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Anna
Abfalterer
,
Javad
Shamsi
,
Dominik J.
Kubicki
,
Christopher N.
Savory
,
James
Xiao
,
Giorgio
Divitini
,
Weiwei
Li
,
Stuart
Macpherson
,
Krzysztof
Gałkowski
,
Judith L.
Macmanus-driscoll
,
David O.
Scanlon
,
Samuel D.
Stranks
Open Access
Abstract: Optoelectronic devices based on lead halide perovskites are processed in facile ways, yet are remarkably efficient. There are extensive research efforts investigating lead-free perovskite and perovskite-related compounds, yet there are challenges to synthesize these materials in forms that can be directly integrated into thin film devices rather than as bulk powders. Here, we report on the colloidal synthesis and characterization of lead-free, antifluorite Cs2ZrX6 (X = Cl, Br) nanocrystals that are readily processed into thin films. We use transmission electron microscopy and powder X-ray diffraction measurements to determine their size and structural properties, and solid-state nuclear magnetic resonance measurements reveal the presence of oleate ligand, together with a disordered distribution of Cs surface sites. Density functional theory calculations reveal the band structure and fundamental band gaps of 5.06 and 3.91 eV for Cs2ZrCl6 and Cs2ZrBr6, respectively, consistent with experimental values. Finally, we demonstrate that the Cs2ZrCl6 and Cs2ZrBr6 nanocrystal thin films exhibit tunable, broad white photoluminescence with quantum yields of 45% for the latter, with respective peaks in the blue and green spectral regions and mixed systems exhibiting properties between them. Our work represents a critical step toward the application of lead-free Cs2ZrX6 nanocrystal thin films into next-generation light-emitting applications.
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Nov 2020
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Abstract: A geometric analysis of the cubic A2BX6 structure commonly formed by metal halides is presented. Using the “hard-sphere” approximation, where the ions are represented by spheres of a fixed radius, we derive four limiting models that each constrain the distances between constituent ions in different ways. We compare the lattice parameters predicted by these four models with experimental data from the Inorganic Crystal Structure Database (ICSD). For the fluorides, the maintenance of the AX bond length at the sum of the A and X radii gives the best approximation of the lattice parameter, leading to structures with widely separated BX6 octahedra. For the heavier halides, a balance between forming an A-site cavity of the correct size and maintaining suitable anion–anion distances determines the lattice parameter. It is found that in many A2BX6 compounds of heavier halides, the neighboring octahedra show very significant anion–anion overlap. We use these models to predict a compound with A-site rattling and use density functional theory (DFT) to confirm this prediction. Finally, we use the geometric models to derive formability criteria for vacancy-ordered double perovskites.
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Nov 2020
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Abstract: The layered perovskite (MA)2PbI2(SCN)2 (MA = CH3NH3+) is a member of an emerging series of compounds derived from hybrid organic–inorganic perovskites. Here, we successfully synthesized (MA)2PbI2–xBrx(SCN)2 (0 ≤ x < 1.6) by using a solid-state reaction. Despite smaller bromide substitution for iodine, 1% linear expansion along the a axis was observed at x ∼ 0.4 due to a change of the orientation of the SCN– anions. Diffuse reflectance spectra reveal that the optical band gap increases by the bromide substitution, which is supported by the DFT calculations. Curiously, bromine-rich compounds where x ≥ 0.8 are light sensitive, leading to partial decomposition after ∼24 h. This study demonstrates that the layered perovskite (MA)2PbI2(SCN)2 tolerates a wide range of bromide substitution toward tuning the band gap energy.
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Nov 2020
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Abstract: Cu2BaSnS4 (CBTS) and Cu2SrSnS4 (CSTS) semiconductors have been recently proposed as potential wide band gap photovoltaic absorbers. Although several measurements indicate that they are less affected by band tailing than their parent compound Cu2ZnSnS4, their photovoltaic efficiencies are still low. To identify possible issues, we characterize CBTS and CSTS in parallel by a variety of spectroscopic methods complemented by first-principles calculations. Two main problems are identified in both materials. The first is the existence of deep defect transitions in low-temperature photoluminescence, pointing to a high density of bulk recombination centers. The second is their low electron affinity, which emphasizes the need for an alternative heterojunction partner and electron contact. We also find a tendency for downward band bending at the surface of both materials. In CBTS, this effect is sufficiently large to cause carrier-type inversion, which may enhance carrier separation and mitigate interface recombination. Optical absorption at room temperature is exciton-enhanced in both CBTS and CSTS. Deconvolution of excitonic effects yields band gaps that are about 100 meV higher than previous estimates based on Tauc plots. Although the two investigated materials are remarkably similar in an idealized, defect-free picture, the present work points to CBTS as a more promising absorber than CSTS for tandem photovoltaics.
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Oct 2020
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Open Access
Abstract: Nine members of two contiguous solid solutions, Ba3Sc2−xInxO5Cu2S2 and Ba3In2O5Cu2S2−ySey (x, y = 0, 0.5, 1, 1.5 and 2), were synthesised at temperatures between 800 °C and 900 °C by stoichiometric combination of binary precursors. Their structures were determined by Rietveld refinement of X-ray powder diffraction data and found to adopt the SmNi3Ge3 structure with I4/mmm symmetry. Approximate Vegard law relationships were found within each solution between the lattice parameters and composition, with an observed cell volume of 466.4 Å3 for Ba3Sc2O5Cu2S2 increasing to 481.0 Å3 for Ba3In2O5Cu2S2 and finally to 499.0 Å3 for Ba3In2O5Cu2Se2. In the first solid solution, this volume increase is driven by the replacement of scandium by the larger indium ion, generating increased strain in the copper chalcogenide layer. In the second solution the substitution into the structure of the larger selenium drives further volume expansion, while relieving the strain in the copper chalcogenide layer. Band gaps were estimated from reflectance spectroscopy and these were determined to be 3.3 eV, 1.8 eV, and 1.3 eV for the three end members Ba3Sc2O5Cu2S2, Ba3In2O5Cu2S2, and Ba3Sc2In2O5Cu2Se2, respectively. For the intermediate compositions a linear relationship between band gap size and composition was observed, driven in the first solution by the introduction of the more electronegative indium lowering the conduction band minimum and in the second solution by the substitution of the electropositive selenium raising the valance band maximum. Photocatalytic activity was observed in all samples under solar simulated light, based on a dye degradation test, with the exception of Ba3In2O5Cu2Se1.5S0.5. The most active sample was found to be Ba3Sc2O5Cu2S2, the material with the largest band gap.
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Oct 2020
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Zewei
Li
,
Seán R.
Kavanagh
,
Mari
Napari
,
Robert G.
Palgrave
,
Mojtaba
Abdi-jalebi
,
Zahra
Andaji-garmaroudi
,
Daniel W.
Davies
,
Mikko
Laitinen
,
Jaakko
Julin
,
Mark A.
Isaacs
,
Richard H.
Friend
,
David O.
Scanlon
,
Aron
Walsh
,
Robert L. Z.
Hoye
Open Access
Abstract: Halide double perovskites have gained significant attention, owing to their composition of low-toxicity elements, stability in air and long charge-carrier lifetimes. However, most double perovskites, including Cs2AgBiBr6, have wide bandgaps, which limits photoconversion efficiencies. The bandgap can be reduced through alloying with Sb3+, but Sb-rich alloys are difficult to synthesize due to the high formation energy of Cs2AgSbBr6, which itself has a wide bandgap. We develop a solution-based route to synthesize phase-pure Cs2Ag(SbxBi1−x)Br6 thin films, with the mixing parameter x continuously varying over the entire composition range. We reveal that the mixed alloys (x between 0.5 and 0.9) demonstrate smaller bandgaps than the pure Sb- and Bi-based compounds. The reduction in the bandgap of Cs2AgBiBr6 achieved through alloying (170 meV) is larger than if the mixed alloys had obeyed Vegard's law (70 meV). Through in-depth computations, we propose that bandgap lowering arises from the type II band alignment between Cs2AgBiBr6 and Cs2AgSbBr6. The energy mismatch between the Bi and Sb s and p atomic orbitals, coupled with their non-linear mixing, results in the alloys adopting a smaller bandgap than the pure compounds. Our work demonstrates an approach to achieve bandgap reduction and highlights that bandgap bowing may be found in other double perovskite alloys by pairing together materials forming a type II band alignment.
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Oct 2020
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Open Access
Abstract: Conventional battery cathodes are limited by the redox capacity of the transition metal components. For example, the delithiation of LiCoO2 involves the formal oxidation from Co(III) to Co(IV). Enhanced capacities can be achieved if the anion also contributes to reversible oxidation. The origins of redox activity in crystals are difficult to quantify from experimental measurements or first-principles materials modelling. We present practical procedures to describe the electrostatic (Madelung potential) and electronic (integrated density of states) contributions, which are applied to the LiMO2 and Li2 MO3 (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, Au) model systems. We discuss how such descriptors could be integrated in a materials design workflow.
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Sep 2020
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Abstract: While the transport of ions and electrons in conventional Li-ion battery cathode materials is well understood, our knowledge of the phonon (heat) transport is still in its infancy. We present a first-principles theoretical investigation of the chemical trends in the phonon frequency dispersion, mode lifetimes, and thermal conductivity in the series of layered lithium transition-metal oxides Li(NixMnyCoz)O2 (x + y + z = 1). The oxidation and spin states of the transition metal cations are found to strongly influence the structural dynamics. Calculations of the thermal conductivity show that LiCoO2 has highest average conductivity of 45.9 W·m–1·K–1 at T = 300 K and the largest anisotropy, followed by LiMnO2 with 8.9 W·m–1·K–1 and LiNiO2 with 6.0 W·m–1·K–1. The much lower thermal conductivity of LiMnO2 and LiNiO2 is found to be due to 1–2 orders of magnitude shorter phonon lifetimes. We further model the properties of binary and ternary transition metal combinations to examine the possible effects of mixing on the thermal transport. These results serve as a guide to ongoing work on the design of multicomponent battery electrodes with more effective thermal management.
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Aug 2020
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I09-Surface and Interface Structural Analysis
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Christopher H.
Don
,
Huw
Shiel
,
Theodore D. C.
Hobson
,
Christopher N.
Savory
,
Jack E. N.
Swallow
,
Matthew J.
Smiles
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Ken
Durose
,
Jonathan D.
Major
,
Vinod R.
Dhanak
,
David O.
Scanlon
,
Tim D.
Veal
Diamond Proposal Number(s):
[21431]
Open Access
Abstract: The presence of a lone pair of 5s electrons at the valence band maximum (VBM) of Sb2Se3 and the resulting band alignments are investigated using soft and hard X-ray photoemission spectroscopy in parallel with density functional theory (DFT) calculations. Vacuum-cleaved and exfoliated bulk crystals of Sb2Se3 are analysed using laboratory and synchrotron X-ray sources to acquire high resolution valence band spectra with both soft and hard X-rays. Utilising the photon-energy dependence of different orbital cross-sections and corresponding DFT calculations, the various orbital contributions to the valence band could be identified, including the 5s orbital's presence at the VBM. The ionization potential is also determined and places the VBM at 5.13 eV below the vacuum level, similar to other materials with 5s2 lone pairs, but far above those of related materials without lone pairs of electrons.
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Aug 2020
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Zilu
Liu
,
Tianjun
Liu
,
Christopher N.
Savory
,
José P.
Jurado
,
Juan Sebastián
Reparaz
,
Jianwei
Li
,
Long
Pan
,
Charl F. J.
Faul
,
Ivan P.
Parkin
,
Gopinathan
Sankar
,
Satoru
Matsuishi
,
Mariano
Campoy‐quiles
,
David O.
Scanlon
,
Martijn A.
Zwijnenburg
,
Oliver
Fenwick
,
Bob C.
Schroeder
Open Access
Abstract: Organometallic coordination polymers (OMCPs) are a promising class of thermoelectric materials with high electrical conductivities and thermal resistivities. The design criteria for these materials, however, remain elusive and so far material modifications have been focused primarily on the nature of the metal cation to tune the thermoelectric properties. Herein, an alternative approach is described by synthesizing new organic ligands for OMCPs, allowing modulation of the thermoelectric properties of the novel OMCP materials over several orders of magnitude, as well as controlling the polarity of the Seebeck coefficient. Extensive material purification combined with spectroscopy experiments and calculations furthermore reveal the charge‐neutral character of the polymer backbones. In the absence of counter‐cations, the OMCP backbones are composed of air‐stable, ligand‐centered radicals. The findings open up new synthetic possibilities for OMCPs by removing structural constraints and putting significant emphasis on the molecular structure of the organic ligands in OMCP materials to tune their thermoelectric properties.
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Jun 2020
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