Publications
559 results found
Moss B, Le H, Corby S, et 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.
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.
Crovetto A, Kim S, Fischer M, et 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.
Gkini K, Balis N, Papadakis M, et al., 2020, Manganese Porphyrin Interface Engineering in Perovskite Solar Cells, ACS APPLIED ENERGY MATERIALS, Vol: 3, Pages: 7353-7363, ISSN: 2574-0962
Rahim W, Skelton JM, Savory CN, et 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.
Park SY, Park J-S, Kim BJ, et 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.
Welch EW, Jung Y-K, Walsh A, et 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.
Hutter EM, Muscarella LA, Wittmann F, et 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.
Davies DW, Morgan BJ, Scanlon DO, et 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.
Golomb MJ, Calbo J, Bristow JK, et 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.
Morita K, Davies DW, Butler KT, et 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.
Park J-S, Li Z, Wilson JN, et al., 2020, Hexagonal stacking faults act as hole-blocking layers in lead halide perovskites, ACS Energy Letters, Vol: 5, Pages: 2231-2233, ISSN: 2380-8195
The transformation between black (corner sharing) and yellow (face sharing) polytypes of lead halide perovskites is a major performance bottleneck. We investigate phase intermixing through the simulation of stacking faults (nanodomains) that reveal a small thermodynamic cost but large electronic consequences in CsPbI3.
Jones LAH, Linhart WM, Fleck N, et al., 2020, Sn 5<i>s</i><SUP>2</SUP> lone pairs and the electronic structure of tin sulphides: A photoreflectance, high-energy photoemission, and theoretical investigation, PHYSICAL REVIEW MATERIALS, Vol: 4, ISSN: 2475-9953
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- Citations: 9
Kim S, Hood SN, Park J-S, et al., 2020, Quick-start guide for first-principles modelling of point defects in crystalline materials, The Journal of High Energy Physics, Vol: 2, Pages: 1-8, ISSN: 1029-8479
Defects influence the properties and functionality of all crystalline materials. For instance, point defects participate in electronic (e.g. carrier generation and recombination) and optical (e.g. absorption and emission) processes critical to solar energy conversion. Solid-state diffusion, mediated by the transport of charged defects, is used for electrochemical energy storage. First-principles calculations of defects based on density functional theory have been widely used to complement, and even validate, experimental observations. In this 'quick-start guide', we discuss the best practice in how to calculate the formation energy of point defects in crystalline materials and analysis techniques appropriate to probe changes in structure and properties relevant across energy technologies.
Morita K, Davies DW, Butler KT, et al., 2020, WMD-group/Dielectric_ML: Repository for submitted paper
This version was created when the associated publication was submitted.
Golomb M, Calbo J, Bristow JK, et 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 N<sub>2</sub>(C<sub>2</sub>H<sub>4</sub>)<sub>3</sub> (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 bandstructure calculations and effective masses of the valence band egde. 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 doping on the bandstructure of MOF-649 are considered. Our findings highlight DMOF-1 as a potential semiconductor with 1D charge carrier mobility along the framework
Rahim W, Skelton J, Savory C, et al., 2020, Polymorph exploration of bismuth stannate using first-principles phonon mode mapping
In this work, we present a new unbiased and efficient quantum chemical method for exploring the potential energy surface of complex crystal structures using theoretically rigorous phonon mode-mapping approach. The method successfully recovers the known experimental phases of the pyrochlore-based Bi<sub>2</sub>Sn<sub>2</sub>O<sub>7</sub>, one of the most difficult cases in structural chemistry, which highlights its utility for searching possible transition pathways and identifying global minima for many other challenging systems.<p></p>
YANG H, Savory C, Morgan B, et al., 2020, Chemical Trends in the Lattice Thermal Conductivity of Li(Ni,Mn,Co)O2 (NMC) Battery Cathodes
<jats:p>While the transport of ions and electrons in conventional Li-ion battery cathodematerials is well understood, our knowledge of the phonon (heat) transport is still in itsinfancy. We present a first-principles theoretical investigation of the chemical trendsin the phonon frequency dispersion, mode lifetimes, and thermal conductivity in theseries of layered lithium transition-metal oxides Li(NixMnyCoz)O2 (x+y+z = 1). Theoxidation and spin states of the transition metal cations are found to strongly influencethe structural dynamics. Calculations of the thermal conductivity show that LiCoO2has highest average conductivity of 45.9W m−1 K−1 at T = 300 K and the largestanisotropy, followed by LiMnO2 with 8.9W m−1 K−1, and LiNiO2 with 6.0W m−1 K−1The 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 and show that the thermal conductivity of NMC is suppressed with decreasing Co content and increasing Ni/Mncontent. The thermal conductivity of commercial NMC622 (LiNi0.6Mn0.2Co0.2O2) and NMC111 (LiNi0.33Mn0.33Co0.33O2) compositions are substantially larger than NMC811LiNi0.8Mn0.1Co0.1O2). These results serve as a guide to ongoing work on the designof multi-component battery electrodes with more effective thermal management.</jats:p><jats:p />
YANG H, Savory C, Morgan B, et al., 2020, Chemical Trends in the Lattice Thermal Conductivity of Li(Ni,Mn,Co)O<sub>2</sub> (NMC) Battery Cathodes
<jats:p><div>While the transport of ions and electrons in conventional Li-ion battery cathode</div><div>materials is well understood, our knowledge of the phonon (heat) transport is still in its</div><div>infancy. We present a first-principles theoretical investigation of the chemical trends</div><div>in the phonon frequency dispersion, mode lifetimes, and thermal conductivity in the</div><div>series of layered lithium transition-metal oxides Li(NixMnyCoz)O2 (x+y+z = 1). The</div><div>oxidation and spin states of the transition metal cations are found to strongly influence</div><div>the structural dynamics. Calculations of the thermal conductivity show that LiCoO2</div><div>has highest average conductivity of 45.9W m−1 K−1 at T = 300 K and the largest</div><div>anisotropy, followed by LiMnO2 with 8.9W m−1 K−1, and LiNiO2 with 6.0W m−1 K−1</div><div>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 and show that the thermal conductivity of NMC is suppressed with decreasing Co content and increasing Ni/Mn</div><div>content. The thermal conductivity of commercial NMC622 (LiNi0.6Mn0.2Co0.2O2) and NMC111 (LiNi0.33Mn0.33Co0.33O2) compositions are substantially larger than NMC811LiNi0.8Mn0.1Co0.1O2). These results serve as a guide to ongoing work on the designof multi-component battery electrodes with more effective thermal management.</div><div><br></div></jats:p>
Rahim W, Skelton J, Savory C, et al., 2020, Polymorph Exploration of Bismuth Stannate Using First-principles Phonon Mode Mapping
<jats:p>In this work, we present a new unbiased and efficient quantum chemical method for exploring the potential energy surface of complex crystal structures using theoretically rigorous phonon mode-mapping approach. The method successfully recovers the known experimental phases of the pyrochlore-based Bi<sub>2</sub>Sn<sub>2</sub>O<sub>7</sub>, one of the most difficult cases in structural chemistry, which highlights its utility for searching possible transition pathways and identifying global minima for many other challenging systems.<p></p></jats:p>
Golomb M, Calbo J, Bristow JK, et al., 2020, Ligand Engineering in Cu(II) Paddle Wheel Metal-Organic Frameworks for Enhanced Electrical Conductivity
<jats:p>We report the electronic structure of two metal-organic frameworks (MOFs) with copper paddle wheel nodes connected by a N<sub>2</sub>(C<sub>2</sub>H<sub>4</sub>)<sub>3</sub> (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 bandstructure calculations and effective masses of the valence band egde. 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 doping on the bandstructure of MOF-649 are considered. Our findings highlight DMOF-1 as a potential semiconductor with 1D charge carrier mobility along the framework</jats:p>
Kim S, Marquez JA, Unold T, et al., 2020, Upper limit to the photovoltaic efficiency of imperfect crystals from first principles, Energy and Environmental Science, Vol: 13, Pages: 1481-1491, ISSN: 1754-5692
The Shockley–Queisser (SQ) limit provides a convenient metric for predicting light-to-electricity conversion efficiency of a solar cell based on the band gap of the light-absorbing layer. In reality, few materials approach this radiative limit. We develop a formalism and computational method to predict the maximum photovoltaic efficiency of imperfect crystals from first principles. The trap-limited conversion efficiency includes equilibrium populations of native defects, their carrier-capture coefficients, and the associated recombination rates. When applied to kesterite solar cells, we reveal an intrinsic limit of 20% for Cu2ZnSnSe4, which falls far below the SQ limit of 32%. The effects of atomic substitution and extrinsic doping are studied, leading to pathways for an enhanced efficiency of 31%. This approach can be applied to support targeted-materials selection for future solar-energy technologies.
Doherty TAS, Winchester AJ, Macpherson S, et al., 2020, Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites, Nature, Vol: 580, Pages: 360-366, ISSN: 0028-0836
Halide perovskite materials have promising performance characteristics for low-cost optoelectronic applications. Photovoltaic devices fabricated from perovskite absorbers have reached power conversion efficiencies above 25 per cent in single-junction devices and 28 per cent in tandem devices1,2. This strong performance (albeit below the practical limits of about 30 per cent and 35 per cent, respectively3) is surprising in thin films processed from solution at low-temperature, a method that generally produces abundant crystalline defects4. Although point defects often induce only shallow electronic states in the perovskite bandgap that do not affect performance5, perovskite devices still have many states deep within the bandgap that trap charge carriers and cause them to recombine non-radiatively. These deep trap states thus induce local variations in photoluminescence and limit the device performance6. The origin and distribution of these trap states are unknown, but they have been associated with light-induced halide segregation in mixed-halide perovskite compositions7 and with local strain8, both of which make devices less stable9. Here we use photoemission electron microscopy to image the trap distribution in state-of-the-art halide perovskite films. Instead of a relatively uniform distribution within regions of poor photoluminescence efficiency, we observe discrete, nanoscale trap clusters. By correlating microscopy measurements with scanning electron analytical techniques, we find that these trap clusters appear at the interfaces between crystallographically and compositionally distinct entities. Finally, by generating time-resolved photoemission sequences of the photo-excited carrier trapping process10,11, we reveal a hole-trapping character with the kinetics limited by diffusion of holes to the local trap clusters. Our approach shows that managing structure and composition on the nanoscale will be essential for optimal performance of halide perovskite devices
Kim S, Hood S, van Gerwen P, et al., 2020, CarrierCapture.jl: anharmonic carrier capture, Journal of Open Source Software, Vol: 5, Pages: 2102-2102, ISSN: 2475-9066
Kim S, Hood SN, van Gerwen P, et al., 2020, CarrierCapture.jl: Anharmonic Carrier Capture
This release includes bug fixes and improvements as suggested as part of the JOSS review process
Kim S, Walsh A, 2020, Comment on "Low-frequency lattice phonons in halide perovskites explain high defect tolerance toward electron-hole recombination", Publisher: arXiv
Halide perovskites exhibit slow rates of non-radiative electron-holerecombination upon illumination. Chu et al. [Sci. Adv. 6 7, eaaw7453 (2020)]use the results of first-principles simulations to argue that this arises fromthe nature of the crystal vibrations and leads to a breakdown ofShockley-Read-Hall theory. We highlight flaws in their methodology and analysisof carrier capture by point defects in crystalline semiconductors.
Davies DW, Savory CN, Frost JM, et al., 2020, Descriptors for electron and hole charge carriers in metal oxides, Journal of Physical Chemistry Letters, Vol: 11, Pages: 438-444, ISSN: 1948-7185
Metal oxides can act as insulators, semiconductors, or metals depending on their chemical composition and crystal structure. Metal oxide semiconductors, which support equilibrium populations of electron and hole charge carriers, have widespread applications including batteries, solar cells, and display technologies. It is often difficult to predict in advance whether these materials will exhibit localized or delocalized charge carriers upon oxidation or reduction. We combine data from first-principles calculations of the electronic structure and dielectric response of 214 metal oxides to predict the energetic driving force for carrier localization and transport. We assess descriptors based on the carrier effective mass, static polaron binding energy, and Fröhlich electron–phonon coupling. Numerical analysis allows us to assign p- and n-type transport of a metal oxide to three classes: (i) band transport with high mobility; (ii) small polaron transport with low mobility; and (iii) intermediate behavior. The results of this classification agree with observations regarding carrier dynamics and lifetimes and are used to predict 10 candidate p-type oxides.
Yang RX, Skelton JM, da Silva EL, et al., 2020, Assessment of dynamic structural instabilities across 24 cubic inorganic halide perovskites, Journal of Chemical Physics, Vol: 152, Pages: 024703-1-024703-9, ISSN: 0021-9606
Metal halide perovskites are promising candidates for next-generation photovoltaic and optoelectronic applications. The flexible nature of the octahedral network introduces complexity when understanding their physical behavior. It has been shown that these materials are prone to decomposition and phase competition, and the local crystal structure often deviates from the average space group symmetry. To make stable phase-pure perovskites, understanding their structure–composition relations is of central importance. We demonstrate, from lattice dynamics calculations, that the 24 inorganic perovskites ABX3 (A = Cs, Rb; B = Ge, Sn, Pb; X = F, Cl, Br, I) exhibit instabilities in their cubic phase. These instabilities include cation displacements, octahedral tilting, and Jahn-Teller distortions. The magnitudes of the instabilities vary depending on the chemical identity and ionic radii of the composition. The tilting instabilities are energetically dominant and reduce as the tolerance factor increases, whereas cation displacements and Jahn-Teller type distortions depend on the interactions between the constituent ions. We further considered representative tetragonal, orthorhombic, and monoclinic perovskite phases to obtain phonon-stable structures for each composition. This work provides insights into the thermodynamic driving force of the instabilities and will help guide computer simulations and experimental synthesis in material screening.
Khenkin MV, Katz EA, Abate A, et al., 2020, Consensus statement for stability assessment and reporting for perovskite photovoltaics based on ISOS procedures, Nature Energy, Vol: 5, Pages: 35-49, ISSN: 2058-7546
Improving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great emphasis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mechanisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing qualification standards, but rather they aim to unify the stability assessment and to understand failure modes. Finally, we identify key procedural information which we suggest reporting in publications to improve reproducibility and enable large data set analysis.
Silva EL, Santos MC, Skelton JM, et al., 2020, Electronic and Phonon Instabilities in Bilayer Graphene under Applied External Bias, Pages: 373-382
We have performed electronic-structure and lattice-dynamics calculations on the AB and AA structures of bilayer graphene. We study the effect of external electric fields and compare results obtained with different levels of theory to existing theoretical and experimental results. Application of an external field to the AB bilayer alters the electronic spectrum, with the bands changing under bias from a parabolic to a "Mexican hat" structure. This results in a semi-metal-To-semiconductor phase transition, with the size of the induced electronic band-gap being tuneable through the field strength. A reduction of continuous symmetry from a hexagonal to a triangular lattice is also evidenced through in-plane electronic charge inhomogeneities between the sublattices. When spin-orbit coupling is turned on for the AB system, we find that the bulk gap decreases, gradually increasing for larger intensities of the bias. Under large bias the energy dispersion recovers the Mexican hat structure, since the energy interaction between the layers balances the coupling interaction. We find that external bias perturbs the harmonic phonon spectra and leads to anomalous behaviour of the out-of-plane flexural ZA and layer-breathing ZO' modes. For the AA system, the electronic and phonon dispersions both remain stable under bias, but the phonon spectrum exhibits zone-center imaginary modes due to layersliding dynamical instabilities.
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