific results of the project contributed to accelerate research on the emerging perovskite solar cells, which feature reduced fabrication costs and high power conversion efficiencies. The project untoubly boosted the central role of Europe in renewable energy transformation, supporting future applications in the photovoltaics industry, and in general, optoelectronics, and the creation of a low-carbon economy by 2050. The computational code, data, and more than 10 scientific publications are all available in open-source platforms. MZ and JE also participated to international conferences and workshops to disseminate the results of this project, as well as uploaded videos and posts on social media platforms to support the rapid dissemination of the project's outcomes.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We directly address one of the most important pending questions on the physics of halide perovskites, which is expected to have a great impact on the fields of photovoltaics and thin films optoelectronics, including low dimensional nanostructures for quantum applications. Our current understanding of the limited carrier transport in halide perovskites is based on standard electron-phonon scattering mechanisms, among which the Fröhlich coupling is assumed to be the only sizeable one [10.1021/acsener-gylett.8b02346] with some further contributions from ferroelectric effects, Rashba-type band splitting, and polaronic transport [10.1021/acs.jpclett.0c00018]. Furthermore, various experimental results have been progressively discussed in terms of carrier localization or exciton self-trapping [10.1126/science.aap8671 10.1021/acs.chemrev.8b00477]. The key to fundamentally advance our understanding of perovskites’ properties hinges on systematic deviations of the lattice dynamics from the idealized phonon picture and on how to fully consider their overwhelming lattice anharmonicity in electron-phonon interactions. In the present pioneering study, we explore the concept of local disorder in ultrasoft cubic perovskites to shed light on these intriguing aspects.

Our discovery: We uncover the crucial role of structural disorder in the description of highly overdamped phonon dynamics and electron-phonon interaction in cubic perovskites. Developing a remarkably efficient methodology [10.1038/s41524-023-01089-2 10.1103/PhysRevB.108.035155,10.48550/arXiv.2506.10402,10.48550/arXiv.2506.09673] we developed a unified treatment of anharmonicity and electron-phonon coupling in locally disordered materials and demonstrate that (i) lattice dynamics in halide perovskites depart severely from the textbook phonon picture, exhibiting extensive broadening and non-dispersive optical vibrations, while preserving acoustic dispersions, (ii) electron-phonon coupling in halide perovskites is dominated by anharmonic optical vibrations, and (iii) the complex potential energy landscape has a central role in the prediction of cubic oxide and halide perovskites’ band gaps, effective masses, phonon-induced band gap renormalization, carrier mobilities, and ultrafast dynamics. Our methodology and data give unprecedented agreement with lattice and optical spectroscopy measurements, calling for revisiting open questions connected to anharmonic and electron-phonon properties of cubic perovskites. Some of the breaking new ground physics explored here are intrinsically related to the extraordinary lattice softness of halide perovskites, and, thus, their ability to sustain a high degree of polymorphism as opposed to conventional semiconductors.

Key achievements
• Multi-well potential energy surface: We elucidate the strong connection of the multi-well potential energy landscape to the degree of local disorder, anharmonicity, and electron-phonon coupling in cubic perovskites.
• Origin of the overdamped phonon dynamics: We show that the multi-well potential energy landscape is at the origin of phonon bunching and overdamping observed in neutron scattering measurements and, thus, the key to understand the complex and unique phenomena manifested in halide perovskites.
• New methodology: We develop and demonstrate a new first-principles approach (A-SDM) that enables very efficient electron-phonon calculations in strongly anharmonic and/or locally disordered materials.
• Role of disorder in anharmonic electron-phonon coupling: We show that local disorder is crucial to correctly describe the anharmonic electron-phonon properties of cubic perovskites, using the example of phonon-induced
band gap renormalization. This finding casts doubt on the archetypal fully-ordered picture of cubic perovskites.
• Multiphonon diffuse scattering: We identify and explain new features in measured diffuse scattering patterns of halide perovskites, arising from highly anharmonic multiphonon excitations.
• Accurate electronic structure calculations: We clarify for the first time that accurate electronic structure calculations of anharmonic cubic crystals require combined corrections due to disorder, spin-orbit coupling, exchange-
correlation functionals of high accuracy, and electron-phonon coupling.
• Workflow: We provide a very robust and simple workflow that opens the way for straightforward calculations of anharmonicity by any electronic structure code.
• High-throughput capability: We evaluate anharmonic phonon dispersions of 12 complex halide perovskites, demonstrating the generality of our approach as well as its high-throughput capability.
• Our methodology and codes were combined successfully to explain phenomena observed in state-of-the-art scattering and spectroscopy experiments with collaborations across the world, as attested by the high-impact scientific publications.

The calculations, data, and code developed for this project were made available via the NOMAD repository and GitLab: https://gitlab.com/epw/q-e/.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Impact on the perovskite community: We propose and confirm a radically new way of conceptualize the lattice dynamics in cubic perovskites and their impact on vibrational and electron-phonon properties. At a fundamental level, our new approach opens the way for elucidating breaking ground physics related to optoelectronic and thermoelectric applications of cubic perovskites. Furthermore, our work will provide a whole new perspective in the study of non-equilibrium phonon and carrier dynamics which might lead to new discoveries in the popular fields of hot-carrier photovoltaics and ultrafast optical communications. We also stress that the method developed is very accurate, efficient, and powerful, that we expect to become the roadmap for future strongly anharmonic and electron-phonon calculations of perovskites. Our code developments are available in Quantum Espresso, a widely used open-source package. Our developments will be extended in the near future to more complex materials, like superionic conductors, multilayered perovskites, as well as 2D/3D perovskite or perovskite/silicon heterostructures, which are of utmost importance as the active zone of durable and highly efficient hybrid photovoltaic devices.

Interest to a broad readership: The results of this project will have a strong appeal to the broad communities of physics, photonics, semiconductor devices, phonon and optical spectroscopy, as well as of experimental material and chemical sciences. These communities put a great effort into studying the peculiar vibrational and electron-phonon properties of highly disordered semiconductors, and our developments will be the key to clarify open questions or unexplored properties. We also feel that our method for anharmonic electron-phonon coupling opens entirely new opportunities in electronic structure calculations, and it has the potential to be adopted by thousands of materials scientists in this area. We also believe that the results of the project lead to significant new understanding of materials fundamentals and the development of emerging computational science approaches.

We also regard the development of the anharmonic special displacement method (ASDM) introduced via this project as a significant step towards very efficient ab initio calculations of anharmonicity, pushing this established field into a new direction. Not only is our method accurate and stable, but it is also simple to the point that it is suitable for both condensed matter theorists and experimentalists. Given its impressive efficiency, we expect that it will become a systematic feature of future electron-phonon calculations in anharmonic systems. It also holds promise to pave the way for calculations of other anharmonic-driven properties, like thermal conductivity, at very low cost. These aspects are of paramount importance for ongoing and future developments of thin films containing soft materials for low-cost applications. Our outcomes are also particularly timely given the rising importance of data quality and fidelity in materials informatics. We demonstrate that realistic, thermally consistent features, rooted in atomic-level disorder and anharmonicity, are essential for accurately capturing the finite-temperature behavior of soft semiconductors like halide perovskites. Our calculations offer a benchmark dataset for validating machine learning force fields, guiding the discovery of new materials, and advancing the next generation of data-driven materials modeling.

Future actions for standardization and community adoption: (i) Establish reference workflows: The ASDM code and associated scripts or Jupyter Notebooks will be published into modular, user-friendly workflows (e.g. with Quantum ESPRESSO, EPWpy, or AiiDA plugins) as well as published as reference cases with datasets. (ii) Create benchmark sets: Define community benchmarks for anharmonic phonon and electron-phonon calculations to compare across methods and codes. (iii) Publish standards in modeling disorder: Advocate for a consistent theoretical and computational framework to treat local disorder, beyond averaging or perturbative approaches. (iv) Benchmark across techniques: Collaborate with neutron/X-ray scattering and ultrafast spectroscopy groups to systematically validate predictions across multiple compounds and temperature ranges. (v) Expand to broader material classes: Apply the developed methodology (A-SDM) to other anharmonic or disordered systems, including chalcogenides, thermoelectrics, and topological materials.

Impact of polymorphism (local disorder) on fundamental mechanisms in energy efficient materials.