Effect of static local distortions vs. dynamic motions on the stability and band gaps of cubic oxide and halide perovskites

Zhao, Xian-Geng; Wang, Zhi; Malyi, Oleksandr I.; Zunger, Alex

doi:10.1016/j.mattod.2021.05.021

Cited by 52 publications

(47 citation statements)

References 68 publications

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“…As the B-cation off-center displacement governs the extent of orbital mixing of the band-edge states, stronger lone pair distortion results in a larger band gap and thus leads to a negative EP term in (MHy)PbBr 3 . This is consistent with previous calculations which showed the band gap increases with increasing magnitude of the B-cation off-center displacement. , Such a structure–band gap relationship can also be observed in 3D AGeI 3 with a series of A-cations (Table S5), showing an increase of the band gap with increasing magnitude of Ge 2+ off-center displacement.…”

Section: Resultssupporting

confidence: 92%

“…This is consistent with previous calculations which showed the band gap increases with increasing magnitude of the B-cation off-center displacement. 17,64 Such a structure−band gap relationship can also be observed in 3D AGeI 3 with a series of A-cations (Table S5), 36 showing an increase of the band gap with increasing magnitude of Ge 2+ off-center displacement.…”

Section: ■ Introductionmentioning

confidence: 56%

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Understanding Electron–Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s² Lone Pairs

Huang

et al. 2022

J. Am. Chem. Soc.

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The electron–phonon (e–ph) interaction in lead halide perovskites (LHPs) plays a role in a variety of physical phenomena. Unveiling how the local lattice distortion responds to charge carriers is a critical step toward understanding the e–ph interaction in LHPs. Herein, we advance a fundamental understanding of the e–ph interaction in LHPs from the perspective of stereochemical activity of 6s2 lone-pair electrons on the Pb2+ cation. We demonstrate a model system based on three LHPs with distinctive lone-pair activities for studying the structure–property relationships. By tuning the A-cation chemistry, we synthesized single-crystal CsPbBr3, (MA0.13EA0.87)PbBr3 (MA+ = methylammonium; EA+ = ethylammonium), and (MHy)PbBr3 (MHy+ = methylhydrazinium), which exhibit stereo-inactive, dynamic stereo-active, and static stereo-active lone pairs, respectively. This gives rise to distinctive local lattice distortions and low-frequency vibrational modes. We find that the e–ph interaction leads to a blue shift of the band gap as temperature increases in the structure with the dynamic stereo-active lone pair but to a red shift in the structure with the static stereo-active lone pair. Furthermore, analyses of the temperature-dependent low-energy photoluminescence tails reveal that the strength of the e–ph interaction increases with increasing lone-pair activity, leading to a transition from a large polaron to a small polaron, which has significant influence on the emission spectra and charge carrier dynamics. Our results highlight the role of the lone-pair activity in controlling the band gap, phonon, and polaronic effect in LHPs and provide guidelines for optimizing the optoelectronic properties, especially for tin-based and germanium-based halide perovskites, where stereo-active lone pairs are more prominent than their lead counterparts.

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Section: Resultssupporting

confidence: 92%

Section: ■ Introductionmentioning

confidence: 56%

Understanding Electron–Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s² Lone Pairs

Huang

et al. 2022

J. Am. Chem. Soc.

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“…Among the most accurate methods for the treatment of the excitation problem in a solid, one can find the GW approach, − which is in general computationally demanding. , The conventional computational methodology is based on density functional theory (DFT). − For the specific case of halide perovskites, several important studies have been reported. − DFT band gaps are obtained from the Kohn–Sham orbitals describing the conduction band maximum and conduction band minimum of a system, which of course represents an approximation of the real optical excitation problem, and excitonic effects are neglected. − It should be said that DFT provides a-thermal band gaps, therefore temperature effects are neglected. This aspect can be addressed by performing ab-initio molecular simulation at finite temperature, − and for this specific and challenging problem, we refer to a seminal work by Pasquarello and co-workers . Despite the mentioned critical aspects, DFT represents a valid compromise between computational cost and accuracy.…”

Section: Introductionmentioning

confidence: 99%

Density Functional Theory Estimate of Halide Perovskite Band Gap: When Spin Orbit Coupling Helps

2022

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The description of the band gap of halide perovskites at the level of density functional theory (DFT) has been subject of several studies but still presents significant problems and deviations from experimental values. Various approaches have been proposed, including the use of system-specific hybrid functionals with a variable amount of exact exchange or the explicit inclusion of spin–orbit coupling (SOC) effects. In this work, we present a pragmatic recipe to compute the band gap of halide perovskites with a minimum average error. The recipe is tested on a set of 36 halide perovskites of the type ABX3 [A = Cs, methyl-ammonium (MA), and formamidinium (FA); B = Ge, Sn, and Pb; and X = Cl, Br, and I] for which experimental estimates of the band gap have been reported in the literature. Upon assessment of the accuracy of commonly used DFT functionals and the analysis of their performances based on error and statistical analysis, we suggest a strategy to compute band gaps in halide perovskites with a single functional. This is based on the use of the hybrid HSE06 functional where SOC is included exclusively for Pb-containing compounds. The results are rationalized in terms of the materials’ chemical nature and are corroborated by the prediction of their expected efficiencies in solar cells. The calculated efficiencies from band gaps obtained with the proposed approach closely follow the experimental trend, demonstrating the importance of adopting a reliable but material-independent computational strategy to screen new halide perovskite materials for solar energy conversion.

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“…Modulated phases could also be in principle seen in ab initio calculations, albeit using very large supercells. In that context we note recent calculations [32,33] which predict that STO and cubic BaTiO 3 should be polymorphic at T = 0, with a disordered arrangement of off-site Ti distortions. A disordered ground state at absolute zero violates the third law, and perhaps is associated instead with the failure to match the supercell to the natural incommensurate period.…”

mentioning

confidence: 59%

Lamellar fluctuations melt ferroelectricity

Guzmán-Verri¹,

H.²,

Littlewood³

2022

Preprint

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We consider a Ginzburg-Landau theory for a ferroelectric phase transition whose primary order parameter (electrical polarization) is coupled to gradients of elastic strain (flexoelectricity). At the harmonic level, such couplings lead to a hybridization of acoustic and optic phonon modes and can lead to phases with modulated lattice structures that precede the symmetry broken state due to level repulsion [1][2][3][4][5]. Here, we show that at the mean field plus Gaussian approximation, the fluctuations of polarization of the parent phase tend to diverge at the onset of the transition and therefore the long-range-ordered modulated phase is avoided. We discuss the implications for the nearly ferroelectric SrTiO 3 and KTaO 3 , and propose that these systems are melted versions of an underlying modulated state which is dominated by non-zero momentum thermal fluctuations except at the very lowest temperatures.

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Effect of static local distortions vs. dynamic motions on the stability and band gaps of cubic oxide and halide perovskites

Cited by 52 publications

References 68 publications

Understanding Electron–Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s² Lone Pairs

Understanding Electron–Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s² Lone Pairs

Density Functional Theory Estimate of Halide Perovskite Band Gap: When Spin Orbit Coupling Helps

Lamellar fluctuations melt ferroelectricity

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Effect of static local distortions vs. dynamic motions on the stability and band gaps of cubic oxide and halide perovskites

Cited by 52 publications

References 68 publications

Understanding Electron–Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s2 Lone Pairs

Understanding Electron–Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s2 Lone Pairs

Density Functional Theory Estimate of Halide Perovskite Band Gap: When Spin Orbit Coupling Helps

Lamellar fluctuations melt ferroelectricity

Contact Info

Product

Resources

About

Understanding Electron–Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s² Lone Pairs

Understanding Electron–Phonon Interactions in 3D Lead Halide Perovskites from the Stereochemical Expression of 6s² Lone Pairs