Publications
559 results found
Davies D, Butler K, Isayev O, et al., 2018, Materials discovery by chemical analogy: role of oxidation states in structure prediction, Faraday Discussions, Vol: 211, Pages: 553-568, ISSN: 1359-6640
The likelihood of an element to adopt a specific oxidation state in a solid, given a certain set of neighbours, might often be obvious to a trained chemist. However, encoding this information for use in high-throughput searches presents a significant challenge. We carry out a statistical analysis of the occurrence of oxidation states in 16,735 ordered, inorganic compounds and show that a large number of cations are only likely to exhibit certain oxidation states in combination with particular anions. We use this data to build a model that ascribes probabilities to the formation of hypothetical compounds, given the proposed oxidation states of its constituent species. The model is then used as part of a high-throughput materials design process, which significantly narrows down the vast compositional search space for new ternary metal halide compounds. Finally, we employ a machine learning analysis of existing compounds to suggest likely structures for a small subset of the candidate compositions. We predict two new compounds, MnZnBr4 and YSnF7, that are thermodynamically stable according to density functional theory, as well as four compounds, MnCdBr4, MnRu2Br8, ScZnF5 and ZnCoBr4, which lie within the window of metastability.
Walsh A, Sokol AA, Buckeridge J, et al., 2018, Oxidation states and ionicity, Nature Materials, Vol: 17, Pages: 958-964, ISSN: 1476-1122
The concepts of oxidation state and atomic charge are entangled in modern materials science. We distinguish between these quantities and consider their fundamental limitations and utility for understanding material properties. We discuss the nature of bonding between atoms and the techniques that have been developed for partitioning electron density. While formal oxidation states help us count electrons (in ions, bonds, lone pairs), variously defined atomic charges are usefully employed in the description of physical processes including dielectric response and electronic spectroscopies. Such partial charges are introduced as quantitative measures in simple mechanistic models of a more complex reality, and therefore may not be comparable or transferable. In contrast, oxidation states are defined to be universal, with deviations constituting exciting challenges as evidenced in mixed-valence compounds, electrides and highly correlated systems. This Perspective covers how these concepts have evolved in recent years, our current understanding and their significance.
Ptak M, Stefańska D, Gągor A, et al., 2018, Heterometallic perovskite-type metal-organic framework with an ammonium cation: structure, phonons, and optical response of [NH4]Na0.5CrxAl0.5-x(HCOO)3 (x = 0, 0.025 and 0.5)., Physical Chemistry Chemical Physics, Vol: 20, Pages: 22284-22295, ISSN: 1463-9076
We report the synthesis, crystal structure, vibrational and luminescence properties of two heterometallic perovskite-type metal-organic frameworks (MOFs) containing the ammonium cation (NH4+, Am+): [NH4][Na0.5Cr0.5(HCOO)3] (AmNaCr) and [NH4][Na0.5Al0.475Cr0.025(HCOO)3] (AmNaAlCr) in comparison to the previously reported [NH4][Na0.5Al0.5(HCOO)3] (AmNaAl). The room-temperature crystal structure of AmNaCr and AmNaAlCr was determined to be R3[combining macron]. The hydrogen bonding (HB) energy calculated using density functional theory (DFT) agrees well with experimental data, and confirms the existence of almost identical H-bonding in AmNaCr and AmNaAl, with three short hydrogen bonds and a longer trifurcated H-bond. Temperature-dependent Raman measurements supported by differential scanning calorimetry show that AmNaCr does not undergo any structural phase transitions in the 80-400 K temperature range. The high-pressure Raman spectra of AmNaCr show the onset of two structural instabilities near 0.5 and 1.5 GPa. The first instability involves weak distortion of the framework, while the second leads to irreversible amorphization of the sample. High-pressure DFT simulations show that the unit cell of the AmNaCr compound contracts along the c axis, which leads to a shortening of the trifurcated H-bond. The optical properties show that both studied crystals exhibit Cr3+-based emission characteristic of intermediate ligand field strength.
Steele JA, Puech P, Keshavarz M, et al., 2018, Giant Electron-Phonon Coupling and Deep Conduction Band Resonance in Metal Halide Double Perovskite, ACS NANO, Vol: 12, Pages: 8081-8090, ISSN: 1936-0851
The room-temperature charge carrier mobility and excitation–emission properties of metal halide perovskites are governed by their electronic band structures and intrinsic lattice phonon scattering mechanisms. Establishing how charge carriers interact within this scenario will have far-reaching consequences for developing high-efficiency materials for optoelectronic applications. Herein we evaluate the charge carrier scattering properties and conduction band environment of the double perovskite Cs2AgBiBr6 via a combinatorial approach; single crystal X-ray diffraction, optical excitation and temperature-dependent emission spectroscopy, resonant and nonresonant Raman scattering, further supported by first-principles calculations. We identify deep conduction band energy levels and that scattering from longitudinal optical phonons—via the Fröhlich interaction—dominates electron scattering at room temperature, manifesting within the nominally nonresonant Raman spectrum as multiphonon processes up to the fourth order. A Fröhlich coupling constant nearing 230 meV is inferred from a temperature-dependent emission line width analysis and is found to be extremely large compared to popular lead halide perovskites (between 40 and 60 meV), highlighting the fundamentally different nature of the two “single” and “double” perovskite materials branches.
Walsh A, Scanlon DO, Wei S-H, 2018, Preface for special topic: Earth abundant materials in solar cells, APL Materials, Vol: 6, ISSN: 2166-532X
Souto M, Romero J, Calbo J, et al., 2018, Breathing-Dependent Redox Activity in a Tetrathiafulvalene-Based Metal-Organic Framework, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 140, Pages: 10562-10569, ISSN: 0002-7863
“Breathing” metal–organic frameworks (MOFs) that involve changes in their structural and physical properties upon an external stimulus are an interesting class of crystalline materials due to their range of potential applications including chemical sensors. The addition of redox activity opens up a new pathway for multifunctional “breathing” frameworks. Herein, we report the continuous breathing behavior of a tetrathiafulvalene (TTF)-based MOF, namely MUV-2, showing a reversible swelling (up to ca. 40% of the volume cell) upon solvent adsorption. Importantly, the planarity of the TTF linkers is influenced by the breathing behavior of the MOF, directly impacting on its electrochemical properties and thus opening the way for the development of new electrochemical sensors. Quantum chemical calculations and Raman spectroscopy have been used to provide insights into the tunability of the oxidation potential.
Walsh A, Stranks SD, 2018, Taking control of ion transport in halide perovskite solar cells, ACS Energy Letters, Vol: 3, Pages: 1983-1990, ISSN: 2380-8195
Lead halide perovskites are mixed electron–ion conductors that support high rates of solid-state ion transport at room temperature, in addition to conventional electron and hole conduction. Mass transport mediated by charged defects is responsible for unusual phenomena such as current–voltage hysteresis in photovoltaic devices, anomalous above-bandgap photovoltages, light-induced lattice expansion and phase separation, self-healing, and rapid chemical conversion between halides. We outline the principles that govern ion transport in perovskite solar cells including intrinsic (point and extended defects) and extrinsic (light, heat, electrical fields, and chemical gradients) factors. These microscopic processes underpin a wide range of reported observations, including photoionic conductivity, and offer valuable directions for both limiting ion transport, where required, and harnessing it to enable new functionality.
Mckechnie S, Frost JM, Pashov D, et al., 2018, Dynamic symmetry breaking and spin splitting in metal halide perovskites, PHYSICAL REVIEW B, Vol: 98, ISSN: 2469-9950
Butler K, Davies D, Hugh C, et al., 2018, Machine learning for molecular and materials science, Nature, Vol: 559, Pages: 547-555, ISSN: 0028-0836
In this Perspective, we outline theprogress andpotential of machine learning for the physical sciences. We envisage a future where the design, synthesis, characterisation, and application of molecules and materialsis accelerated by artificial intelligence.
Wei W, Li W, Butler KT, et al., 2018, An Unusual Phase Transition Driven by Vibrational Entropy Changes in a Hybrid Organic-Inorganic Perovskite, ANGEWANDTE CHEMIE-INTERNATIONAL EDITION, Vol: 57, Pages: 8932-8936, ISSN: 1433-7851
Park J, Kim S, Xie Z, et al., 2018, Point defect engineering in thin-film solar cells, Nature Reviews Materials, Vol: 3, Pages: 194-210, ISSN: 2058-8437
Control of defect processes in photovoltaic materials is essential forrealising high-efficiency solar cells and related optoelectronic devices. The concentrations of native defects and extrinsic dopants tune the Fermi level and enablesemiconducting p-n junctions; however, fundamental limits to doping exist in many compounds. Optical transitions involving defect states can enhance photocurrent generation through sub-bandgap absorption; however, such states are often responsible for carrier trapping and non-radiative recombination events that limit open-circuit voltage. Many classes of materials –including metal oxides, chalcogenides,and halides –are being examined for next-generation solar energyapplications, and each technology faces distinct challengesthat could benefit from point defect engineering. We review the evolution in point defect behaviour from Si-based photovoltaics to thin-film CdTe and Cu(In,Ga)Se2technologies, through to the latest generation halide perovskite (CH3NH3PbI3) and kesterite (Cu2ZnSnS4) devices. We focus on the chemical bonding that underpins the defect chemistry, and the atomistic processes associated with the photophysics of charge carrier generation, trapping,and recombination in solar cells. Finally, we outlinegeneral principles to enable defect control incomplex semiconducting materials.
Xie Z, Sui Y, Buckeridge J, et al., 2018, Prediction of multiband luminescence due to the gallium vacancy-oxygen defect complex in GaN, Applied Physics Letters, Vol: 112, ISSN: 1077-3118
Oxygen is the most common unintentional impurity found in GaN. We study the interaction between substitutional oxygen (ON) and the gallium vacancy (VGa) to form a point defect complex in GaN. The formation energy of the gallium vacancy is largely reduced in n-type GaN by complexing with oxygen, while thermodynamic and optical transition levels remain within the bandgap. We study the spectroscopy of this complex using a hybrid quantum-mechanical molecular-mechanical embedded-cluster approach. We reveal how a single defect center can be responsible for multiband luminescence, including possible contributions to the ubiquitous yellow luminescence signatures observed in n-type GaN, owing to the coexistence of diffuse (extended) and compact (localized) holes.
Buckeridge J, Catlow CRA, Farrow MR, et al., 2018, Deep vs shallow nature of oxygen vacancies and consequent n-type carrier concentrations in transparent conducting oxides, Physical Review Materials, Vol: 2, ISSN: 2475-9953
The source of n-type conductivity in undoped transparent conducting oxides has been a topic of debate for several decades. The point defect of most interest in this respect is the oxygen vacancy, but there are many conflicting reports on the shallow versus deep nature of its related electronic states. Here, using a hybrid quantum mechanical/molecular mechanical embedded cluster approach, we have computed formation and ionization energies of oxygen vacancies in three representative transparent conducting oxides: In2O3,SnO2, and ZnO. We find that, in all three systems, oxygen vacancies form well-localized, compact donors. We demonstrate, however, that such compactness does not preclude the possibility of these states being shallow in nature, by considering the energetic balance between the vacancy binding electrons that are in localized orbitals or in effective-mass-like diffuse orbitals. Our results show that, thermodynamically, oxygen vacancies in bulk In2O3 introduce states above the conduction band minimum that contribute significantly to the observed conductivity properties of undoped samples. For ZnO and SnO2, the states are deep, and our calculated ionization energies agree well with thermochemical and optical experiments. Our computed equilibrium defect and carrier concentrations, however, demonstrate that these deep states may nevertheless lead to significant intrinsic n-type conductivity under reducing conditions at elevated temperatures. Our study indicates the importance of oxygen vacancies in relation to intrinsic carrier concentrations not only in In2O3, but also in SnO2 and ZnO.
Wallace S, Frost JM, Walsh A, 2018, Thermodynamically Limited Cu-Zn Order in Cu2ZnSnS4 from Monte Carlo Simulations
<jats:p>Kesterite-structured Cu2ZnSnS4 (CZTS) is a 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, despite the promising sunlight-matched optical band gap. Disorder in the Cu-Zn sub-lattice has been observed and is proposed as one possible 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 provideatomistic insight into Cu-Zn disorder we have developed a Monte Carlo (MC) model based on pairwise interactions. We utilise two order parameters to relate Cu-Zn disorder to the processing temperature for stoichiometric systems: one based on cation site occupancies (the Q order parameter) and the other based on cation pair-correlation functions. Our model predicts that the order parameters reach a plateau at experimentally relevant low temperatures, indicating that Cu-Zn order in stoichiometric CZTS is thermodynamically limited. Around room temperature, we predict a minimum of 10% disorder in the cation site occupancywithin (001) Cu-Zn planes.</jats:p>
Kye Y-H, Yu C-J, Jong U-G, et al., 2018, Critical Role of Water in Defect Aggregation and Chemical Degradation of Perovskite Solar Cells, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, Vol: 9, Pages: 2196-2201, ISSN: 1948-7185
The chemical stability of methylammonium lead iodide (MAPbI3) under humid conditions remains the primary challenge facing halide perovskite solar cells. We investigate defect processes in the water-intercalated iodide perovskite (MAPbI3_H2O) and monohydrated phase (MAPbI3·H2O) within a first-principles thermodynamic framework. We consider the formation energies of isolated and aggregated vacancy defects with different charge states under I-rich and I-poor conditions. It is found that a PbI2 (partial Schottky) vacancy complex can be formed readily, while the MAI vacancy complex is difficult to form in the hydrous compounds. Vacancies in the hydrous phases create deep charge transition levels, indicating the degradation of the lead halide perovskite upon exposure to moisture. Electronic structure analysis supports a mechanism of water-mediated vacancy pair formation.
Monserrat B, Park J, Kim S, et al., 2018, Role of electron-phonon coupling and thermal expansion on band gaps, carrier mobility, and interfacial offsets in kesterite thin-film solar cells., Applied Physics Letters, Vol: 112, ISSN: 0003-6951
The efficiencies of solar cells based on kesterite Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) are limited by a low open-circuit voltage due to high rates of non-radiative electron-hole recombination. To probe the origin of this bottleneck, we calculate the band offset of CZTS(Se) with CdS, confirming a weak spike of 0.1 eV for CZTS/wurtzite-CdS and a strong spike of 0.4 eV for CZTSe/wurtzite-CdS. We also consider the effects of temperature on the band alignment, finding that increasing temperature significantly enhances the spike-type offset. We further resolve an outstanding discrepancy between the measured and calculated phonon frequencies for the kesterites, and use these to estimate the upper limit of electron and hole mobilities based on optic phonon Fröhlich scattering, which uncovers an intrinsic asymmetry with faster (minority carrier) electron mobility.
Svane K, Bristow J, Gale J, et al., 2018, Vacancy defect configurations in the metal– organic framework UiO-66: energetics and electronic structure, Journal of Materials Chemistry A, Vol: 6, Pages: 8507-8513, ISSN: 2050-7496
Vacancy lattice sites in the metal–organic framework UiO-66 are known to have a profound effect on the material properties. Here we use density functional theory to compare the energies of defect arrangements containing missing linkers and missing metal clusters for different choices of charge compensation. Our results show that the preference for missing metal clusters or missing linker defects depends on the charge compensation as well as the overall concentration of defects in the crystal. Both regimes can be experimentally accessible depending on the synthesis conditions. We investigate the electronic structure of the different types of defects, showing that, despite some changes in the localisation of the frontier orbitals, the electronic energy levels are only weakly affected by the presence of point defects.
Kim S, Park J-S, Walsh A, 2018, Correction to “Identification of Killer Defects in Kesterite Thin-Film Solar Cells”, ACS ENERGY LETTERS, Vol: 3, Pages: 1077-1077, ISSN: 2380-8195
Nonradiative electron–hole recombination is the bottleneck to efficient kesterite thin-film solar cells. We have performed a search for active point defect recombination centers using first-principles calculations. We show that the anion vacancy in Cu2ZnSnS4 (CZTS) is electrically benign without a donor level in the band gap. VS can still act as an efficient nonradiative site through the aid of an intermediate excited state involving electron capture by Sn. The bipolaron associated with Sn4+ to Sn2+ two-electron reduction stabilizes the neutral sulfur vacancy over the charged states; however, we demonstrate a mechanism whereby nonradiative recombination can occur via multiphonon emission. Our study highlights that defect-mediated recombination does not require a charge transition level deep in the band gap of a semiconductor. We further identify SnZn as the origin of persistent electron trapping/detrapping in kesterite photovoltaic devices, which is suppressed in the selenide compound.
Ganose A, Butler K, Walsh A, et al., 2018, Bismuth chalcohalides as earth-abundant and non-toxic photovoltaics, 255th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nexus of Food, Energy, and Water, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
Zhang YY, Chen S, Xu P, et al., 2018, Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH3NH3PbI3, Chinese Physics Letters, Vol: 35, ISSN: 0256-307X
© 2018 Chinese Physical Society and IOP Publishing Ltd. The organic-inorganic hybrid perovskite CH 3 NH 3 PbI 3 has attracted significant interest for its high performance in converting solar light into electrical power with an efficiency exceeding 20%. Unfortunately, chemical stability is one major challenge in the development of CH 3 NH 3 PbI 3 solar cells. It was commonly assumed that moisture or oxygen in the environment causes the poor stability of hybrid halide perovskites, however, here we show from the first-principles calculations that the room-temperatu re tetragonal phase of CH 3 NH 3 PbI 3 is thermodynamically unstable with respect to the phase separation into CH 3 NH 3 I+PbI 2 , i.e., the disproportionation is exothermic, independent of the humidity or oxygen in the atmosphere. When the structure is distorted to the low-temperature orthorhombic phase, the energetic cost of separation increases, but remains small. Contributions from vibrational and configurational entropy at room temperature have been considered, but the instability of CH 3 NH 3 PbI 3 is unchanged. When I is replaced by Br or Cl, Pb by Sn, or the organic cation CH3NH3 by inorganic Cs, the perovskites become more stable and do not phase-separate spontaneously. Our study highlights that the poor chemical stability is intrinsic to CH 3 NH 3 PbI 3 and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells.
Jong U-G, Yu C-J, Ri G-C, et al., 2018, Influence of water intercalation and hydration on chemical decomposition and ion transport in methylammonium lead halide perovskites, Publisher: ROYAL SOC CHEMISTRY
- Author Web Link
- Open Access Link
- Cite
- Citations: 82
Park J, Kim S, Walsh A, 2018, Opposing effects of stacking faults and antisite domain boundaries on the conduction band edge in kesterite quaternary semiconductors, Physical Review Materials, Vol: 2, ISSN: 2475-9953
We investigated stability and the electronic structure of extended defects including antisite domain boundaries and stacking faults in the kesterite-structured semiconductors, Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe). Our hybrid density functional theory calculations show that stacking faults in CZTS and CZTSe induce a higher conduction band edge than the bulk counterparts, and thus the stacking faults act as electron barriers. Antisite domain boundaries, however, accumulate electrons as the conduction band edge is reduced in energy, having an opposite role. An Ising model was constructed to account for the stability of stacking faults, which shows the nearest-neighbor interaction is stronger in the case of the selenide.
Svane KL, Forse AC, Grey CP, et al., 2017, How Strong Is the Hydrogen Bond in Hybrid Perovskites?, Journal of Physical Chemistry Letters, Vol: 8, Pages: 6154-6159, ISSN: 1948-7185
Hybrid organic–inorganic perovskites represent a special class of metal–organic framework where a molecular cation is encased in an anionic cage. The molecule–cage interaction influences phase stability, phase transformations, and the molecular dynamics. We examine the hydrogen bonding in four AmBX3 formate perovskites: [Am]Zn(HCOO)3, with Am+ = hydrazinium (NH2NH3+), guanidinium (C(NH2)3+), dimethylammonium (CH3)2NH2+, and azetidinium (CH2)3NH2+. We develop a scheme to quantify the strength of hydrogen bonding in these systems from first-principles, which separates the electrostatic interactions between the amine (Am+) and the BX3– cage. The hydrogen-bonding strengths of formate perovskites range from 0.36 to 1.40 eV/cation (8–32 kcalmol–1). Complementary solid-state nuclear magnetic resonance spectroscopy confirms that strong hydrogen bonding hinders cation mobility. Application of the procedure to hybrid lead halide perovskites (X = Cl, Br, I, Am+ = CH3NH3+, CH(NH2)2+) shows that these compounds have significantly weaker hydrogen-bonding energies of 0.09 to 0.27 eV/cation (2–6 kcalmol–1), correlating with lower order–disorder transition temperatures.
Jong UG, Yu CJ, Ri GC, et al., 2017, Influence of water intercalation and hydration on chemical decomposition and ion transport in methylammonium lead halide perovskites, Journal of Materials Chemistry A, Vol: 6, Pages: 1067-1074, ISSN: 2050-7496
The application of methylammonium (MA) lead halide perovskites, CH3NH3PbX3 (X = I, Br, Cl), in perovskite solar cells has made great recent progress in performance efficiency during recent years. However, the rapid decomposition of these materials in humid environments hinders outdoor application, and thus, a comprehensive understanding of the degradation mechanism is required. We investigate the effect of water intercalation and hydration of the decomposition and ion migration of CH3NH3PbX3 using first-principles calculations. We find that water interacts with PbX6 and MA through hydrogen bonding, and the former interaction increases gradually, while the latter hardly changes when going from X = I to Br and to Cl. Thermodynamic calculations indicate that water exothermically intercalates into the perovskite, and suggest that the water intercalated and monohydrated compounds are stable with respect to decomposition. More importantly, the water intercalation reduces the activation energies for vacancy-mediated ion migration, which become higher going from X = I to Br and to Cl. Our work indicates that hydration of halide perovskites must be avoided to prevent the degradation of solar cells upon moisture exposure.
Bryant MJ, Skelton JM, Hatcher LE, et al., 2017, A rapidly-reversible absorptive and emissive vapochromic Pt(II) pincer-based chemical sensor., Nature Communications, Vol: 8, Pages: 1800-1800, ISSN: 2041-1723
Selective, robust and cost-effective chemical sensors for detecting small volatile-organic compounds (VOCs) have widespread applications in industry, healthcare and environmental monitoring. Here we design a Pt(II) pincer-type material with selective absorptive and emissive responses to methanol and water. The yellow anhydrous form converts reversibly on a subsecond timescale to a red hydrate in the presence of parts-per-thousand levels of atmospheric water vapour. Exposure to methanol induces a similarly-rapid and reversible colour change to a blue methanol solvate. Stable smart coatings on glass demonstrate robust switching over 104 cycles, and flexible microporous polymer membranes incorporating microcrystals of the complex show identical vapochromic behaviour. The rapid vapochromic response can be rationalised from the crystal structure, and in combination with quantum-chemical modelling, we provide a complete microscopic picture of the switching mechanism. We discuss how this multiscale design approach can be used to obtain new compounds with tailored VOC selectivity and spectral responses.
Jung Y-K, Butler KT, Walsh A, 2017, Halide Perovskite Heteroepitaxy: Bond Formation and Carrier Confinement at the PbS-CsPbBr3 Interface, Journal of Physical Chemistry C, Vol: 121, Pages: 27351-27356, ISSN: 1932-7447
Control of the stability, transport, and confinement of charge carriers (electrons and holes) at interfaces is a key requirement to realize robust halide perovskite devices. The PbS–CsPbBr3 interface is atomically matched with low lattice strain, opening the potential for epitaxial growth. We assess the atomic nature of the interface using first-principles density functional theory calculations to identify (1) the thermodynamically stable (100) surface termination of the halide perovskite; (2) the most favorable (100)|(100) contact geometry; (3) the strong interfacial chemical bonding between PbS and CsPbBr3; (4) the type I (straddling) band alignment that enables electron and hole confinement in the lead sulfide layer. The combination of metal halide perovskites and IV–VI semiconductors represents an important platform for probing interfacial chemical processes and realizing new functionality.
Santaclara JG, Olivos-Suarez AI, Gonzalez-Nelson A, et al., 2017, Revisiting the Incorporation of Ti(IV) in UiO-type Metal-Organic Frameworks: Metal Exchange versus Grafting and Their Implications on Photocatalysis, CHEMISTRY OF MATERIALS, Vol: 29, Pages: 8963-8967, ISSN: 0897-4756
Zhao D, Skelton JM, Hu H, et al., 2017, Low-frequency optical phonon modes and carrier mobility in the halide perovskite CH3NH3PbBr3 using terahertz time-domain spectroscopy, APPLIED PHYSICS LETTERS, Vol: 111, ISSN: 0003-6951
As a light absorber in photovoltaic applications, hybrid organic-inorganic halide perovskites shouldhave long and balanced diffusion lengths for both the separated electrons and holes before recombi-nation, which necessitates high carrier mobility. In polar semiconductors, the room-temperaturecarrier mobility is often limited by the scattering between carriers and the lowest-frequency opticalphonon modes. Using terahertz time-domain spectroscopy, we examine the temperature evolutionof these phonon modes in CH3NH3PbBr3and obtained high carrier mobility values usingFeynman’s polaron theory. This method allows us to estimate the upper limit of carrier mobilitieswithout the need to create photogenerated free carriers, and can be applied to other heteropolarsemiconductor systems with large polarons.
Whalley L, Crespo-Otero R, Walsh A, 2017, H-Center and V-Center Defects in Hybrid Halide Perovskites, ACS Energy Letters, Vol: 2, Pages: 2713-2714, ISSN: 2380-8195
The self-trapping of holes with the formation of a molecular X2– anion is a well-established process in metal halide (MX) crystals, but V-center (2X– + h+ → X2–) and H-center (X– + Xi– + h+ → X2–) defects have not yet been confirmed in halide perovskite semiconductors. The I2– split-interstitial defect is predicted to be a spin radical in CH3NH3PbI3 with an optically excited state in the semiconductor band gap.
Frost JM, Whalley LD, Walsh A, 2017, Slow cooling of hot polarons in halide perovskite solar cells, ACS Energy Letters, Vol: 2, Pages: 2647-2652, ISSN: 2380-8195
Halide perovskites show unusual thermalisation kinetics for above bandgapphoto-excitation. We explain this as a consequence of excess energy beingdeposited into discrete large polaron states. The cross-over betweenlow-fluence and high-fluence `phonon bottleneck' cooling is due to a Motttransition where the polarons overlap ($n \ge 10^{18}/\mathrm{cm}^3$) and thephonon sub-populations are shared. We calculate the initial rate of cooling(thermalisation) from the scattering time in the Fr\"ohlich polaron model to be78 meVps$^{-1}$ for $\mathrm{CH}_3\mathrm{NH}_3\mathrm{PbI}_3$. This rapidinitial thermalisation involves heat transfer into optical phonon modes coupledby a polar dielectric interaction. Further cooling to equilibrium over hundredsof picoseconds is limited by the ultra-low thermal conductivity of theperovskite lattice.
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.