Publications
559 results found
Jung Y-K, Calbo J, Park J-S, et al., 2019, Intrinsic doping limit and defect-assisted luminescence in Cs4PbBr6
<jats:p>Cs<jats:sub>4</jats:sub>PbBr<jats:sub>6 </jats:sub>is a member of the halide perovskite family that is built from isolated (zero-dimensional) PbBr<jats:sub>6</jats:sub><jats:sup>4-</jats:sup> octahedra with Cs<jats:sup>+</jats:sup> counter ions. The material exhibits anomalous optoelectronic properties: optical absorption and weak emission in the deep ultraviolet (310 - 375 nm) with efficient luminescence in the green region (~ 540 nm). Several hypotheses have been proposed to explain the giant Stokes shift including: (i) phase impurities; (ii) self-trapped exciton; (iii) defect emission. We explore, using first-principles theory and self-consistent Fermi level analysis, the unusual defect chemistry and physics of Cs<jats:sub>4</jats:sub>PbBr<jats:sub>6</jats:sub>. We find a heavily compensated system where the room-temperature carrier concentrations (< 10<jats:sup>9</jats:sup> cm<jats:sup>-3</jats:sup>) are more than one million times lower than the defect concentrations. We show that the low-energy Br-on-Cs antisite results in the formation of a polybromide (Br<jats:sub>3</jats:sub>) species that can exist in a range of charge states. We further demonstrate from excited-state calculations that tribromide moieties are photoresponsive and can contribute to the observed green luminescence. Photoactivity of polyhalide molecules is expected to be present in other halide perovskite-related compounds where they can influence light absorption and emission. </jats:p><jats:p />
Kye YH, Yu CJ, Jong UG, et al., 2019, Vacancy-driven stabilization of the cubic perovskite polymorph of CsPbI3, The Journal of Physical Chemistry C, Vol: 123, Pages: 9735-9744, ISSN: 1932-7447
The inorganic halide perovskite CsPbI 3 has shown great promise for efficient solar cells, but the instability of its cubic phase remains a major challenge. We present a route for stabilizing the cubic α-phase of CsPbI 3 through the control of vacancy defects. Analysis of the ionic chemical potentials is performed within an ab initio thermodynamic formalism, including the effect of solution. It is found that cation vacancies lead to weakening of the interaction between Cs and PbI 6 octahedra in CsPbI 3 , with a decrease in the energy difference between the α- A nd Î-phases. Under I-rich growth conditions, which can be realized experimentally, we predict that the formation of cation vacancies can be controlled. Other synthetic strategies for cubic-phase stabilization include the growth of nanocrystals, surface capping ligands containing reductive functional groups, and extrinsic doping. Our analysis reveals mechanisms for polymorph stabilization that open a new pathway for structural control of halide perovskites.
Zhang F, Kim DH, Lu H, et al., 2019, Enhanced charge transport in 2D Perovskites via fluorination of organic cation, Journal of the American Chemical Society, Vol: 141, Pages: 5972-5979, ISSN: 1520-5126
Organic-inorganic halide perovskites incorporating two-dimensional (2D) structures have shown promise for enhancing the stability of perovskite solar cells (PSCs). However, the bulky spacer cations often limit charge transport. Here, we report on a simple approach based on molecular design of the organic spacer to improve the transport properties of 2D perovskites, and we use phenethylammonium (PEA) as an example. We demonstrate that by fluorine substitution on the para position in PEA to form 4-fluorophenethylammonium (F-PEA), the average phenyl ring centroid-centroid distances in the organic layer become shorter with better aligned stacking of perovskite sheets. The impact is enhanced orbital interactions and charge transport across adjacent inorganic layers as well as increased carrier lifetime and reduced trap density. Using a simple perovskite deposition at room temperature without using any additives, we obtained a power conversion efficiency of >13% for (F-PEA)2MA4Pb5I16-based PSCs. In addition, the thermal stability of 2D PSCs based on F-PEA is significantly enhanced compared to those based on PEA.
Shin M, Kim J, Jung Y-K, et al., 2019, Low-dimensional formamidinium lead perovskite architectures via controllable solvent intercalation, Journal of Materials Chemistry C, Vol: 7, Pages: 3945-3951, ISSN: 2050-7526
We report the formation of a new class of solvent-intercalated two-dimensional (SI-2D) formamidinium lead halide perovskites. They can be mixed with three-dimensional (3D) stoichiometric perovskites by controlling the ratio of the precursor solutions. The composite leads to greatly improved photoluminescence quantum yield (PLQY) over the 3D compound. The enhanced PLQY is attributed to a type-I band alignment between the 3D and SI-2D, as revealed by first-principles calculations, which results in confined excitons with enhanced radiative recombination. The films exhibited excellent thermal and air stability retaining PLQY > 20% over 2 months in ambient conditions. Assemblies of halide perovskites with mixed dimensionality offer a pathway to enhance optoelectronic performance and device lifetimes.
Kim S, Hood SN, Walsh A, 2019, Anharmonic lattice relaxation during non-radiative carrier capture, Publisher: arXiv
Lattice vibrations of point defects are essential for understandingnon-radiative electron and hole capture in semiconductors as they governproperties including persistent photoconductivity and Shockley-Read-Hallrecombination rate. Although the harmonic approximation is sufficient todescribe a defect with small lattice relaxation, for cases of large latticerelaxation it is likely to break down. We describe a first-principles procedureto account for anharmonic carrier capture and apply it to the important case ofthe \textit{DX} center in GaAs. This is a system where the harmonicapproximation grossly fails. Our treatment of the anharmonic Morse-likepotentials accurately describes the observed electron capture barrier,predicting the absence of quantum tunnelling at low temperature, and a highhole capture rate that is independent of temperature. The model also explainsthe origin of the composition-invariant electron emission barrier. Theseresults highlight an important shortcoming of the standard approach fordescribing point defect ionization that is accompanied by large latticerelaxation, where charge transfer occurs far from the equilibriumconfiguration.
Kim S, PaleBlueSam, Walsh A, 2019, WMD-group/CarrierCapture.jl: CarrierCapture.jl
Package to compute trap-assisted electron-hole recombination in semiconducting compounds.
Aspuru-Guzik A, Baik M-H, Balasubramanian S, et al., 2019, Charting a course for chemistry, NATURE CHEMISTRY, Vol: 11, Pages: 286-294, ISSN: 1755-4330
Jones CL, Skelton JM, Parker SC, et al., 2019, Living in the salt-cocrystal continuum: indecisive organic complexes with thermochromic behaviour, CrystEngComm, Vol: 21, Pages: 1626-1634, ISSN: 1466-8033
A family of multicomponent haloaniline/3,5-dinitrobenzoic acid molecular crystals with striking red-to-colourless temperature-induced thermochromism is identified and characterised. Four thermochromic pairs of 1 : 1 neutral cocrystals and ionic salts are identified which, unusually, grow concomitantly under the same conditions. The coloured cocrystals are found to be metastable, kinetically trapped during crystallisation, and convert via proton transfer to the more stable salt forms on heating. The colour of the neutral form and the temperature of the transition can be tuned through the halogen and by chemical substitution on the aniline component. From structural characterisation and first-principles modelling, we elucidate the origin of the metastability of the cocrystals and link structural changes through the phase transition to the striking visible colour change. By deliberately exploiting the uncertainty of the salt-cocrystal continuum, where the small pKa difference between components enables significant solid-state structural rearrangements induced by proton transfer, this work highlights a novel design paradigm for engineering new organic thermochromics with tailored physical properties.
Ptak M, Svane KL, Walsh A, et al., 2019, Stability and flexibility of heterometallic formate perovskites with the dimethylammonium cation: pressure-induced phase transitions, Physical Chemistry Chemical Physics, Vol: 21, Pages: 4200-4208, ISSN: 1463-9076
We report the high-pressure properties of two heterometallic perovskite-type metal-organic frameworks (MOFs) templated by dimethylammonium (NH2(CH3)2, DMA+) with the general formula [DMA]MI0.5CrIII0.5(HCOO)3, where MI = Na+ (DMANaCr) and K+ (DMAKCr). The high-pressure Raman scattering studies show crystal instabilities in the 4.0-4.4 GPa and 2.0-2.5 GPa ranges for DMANaCr and DMAKCr, respectively. The mechanism is similar in the two compounds and involves strong deformation of the metal-formate framework, especially pronounced for the subnetwork of CrO6 octahedra, accompanied by substantial compressibility of the DMA+ cations. Comparison with previous high-pressure Raman studies of sodium-chromium heterometallic MOFs show that the stability depends on the templated cation and increases as follows: ammonium < imidazolium < DMA+. Density functional theory (DFT) calculations are performed to get a better understanding of the structural properties leading to the existence of phase transitions. We calculate the energy of the hydrogen bonds (HBs) between the DMA+ cation and the metal formate cage, revealing a stronger interaction in the DMAKCr compound due to a HB arrangement that primarily involves the energetically preferred bonding to KO6 octahedra. This material however also has a smaller structural tolerance factor (TF) and a higher vibrational entropy than DMANaCr. This indicates a more flexible crystal structure, explaining the lower phase transition pressure, as well as the previously observed phase transition at 190 K, which is absent in the DMANaCr compound. The DFT high-pressure simulations show the largest contraction to be along the trigonal axis, leading to a minimal distortion of the HBs formed between the DMA+ cations and the metal-formate sublattice.
Whalley LD, Frost JM, Morgan BJ, et al., 2019, Impact of nonparabolic electronic band structure on the optical and transport properties of photovoltaic materials, Physical review B: Condensed matter and materials physics, Vol: 99, ISSN: 1098-0121
The effective mass approximation (EMA) models the response to an external perturbation of an electron in a periodic potential as the response of a free electron with a renormalized mass. For semiconductors used in photovoltaic devices, the EMA allows calculation of important material properties from first-principles calculations, including optical properties (e.g., exciton binding energies), defect properties (e.g., donor and acceptor levels), and transport properties (e.g., polaron radii and carrier mobilities). The conduction and valence bands of semiconductors are commonly approximated as parabolic around their extrema, which gives a simple theoretical description but ignores the complexity of real materials. In this work, we use density functional theory to assess the impact of band nonparabolicity on four common thin-film photovoltaic materials—GaAs, CdTe, Cu2ZnSnS4 and CH3NH3PbI3—at temperatures and carrier densities relevant for real-world applications. First, we calculate the effective mass at the band edges. We compare finite-difference, unweighted least-squares and thermally weighted least-squares approaches. We find that the thermally weighted least-squares method reduces sensitivity to the choice of sampling density. Second, we employ a Kane quasilinear dispersion to quantify the extent of nonparabolicity and compare results from different electronic structure theories to consider the effect of spin-orbit coupling and electron exchange. Finally, we focus on the halide perovskite CH3NH3PbI3 as a model system to assess the impact of nonparabolicity on calculated electron transport and optical properties at high carrier concentrations. We find that at a concentration of 1020cm−3 the optical effective mass increases by a factor of two relative to the low carrier-concentration value, and the polaron mobility decreases by a factor of three. Our work suggests that similar adjustments should be made to the predicted optical and transport proper
Kim S, Park J-S, Hood S, et al., 2019, Lone-pair effect on carrier capture in Cu2ZnSnS4 solar cells, Journal of Materials Chemistry A, Vol: 7, Pages: 2686-2693, ISSN: 2050-7496
The performance of kesterite thin-film solar cells is limited by a low open-circuit voltage due to defect-mediated electron–hole recombination. We calculate the non-radiative carrier-capture cross sections and Shockley–Read–Hall recombination coefficients of deep-level point defects in Cu2ZnSnS4 (CZTS) from first-principles. While the oxidation state of Sn is +4 in stoichiometric CZTS, inert lone pair (5s2) formation lowers the oxidation state to +2. The stability of the lone pair suppresses the ionization of certain point defects, inducing charge transition levels deep in the band gap. We find large lattice distortions associated with the lone-pair defect centers due to the difference in ionic radii between Sn(II) and Sn(IV). The combination of a deep trap level and large lattice distortion facilitates efficient non-radiative carrier capture, with capture cross-sections exceeding 10−12 cm2. The results highlight a connection between redox active cations and ‘killer’ defect centres that form giant carrier traps. This lone pair effect will be relevant to other emerging photovoltaic materials containing ns2 cations.
Wallace SK, Butler KT, Hinuma Y, et al., 2019, Finding a junction partner for candidate solar cell absorbers enargite and bournonite from electronic band and lattice matching, Journal of Applied Physics, Vol: 125, ISSN: 0021-8979
An essential step in the development of a new photovoltaic (PV) technology is choosing appropriate electron and hole extraction layers to make an efficient device. We recently proposed the minerals enargite (Cu3AsS4) and bournonite (CuPbSbS3) as materials that are chemically stable with desirable optoelectronic properties for use as the absorber layer in a thin-film PV device. For these compounds, spontaneous lattice polarization with internal electric fields—and potential ferroelectricity—may allow for enhanced carrier separation and novel photophysical effects. In this work, we calculate the ionization potentials for non-polar surface terminations and propose suitable partners for forming solar cell heterojunctions by matching the electronic band edges to a set of candidate electrical materials. We then further screen these candidates by matching the lattice constants and identify those that are likely to minimise strain and achieve epitaxy. This two-step screening procedure identified a range of unconventional candidate junction partners including SnS2, ZnTe, WO3, and Bi2O3.
Wahila MJ, Lebens-Higgins ZW, Butler KT, et al., 2019, Accelerated optimization of transparent, amorphous zinc-tin-oxide thin films for optoelectronic applications, APL Materials, Vol: 7, ISSN: 2166-532X
In the last decade, transparent amorphous oxide semiconductors (TAOS) have become an essential component of many electronics, from ultra high resolution displays to solar cells. However, these disordered oxides typically rely on expensive component metals like indium to provide sufficient charge carrier conduction, and their optoelectronic properties are not as predictable and well-described as those of traditional, crystalline semiconductors. Herein we report on our comprehensive study of the amorphous zinc-tin-oxide (a-ZTO) system for use as an indium-free, n-type TAOS. Using a combination of high-throughput co-deposition growth, high resolution spectral mapping, and atomistic calculations, we explain the development of disorder-related subgap states in SnO2-like a-ZTO and optical bandgap reduction in ZnO-like a-ZTO. In addition, we report on a composition-induced electronic and structural transition in ZnO-like a-ZTO resulting in an exceptionally high figure of merit, comparable to that of amorphous indium-gallium-zinc-oxide. Our results accelerate the development of a-ZTO and similar systems as indium-free TAOS materials.
Jones TW, Osherov A, Alsari M, et al., 2019, Lattice strain causes non-radiative losses in halide perovskites, Energy and Environmental Science, Vol: 12, Pages: 596-606, ISSN: 1754-5692
Halide perovskites are promising semiconductors for inexpensive, high-performance optoelectronics. Despite a remarkable defect tolerance compared to conventional semiconductors, perovskite thin films still show substantial microscale heterogeneity in key properties such as luminescence efficiency and device performance. However, the origin of the variations remains a topic of debate, and a precise understanding is critical to the rational design of defect management strategies. Through a multi-scale investigation – combining correlative synchrotron scanning X-ray diffraction and time-resolved photoluminescence measurements on the same scan area – we reveal that lattice strain is directly associated with enhanced defect concentrations and non-radiative recombination. The strain patterns have a complex heterogeneity across multiple length scales. We propose that strain arises during the film growth and crystallization and provides a driving force for defect formation. Our work sheds new light on the presence and influence of structural defects in halide perovskites, revealing new pathways to manage defects and eliminate losses.
Park J-S, Walsh A, 2019, Embrace your defects, Nature Energy, Vol: 4, Pages: 95-96, ISSN: 1520-8524
Control of electron and hole concentrations in semiconductors is a longstanding challenge. Now, by managing defect populations, a p–n homojunction solar cell has been fabricated, opening a new avenue for metal halide perovskite devices.
Wilson JN, Frost JM, Wallace SK, et al., 2019, Dielectric and ferroic properties of metal halide perovskites, APL Materials, Vol: 7, ISSN: 2166-532X
Halide perovskite semiconductors and solar cells respond to electric fields in a way that varies across time and length scales. We discuss the microscopic processes that give rise to the macroscopic polarization of these materials, ranging from the optical and vibrational response to the transport of ions and electrons. The strong frequency dependence of the dielectric permittivity can be understood by separating the static dielectric constant into its constituents, including the orientational polarization due to rotating dipoles, which connects theory with experimental observations. The controversial issue of ferroelectricity is addressed, where we highlight recent progress in materials and domain characterization but emphasize the challenge associated with isolating spontaneous lattice polarization from other processes such as charged defect formation and transport. We conclude that CH3NH3PbI3 exhibits many features characteristic of a ferroelastic electret, where a spontaneous lattice strain is coupled to long-lived metastable polarization states.
Jung Y-K, Calbo J, Park J-S, et al., 2019, Luminescence of Polybromide Defects in Cs4PbBr6
<jats:p>Cs<jats:sub>4</jats:sub>PbBr<jats:sub>6</jats:sub> is a member of the halide perovskite family that is built from isolated (zero-dimensional) PbBr<jats:sub>6</jats:sub><jats:sup>4-</jats:sup> octahedra with Cs<jats:sup>+</jats:sup> counter ions. The material exhibits anomalous optoelectronic properties: optical absorption and weak emission in the deep ultraviolet (310 - 375 nm) with efficient luminescence in the green region (540 nm). Several hypotheses have been proposed to explain the giant Stokes shift including: (i) CsPbBr<jats:sub>3</jats:sub> phase impurities; (ii) self-trapped exciton; (iii) defect emission. We show -- within the modern first-principles theory of defects -- that many of the low energy point defects in Cs<jats:sub>4</jats:sub>PbBr<jats:sub>6</jats:sub> lead to the formation of polybromide (Br<jats:sub>3</jats:sub>) species that exist in a range of charge states. We further demonstrate from excited-state calculations that tribromide moieties are photoresponsive and can contribute to the observed green luminescence. Photoactivity of polyhalide molecules is expected to be present in other halide perovskite-related compounds.</jats:p>
Jung Y-K, Calbo J, Park J-S, et al., 2019, Luminescence of Polybromide Defects in Cs4PbBr6
<jats:p>Cs<jats:sub>4</jats:sub>PbBr<jats:sub>6</jats:sub> is a member of the halide perovskite family that is built from isolated (zero-dimensional) PbBr<jats:sub>6</jats:sub><jats:sup>4-</jats:sup> octahedra with Cs<jats:sup>+</jats:sup> counter ions. The material exhibits anomalous optoelectronic properties: optical absorption and weak emission in the deep ultraviolet (310 - 375 nm) with efficient luminescence in the green region (540 nm). Several hypotheses have been proposed to explain the giant Stokes shift including: (i) CsPbBr<jats:sub>3</jats:sub> phase impurities; (ii) self-trapped exciton; (iii) defect emission. We show -- within the modern first-principles theory of defects -- that many of the low energy point defects in Cs<jats:sub>4</jats:sub>PbBr<jats:sub>6</jats:sub> lead to the formation of polybromide (Br<jats:sub>3</jats:sub>) species that exist in a range of charge states. We further demonstrate from excited-state calculations that tribromide moieties are photoresponsive and can contribute to the observed green luminescence. Photoactivity of polyhalide molecules is expected to be present in other halide perovskite-related compounds.</jats:p>
Davies DW, Walsh A, Mudd JJ, et al., 2019, Identification of lone-pair surface states on indium oxide, Journal of Physical Chemistry C, Vol: 123, Pages: 1700-1709, ISSN: 1932-7447
Indium oxide is widely used as a transparent electrode in optoelectronic devices and as a photocatalyst with activity for reduction of CO2. However, very little is known about the structural and electronic properties of its surfaces, particularly those prepared under reducing conditions. In this report, directional “lone-pair” surface states associated with filled 5s2 orbitals have been identified on vacuum-annealed In2O3(111) through a combination of hard and soft X-ray photoemission spectroscopy and density functional theory calculations. The lone pairs reside on indium ad-atoms in a formal +1 oxidation state, each of which traps two electrons into a localized hybrid orbital protruding away from the surface and lying just above the valence band maximum in photoemission spectra. The third electron associated with the ad-atoms is delocalized into the conduction band, thus producing the surface electron accumulation layer identified previously on vacuum-annealed In2O3(111) (1 × 1) surfaces. The surface structure is further supported by low-energy electron diffraction, but there is no chemical shift in indium core level X-ray photoelectron spectra between surface In(I) ad-atoms and bulk In(III). The 5s2 lone pairs confer Lewis basicity on the surface In sites and may have a pronounced impact on the catalytic or photocatalytic activity of reduced In2O3.
Wallace SK, Frost JM, Walsh A, 2019, Atomistic insights into the order-disorder transition in Cu<inf>2</inf>ZnSnS<inf>4</inf> solar cells from Monte Carlo simulations, Journal of Materials Chemistry A, Vol: 7, Pages: 312-321, ISSN: 2050-7488
Kesterite-structured Cu2ZnSnS4 (CZTS) is an earth-abundant and non-toxic semiconductor that is being studied for use as the absorber layer in thin-film solar cells. Currently, the power-conversion efficiencies of this technology fall short of the requirements for commercialisation. Disorder in the Cu-Zn sub-lattice has been observed and is proposed as one explanation for the shortcomings of CZTS solar cells. Cation site disorder averaged over a macroscopic sample does not provide insights into the microscopic cation distribution that will interact with photogenerated electrons and holes. To provide atomistic insight into Cu/Zn disorder, we have developed a Monte Carlo (MC) model based on pairwise electrostatic interactions. Substitutional disorder amongst Cu and Zn ions in Cu-Zn (001) planes on the 2c and 2d Wyckoff sites-2D disorder-has been proposed as the dominant form of Cu/Zn disorder in near-stoichiometric crystals. We use our model to study the Cu/Zn order-disorder transition in 2D but also allow Zn to substitute onto the Cu 2a site-3D disorder-including Cu-Sn (001) planes. We find that defects are less concentrated in Cu-Sn (001) planes but that Zn ions readily substitute onto the Cu 2a site and that the critical temperature is lowered for 3D disorder.
Alberi K, Nardelli MB, Zakutayev A, et al., 2019, The 2019 materials by design roadmap, Journal of Physics D: Applied Physics, Vol: 52, ISSN: 0022-3727
Advances in renewable and sustainable energy technologies critically depend on our ability to design and realize materials with optimal properties. Materials discovery and design efforts ideally involve close coupling between materials prediction, synthesis and characterization. The increased use of computational tools, the generation of materials databases, and advances in experimental methods have substantially accelerated these activities. It is therefore an opportune time to consider future prospects for materials by design approaches. The purpose of this Roadmap is to present an overview of the current state of computational materials prediction, synthesis and characterization approaches, materials design needs for various technologies, and future challenges and opportunities that must be addressed. The various perspectives cover topics on computational techniques, validation, materials databases, materials informatics, high-throughput combinatorial methods, advanced characterization approaches, and materials design issues in thermoelectrics, photovoltaics, solid state lighting, catalysts, batteries, metal alloys, complex oxides and transparent conducting materials. It is our hope that this Roadmap will guide researchers and funding agencies in identifying new prospects for materials design.
Butler KT, Davies DW, Walsh A, 2019, Computational Design of Photovoltaic Materials, COMPUTATIONAL MATERIALS DISCOVERY, Editors: Oganov, Saleh, Kvashnin, Publisher: ROYAL SOC CHEMISTRY, Pages: 176-197, ISBN: 978-1-78262-961-0
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del Olmo L, Dommett M, Oevreeide IH, et al., 2018, Water oxidation catalysed by quantum- sized BiVO4+, Journal of Materials Chemistry A, Vol: 6, Pages: 24965-24970, ISSN: 2050-7496
Bismuth vanadate BiVO4 is one of the most promising materials for photoelectrochemical water splitting, with recent work highlighting the improved photocatalytic activity of quantum sized BiVO4 compared with the crystalline phase. Herein, we report a theoretical investigation of the structural, optical and catalytic properties of the (BiVO4)4 clusters through a combination of density functional theory methods (ab initio molecular dynamics, time-dependent density functional theory, transition state theory). The enhanced solar water oxidation efficiency of BiVO4 quantum-sized clusters is linked with the localisation of the spin density on the cluster surface, and the dramatic reduction, compared with the crystalline BiVO4 phase, of the Gibbs energy of activation and Gibbs energy of reaction associated with the hydrogen transfer process between water and BiVO4. Our results illustrate the main effects associated with the reduction of dimensions (from bulk to quantum-size) on the main steps of water oxidation mechanisms. This understanding can contribute to the design of efficient BiVO4 quantum sized water-splitting photocatalysts.
Kieslich G, Skelton JM, Armstrong J, et al., 2018, Hydrogen Bonding versus Entropy: Revealing the Underlying Thermodynamics of the Hybrid Organic-Inorganic Perovskite [CH3NH3]PbBr3, CHEMISTRY OF MATERIALS, Vol: 30, Pages: 8782-8788, ISSN: 0897-4756
Davies DW, Butler KT, Skelton JM, et al., 2018, Computer-aided design of metal chalcohalide semiconductors: from chemical composition to crystal structure, Chemical Science, Vol: 9, Pages: 1022-1030, ISSN: 2041-6520
The standard paradigm in computational materials science is INPUT: STRUCTURE; OUTPUT: PROPERTIES, which has yielded many successes but is ill-suited for exploring large areas of chemical and configurational hyperspace. We report a high-throughput screening procedure that uses compositional descriptors to search for new photoactive semiconducting compounds. We show how feeding high-ranking element combinations to structure prediction algorithms can constitute a pragmatic computer-aided materials design approach. Techniques based on structural analogy (data mining of known lattice types) and global searches (direct optimisation using evolutionary algorithms) are combined for translating between chemical composition and crystal structure. The properties of four novel chalcohalides (Sn5S4Cl2, Sn4SF6, Cd5S4Cl2 and Cd4SF6) are predicted, of which two are calculated to have bandgaps in the visible range of the electromagnetic spectrum.
Gold-Parker A, Gehring PM, Skelton JM, et al., 2018, Acoustic phonon lifetimes limit thermal transport in methylammonium lead iodide, Proceedings of the National Academy of Sciences of the United States of America, Vol: 115, Pages: 11905-11910, ISSN: 0027-8424
Hybrid organic–inorganic perovskites (HOIPs) have become an important class of semiconductors for solar cells and other optoelectronic applications. Electron–phonon coupling plays a critical role in all optoelectronic devices, and although the lattice dynamics and phonon frequencies of HOIPs have been well studied, little attention has been given to phonon lifetimes. We report high-precision momentum-resolved measurements of acoustic phonon lifetimes in the hybrid perovskite methylammonium lead iodide (MAPI), using inelastic neutron spectroscopy to provide high-energy resolution and fully deuterated single crystals to reduce incoherent scattering from hydrogen. Our measurements reveal extremely short lifetimes on the order of picoseconds, corresponding to nanometer mean free paths and demonstrating that acoustic phonons are unable to dissipate heat efficiently. Lattice-dynamics calculations using ab initio third-order perturbation theory indicate that the short lifetimes stem from strong three-phonon interactions and a high density of low-energy optical phonon modes related to the degrees of freedom of the organic cation. Such short lifetimes have significant implications for electron–phonon coupling in MAPI and other HOIPs, with direct impacts on optoelectronic devices both in the cooling of hot carriers and in the transport and recombination of band edge carriers. These findings illustrate a fundamental difference between HOIPs and conventional photovoltaic semiconductors and demonstrate the importance of understanding lattice dynamics in the effort to develop metal halide perovskite optoelectronic devices.
Park J-S, Jung Y-K, Butler KT, et al., 2018, Quick-start guide for first-principles modelling of semiconductor interfaces, JPhys Energy, Vol: 1, ISSN: 2515-7655
Interfaces between dissimilar materials control the transport of energy in a range of technologies including solar cells (electron transport), batteries (ion transport), and thermoelectrics (heat transport). Advances in computer power and algorithms mean that first-principles models of interfacial processes in realistic systems are now possible using accurate approaches such as density functional theory. In this 'quick-start guide', we discuss the best practice in how to construct atomic models between two materials and analysis techniques appropriate to probe changes in local bonding and electronic band offsets. A number of examples are given related to perovskite solar cells.
Park J-S, Kim S, Hood SN, et al., 2018, Open-circuit voltage deficit in Cu2ZnSnS4 solar cells by interface bandgap narrowing, Applied Physics Letters, Vol: 113, ISSN: 1077-3118
There is evidence that interface recombination in Cu2ZnSnS4 solar cells contributes to the opencircuit voltage deficit. Our hybrid density functional theory calculations suggest that electron-holerecombination at the Cu2ZnSnS4/CdS interface is caused by a deeper conduction band that slowselectron extraction. In contrast, the bandgap is not narrowed for the Cu2ZnSnSe4/CdS interface,consistent with a lower open-circuit voltage deficit.
Wallace S, Frost J, Walsh A, 2018, Order-disorder transition in Cu2ZnSnS4 solar cells from Monte Carlo simulations
<jats:p>Kesterite-structured Cu2ZnSnS4 (CZTS) is an earth-abundant and non-toxic semiconductor that is being studied for use as the absorber layer in thin-film solar cells. Currently, the power-conversion efficiencies of this technology fall short of the requirements for commercialisation. Disorder in the Cu-Zn sub-lattice has been observed and is proposed as one explanation for the shortcomings of CZTS solar cells. Cation site disorder averaged over a macroscopic sample does not provide insights into the microscopic cation distribution that will interact with photogenerated electrons and holes. To provide atomistic insight into Cu-Zn disorder, we have developed a Monte Carlo (MC) model based on pairwise electrostatic interactions. Substitutional disorder amongst Cu and Zn ions in Cu-Zn (001) planes on the 2\textit{c} and 2\textit{d} Wyckoff sites -- 2D disorder -- has been proposed as the dominant form of Cu/Zn disorder in near-stoichiometric crystals. We use our model to study the Cu/Zn order-disorder transition in 2D but also allow Zn to substitute onto the Cu 2\textit{a} site -- 3D disorder -- including Cu-Sn (001) planes. We find that defects are less concentrated in Cu-Sn (001) planes but that Zn ions readily substitute onto the Cu 2\textit{a} site and that the critical temperature is lowered for 3D disorder.</jats:p>
Park J-S, Kim S, Walsh A, 2018, Stability and electronic properties of planar defects in quaternary I2-II-IV-VI4 semiconductors, Publisher: arXiv
Extended defects such as stacking faults and anti-site domain boundaries can perturb the band edges in Cu2ZnSnS4 and Cu2ZnSnSe4, acting as a weak electron barrier or a source for electron capture, respectively. In order to find ways to prohibit the formation of planar defects, we investigated the effect of chemical substitution on the stability of the intrinsic stacking fault and metastable polytypes and analyze their electrical properties. Substitution of Ag for Cu makes stacking faults less stable, whereas the other substitutions (Cd and Ge) promote their formation. Ge substitution has no effect on the electron barrier of the intrinsic stacking fault, but Cd substitution reduces the barrier energy and Ag substitution makes the stacking fault electron capture. While Cd substitution stabilizes the stannite structure, chemical substitutions make the primitive-mixed CuAu (PMCA) structure less stable with respect to the ground-state kesterite structure.
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