Imperial College London

ProfessorAronWalsh

Faculty of EngineeringDepartment of Materials

Chair in Materials Design
 
 
 
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Contact

 

+44 (0)20 7594 1178a.walsh Website

 
 
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Location

 

2.10Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

426 results found

Bainglass E, Walsh A, Huda MN, 2021, BiSbWO6: Properties of a mixed 5s/6s lone-pair-electron system, Chemical Physics, Vol: 544, Pages: 1-8, ISSN: 0301-0104

We investigate the behavior of lone-pair electrons in a mixed Sb(5s)/Bi(6s) crystal-environment. Density functional theory is used to calculate the electronic properties of Sb-alloyed-Bi2WO6 and to study the effects of introducing Sb 5s orbitals to the band structure. The band edge positions, partial charge analyses, and band decomposed charge densities of BiSbWO6 are used to explain the observed trends in relative stabilities and band edge shifts. To isolate the role of the mixed lone-pair, we considered WO3 as a control model. We find that local distortions caused by Sb 5s lone-pair electrons lead to upshifts in both valence and conduction band edges.

Journal article

Jedlicka E, Wang J, Mutch J, Jung Y-K, Went P, Mohammed J, Ziffer M, Giridharagopal R, Walsh A, Chu J-H, Ginger DSet al., 2021, Bismuth doping alters structural phase transitions in methylammonium lead tribromide single crystals., Journal of Physical Chemistry Letters, Vol: 12, Pages: 2749-2755, ISSN: 1948-7185

We study the effects of bismuth doping on the crystal structure and phase transitions in single crystals of the perovskite semiconductor methylammonium lead tribromide, MAPbBr3. By measuring the temperature-dependent specific heat capacity (C p ), we find that as the Bi doping increases, the phase transition assigned to the cubic to tetragonal phase boundary decreases in temperature. Furthermore, after doping we observe one phase transition between 135 and 155 K, in contrast to two transitions observed in the undoped single crystal. These results appear strikingly similar to previously reported effects of mechanical pressure on perovskite crystal structure. Using X-ray diffraction, we show that the lattice constant decreases as Bi is incorporated into the crystal, as predicted by density functional theory. We propose that bismuth substitutional doping on the lead site is dominant, resulting in BiPb+ centers that induce compressive chemical strain that alters the crystalline phase transitions.

Journal article

Kavanagh SR, Walsh A, Scanlon DO, 2021, Rapid Recombination by Cadmium Vacancies in CdTe, ACS Energy Letters, Pages: 1392-1398, ISSN: 2380-8195

Journal article

Protesescu L, Calbo J, Williams K, Tisdale W, Walsh A, Dinca Met al., 2021, Colloidal nano-MOFs nucleate and stabilize ultra-small quantum dots of lead bromide perovskites dagger, CHEMICAL SCIENCE, ISSN: 2041-6520

Journal article

Park JS, Walsh A, 2021, Modeling Grain Boundaries in Polycrystalline Halide Perovskite Solar Cells, Annual Review of Condensed Matter Physics, Vol: 12, Pages: 95-109, ISSN: 1947-5454

Solar cells are semiconductor devices that generate electricity through charge generation upon illumination. For optimal device efficiency, the photogenerated carriers must reach the electrical contact layers before they recombine. A deep understanding of the recombination process and transport behavior is essential to design better devices. Halide perovskite solar cells are commonly made of a polycrystalline absorber layer, but there is no consensus on the nature and role of grain boundaries. This review concerns theoretical approaches for the investigation of extended defects. We introduce recent computational studies on grain boundaries, and their influence on point-defect distributions, in halide perovskite solar cells. We conclude with a discussion of future research directions.

Journal article

Jaskaniec S, Kavanagh SR, Coelho J, Ryan S, Hobbs C, Walsh A, Scanlon DO, Nicolosi Vet al., 2021, Solvent engineered synthesis of layered SnO for high-performance anodes, NPJ 2D MATERIALS AND APPLICATIONS, Vol: 5

Journal article

Zakutayev A, Major JD, Hao X, Walsh A, Tang J, Todorov TK, Wong LH, Saucedo Eet al., 2021, Emerging inorganic solar cell Efficiency Tables (version 2), Journal of Physics: Energy

Journal article

Rigter SA, Quinn XL, Kumar RE, Fenning DP, Massonnet P, Ellis SR, Heeren RMA, Svane KL, Walsh A, Garnett ECet al., 2021, Passivation properties and formation mechanism of amorphous halide perovskite thin films, Advanced Functional Materials, Vol: 31, Pages: 1-10, ISSN: 1616-301X

Lead halide perovskites are among the most exciting classes of optoelectronic materials due to their unique ability to form high‐quality crystals with tunable bandgaps in the visible and near‐infrared using simple solution precipitation reactions. This facile crystallization is driven by their ionic nature; just as with other salts, it is challenging to form amorphous halide perovskites, particularly in thin‐film form where they can most easily be studied. Here, rapid desolvation promoted by the addition of acetate precursors is shown as a general method for making amorphous lead halide perovskite films with a wide variety of compositions, including those using common organic cations (methylammonium and formamidinium) and anions (bromide and iodide). By controlling the amount of acetate, it is possible to tune from fully crystalline to fully amorphous films, with an interesting intermediate state consisting of crystalline islands embedded in an amorphous matrix. The amorphous lead halide perovskite has a large and tunable optical bandgap. It improves the photoluminescence quantum yield and lifetime of incorporated crystalline perovskite, opening up the intriguing possibility of using amorphous perovskite as a passivating contact, as is currently done in record efficiency silicon solar cells.

Journal article

Okenyi MTO, Ratcliff LE, Walsh A, 2021, Multi-phonon proton transfer pathway in a molecular organic ferroelectric crystal, Physical Chemistry Chemical Physics, Vol: 23, Pages: 2885-2890, ISSN: 1463-9076

While the majority of ferroelectrics research has been focused on inorganic ceramics, molecular ferroelectrics can also combine large spontaneous polarization with high Curie temperatures. However, the microscopic mechanism of their ferroelectric switching is not fully understood. We explore proton tautomerism in the prototypical case of croconic acid, C5O5H2. In order to determine how efficiently ferroelectricity in croconic acid is described in terms of its Γ-point phonon modes, the minimum energy path between its structural ground states is approximated by projection onto reduced basis sets formed from subsets of these modes. The potential energy curve along the minimum energy path was found to be sensitive to the order of proton transfer, which requires a large subset (≳8) of the modes to be approximated accurately. Our findings suggest rules for the construction of effective Hamiltonians to describe proton transfer ferroelectrics.

Journal article

Huang Y-T, Kavanagh S, Scanlon D, Walsh A, Hoye Ret al., 2021, Perovskite-inspired materials for photovoltaics and beyond – from design to devices, Nanotechnology, Vol: 32, Pages: 1-60, ISSN: 0957-4484

NanotechnologyACCEPTED MANUSCRIPT • The following article is Open accessPerovskite-Inspired Materials for Photovoltaics and Beyond – From Design to DevicesYi-Teng Huang1, Seán R. Kavanagh2, David O Scanlon3, Aron Walsh4 and Robert Hoye5Accepted Manuscript online 1 December 2020 • © 2020 The Author(s). Published by IOP Publishing Ltd.What is an Accepted Manuscript?Download Accepted Manuscript PDFDownload PDFArticle has an altmetric score of 6Turn on MathJaxShare this article Share this content via email Share on Facebook Share on Twitter Share on Google+ Share on MendeleyArticle informationAbstractLead-halide perovskites have demonstrated astonishing increases in power conversion efficiency in photovoltaics over the last decade. The most efficient perovskite devices now outperform industry-standard multi-crystalline silicon solar cells, despite the fact that perovskites are typically grown at low temperature using simple solution-based methods. However, the toxicity of lead and its ready solubility in water are concerns for widespread implementation. These challenges, alongside the many successes of the perovskites, have motivated significant efforts across multiple disciplines to find lead-free and stable alternatives which could mimic the ability of the perovskites to achieve high performance with low temperature, facile fabrication methods. This Review discusses the computational and experimental approaches that have been taken to discover lead-free perovskite-inspired materials, and the recent successes and challenges in synthesizing these compounds. The atomistic origins of the extraordinary performance exhibited by lead-halide perovskites in photovoltaic devices is discussed, alongside the key challenges in engineering such high-performance in alternative, next-generation materials. Beyond photovoltaics, this Review discusses the impact perovskite-inspired materials have had in spurring efforts to apply new materials i

Journal article

Bibi A, Lee I, Nah Y, Allam O, Kim H, Quan LN, Tang J, Walsh A, Jang SS, Sargent EH, Kim DHet al., 2021, Lead-free halide double perovskites: Toward stable and sustainable optoelectronic devices, Materials Today, ISSN: 1369-7021

In recent years, metal halide perovskites (MHPs) have attracted attention as semiconductors that achieve desirable properties for optoelectronic devices. However, two challenges—instability and the regulated nature of Pb —remain to be addressed with commercial applications. The development of Pb-free halide double perovskite (HDP) materials has gained interest and attention as a result. This family offers potential in the field of optoelectronic devices through flexible material designs and compositional adjustments. We highlight recent progress and development in halide double perovskites and encompass the synthesis, optoelectronic properties, and engineering of the electronic structures of these materials along with their applications in optoelectronic devices. Computational and data-driven statistical methods can also be used to explore mechanisms and discover promising candidate double perovskites.

Journal article

Reichert S, An Q, Woo Y-W, Walsh A, Vaynzof Y, Deibel Cet al., 2020, Probing the ionic defect landscape in halide perovskite solar cells, Nature Communications, Vol: 11, Pages: 1-10, ISSN: 2041-1723

Point defects in metal halide perovskites play a critical role in determining their properties and optoelectronic performance; however, many open questions remain unanswered. In this work, we apply impedance spectroscopy and deep-level transient spectroscopy to characterize the ionic defect landscape in methylammonium lead triiodide (MAPbI3) perovskites in which defects were purposely introduced by fractionally changing the precursor stoichiometry. Our results highlight the profound influence of defects on the electronic landscape, exemplified by their impact on the device built-in potential, and consequently, the open-circuit voltage. Even low ion densities can have an impact on the electronic landscape when both cations and anions are considered as mobile. Moreover, we find that all measured ionic defects fulfil the Meyer–Neldel rule with a characteristic energy connected to the underlying ion hopping process. These findings support a general categorization of defects in halide perovskite compounds.

Journal article

Kobayashi Y, Hirata K, Hood SN, Yang H, Walsh A, Matsushita Y, Ishioka Ket al., 2020, Crystal structure and metallization mechanism of the pi-radical metal TED (vol 11, pg 11699, 2020), Chemical Science, Vol: 11, Pages: 11945-11946, ISSN: 2041-6520

Correction for ‘Crystal structure and metallization mechanism of the π-radical metal TED’ by Yuka Kobayashi et al., Chem. Sci., 2020, DOI: 10.1039/d0sc03521a.

Journal article

Li Z, Kavanagh S, Napari M, Palgrave RG, Abdi-Jalebi M, Andaji-Garmaroudi Z, Davies DW, Laitinen M, Julin J, Isaacs MA, Friend RH, Scanlon DO, Walsh A, Hoye Ret al., 2020, Bandgap lowering in mixed alloys ofCs2Ag(SbxBi1-x)Br6 double perovskite thin films, Journal of Materials Chemistry A, Vol: 8, Pages: 21780-21788, ISSN: 2050-7488

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.

Journal article

Muscarella LA, Hutter EM, Wittmann F, Woo YW, Jung Y-K, McGovern L, Versluis J, Walsh A, Bakker HJ, Ehrler Bet al., 2020, Lattice compression increases the activation barrier for phase segregation in mixed-halide perovskites, ACS Energy Letters, Vol: 5, Pages: 3152-3158, ISSN: 2380-8195

The bandgap tunability of mixed-halide perovskites makes them promising candidates for light-emitting diodes and tandem solar cells. However, illuminating mixed-halide perovskites results in the formation of segregated phases enriched in a single halide. This segregation occurs through ion migration, which is also observed in single-halide compositions, and whose control is thus essential to enhance the lifetime and stability. Using pressure-dependent transient absorption spectroscopy, we find that the formation rates of both iodide- and bromide-rich phases in MAPb(BrxI1–x)3 reduce by 2 orders of magnitude on increasing the pressure to 0.3 GPa. We explain this reduction from a compression-induced increase of the activation energy for halide migration, which is supported by first-principle calculations. A similar mechanism occurs when the unit cell volume is reduced by incorporating a smaller cation. These findings reveal that stability with respect to halide segregation can be achieved either physically through compressive stress or chemically through compositional engineering.

Journal article

Simenas M, Balciunas S, Wilson JN, Svirskas S, Kinka M, Garbaras A, Kalendra V, Gagor A, Szewczyk D, Sieradzki A, Maczka M, Samulionis V, Walsh A, Grigalaitis R, Banys Jet al., 2020, Suppression of phase transitions and glass phase signatures in mixed cation halide perovskites, Nature Communications, Vol: 11, ISSN: 2041-1723

Cation engineering provides a route to control the structure and properties of hybrid halide perovskites, which has resulted in the highest performance solar cells based on mixtures of Cs, methylammonium, and formamidinium. Here, we present a multi-technique experimental and theoretical study of structural phase transitions, structural phases and dipolar dynamics in the mixed methylammonium/dimethylammonium MA1-xDMAxPbBr3 hybrid perovskites (0 ≤ x ≤ 1). Our results demonstrate a significant suppression of the structural phase transitions, enhanced disorder and stabilization of the cubic phase even for a small amount of dimethylammonium cations. As the dimethylammonium concentration approaches the solubility limit in MAPbBr3, we observe the disappearance of the structural phase transitions and indications of a glassy dipolar phase. We also reveal a significant tunability of the dielectric permittivity upon mixing of the molecular cations that arises from frustrated electric dipoles.

Journal article

Walsh A, Boettcher S, Lu J, Seger Bet al., 2020, Energy Spotlight, ACS Energy Letters, Vol: 5, Pages: 3265-3267, ISSN: 2380-8195

Journal article

Kobayashi Y, Hirata K, Hood SN, Yang H, Walsh A, Matsushita Y, Ishioka Ket al., 2020, Crystal structure and metallization mechanism of the π-radical metal TED, Chemical Science, Vol: 11, Pages: 11699-11704, ISSN: 2041-6520

Radical electrons tend to localize on individual molecules, resulting in an insulating (Mott–Hubbard) bandgap in the solid state. Herein, we report the crystal structure and intrinsic electronic properties of the first single crystal of a π-radical metal, tetrathiafulvalene-extended dicarboxylate (TED). The electrical conductivity is up to 30 000 S cm−1 at 2 K and 2300 S cm−1 at room temperature. Temperature dependence of resistivity obeys a T3 power-law above T > 100 K, indicating a new type of metal. X-ray crystallographic analysis clarifies the planar TED molecule, with a symmetric intramolecular hydrogen bond, is stacked along longitudinal (the a-axis) and transverse (the b-axis) directions. The π-orbitals are distributed to avoid strong local interactions. First-principles electronic calculations reveal the origin of the metallization giving rise to a wide bandwidth exceeding 1 eV near the Fermi level. TED demonstrates the effect of two-dimensional stacking of π-orbitals on electron delocalization, where a high carrier mobility of 31.6 cm2 V−1 s−1 (113 K) is achieved.

Journal article

Yang H, Savory CN, Morgan BJ, Scanlon DO, Skelton JM, Walsh Aet al., 2020, Chemical trends in the lattice thermal conductivity of Li(Ni, Mn, Co)O-2 (NMC) battery cathodes, Chemistry of Materials, Vol: 32, Pages: 7542-7550, ISSN: 0897-4756

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.

Journal article

Moss B, Le H, Corby S, Morita K, Selim S, Sotelo-Vazquez C, Chen Y, Borthwick A, Wilson A, Blackman C, Durrant JR, Walsh A, Kafizas Aet al., 2020, Anisotropic electron transport limits performance of Bi2WO6 photoanodes, The Journal of Physical Chemistry C, Vol: 124, Pages: 18859-18867, ISSN: 1932-7447

Bi2WO6 is one of the simplest members of the versatile Aurivillius oxide family of materials. As an intriguing model system for Aurivillius oxides, BiVO4 exhibits low water oxidation onset potentials (∼0.5–0.6 VRHE) for driven solar water oxidation. Despite this, Bi2WO6 also produces low photocurrents in comparison to other metal oxides. Due to a lack of in situ studies, the reasons for such poor performance are not understood. In this study, Bi2WO6 photoanodes are synthesized by aerosol-assisted chemical vapor deposition. The charge carrier dynamics of Bi2WO6 are studied in situ under water oxidation conditions, and the rate of both bulk recombination and water oxidation is found to be comparable to other metal oxide photoanodes. However, the rate of electron extraction is at least 10 times slower than the slowest kinetics previously reported in an oxide photoanode. First-principles analysis indicates that the slow electron extraction kinetics are linked to a strong anisotropy in the conduction band. Preferred or epitaxial growth along the conductive axes is a strategy to overcome slow electron transport and low photocurrent densities in layered materials such as Bi2WO6.

Journal article

Walsh A, Park J-S, 2020, The holey grail of transparent electronics, Matter, Vol: 3, Pages: 604-606, ISSN: 2590-2385

For decades, researchers have attempted to discover transparent conductors that transport holes. A robust p-type material would usher in a new era of technologies. Many reports have failed to live up to their hype; however, the screening by Williamson et al. has predicted [Cu2S2][Ba3Sc2O5] to support a conductivity exceeding 2,000 S cm−1.

Journal article

Crovetto A, Kim S, Fischer M, Stenger N, Walsh A, Chorkendorff I, Vesborg PCKet al., 2020, Assessing the defect tolerance of kesterite-inspired solar absorbers, Energy & Environmental Science, Vol: 13, Pages: 3489-3503, ISSN: 1754-5692

Various thin-film I2–II–IV–VI4 photovoltaic absorbers derived from kesterite Cu2ZnSn(S,Se)4 have been synthesized, characterized, and theoretically investigated in the past few years. The availability of this homogeneous materials dataset is an opportunity to examine trends in their defect properties and identify criteria to find new defect-tolerant materials in this vast chemical space. We find that substitutions on the Zn site lead to a smooth decrease in band tailing as the ionic radius of the substituting cation increases. Unfortunately, this substitution strategy does not ensure the suppression of deeper defects and non-radiative recombination. Trends across the full dataset suggest that Gaussian and Urbach band tails in kesterite-inspired semiconductors are two separate phenomena caused by two different antisite defect types. Deep Urbach tails are correlated with the calculated band gap narrowing caused by the (2III + IVII) defect cluster. Shallow Gaussian tails are correlated with the energy difference between the kesterite and stannite polymorphs, which points to the role of (III + III) defect clusters involving Group IB and Group IIB atoms swapping across different cation planes. This finding can explain why in-plane cation disorder and band tailing are uncorrelated in kesterites. Our results provide quantitative criteria for discovering new kesterite-inspired photovoltaic materials with low band tailing.

Journal article

Gkini K, Balis N, Papadakis M, Verykios A, Skoulikidou M-C, Drivas C, Kennou S, Golomb M, Walsh A, Coutsolelos AG, Vasilopoulou M, Falaras Pet al., 2020, Manganese Porphyrin Interface Engineering in Perovskite Solar Cells, ACS APPLIED ENERGY MATERIALS, Vol: 3, Pages: 7353-7363, ISSN: 2574-0962

Journal article

Rahim W, Skelton JM, Savory CN, Evans IR, Evans JSO, Walsh A, Scanlon DOet al., 2020, Polymorph exploration of bismuth stannate using first-principles phonon mode mapping, Chemical Science, Vol: 11, Pages: 7904-7909, ISSN: 2041-6520

Accurately modelling polymorphism in crystalline solids remains a key challenge in computational chemistry. In this work, we apply a theoretically-rigorous phonon mode-mapping approach to understand the polymorphism in the ternary metal oxide Bi2Sn2O7. Starting from the high-temperature cubic pyrochlore aristotype, we systematically explore the structural potential-energy surface and recover the two known low-temperature phases alongside three new metastable phases, together with the transition pathways connecting them. This first-principles lattice-dynamics method is completely general and provides a practical means to identify and characterise the stable polymorphs and phase transitions in materials with complex crystal structures.

Journal article

Park SY, Park J-S, Kim BJ, Lee H, Walsh A, Zhu K, Kim DH, Jung HSet al., 2020, Sustainable lead management in halide perovskite solar cells, Nature Sustainability, Vol: 3, Pages: 1044-1051, ISSN: 2398-9629

Despite the rapid development of perovskite solar cells (PSCs) toward commercialization, the toxic lead (Pb) ions in PSCs pose a potential threat to the environment, health and safety. Managing Pb via recycling represents a promising approach to mitigating its toxicity. However, managing Pb from commonly used organic solvents has been challenging due to the lack of suitable Pb adsorbents. Here, we report a new adsorbent for both separation and recovery of Pb from PSC pollutants. The synthesized iron-incorporated hydroxyapatite possesses a strongly negatively charged surface that improves electrostatic interaction through surface-charge delocalization, thus leading to enhanced Pb adsorption. We demonstrate the feasibility of a complete Pb management process, including the purification of Pb-containing non-aqueous solvents below 15 parts per 109, a level compliant with the standards of the US Environmental Protection Agency, as well as recycling of 99.97% of Pb ions by forming lead iodide.

Journal article

Welch EW, Jung Y-K, Walsh A, Scolfaro L, Zakhidov Aet al., 2020, A density functional theory study on the interface stability between CsPbBr3 and CuI, AIP Advances, Vol: 10, Pages: 1-6, ISSN: 2158-3226

This paper assesses the interface stability of the perovskite CsPbBr3 and transport layer CuI using density functional theory and band offset calculations. As a low-cost, more stable alternative to current hole transport materials, CuI may be used to template the epitaxial growth of perovskites such as CsPbBr3 owing to a 1% lattice constant mismatch and larger bulk modulus. We compare all eight atomic terminations of the interfaces between the (100) low-energy facet for both CsPbBr3 and CuI, increasing material thickness to consider charge density redistribution and bonding characteristics between surface and bulk-like regions. A low energy atomic termination is found to exist between these materials where alternating charge accumulation and depletion regions stabilize bonds at the interface. Band offset calculations reveal a type I straddling gap offset in the bulk shifting to a type II staggered gap offset as the thickness of the materials is increased, where the built-in potential changes as layer thickness increases, indicating the tunability of charge separation at the interface. CuI may, thus, be used as an alternative hole transport layer material in CsPbBr3 optoelectronic devices.

Journal article

Hutter EM, Muscarella LA, Wittmann F, Versluis J, McGovern L, Bakker HJ, Woo Y-W, Jung Y-K, Walsh A, Ehrler Bet al., 2020, Thermodynamic stabilization of mixed-halide perovskites against phase segregation, Cell Reports Physical Science, Vol: 1, Pages: 1-11, ISSN: 2666-3864

Mixing iodide and bromide in halide perovskite semiconductors is an effective strategy to tune their band gap; therefore, mixed-halide perovskites hold great promise for color-tunable LEDs and tandem solar cells. However, the band gap of mixed-halide perovskites is unstable under (sun-)light, since the halides segregate into domains of different band gaps. Using pressure-dependent ultrafast transient absorption spectroscopy, we find that high external pressure increases the range of stable halide mixing ratios. Chemical compression, by inserting a smaller cation, has the same effect, which means that any iodide:bromide ratio can be stabilized by tuning the crystal volume and compressibility. We interpret these findings as an increased thermodynamic stabilization through alteration of the Gibbs free energy via the largely overlooked PΔV term.

Journal article

Davies DW, Morgan BJ, Scanlon DO, Walsh Aet al., 2020, Low-cost descriptors of electrostatic and electronic contributions to anion redox activity in batteries, IOP SciNotes, Vol: 1, Pages: 1-7, ISSN: 2633-1357

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 Li2MO3 (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.

Journal article

Golomb MJ, Calbo J, Bristow JK, Walsh Aet al., 2020, Ligand engineering in Cu(ii) paddle wheel metal-organic frameworks for enhanced semiconductivity, Journal of Materials Chemistry A, Vol: 8, Pages: 13160-13165, ISSN: 2050-7488

We report the electronic structure of two metal–organic frameworks (MOFs) with copper paddle wheel nodes connected by a N2(C2H4)3 (DABCO) ligand with accessible nitrogen lone pairs. The coordination is predicted, from first-principles density functional theory, to enable electronic pathways that could facilitate charge carrier mobility. Calculated frontier crystal orbitals indicate extended electronic communication in DMOF-1, but not in MOF-649. This feature is confirmed by band structure calculations and effective masses of the valence band edge. We explain the origin of the frontier orbitals of both MOFs based on the energy and symmetry alignment of the underlying building blocks. The effects of isovalent substitution on the band structure of MOF-649 are considered. Our findings highlight DMOF-1 as a potential semiconductor with enhanced 1D charge carrier mobility along the framework.

Journal article

Morita K, Davies DW, Butler KT, Walsh Aet al., 2020, Modeling the dielectric constants of crystals using machine learning, Journal of Chemical Physics, Vol: 153, Pages: 1-9, ISSN: 0021-9606

The relative permittivity of a crystal is a fundamental property that links microscopic chemical bonding to macroscopic electromagnetic response. Multiple models, including analytical, numerical, and statistical descriptions, have been made to understand and predict dielectric behavior. Analytical models are often limited to a particular type of compound, whereas machine learning (ML) models often lack interpretability. Here, we combine supervised ML, density functional perturbation theory, and analysis based on game theory to predict and explain the physical trends in optical dielectric constants of crystals. Two ML models, support vector regression and deep neural networks, were trained on a dataset of 1364 dielectric constants. Analysis of Shapley additive explanations of the ML models reveals that they recover correlations described by textbook Clausius–Mossotti and Penn models, which gives confidence in their ability to describe physical behavior, while providing superior predictive power.

Journal article

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