J.A. Steele

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

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
A1

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

An embedded interfacial network stabilizes inorganic CsPbI3 perovskite thin films

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
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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.

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

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
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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

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

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
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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 CsPbI2.85Br0.15 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) CsPbI3-xBrx 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-CsPbI3-xBrx 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.

Thermal unequilibrium of strained black CsPbI3 thin films

J.A. Steele, H. Jin, I. Dovgaliuk, R.F. Berger, T. Braeckevelt, H. Yuan, C. Martin, E. Solano, K. Lejaeghere, S.M.J. Rogge, C. Notebaert, W. Vandezande, K.P.F. Janssen, B. Goderis, E. Debroye, Y.-K. Wang, Y. Dong, D. Ma, M. Saidaminov, H. Tan, Z. Lu, V. Dyadkin, D. Chernyshov, V. Van Speybroeck, E.H. Sargent, J. Hofkens, M. Roeffaers
Science
365 (6454), 679-684
2019
A1

Abstract 

The high-temperature all-inorganic CsPbI3 perovskite black phase is metastable relative to its yellow non-perovskite phase, at room temperature. Since only the black phase is optically active, this represents an impediment for the use of CsPbI3 in optoelectronic devices. We report the use of substrate clamping and biaxial strain to render stable, at room temperature, black phase CsPbI3 thin films. We used synchrotron-based grazing incidence wide angle x-ray scattering to track the introduction of crystal distortions and strain-driven texture formation within black CsPbI3 thin films when they were cooled following annealing at 330°C. The thermal stability of black CsPbI3 thin films is vastly improved by the strained interface, a response verified by ab initio thermodynamic modelling.

Open Access version available at UGent repository
Gold Open Access

Accurately determining the transition temperature of metal halide perovskites via RPA calculations and phase-transferable machine learning potentials

ISBN/ISSN:
Talk

Conference / event / venue 

DFT2022
Brussels, Belgium
Monday, 29 August, 2022 to Friday, 2 September, 2022

Stabilizing the perovskite phase of cesium lead iodide thin films via interfacial strains

ISBN/ISSN:
Poster

Conference / event / venue 

5th International Conference on Perovskite Solar Cells and Optoelectronics (PSCO19)
Lausanne, Switzerland
Monday, 30 September, 2019 to Wednesday, 2 October, 2019
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