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

495 results found

Kavanagh SR, Savory CN, Liga SM, Konstantatos G, Walsh A, Scanlon DOet al., 2022, Frenkel Excitons in Vacancy-Ordered Titanium Halide Perovskites (Cs2TiX6)., J Phys Chem Lett, Pages: 10965-10975

Low-cost, nontoxic, and earth-abundant photovoltaic materials are long-sought targets in the solar cell research community. Perovskite-inspired materials have emerged as promising candidates for this goal, with researchers employing materials design strategies including structural, dimensional, and compositional transformations to avoid the use of rare and toxic elemental constituents, while attempting to maintain high optoelectronic performance. These strategies have recently been invoked to propose Ti-based vacancy-ordered halide perovskites (A2TiX6; A = CH3NH3, Cs, Rb, or K; X = I, Br, or Cl) for photovoltaic operation, following the initial promise of Cs2SnX6 compounds. Theoretical investigations of these materials, however, consistently overestimate their band gaps, a fundamental property for photovoltaic applications. Here, we reveal strong excitonic effects as the origin of this discrepancy between theory and experiment, a consequence of both low structural dimensionality and band localization. These findings have vital implications for the optoelectronic application of these compounds while also highlighting the importance of frontier-orbital character for chemical substitution in materials design strategies.

Journal article

Agbenyeke R, Andreasen JW, Benhaddou N, Bowers JW, Breternitz J, Bar M, Dimitrievska M, Fermin DJ, Ganose A, Hawkins C, Jaramillo R, Kavanagh SR, Kondrotas R, Major JD, Mandati S, Nicolson A, Bjorkman CP, Savory C, Scanlon DO, Schorr S, Scragg JJS, Sheppard A, Shin B, Siebentritt S, Sood M, Sopiha K, Spalatu N, Tang J, Walsh A, Weiss TP, Woods-Robinson R, Yetkin HAet al., 2022, Materials design and bonding: general discussion, FARADAY DISCUSSIONS, Vol: 239, Pages: 375-404, ISSN: 1359-6640

Journal article

Andreasen JW, Arca E, Bowers JW, Bar M, Breternitz J, Dale PJ, Dimitrievska M, Fermin DJ, Ganose A, Hages CJ, Hobson T, Jaramillo R, Kavanagh SR, Kayastha P, Kondrotas R, Lee J, Major JD, Mandati S, Mitzi DB, Scanlon DO, Schorr S, Scragg JJS, Shin B, Siebentritt S, Smiles M, Sood M, Sopiha K, Spalatu N, Sutton M, Unold T, Valdes M, Walsh A, Wang M, Wang X, Weiss TP, Woo YW, Woods-Robinson R, Tiwari Det al., 2022, Novel chalcogenides, pnictides and defect-tolerant semiconductors: general discussion, FARADAY DISCUSSIONS, Vol: 239, Pages: 287-316, ISSN: 1359-6640

Journal article

Andreasen JW, Breternitz J, Baer M, Dale PJ, Dimitrievska M, Fermin DJ, Fleck N, Hages CJ, Havryliuk Y, Hawkins C, Jaramillo R, Kavanagh SR, Kayastha P, Kondrotas R, Lapalikar V, Mandati S, Mitzi DB, Bjoerkman CP, Savory C, Scragg JJS, Shin B, Siebentritt S, Sood M, Tiwari D, Valdes M, Walsh A, Weiss TP, Woo YW, Woods-Robinson R, Yetkin HAet al., 2022, Bulk and surface characterisation techniques of solar absorbers: general discussion, FARADAY DISCUSSIONS, Vol: 239, Pages: 180-201, ISSN: 1359-6640

Journal article

Huang Y-T, Kavanagh S, Righetto M, Rusu M, Levine I, Unold T, Zelewiski S, Sneyd A, Zhang K, Dai L, Britton A, Ye J, Julin J, Napari M, Zhang Z, Xiao J, Laitinen M, Torrente-Murciano L, Stranks S, Rao A, Herz L, Scanlon D, Walsh A, Hoye Ret al., 2022, Strong absorption and ultrafast localisation in NaBiS2 nanocrystals with slow charge-carrier recombination, Nature Communications, Vol: 13, ISSN: 2041-1723

I-V-VI2 ternary chalcogenides are gaining attention as earth-abundant, nontoxic, and air-stable absorbers for photovoltaic applications. However, the semiconductors explored thus far have slow absorption onsets, and their charge-carrier transport is not well understood yet. Herein, we investigate cation-disordered NaBiS2 nanocrystals, which have a steep absorption onset, with absorption coefficients reaching >105 cm-1 just above its pseudo-direct bandgap of 1.4 eV. Surprisingly, we also observe an ultrafast (at picosecond-time scale) photoconductivity decay and long-lived charge-carrier population persisting for over one microsecond in NaBiS2 nanocrystals. These unusual features arise because of the localised, non-bonding S p character of the upper valence band, which leads to a high density of electronic states at the band edges, ultrafast localisation of spatially-separated electrons and holes, as well as the slow decay of trapped holes. This work reveals the critical role of cation disorder in these systems on both absorption characteristics and charge-carrier kinetics.

Journal article

Walsh A, 2022, Concluding remarks: emerging inorganic materials in thin-film photovoltaics, FARADAY DISCUSSIONS, Vol: 239, Pages: 405-412, ISSN: 1359-6640

Journal article

Jung Y-K, Abdulla M, Friend RH, Stranks SD, Walsh Aet al., 2022, Pressure-induced non-radiative losses in halide perovskite light-emitting diodes, JOURNAL OF MATERIALS CHEMISTRY C, Vol: 10, Pages: 12560-12568, ISSN: 2050-7526

Journal article

Wang X, Ganose AM, Kavanagh SR, Walsh Aet al., 2022, Band versus Polaron: Charge Transport in Antimony Chalcogenides, ACS ENERGY LETTERS, Vol: 7, Pages: 2954-2960, ISSN: 2380-8195

Journal article

Ganose A, Scanlon D, Walsh A, Hoye R, Ganose A, Scanlon D, Walsh A, Hoye Ret al., 2022, The defect challenge of wide-bandgap semiconductors for photovoltaics and beyond, Nature Communications, Vol: 13, ISSN: 2041-1723

The optoelectronic performance of wide-bandgap semiconductors often cannot compete with that of their defect-tolerant small-bandgap counterpart. Here, the authors outline three main challenges to overcome for mitigating the impact of defects in wide-bandgap semiconductors.

Journal article

Squires AG, Davies DW, Kim S, Scanlon DO, Walsh A, Morgan BJet al., 2022, Low electronic conductivity of Li7La3Zr2O12 solid electrolytes from first principles, PHYSICAL REVIEW MATERIALS, Vol: 6, ISSN: 2475-9953

Journal article

Wang S, Huang M, Wu YN, Chu W, Zhao J, Walsh A, Gong XG, Wei SH, Chen Set al., 2022, Effective lifetime of non-equilibrium carriers in semiconductors from non-adiabatic molecular dynamics simulations, Nature Computational Science, Vol: 2, Pages: 486-493

The lifetimes of non-equilibrium charge carriers in semiconductors calculated using non-adiabatic molecular dynamics often differ from experimental results by orders of magnitude. By revisiting the definition of carrier lifetime, we report a systematic procedure for calculating the effective carrier lifetime in semiconductor crystals under realistic conditions. The consideration of all recombination mechanisms and the use of appropriate carrier and defect densities are crucial to bridging the gap between modeling and measurements. Our calculated effective carrier lifetime of CH3NH3PbI3 agrees with experiments, and is limited by band-to-band radiative recombination and Shockley–Read–Hall defect-assisted non-radiative recombination, whereas the band-to-band non-radiative recombination is found to be negligible. The procedure is further validated by application to the compound semiconductors CdTe and GaAs, and thus can be applied in carrier lifetime simulations in other material systems.

Journal article

Moriarty A, Morita K, Butler KT, Walsh Aet al., 2022, UnlockNN: Uncertainty quantification for neural networkmodels of chemical systems, Journal of Open Source Software, Vol: 7, Pages: 3700-3700

Journal article

Morita K, Kumagai Y, Oba F, Walsh Aet al., 2022, Switchable Electric Dipole from Polaron Localization in Dielectric Crystals, PHYSICAL REVIEW LETTERS, Vol: 129, ISSN: 0031-9007

Journal article

Liu T, Zhang X, Guan J, Catlow CRA, Walsh A, Sokol AA, Buckeridge Jet al., 2022, Insight into the Fergusonite-Scheelite Phase Transition of ABO(4)-Type Oxides by Density Functional Theory: A Case Study of the Subtleties of the Ground State of BiVO4, CHEMISTRY OF MATERIALS, Vol: 34, Pages: 5334-5343, ISSN: 0897-4756

Journal article

Huang J, Golomb MJ, Kavanagh SR, Tolborg K, Ganose AM, Walsh Aet al., 2022, Band gap opening from displacive instabilities in layered covalent-organic frameworks, Journal of Materials Chemistry A, Vol: 10, Pages: 13500-13507, ISSN: 2050-7488

Covalent organic frameworks (COFs) offer a high degree of chemical and structural flexibility. There is a large family of COFs built from 2D sheets that are stacked to form extended crystals. While it has been common to represent the stacking as eclipsed with one repeating layer (“AA”), there is growing evidence that a more diverse range of stacking sequences is accessible. Herein, we report a computational study using density functional theory of layer stacking in two prototypical COFs, Tp-Azo and DAAQ-TFP, which have shown high performance as Li-ion battery electrodes. We find a striking preference for slipped structures with horizontal offsets between layers ranging from 1.7 Å to 3.5 Å in a potential energy minimum that forms a low energy ring. The associated symmetry breaking results in a pronounced change in the underlying electronic structure. A band gap opening of 0.8–1.4 eV is found due to modifications of the underlying valence and conduction band dispersion as explained from changes in the π orbital overlap. The implications for the screening and selection of COF for energy applications are discussed.

Journal article

Ma S, Jung Y-K, Ahn J, Kyhm J, Tan J, Lee H, Jang G, Lee CU, Walsh A, Moon Jet al., 2022, Elucidating the origin of chiroptical activity in chiral 2D perovskites through nano-confined growth, NATURE COMMUNICATIONS, Vol: 13

Journal article

Pastor E, Sachs M, Selim S, Durrant JR, Bakulin AA, Walsh Aet al., 2022, Electronic defects in metal oxide photocatalysts, NATURE REVIEWS MATERIALS, Vol: 7, Pages: 503-521, ISSN: 2058-8437

Journal article

Rakowski R, Fisher W, Calbo J, Mokhtar MZ, Liang X, Ding D, Frost JM, Haque SA, Walsh A, Barnes PRF, Nelson J, van Thor JJet al., 2022, High Power Irradiance Dependence of Charge Species Dynamics in Hybrid Perovskites and Kinetic Evidence for Transient Vibrational Stark Effect in Formamidinium, NANOMATERIALS, Vol: 12

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

Kavanagh SR, Scanlon DO, Walsh A, Freysoldt Cet al., 2022, Impact of metastable defect structures on carrier recombination in solar cells, FARADAY DISCUSSIONS, Vol: 239, Pages: 339-356, ISSN: 1359-6640

Journal article

Yun J, Tan J, Jung Y-K, 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, Vol: 7, Pages: 1392-1402, ISSN: 2380-8195

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, Vol: 16, Pages: 396-396, 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, Vol: 9

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, Vol: 10, Pages: 9582-9591, 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, Vol: 2022, 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

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