Center for Molecular Modeling - T. Braeckevelt

Generating a Stable Higher-Symmetry CsPbI3 Perovskite Phase in Ambient Conditions: Unveiling the Stabilization Mechanism

tbraeckevelt — Mon, 26 Jan 2026 12:20:59 +0000

R.A. Saha, A. Papadopoulou, R. Ariza, G. Degutis, I. Skvortsova, T. Braeckevelt, F. De Angelis, E. Solano, J. P. de Sousa Gouveia dos Anjos, M. I. Pintor Monroy, I. Mongilyov, B. God, J. Rubio-Zuazo, J. Genoe, C. Meneghini, J.A. Steele, S. Bals, V. Van Speybroeck, J. Hofkens

ACS Nano

19, 31, 28540–28553

2025

Abstract

Black-phase cesium lead iodide (CsPbI₃) is a promising candidate for high-efficiency perovskite optoelectronics, but its instability under ambient conditions remains a major challenge. Among several strategies, dimethylammonium iodide (DMAI) has emerged as a potential stabilizer; however, inconsistencies in phase stability (3–7 days) and lower solar power conversion efficiencies (∼20 vs ∼27% for hybrid perovskites) highlight the need for further improvements. This study not only demonstrates enhanced stabilization of the high-symmetry black phase of CsPbI₃ and improved film morphology through optimized composition and annealing conditions but also more importantly provides detailed mechanistic insights obtained from comprehensive experimental and theoretical analyses. Systematic tuning of the DMAI concentration (1.2 M), annealing temperature (200 °C, 1 min), and Cs⁺ substitution (12–15%) significantly extends phase stability to 7 days under ambient conditions (35–52% relative humidity) and maintains stability even after 16 months in a drybox environment by reducing orthorhombic strain and octahedral tilting. Additionally, a minor (∼5%) zero-dimensional (0D) Cs₄PbI₆ phase fills pinholes, enhancing the film quality. Optimized photodiodes exhibit a low dark current (∼1 μA/cm²), high external quantum efficiency (∼80% at −2 V), and a ≥100 dB linear dynamic range. These findings provide mechanistic insights into the stabilization of the black phase of CsPbI₃, advancing the development of more stable and efficient perovskite-based optoelectronic devices.

DOI

http://dx.doi.org/

Quantitative Description of Strongly Correlated Materials by Combining Downfolding Techniques and Tensor Networks

leen — Mon, 18 Aug 2025 07:56:32 +0000

D. Vrancken, S. Ganne, D. Verraes, T. Braeckevelt, L. Devos, L. Vanderstraeten, J. Haegeman, V. Van Speybroeck

Journal of Chemical Theory and Computation

21, 16, 7830-7844

2025

Abstract

We present a high-accuracy procedure for electronic structure calculations of strongly correlated materials. To address limitations in current electronic structure methods, we employ density functional theory in combination with the constrained random phase approximation to construct an effective multiband Hubbard model, which is subsequently solved using tensor networks. Our work focuses on one-dimensional and quasi-one-dimensional materials, for which we employ the machinery of matrix product states. We apply this framework to the conjugated polymers trans-polyacetylene and polythiophene, as well as the quasi-one-dimensional charge-transfer insulator Sr₂CuO₃. The predicted band gaps show quantitative agreement with state-of-the-art computational techniques and experimental measurements. Beyond band gaps, tensor networks provide access to a wide range of physically relevant properties, including spin magnetization and various excitation energies. Their flexibility supports the implementation of complex Hamiltonians with longer-range interactions, while the bond dimension enables systematic control over accuracy. Furthermore, the computational cost scales efficiently with system size, demonstrating the framework’s scalability.

DOI

http://dx.doi.org/10.1021/acs.jctc.5c00796

Private attachment

vrancken-et-al-2025-quantitative-description-of-strongly-correlated-materials-by-combining-downfolding.pdf

Increasing the Phase Stability of CsPbI3 Nanocrystals by Zn2+ and Cd2+ Addition: Synergy of Transmission Electron Microscopy and Molecular Dynamics

leen — Thu, 26 Jun 2025 11:39:11 +0000

I. Skvortsova, T. Braeckevelt, A. De Backer, N. Schrenker, B. Pradhan, J. Hofkens, S. Van Aert, V. Van Speybroeck, S. Bals

ACS Nano

19, 18, 17698-17708

2025

Abstract

Metal halide perovskites (MHPs) are emerging as promising materials for optoelectronic and photovoltaic applications due to their favorable electronic properties, including a tunable bandgap. However, achieving high stability for these materials remains a critical challenge, particularly for CsPbI₃, whose photoactive phases spontaneously convert into a nonphotoactive yellow orthorhombic δ-phase under ambient conditions. This transformation results in a significant increase in bandgap and a loss of photoactive functionality. In this study, we investigate the impact of Zn²⁺ and Cd²⁺ dopants on the phase stability of CsPbI₃ nanocrystals (NCs), emphasizing the formation of Ruddlesden–Popper (RP) planar defects, which are frequently observed during compositional tuning. Using transmission electron microscopy (TEM), we follow the temporal evolution of the phase transformation, where black-phase NCs agglomerate and form elongated microtubes with a yellow-phase crystal structure. Our observations demonstrate that doped samples are significantly more stable, while the dopants are key factors in the formation of the RP-like defects with specific atomic arrangements. Using a combination of quantitative TEM and molecular dynamics (MD) simulations we characterize the structure and composition of as-found RP-like defects and elucidate their role in stabilizing the photoactive phases of CsPbI₃ through decreased phase transition kinetics.

DOI

http://dx.doi.org/10.1021/acsnano.5c01825

Private attachment

skvortsova-et-al-2025-increasing-the-phase-stability-of-cspbi3-nanocrystals-by-zn2-and-cd2-addition-synergy-of.pdf

Investigation of the Octahedral Network Structure in Formamidinium Lead Bromide Nanocrystals by Low-Dose Scanning Transmission Electron Microscopy

leen — Thu, 22 Aug 2024 09:05:30 +0000

N. J. Schrenker, T. Braeckevelt, A. De Backer, N. Livakas, C.-P. Yu, T. Friedrich, M.B.J. Roeffaers, J. Hofkens, J. Verbeeck, L. Manna, V. Van Speybroeck, S. Van Aert, S. Bals

Nano Letters

24, 35, 10936-10942

2024

Abstract

Metal halide perovskites (MHP) are highly promising semiconductors. In this study, we focus on FAPbBr₃ nanocrystals, which are of great interest for green light-emitting diodes. Structural parameters significantly impact the properties of MHPs and are linked to phase instability, which hampers long-term applications. Clearly, there is a need for local and precise characterization techniques at the atomic scale, such as transmission electron microscopy. Because of the high electron beam sensitivity of MHPs, these investigations are extremely challenging. Here, we applied a low-dose method based on four-dimensional scanning transmission electron microscopy. We quantified the observed elongation of the projections of the Br atomic columns, suggesting an alternation in the position of the Br atoms perpendicular to the Pb–Br–Pb bonds. Together with molecular dynamics simulations, these results remarkably reveal local distortions in an on-average cubic structure. Additionally, this study provides an approach to prospectively investigating the fundamental degradation mechanisms of MHPs.

This publication is licensed under the terms of your institutional subscription. Request reuse permissions.

DOI

https://doi.org/10.1021/acs.nanolett.4c02811

Private attachment

schrenker-et-al-2024-investigation-of-the-octahedral-network-structure.pdf

Additivity of atomic strain fields as a tool to strain-engineering phase-stabilized CsPbI3 perovskites

leen — Mon, 13 Nov 2023 10:15:01 +0000

J. Teunissen, T. Braeckevelt, I. Skvortsova, J. Guo, B. Pradhan, E. Debroye, M. Roeffaers, J. Hofkens, S. Van Aert, S. Bals, S.M.J. Rogge, V. Van Speybroeck

The Journal of Physical Chemistry C

127, 48, 23400-23411

2023

Abstract

CsPbI₃ is a promising perovskite material for photovoltaic applications in its photoactive perovskite or black phase. However, the material degrades to a photovoltaically inactive or yellow phase at room temperature. Various mitigation strategies are currently being developed to increase the lifetime of the black phase, many of which rely on inducing strains in the material that hinder the black-to-yellow phase transition. Physical insight into how these strategies exactly induce strain as well as knowledge of the spatial extent over which these strains impact the material is crucial to optimize these approaches but is still lacking. Herein, we combine machine learning potential-based molecular dynamics simulations with our in silico strain engineering approach to accurately quantify strained large-scale atomic structures on a nanosecond time scale. To this end, we first model the strain fields introduced by atomic substitutions as they form the most elementary strain sources. We demonstrate that the magnitude of the induced strain fields decays exponentially with the distance from the strain source, following a decay rate that is largely independent of the specific substitution. Second, we show that the total strain field induced by multiple strain sources can be predicted to an excellent approximation by summing the strain fields of each individual source. Finally, through a case study, we illustrate how this additive character allows us to explain how complex strain fields, induced by spatially extended strain sources, can be predicted by adequately combining the strain fields caused by local strain sources. Hence, the strain additivity proposed here can be adopted to further our insight into the complex strain behavior in perovskites and to design strain from the atomic level onward to enhance their sought-after phase stability.

Open Access version available at UGent repository

Gold Open Access

DOI

http://dx.doi.org/10.1021/acs.jpcc.3c05770

Private attachment

teunissen-et-al-2023-additivity-of-atomic-strain-fields-as-a-tool-to-strain-engineering-phase-stabilized-cspbi3.pdf

Understanding the phase transition mechanism in the lead halide perovskite CsPbBr₃ via theoretical and experimental GIWAXS and Raman spectroscopy

leen — Thu, 06 Apr 2023 07:27:01 +0000

A.E.J. Hoffman, R.A. Saha, S. Borgmans, P. Puech, T. Braeckevelt, M.B.J. Roeffaers, J.A. Steele, J. Hofkens, V. Van Speybroeck

APL Materials

Volume 11, Issue 4, article number 041124

2023

Abstract

Metal-halide perovskites (MHPs) exhibit excellent properties for application in optoelectronic devices. The bottleneck for their incorporation is the lack of long-term stability such as degradation due to external conditions (heat, light, oxygen, moisture, and mechanical stress), but the occurrence of phase transitions also affects their performance. Structural phase transitions are often influenced by phonon modes. Hence, an insight into both the structure and lattice dynamics is vital to assess the potential of MHPs. In this study, GIWAXS and Raman spectroscopy are applied, supported by density functional theory calculations, to investigate the apparent manifestation of structural phase transitions in the MHP CsPbBr₃. Macroscopically, CsPbBr₃ undergoes phase transitions between a cubic (α), tetragonal (β), and orthorhombic (γ) phase with decreasing temperature. However, microscopically, it has been argued that only the γ phase exists, while the other phases exist as averages over length and time scales within distinct temperature ranges. Here, direct proof is provided for this conjecture by analyzing both theoretical diffraction patterns and the evolution of the tilting angle of the PbBr₆ octahedra from molecular dynamics simulations. Moreover, sound agreement between experimental and theoretical Raman spectra allowed to identify the Raman active phonon modes and to investigate their frequency as a function of temperature. As such, this work increases the understanding of the structure and lattice dynamics of CsPbBr₃ and similar MHPs.

Gold Open Access

DOI

http://dx.doi.org/10.1063/5.0144344

Private attachment

041124_1_5.0144344.pdf

An embedded interfacial network stabilizes inorganic CsPbI3 perovskite thin films

tbraeckevelt — Wed, 07 Dec 2022 10:38:53 +0000

J.A. Steele, T. Braeckevelt, V. Prakasam, G. Degutis, H. Yuan, H. Jin, E. Solano, P. Puech, S. Basak, M.I. Pintor-Monroy, H. Van Gorp, G. Fleury, R.X. Yang, Z. Lin, H. Huang, E. Debroye, D. Chernyshov, B. Chen, M. Wei, Y. Hou, R. Gehlhaar, J. Genoe, S. De Feyter, S.M.J. Rogge, A. Walsh, E.H. Sargent, P. Yang, J. Hofkens, V. Van Speybroeck, M.B.J. Roeffaers

Nature Communications

13, 7513

2022

Abstract

The black perovskite phase of CsPbI₃ is promising for optoelectronic applications; however, it is unstable under ambient conditions, transforming within minutes into an optically inactive yellow phase, a fact that has so far prevented its widespread adoption. Here we use coarse photolithography to embed a PbI₂-based interfacial microstructure into otherwise-unstable CsPbI₃ perovskite thin films and devices. Films fitted with a tessellating microgrid are rendered resistant to moisture-triggered decay and exhibit enhanced long-term stability of the black phase (beyond 2.5 years in a dry environment), due to increasing the phase transition energy barrier and limiting the spread of potential yellow phase formation to structurally isolated domains of the grid. This stabilizing effect is readily achieved at the device level, where unencapsulated CsPbI₃ perovskite photodetectors display ambient-stable operation. These findings provide insights into the nature of phase destabilization in emerging CsPbI₃ perovskite devices and demonstrate an effective stabilization procedure which is entirely orthogonal to existing approaches.

DOI

https://doi.org/10.1038/s41467-022-35255-9

Private attachment

s41467-022-35255-9 (1).pdf

How the Layer Alignment in Two-dimensional Nanoporous Covalent Organic Frameworks Impacts Its Electronic Properties

tbraeckevelt — Sat, 11 Jun 2022 18:58:18 +0000

K. S. Rawat, S. Borgmans, T. Braeckevelt, C.V. Stevens, P. Van der Voort, V. Van Speybroeck

ACS Applied Nano Materials

5, 10, 14377-14387

2022

Abstract

Two-dimensional nanoporous covalent organic frame-works (2D COFs) have gathered significant interest due to their wide range of applications. Due to the lack of strong covalent interlayer interactions, their layers can be stacked in countless ways, each resulting in unique nanoscale characteristics impacting the structural, chemical, and electronic properties. To characterize and understand the layer stacking in 2D COFs and its effect on the structural and electronic properties, we carried out a detailed density functional theory investigation on four materials, CTF-1, COF-1, COF-5, and Pc-PBBA. This entailed an in-depth evaluation of the potential energy as a function of the interlayer distance and offset, the powder X-ray diffraction (PXRD) pattern, and the electronic properties. From the potential energy surfaces, the typical slipped AA-stacking configuration was confirmed as optimal for each of the 2D COFs, with a slight offset from a perfect alignment of the layers. The statically calculated PXRD patterns based on these optimized stacking configurations showed discrepancies when compared to experimental data. Instead, when properly accounting for dynamic fluctuations by calculating the average diffraction pattern over the course of a molecular dynamics simulation, a better agreement with the experiment is obtained. Different stacking configurations also profoundly affected the electronic band structure of COFs as the interlayer pi-pi interactions are significantly impacted by the layer offset. Evidently, with decreasing layer offsets, the pi-pi interactions increase due to the layer alignment, leading to a decrease in the band gap and an increase in interlayer charge mobility. Our study highlights the need for accurate modeling of the stacking configuration in 2D COFs as a small-scale deviation in the adjacent layer position can significantly affect the structural and electronic properties.

DOI

https://doi.org/10.1021/acsanm.2c02647

Private attachment

acsanm.2c02647.pdf

Accurately Determining the Phase Transition Temperature of CsPbI3 via Random-Phase Approximation Calculations and Phase-Transferable Machine Learning Potentials

tbraeckevelt — Sat, 11 Jun 2022 18:44:03 +0000

T. Braeckevelt, R. Goeminne, S. Vandenhaute, S. Borgmans, T. Verstraelen, J.A. Steele, M. Roeffaers, J. Hofkens, S.M.J. Rogge, V. Van Speybroeck

Chemistry of Materials

34, 19, 8561–8576

2022

Abstract

While metal halide perovskites (MHPs) have shown great potential for various optoelectronic applications, their widespread adoption in commercial photovoltaic cells or photosensors is currently restricted, given that MHPs such as CsPbI₃ and FAPbI₃ spontaneously transition to an optically inactive nonperovskite phase at ambient conditions. Herein, we put forward an accurate first-principles procedure to obtain fundamental insight into this phase stability conundrum. To this end, we computationally predict the Helmholtz free energy, composed of the electronic ground state energy and thermal corrections, as this is the fundamental quantity describing the phase stability in polymorphic materials. By adopting the random phase approximation method as a wave function-based method that intrinsically accounts for many-body electron correlation effects as a benchmark for the ground state energy, we validate the performance of different exchange-correlation functionals and dispersion methods. The thermal corrections, accessed through the vibrational density of states, are accessed through molecular dynamics simulations, using a phase-transferable machine learning potential to accurately account for the MHPs’ anharmonicity and mitigate size effects. The here proposed procedure is critically validated on CsPbI₃, which is a challenging material as its phase stability changes slowly with varying temperature. We demonstrate that our procedure is essential to reproduce the experimental transition temperature, as choosing an inadequate functional can easily miss the transition temperature by more than 100 K. These results demonstrate that the here validated methodology is ideally suited to understand how factors such as strain engineering, surface functionalization, or compositional engineering could help to phase-stabilize MHPs for targeted applications.

Open Access version available at UGent repository

Gold Open Access

DOI

https://doi.org/10.1021/acs.chemmater.2c01508

Private attachment

acs.chemmater.2c01508.pdf

Texture Formation in Polycrystalline Thin Films of All-Inorganic Lead Halide Perovskite

sven — Sat, 16 Jan 2021 12:42:12 +0000

J.A. Steele, E. Solano, H. Jin, V. Prakasam, T. Braeckevelt, H. Yuan, Z. Lin, R. de Kloe, Q. Wang, S.M.J. Rogge, V. Van Speybroeck, D. Chernyshov, J. Hofkens, M. Roeffaers

Advanced Materials

33 (13), 2007224

2021

Abstract

Controlling grain orientations within polycrystalline all-inorganic halide perovskite solar cells can help increase conversion efficiencies toward their thermodynamic limits, however the forces governing texture formation are ambiguous. Using synchrotron X-ray diffraction, we report meso-structure formation within polycrystalline CsPbI_2.85Br_0.15 powders as they cool from a high-temperature cubic perovskite (α-phase). Tetragonal distortions (β-phase) trigger preferential crystallographic alignment within polycrystalline ensembles, a feature we suggest is coordinated across multiple neighboring grains via interfacial forces that select for certain lattice distortions over others. External anisotropy is then imposed on polycrystalline thin films of orthorhombic (γ-phase) CsPbI_3-xBr_x perovskite via substrate clamping, revealing two fundamental uniaxial texture formations; (i) I-rich films possess orthorhombic-like texture (<100> out-of-plane; <010> and <001> in-plane), while (ii) Br-rich films form tetragonal-like texture (<110> out-of-plane; <1-10> and <001> in-plane). In contrast to relatively uninfluential factors like the choice of substrate, film thickness and annealing temperature, Br incorporation modifies the γ-CsPbI_3-xBr_x crystal structure by reducing the orthorhombic lattice distortion (making it more tetragonal-like) and governs the formation of the different, energetically favored textures within polycrystalline thin films.

DOI

http://dx.doi.org/10.1002/adma.202007224