| 1. |
- Lan, Zhenyun, et al.
(författare)
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Exploring the Intrinsic Point Defects in Cesium Copper Halides
- 2021
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:2, s. 1592-1598
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Tidskriftsartikel (refereegranskat)abstract
- Cesium copper halides Cs3Cu2X5 (X = Cl, Br, and I) have attracted much attention for optoelectronic applications because of their self-trap excitons and high photoluminescence quantum yield. Intrinsic point defects play a critical role in the optoelectronic performance of these materials by affecting fundamental properties, such as carrier mobility, lifetime, and recombination rate. In this work, we have calculated, by means of quantum mechanical calculations, formation energies and transition levels of all possible intrinsic point defects in Cs3Cu2X5. We have found that only Xi and XCs defects show simultaneously, deep transition energy levels and negative formation energies. Interestingly, the dominant defect under halide-rich growth conditions exhibits much higher concentration than that under halide-poor conditions. Thus, avoiding the halide-rich conditions could help in reducing the defect concentration.
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| 2. |
- Liang, Mingli, et al.
(författare)
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Electronic Structure and Trap States of Two-Dimensional Ruddlesden–Popper Perovskites with the Relaxed Goldschmidt Tolerance Factor
- 2020
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Ingår i: ACS Applied Electronic Materials. - : American Chemical Society (ACS). - 2637-6113. ; 2:5, s. 1402-1412
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional Ruddlesden–Popper perovskites (2D RPPs) have been considered as promising building blocks for optoelectronic applications owing to optical properties comparable to the ones of 3D perovskites, together with superior stability. In addition, the more flexible structure adopted by such perovskites leads to a relaxation of the Goldschmidt tolerance factor (τ) requirement. Herein, we compare the crystalline and electronic structures, as well as the photophysics of two 2D perovskite single crystals (n-BA)2(MA)2Pb3I10 (BMAPI) and (n-BA)2(EA)2Pb3I10 (BEAPI) (n-BA = n-butylamine) containing small A-cations (MA, methylammonium) and large A-cations (EA, ethylammonium), respectively. The latter presents a relaxed τ (τEA > 1) compared with the requirement of a stable phase in 3D perovskites (τ < 1). Such relaxed τ is beneficial from the structural flexibility of the long organic cation bilayer and the pronounced lattice distortions in the 2D perovskite structures. We further elucidate how the greater lattice distortions concurrently modulate the electronic structure as well as trap densities in these 2D RPPs. The electronic band gap (Eg) of BEAPI (2.08 ± 0.03 eV) is ∼0.17 eV larger than the one of BMAPI (1.91 ± 0.03 eV). This is mainly because of a shift in the valence band maximum associated with the expansion of the Pb–I bond length in BEAPI. In addition, the overall trap state densities for BMAPI and BEAPI are calculated to be ∼2.18 × 1016 and ∼3.76 × 1016 cm–3, respectively, as extracted from the time-resolved photoluminescence studies. The larger trap density in BEAPI can be attributed to the stronger interfacial lattice distortion that sets in when large EA cations are contained into the inorganic crystal lattice.
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| 3. |
- Liang, Mingli, et al.
(författare)
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Free Carriers versus Self-Trapped Excitons at Different Facets of Ruddlesden-Popper Two-Dimensional Lead Halide Perovskite Single Crystals
- 2021
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 12:20, s. 4965-4971
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Tidskriftsartikel (refereegranskat)abstract
- The physical origin of sub-band gap photoluminescence in Ruddlesden-Poppers two-dimensional (2D) lead halide perovskites (LHPs) is still under debate. In this paper, we studied the photoluminescence features from two different facets of 2D LHP single crystals: the in-plane facet (IF) containing the 2D inorganic layers and the facet perpendicular to the 2D layers (PF). At the IF, the free carriers (FCs) dominate due to the weak electron-phonon coupling in a symmetric lattice. At the PF, the strain accumulation along the 2D layers enhances the electron-phonon coupling and facilitates self-trapped exciton (STE) formation. The time-resolved PL studies indicate that free carriers (FCs) at the IF can move freely and display the trapping by the intrinsic defects. The STEs at the PF are not likely trapped by the defects due to the reduced mobility. However, with increasing STE density, the STE transport is promoted, enabling the trapping of STE by the intrinsic defects.
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| 4. |
- Meng, Jie, et al.
(författare)
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Atomic-Scale Observation of Oxygen Vacancy-Induced Step Reconstruction in WO3
- 2021
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 125:15, s. 8456-8460
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Tidskriftsartikel (refereegranskat)abstract
- As the most common type of defects in WO3, surface oxygen deficiencies have been widely investigated in terms of their structure as well as physicochemical properties. In general, oxygen vacancy on the surface may lead to the reconstruction of the surface structure due to the need for surface energy minimization, especially for the area with high oxygen vacancy concentration. However, such a phenomenon has not been directly demonstrated or observed from the experiment. In the paper, we observed such surface reconstruction in the step area or high-index facets in WO3 using the spherical aberration-corrected scanning transmission electron microscope (Cs-STEM) with atomic-scale resolution. This surface reconstruction results in a structure of 5-fold symmetry pentagonal columns, which is first reported. According to the DFT calculation, the formation energy of oxygen vacancy in the step edge is much lower than the smooth surface. Furthermore, the formation energy of oxygen vacancy in the higher index (110) facet is lower than the low index (100) and (010) facets. Our experimental results support these findings, i.e., that the reconstruction is pronounced in the step or high-index facets with a high concentration of oxygen vacancy. Therefore, the high concentration of oxygen vacancy in the step or high-index facet area contributes to the formation of pentagonal column structures. This work could provide a novel insight into the correlation between intrinsic defect and surface structure formation in these materials.
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| 5. |
- Meng, Jie, et al.
(författare)
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Modulating Charge-Carrier Dynamics in Mn-Doped All-Inorganic Halide Perovskite Quantum Dots through the Doping-Induced Deep Trap States
- 2020
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 11:9, s. 3705-3711
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Tidskriftsartikel (refereegranskat)abstract
- Transition-metal ion doping has been demonstrated to be effective for tuning the photoluminescence properties of perovskite quantum dots (QDs). However, it would inevitably introduce defects in the lattice. As the Mn concentration increases, the Mn dopant photoluminescence quantum yield (PLQY) first increases and then decreases. Herein the influence of the dopant and the defect states on the photophysics in Mn-doped CsPbCl3 QDs was studied by time-resolved spectroscopies, whereas the energy levels of the possible defect states were analyzed by density functional theory calculations. We reveal the formation of deep interstitials defects (Cli) by Mn2+ doping. The depopulation of initial QD exciton states is a competition between exciton-dopant energy transfer and defect trapping on an early time scale (<100 ps), which determines the final PLQY of the QDs. The present work establishes a robust material optimization guideline for all of the emerging applications where a high PLQY is essential.
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| 6. |
- Meng, Jie, et al.
(författare)
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Optimizing the quasi-equilibrium state of hot carriers in all-inorganic lead halide perovskite nanocrystals through Mn doping : fundamental dynamics and device perspectives
- 2022
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Ingår i: Chemical Science. - : Royal Society of Chemistry (RSC). - 2041-6520 .- 2041-6539. ; 13:6, s. 1734-1745
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Tidskriftsartikel (refereegranskat)abstract
- Hot carrier (HC) cooling accounts for the significant energy loss in lead halide perovskite (LHP) solar cells. Here, we study HC relaxation dynamics in Mn-doped LHP CsPbI3 nanocrystals (NCs), combining transient absorption spectroscopy and density functional theory (DFT) calculations. We demonstrate that Mn2+ doping (1) enlarges the longitudinal optical (LO)-acoustic phonon bandgap, (2) enhances the electron-LO phonon coupling strength, and (3) adds HC relaxation pathways via Mn orbitals within the bands. The spectroscopic study shows that the HC cooling process is decelerated after doping under band-edge excitation due to the dominant phonon bandgap enlargement. When the excitation photon energy is larger than the optical bandgap and the Mn2+ transition gap, the doping accelerates the cooling rate owing to the dominant effect of enhanced carrier-phonon coupling and relaxation pathways. We demonstrate that such a phenomenon is optimal for the application of hot carrier solar cells. The enhanced electron-LO phonon coupling and accelerated cooling of high-temperature hot carriers efficiently establish a high-temperature thermal quasi-equilibrium where the excessive energy of the hot carriers is transferred to heat the cold carriers. On the other hand, the enlarged phononic band-gap prevents further cooling of such a quasi-equilibrium, which facilitates the energy conversion process. Our results manifest a straightforward methodology to optimize the HC dynamics for hot carrier solar cells by element doping. This journal is
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