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

474 results found

Pastor E, Sachs M, Selim S, Durrant JR, Bakulin AA, Walsh Aet al., 2022, Electronic defects in metal oxide photocatalysts, Nature Reviews Materials

Journal article

Gu GH, Jang J, Noh J, Walsh A, Jung Yet al., 2022, Perovskite synthesizability using graph neural networks, NPJ COMPUTATIONAL MATERIALS, Vol: 8

Journal article

Verma S, Rivera M, Scanlon DO, Walsh Aet al., 2022, Machine learned calibrations to high-throughput molecular excited state calculations., Journal of Chemical Physics, Vol: 156, Pages: 134116-134116, ISSN: 0021-9606

Understanding the excited state properties of molecules provides insight into how they interact with light. These interactions can be exploited to design compounds for photochemical applications, including enhanced spectral conversion of light to increase the efficiency of photovoltaic cells. While chemical discovery is time- and resource-intensive experimentally, computational chemistry can be used to screen large-scale databases for molecules of interest in a procedure known as high-throughput virtual screening. The first step usually involves a high-speed but low-accuracy method to screen large numbers of molecules (potentially millions), so only the best candidates are evaluated with expensive methods. However, use of a coarse first-pass screening method can potentially result in high false positive or false negative rates. Therefore, this study uses machine learning to calibrate a high-throughput technique [eXtended Tight Binding based simplified Tamm-Dancoff approximation (xTB-sTDA)] against a higher accuracy one (time-dependent density functional theory). Testing the calibration model shows an approximately sixfold decrease in the error in-domain and an approximately threefold decrease in the out-of-domain. The resulting mean absolute error of ∼0.14 eV is in line with previous work in machine learning calibrations and out-performs previous work in linear calibration of xTB-sTDA. We then apply the calibration model to screen a 250k molecule database and map inaccuracies of xTB-sTDA in chemical space. We also show generalizability of the workflow by calibrating against a higher-level technique (CC2), yielding a similarly low error. Overall, this work demonstrates that machine learning can be used to develop a cost-effective and accurate method for large-scale excited state screening, enabling accelerated molecular discovery across a variety of disciplines.

Journal article

Wang Y, Kavanagh SR, Burgues-Ceballos I, Walsh A, Scanlon DO, Konstantatos Get al., 2022, Cation disorder engineering yields AgBiS2 nanocrystals with enhanced optical absorption for efficient ultrathin solar cells (vol 16, pg 235, 2022), NATURE PHOTONICS, ISSN: 1749-4885

Journal article

Twyman NM, Walsh A, Buonassisi T, 2022, Environmental stability of crystals: a greedy screening, Chemistry of Materials, Vol: 34, Pages: 2545-2552, ISSN: 0897-4756

Discovering materials that are environmentally stable and also exhibit the necessary collection of properties required for a particular application is a perennial challenge in materials science. Herein, we present an algorithm to rapidly screen materials for their thermodynamic stability in a given environment, using a greedy approach. The performance was tested against the standard energy above the hull stability metric for inert conditions. Using data of 126 320 crystals, the greedy algorithm was shown to estimate the driving force for decomposition with a mean absolute error of 39.5 meV/atom, giving it sufficient resolution to identify stable materials. To demonstrate the utility outside of a vacuum, the in-oxygen stability of 39 654 materials was tested. The enthalpy of oxidation was found to be largely exothermic. Further analysis showed that 1438 of these materials fall into the range required for self-passivation based on the Pilling–Bedworth ratio.

Journal article

Yang J-M, Lee J-H, Jung Y-K, Kim S-Y, Kim J-H, Kim S-G, Kim J-H, Seo S, Park D-A, Lee J-W, Walsh A, Park J-H, Park N-Get al., 2022, Mixed-Dimensional Formamidinium Bismuth Iodides Featuring In-Situ Formed Type-I Band Structure for Convolution Neural Networks, ADVANCED SCIENCE

Journal article

Michaels H, Golomb MJ, Kim BJ, Edvinsson T, Cucinotta F, Waddell PG, Probert MR, Konezny SJ, Boschloo G, Walsh A, Freitag Met al., 2022, Copper coordination polymers with selective hole conductivity, Journal of Materials Chemistry A, ISSN: 2050-7488

Emerging technologies in solar energy will be critical in enabling worldwide society in overcoming the present energy challenges and reaching carbon net zero. Inefficient and unstable charge transport materials limit the current emerging energy conversion and storage technologies. Low-dimensional coordination polymers represent an alternative, unprecedented class of charge transport materials, comprised of molecular building blocks. Here, we provide a comprehensive study of mixed-valence coordination polymers from an analysis of the charge transport mechanism to their implementation as hole-conducting layers. CuII dithiocarbamate complexes afford morphology control of 1D polymer chains linked by (CuI2X2) copper halide rhombi. Concerted theoretical and experimental efforts identified the charge transport mechanism in the transition to band-like transport with a modeled effective hole mass of 6me. The iodide-bridged coordination polymer showed an excellent conductivity of 1 mS cm−1 and a hole mobility of 5.8 10−4 cm2 (V s)−1 at room temperature. Nanosecond selective hole injection into coordination polymer thin films was captured by nanosecond photoluminescence of halide perovskite films. Coordination polymers constitute a sustainable, tunable alternative to the current standard of heavily doped organic hole conductors.

Journal article

Wang X, Li Z, Kavanagh S, Ganose A, Walsh Aet al., 2022, Lone pair driven anisotropy in antimony chalcogenide semiconductors, Physical Chemistry Chemical Physics, ISSN: 1463-9076

Antimony sulfide (Sb2S3) and selenide (Sb2Se3) have emerged as promising earth-abundant alternatives among thin-film photovoltaic compounds. A distinguishing feature of these materials is their anisotropic crystal structures, which are composed of quasi-one-dimensional (1D) [Sb4X6]n ribbons. The interaction between ribbons has been reported to be van der Waals (vdW) in nature and Sb2X3 are thus commonly classified in the literature as 1D semiconductors. However, based on first-principles calculations, here we show that inter-ribbon interactions are present in Sb2X3 beyond the vdW regime. The origin of the anisotropic structures is related to the stereochemical activity of the Sb 5s lone pair according to electronic structure analysis. The impacts of structural anisotropy on the electronic, dielectric and optical properties relevant to solar cells are further examined, including the presence of higher dimensional Fermi surfaces for charge carrier transport. Our study provides guidelines for optimising the performance of Sb2X3-based photovoltaics via device structuring based on the underlying crystal anisotropy.

Journal article

Wang S, Huang M, Wu Y-N, Chu W, Zhao J, Walsh A, Gong X-G, Wei S-H, Chen Set al., 2022, Effective Lifetime of Non-Equilibrium Carriers in Semiconductors from Non-Adiabatic Molecular Dynamics Simulations

<jats:title>Abstract</jats:title> <jats:p>The lifetime of non-equilibrium electrons and holes in semiconductors is crucial for solar cell and optoelectronic applications. Non-adiabatic molecular dynamics (NAMD) simulations based on time-dependent density functional theory (TDDFT) are widely used to study excited-state carrier dynamics. However, the calculated carrier lifetimes are often different from experimental results by orders of magnitude. In this work, by revisiting the definition of carrier lifetime and considering different recombination mechanisms, we report a systematic procedure for calculating the effective carrier lifetime in realistic semiconductor crystals that can be compared directly to experimental measurements. The procedure shows that considering all recombination mechanisms and using reasonable densities of carriers and defects are crucial in calculating the effective lifetime. When NAMD simulations consider only Shockey-Read-Hall (SRH) defect-assisted and band-to-band non-radiative recombination while neglect band-to-band radiative recombination, and the densities of non-equilibrium carriers and defects in supercell simulations are much higher than those in realistic semiconductors under solar illumination, the calculated lifetimes are ineffective and thus differ from experiments. Using our procedure, the calculated effective lifetime of the halide perovskite CH<jats:sub>3</jats:sub>NH<jats:sub>3</jats:sub>PbI<jats:sub>3</jats:sub> agrees with experiments. It is mainly determined by band-to-band radiative and defect-assisted non-radiative recombination, while band-to-band non-radiative recombination is negligible. These results indicate that it is possible to calculate carrier lifetimes accurately based on NAMD simulations, but the directly calculated values should be converted to effective lifetimes for comparison to experiments. The revised procedure can be widely applied in future

Journal article

Wang Y, Kavanagh SR, Burgues-Ceballos I, Walsh A, Scanlon D, Konstantatos Get al., 2022, Cation disorder engineering yields AgBiS2 nanocrystals with enhanced optical absorption for efficient ultrathin solar cells, NATURE PHOTONICS, Vol: 16, Pages: 235-+, ISSN: 1749-4885

Journal article

Kim C-E, Lee J, Walsh A, Lordi V, Bahr DFet al., 2022, Role of ripples in altering the electronic and chemical properties of graphene, Journal of Chemical Physics, Vol: 156, Pages: 1-6, ISSN: 0021-9606

Ripples of graphene are known to manipulate electronic and hydrogenation properties of graphitic materials. More detailed work is needed to elucidate the structure–property relationship of these systems. In this work, the density functional theory is used to compute the energy and electronic structure of the graphene models with respect to variable curvatures and hydrogen adsorption sites. The magnitude of finite bandgap opening depends on the orientation of ripples, and the hydrogen adsorption energy depends on the local curvature of graphene. An adsorbed hydrogen alters the local curvature, resulting in relatively weakened adsorption on the neighboring three sites, which gives a rationale to experimentally observed dynamic equilibrium stoichiometry (H:C = 1:4) of hydrogenated graphene. The surface diffusion transition state energy of adsorbed hydrogen is computed, which suggests that the Eley–Rideal surface recombination mechanism may be important to establish the dynamic equilibrium, instead of the commonly assumed Langmuir–Hinshelwood mechanism.

Journal article

Morgan LM, Islam MM, Yang H, O'Regan K, Patel AN, Ghosh A, Kendrick E, Marinescu M, Offer GJ, Morgan BJ, Islam MS, Edge J, Walsh Aet al., 2022, From Atoms to Cells: Multiscale Modeling of a LiNixMnyCozO2 Cathodes for Li-Ion Batteries, ACS ENERGY LETTERS, Vol: 7, Pages: 108-122, ISSN: 2380-8195

Journal article

Morita K, Davies D, Butler K, Walsh Aet al., 2022, Breaking the aristotype: featurisation of polyhedral distortions in perovskite crystals, Chemistry of Materials, Vol: 34, ISSN: 0897-4756

While traditional crystallographic representations of structure play an important role in materials science, they are unsuitable for efficient machine learning. A range of effective numerical descriptors have been developed for molecular and crystal structures. We are interested in a special case, where distortions emerge relative to an ideal high symmetry parent structure. We demonstrate that irreducible representations form anefficient basis for the featurisation of polyhedral deformations with respect to such an aristotype. Applied to a dataset of 552 octahedra in ABO3 perovskite-type materials, we use unsupervised machine learning with irreducible representation descriptors to identify four distinct classes of behaviour, associated with predominately corner, edge, face, and mixed connectivity between neighbouring octahedral units. Through this analysis, we identify SrCrO3 as a material with tuneable multiferroic behaviour. We further show, through supervised machine learning, that thermally activated structural distortions of CsPbI3 are well described by this approach.

Journal article

Morgan LM, Mercer MP, Bhandari A, Peng C, Islam MM, Yang H, Holland J, Coles SW, Sharpe R, Walsh A, Morgan BJ, Kramer D, Islam MS, Hoster HE, Edge JS, Skylaris C-Ket al., 2022, Pushing the boundaries of lithium battery research with atomistic modelling on different scales, Progress in Energy, Vol: 4, Pages: 012002-012002

<jats:title>Abstract</jats:title> <jats:p>Computational modelling is a vital tool in the research of batteries and their component materials. Atomistic models are key to building truly physics-based models of batteries and form the foundation of the multiscale modelling chain, leading to more robust and predictive models. These models can be applied to fundamental research questions with high predictive accuracy. For example, they can be used to predict new behaviour not currently accessible by experiment, for reasons of cost, safety, or throughput. Atomistic models are useful for quantifying and evaluating trends in experimental data, explaining structure-property relationships, and informing materials design strategies and libraries. In this review, we showcase the most prominent atomistic modelling methods and their application to electrode materials, liquid and solid electrolyte materials, and their interfaces, highlighting the diverse range of battery properties that can be investigated. Furthermore, we link atomistic modelling to experimental data and higher scale models such as continuum and control models. We also provide a critical discussion on the outlook of these materials and the main challenges for future battery research.</jats:p>

Journal article

Yun J, Tan J, Jung YK, Yang W, Lee H, Ma S, Park YS, Lee CU, Niu W, Lee J, Kim K, Tilley SD, Walsh A, Moon Jet al., 2022, Interfacial Dipole Layer Enables High-Performance Heterojunctions for Photoelectrochemical Water Splitting, ACS Energy Letters, Pages: 1392-1402

TiO2 has been widely used as an n-type overlayer, simultaneously serving as a protective layer for photocathodes. However, the photovoltage generated from a TiO2 junction with p-type absorbers, such as p-Si, Sb2Se3, SnS, and Cu2O, is insufficient. We report a dipole reorientation strategy to overcome this limitation by inserting a polyethylenimine ethoxylated (PEIE) layer between a p-type absorber and TiO2. Furthermore, we demonstrate that the PEIE dipole orientation can be rearranged by increasing the layer thickness, leading to an upward shift of the TiO2 band edge. The magnitude of band shift induced by the dipole effect depends on the TiO2 layer thickness. Using this approach, the onset potential was significantly improved to 0.5 V versus the reversible hydrogen electrode (VRHE) in a p-Si/PEIE/TiO2/Pt device. The versatility of the effective dipole reorientation strategy was demonstrated by application to a range of TiO2-protected heterojunction photocathodes based on Sb2Se3, Cu2O, and SnS.

Journal article

Kavanagh SR, Scanlon DO, Walsh A, Freysoldt Cet al., 2022, Impact of metastable defect structures on carrier recombination in solar cells, Faraday Discussions, ISSN: 1359-6640

<jats:p>The efficiency of a solar cell is often limited by electron-hole recombination mediated by defect states within the band gap of the photovoltaic (PV) semiconductor. The Shockley-Read-Hall (SRH) model considers...</jats:p>

Journal article

Doherty TAS, Nagane S, Kubicki DJ, Jung Y-K, Johnstone DN, Iqbal AN, Guo D, Frohna K, Danaie M, Tennyson EM, Macpherson S, Abfalterer A, Anaya M, Chiang Y-H, Crout P, Ruggeri FS, Collins S, Grey CP, Walsh A, Midgley PA, Stranks SDet al., 2021, Stabilized tilted-octahedra halide perovskites inhibit local formation of performance-limiting phases, SCIENCE, Vol: 374, Pages: 1598-+, ISSN: 0036-8075

Journal article

Hou Q, Buckeridge J, Walsh A, Xie Z, Lu Y, Keal TW, Guan J, Woodley SM, Catlow CRA, Sokol AAet al., 2021, The Interplay of Interstitial and Substitutional Copper in Zinc Oxide, FRONTIERS IN CHEMISTRY, Vol: 9, ISSN: 2296-2646

Journal article

Walsh A, 2021, Atomistic models of metal halide perovskites, MATTER, Vol: 4, Pages: 3867-3873, ISSN: 2590-2393

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, Vol: 49, Pages: 123-144, ISSN: 1369-7021

Journal article

Rondiya SR, Jagt RA, MacManus-Driscoll JL, Walsh A, Hoye RLZet al., 2021, Self-trapping in bismuth-based semiconductors: opportunities and challenges from optoelectronic devices to quantum technologies, Applied Physics Letters, Vol: 119, ISSN: 0003-6951

Semiconductors based on bismuth halides have gained attention for a wide range of electronic applications, including photovoltaics, light-emitting diodes, and radiation detectors. Their appeal is due to their low toxicity, high environmental stability under ambient conditions, and easy processability by a wide range of scalable methods. The performance of Bi-based semiconductors is dictated by electron–phonon interactions, which limit carrier mobilities and can also influence optoelectronic performance, for example, by giving rise to a large Stokes shift for photoluminescence, unavoidable energy loss channels, or shallow optical absorption onsets. In this Perspective, we discuss the recent understanding of how polarons and self-trapped excitons/carriers form in Bi-based semiconductors (particularly for the case of Cs2AgBiBr6), their impact on the optoelectronic properties of the materials, and the consequences on device performance. Finally, we discuss the opportunities that control of electron–phonon coupling enables, including stable solid-state white lighting, and the possibilities of exploiting the strong coupling found in bipolarons for quantum technologies.

Journal article

Krenzer G, Kim C-E, Tolborg K, Morgan BJ, Walsh Aet al., 2021, Anharmonic lattice dynamics of superionic lithium nitride, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 10, Pages: 2295-2304, ISSN: 2050-7488

Journal article

Yang J-M, Jung Y-K, Lee J-H, Kim YC, Kim S-Y, Seo S, Park D-A, Kim J-H, Jeong S-Y, Han I-T, Park J-H, Walsh A, Park N-Get al., 2021, Asymmetric carrier transport in flexible interface-type memristor enables artificial synapses with sub-femtojoule energy consumption, NANOSCALE HORIZONS, Vol: 6, Pages: 987-997, ISSN: 2055-6756

Journal article

Krajewska CJ, Kavanagh SR, Zhang L, Kubicki DJ, Dey K, Galkowski K, Grey CP, Stranks SD, Walsh A, Scanlon DO, Palgrave RGet al., 2021, Enhanced visible light absorption in layered Cs3Bi2Br9 through mixed-valence Sn(ii)/Sn(iv) doping, Chemical Science, Vol: 12, Pages: 14686-14699, ISSN: 2041-6520

Lead-free halides with perovskite-related structures, such as the vacancy-ordered perovskite Cs3Bi2Br9, are of interest for photovoltaic and optoelectronic applications. We find that addition of SnBr2 to the solution-phase synthesis of Cs3Bi2Br9 leads to substitution of up to 7% of the Bi(III) ions by equal quantities of Sn(II) and Sn(IV). The nature of the substitutional defects was studied by X-ray diffraction, 133Cs and 119Sn solid state NMR, X-ray photoelectron spectroscopy and density functional theory calculations. The resulting mixed-valence compounds show intense visible and near infrared absorption due to intervalence charge transfer, as well as electronic transitions to and from localised Sn-based states within the band gap. Sn(II) and Sn(IV) defects preferentially occupy neighbouring B-cation sites, forming a double-substitution complex. Unusually for a Sn(II) compound, the material shows minimal changes in optical and structural properties after 12 months storage in air. Our calculations suggest the stabilisation of Sn(II) within the double substitution complex contributes to this unusual stability. These results expand upon research on inorganic mixed-valent halides to a new, layered structure, and offer insights into the tuning, doping mechanisms, and structure–property relationships of lead-free vacancy-ordered perovskite structures.

Journal article

Huang Y-T, Kavanagh SR, Scanlon DO, Walsh A, Hoye RLZet al., 2021, Perovskite-inspired materials for photovoltaics and beyond-from design to devices (vol 32, 132004, 2021), NANOTECHNOLOGY, Vol: 32, ISSN: 0957-4484

Journal article

Jung Y-K, Kim S, Kim YC, Walsh Aet al., 2021, Low Barrier for Exciton Self-Trapping Enables High Photoluminescence Quantum Yield in Cs3Cu2I5, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, Vol: 12, Pages: 8447-8452, ISSN: 1948-7185

Journal article

Li Z, Park J-S, Walsh A, 2021, Evolutionary exploration of polytypism in lead halide perovskites, CHEMICAL SCIENCE, Vol: 12, Pages: 12165-12173, ISSN: 2041-6520

Journal article

Simenas M, Balciunas S, Svirskas S, Kinka M, Ptak M, Kalendra V, Gagor A, Szewczyk D, Sieradzki A, Grigalaitis R, Walsh A, Maczka M, Banys Jet al., 2021, Phase Diagram and Cation Dynamics of Mixed MA(1-x)FA(x)PbBr(3) Hybrid Perovskites, CHEMISTRY OF MATERIALS, Vol: 33, Pages: 5926-5934, ISSN: 0897-4756

Journal article

Harnett-Caulfield L, Walsh A, 2021, Assessment of interstitial potentials for rapid prediction of absolute band energies in crystals, JOURNAL OF CHEMICAL PHYSICS, Vol: 155, ISSN: 0021-9606

Journal article

Kavanagh SR, Savory CN, Scanlon DO, Walsh Aet al., 2021, Hidden spontaneous polarisation in the chalcohalide photovoltaic absorber Sn2SbS2I3, MATERIALS HORIZONS, Vol: 8, ISSN: 2051-6347

Journal article

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