Publications
80 results found
Yang Y, Rice B, Shi X, et al., 2018, Emergent Properties of an Organic Semiconductor Driven by its Molecular Chirality (vol 11, pg 8329, 2017), ACS NANO, Vol: 12, Pages: 6343-6343, ISSN: 1936-0851
Chiral molecules exist as pairs of nonsuperimposable mirror images; a fundamental symmetry property vastly underexplored in organic electronic devices. Here, we show that organic field-effect transistors (OFETs) made from the helically chiral molecule 1-aza[6]helicene can display up to an 80-fold difference in hole mobility, together with differences in thin-film photophysics and morphology, solely depending on whether a single handedness or a 1:1 mixture of left- and right-handed molecules is employed under analogous fabrication conditions. As the molecular properties of either mirror image isomer are identical, these changes must be a result of the different bulk packing induced by chiral composition. Such underlying structures are investigated using crystal structure prediction, a computational methodology rarely applied to molecular materials, and linked to the difference in charge transport. These results illustrate that chirality may be used as a key tuning parameter in future device applications.
Leventis A, Royakkers J, Rapidis AG, et al., 2018, Highly Luminescent Encapsulated Narrow Bandgap Polymers Based on Diketopyrrolopyrrole, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 140, Pages: 1622-1626, ISSN: 0002-7863
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- Citations: 61
Frost JM, 2017, Calculating polaron mobility in halide perovskites, Physical Review B, Vol: 96, ISSN: 2469-9950
Lead halide perovskite semiconductors are soft, polar materials. The strong driving force for polaron formation (the dielectric electron-phonon coupling) is balanced by the light band effective masses, leading to a strongly-interacting large polaron. A first-principles prediction of mobility would help understand the fundamental mobility limits. Theories of mobility need to consider the polaron (rather than free-carrier) state due to the strong interactions. In this material we expect that at room temperature polar-optical phonon mode scattering will dominate and so limit mobility. We calculate the temperature-dependent polaron mobility of hybrid halide perovskites by variationally solving the Feynman polaron model with the finite-temperature free energies of Ōsaka. This model considers a simplified effective-mass band structure interacting with a continuum dielectric of characteristic response frequency. We parametrize the model fully from electronic-structure calculations. In methylammonium lead iodide at 300K we predict electron and hole mobilities of 133 and 94cm2V-1s-1, respectively. These are in acceptable agreement with single-crystal measurements, suggesting that the intrinsic limit of the polaron charge carrier state has been reached. Repercussions for hot-electron photoexcited states are discussed. As well as mobility, the model also exposes the dynamic structure of the polaron. This can be used to interpret impedance measurements of the charge-carrier state. We provide the phonon-drag mass renormalization and scattering time constants. These could be used as parameters for larger-scale device models and band-structure dependent mobility simulations.
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.
Xi R, Skelton JM, da Silva EL, et al., 2017, Spontaneous Octahedral Tilting in the Cubic Inorganic Cesium Halide Perovskites CsSnX3 and CsPbX3 (X = F, CI, Br, I), JOURNAL OF PHYSICAL CHEMISTRY LETTERS, Vol: 8, Pages: 4720-4726, ISSN: 1948-7185
The local crystal structures of many perovskite-structured materials deviate from the average space-group symmetry. We demonstrate, from lattice-dynamics calculations based on quantum chemical force constants, that all of the cesium–lead and cesium–tin halide perovskites exhibit vibrational instabilities associated with octahedral titling in their high-temperature cubic phase. Anharmonic double-well potentials are found for zone-boundary phonon modes in all compounds with barriers ranging from 108 to 512 meV. The well depth is correlated with the tolerance factor and the chemistry of the composition, but is not proportional to the imaginary harmonic phonon frequency. We provide quantitative insights into the thermodynamic driving forces and distinguish between dynamic and static disorder based on the potential-energy landscape. A positive band gap deformation (spectral blue shift) accompanies the structural distortion, with implications for understanding the performance of these materials in applications areas including solar cells and light-emitting diodes.
Freeman DME, Musser AJ, Fros JM, et al., 2017, Synthesis and Exciton Dynamics of Donor-Orthogonal Acceptor Conjugated Polymers: Reducing the Singlet-Triplet Energy Gap, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 139, Pages: 11073-11080, ISSN: 0002-7863
Whalley LD, Frost JM, Jung YK, et al., 2017, Perspective: theory and simulation of hybrid halide perovskites, Journal of Chemical Physics, Vol: 146, ISSN: 1089-7690
Organic-inorganic halide perovskites present a number of challenges for first-principles atomistic materials modeling. Such “plastic crystals” feature dynamic processes across multiple length and time scales. These include the following: (i) transport of slow ions and fast electrons; (ii) highly anharmonic lattice dynamics with short phonon lifetimes; (iii) local symmetry breaking of the average crystallographic space group; (iv) strong relativistic (spin-orbit coupling) effects on the electronic band structure; and (v) thermodynamic metastability and rapid chemical breakdown. These issues, which affect the operation of solar cells, are outlined in this perspective. We also discuss general guidelines for performing quantitative and predictive simulations of these materials, which are relevant to metal-organic frameworks and other hybrid semiconducting, dielectric and ferroelectric compounds.
Selig O, Sadhanala A, Müller C, et al., 2017, Organic cation rotation and immobilisation in pure and mixed methylammonium lead-halide perovskites, Journal of the American Chemical Society, Vol: 139, Pages: 4068-4074, ISSN: 1520-5126
Three-dimensional lead-halide perovskites have attracted a lot of attention due to their ability to combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder resulting in sharp band edges and narrow distributions of the electronic energies. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic methylammonium (MA) cation orientation in a range of pure and mixed trihalide perovskite materials. For pure MAPbX3 (X = I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. This acceleration is surprising given the expected strengthening of the hydrogen bonds between the MA and the smaller halide anions, but can be explained by the increase in the polarizability with the size of halide. Much slower dynamics, up to partial immobilization of the organic cation, are observed in the mixed MAPb(ClxBr1–x)3 and MAPb(BrxI1–x)3 alloys, which we associate with symmetry breaking within the perovskite unit cell. The observed dynamics are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronic systems.
Whalley LD, Skelton JM, Frost JM, et al., 2016, Phonon anharmonicity, lifetimes, and thermal transport in CH3NH3PbI3 from many-body perturbation theory, Physical Review B, Vol: 94, ISSN: 1550-235X
Lattice vibrations in CH3NH3PbI3 are strongly interacting, with double-well instabilities present at the Brillouin zone boundary. Analysis within a first-principles lattice-dynamics framework reveals anharmonic potentials with short phonon quasiparticle lifetimes and mean free paths. The phonon behavior is distinct from the inorganic semiconductors GaAs and CdTe where three-phonon interaction strengths are three orders of magnitude smaller. The implications for the applications of hybrid halide perovskites arising from thermal conductivity, band-gap deformation, and charge-carrier scattering through electron-phonon coupling, are presented.
Davies DW, Butler KT, Jackson AJ, et al., 2016, Computational screening of all stoichiometric inorganic materials, Chem, Vol: 1, Pages: 617-627
Forming a four-component compound from the first 103 elements of the periodic table results in more than 10(12) combinations. Such a materials space is intractable to high-throughput experiment or first-principle computation. We introduce a framework to address this problem and quantify how many materials can exist. We apply principles of valency and electronegativity to filter chemically implausible compositions, which reduces the inorganic quaternary space to 10(10) combinations. We demonstrate that estimates of band gaps and absolute electron energies can be made simply on the basis of the chemical composition and apply this to the search for new semiconducting materials to support the photoelectrochemical splitting of water. We show the applicability to predicting crystal structure by analogy with known compounds, including exploration of the phase space for ternary combinations that form a perovskite lattice. Computer screening reproduces known perovskite materials and predicts the feasibility of thousands more. Given the simplicity of the approach, large-scale searches can be performed on a single workstation.
Beecheri AN, Semonin OE, Skelton JM, et al., 2016, Direct observation of dynamic symmetry breaking above room temperature in methylammonium lead iodide perovskite, ACS Energy Letters, Vol: 1, Pages: 880-887, ISSN: 2380-8195
Lead halide perovskites such as methylammonium lead triiodide (CH3NH3PbI3) have outstanding optical and electronic properties for photovoltaic applications, yet a full understanding of how this solution-processable material works so well is currently missing. Previous research has revealed that CH3NH3PbI3 possesses multiple forms of static disorder regardless of preparation method, which is surprising in light of its excellent performance. Using high energy resolution inelastic X-ray (HERIX) scattering, we measure phonon dispersions in CH3NH3PbI3 and find direct evidence for another form of disorder in single crystals: large-amplitude anharmonic zone edge rotational instabilities of the PbI6 octahedra that persist to room temperature and above, left over from structural phase transitions that take place tens to hundreds of degrees below. Phonon calculations show that the orientations of the methylammonium (CH3NH3+) couple strongly and cooperatively to these modes. The result is a noncentrosymmetric, instantaneous local structure, which we observe in atomic pair distribution function (PDF) measurements. This local symmetry breaking is unobservable by Bragg diffraction but can explain key material properties such as the structural phase sequence, ultralow thermal transport, and large minority charge carrier lifetimes despite moderate carrier mobility. From the PDF we estimate the size of the fluctuating symmetry broken domains to be between 1 and 3 nm in diameter.
Azarhoosh P, McKechnie S, Frost JM, et al., 2016, Research Update: Relativistic origin of slow electron-hole recombination in hybrid halide perovskite solar cells, APL Materials, Vol: 4, ISSN: 2166-532X
The hybrid perovskite CH3NH3PbI3 (MAPI) exhibits long minority-carrier lifetimes and diffusion lengths. We show that slow recombination originates from a spin-split indirect-gap. Large internal electric fields act on spin-orbit-coupled band extrema, shifting band-edges to inequivalent wavevectors, making the fundamental gap indirect. From a description of photoluminescence within the quasiparticle self-consistent GW approximation for MAPI, CdTe, and GaAs, we predict carrier lifetime as a function of light intensity and temperature. At operating conditions we find radiative recombination in MAPI is reduced by a factor of more than 350 compared to direct gap behavior. The indirect gap is retained with dynamic disorder.
Leguy AM, Goñi AR, Frost JM, et al., 2016, Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites, Physical Chemistry Chemical Physics, Vol: 18, Pages: 27051-27066, ISSN: 1463-9084
We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations. Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm(-1). Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamic effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity. We show that optical rather than acoustic phonon scattering is likely to prevail at room temperature in these materials.
Butler KT, Frost JM, Skelton JM, et al., 2016, Computational materials design of crystalline solids, Chemical Society Reviews, Vol: 45, Pages: 6138-6146, ISSN: 1460-4744
The modelling of materials properties and processes from first principles is becoming sufficiently accurate as to facilitate the design and testing of new systems in silico. Computational materials science is both valuable and increasingly necessary for developing novel functional materials and composites that meet the requirements of next-generation technology. A range of simulation techniques are being developed and applied to problems related to materials for energy generation, storage and conversion including solar cells, nuclear reactors, batteries, fuel cells, and catalytic systems. Such techniques may combine crystal-structure prediction (global optimisation), data mining (materials informatics) and high-throughput screening with elements of machine learning. We explore the development process associated with computational materials design, from setting the requirements and descriptors to the development and testing of new materials. As a case study, we critically review progress in the fields of thermoelectrics and photovoltaics, including the simulation of lattice thermal conductivity and the search for Pb-free hybrid halide perovskites. Finally, a number of universal chemical-design principles are advanced.
Frost JM, Walsh A, 2016, What Is Moving in Hybrid Halide Perovskite Solar Cells?, Accounts of Chemical Research, Vol: 49, Pages: 528-535, ISSN: 1520-4898
Frost JM, Walsh A, 2016, Molecular motion and dynamic crystal structures of hybrid halide perovskites, Organic-Inorganic Halide Perovskite Photovoltaics: From Fundamentals to Device Architectures, Pages: 1-17, ISBN: 9783319351124
Hybrid halide perovskites are semiconductors with a twist. Their structures are highly dynamic with disorder occurring across a range of length and time scales. Herein, we discuss the atomic processes that underpin this behaviour and how they are linked to photovoltaic performance.
Vaissier V, Frost JM, Barnes PRF, et al., 2015, Influence of Intermolecular Interactions on the Reorganization Energy of Charge Transfer between Surface-Attached Dye Molecules, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 119, Pages: 24337-24341, ISSN: 1932-7447
The parameters controlling the kinetics ofintermolecular charge transfer are traditionally estimatedfrom electronic structure calculations on the charge donorand charge acceptor in isolation. Here, we show that thisprocedure results in inaccuracies for hole transfer between apair of organic dye molecules by comparing charge-constraineddensity functional theory (DFT) calculations on a dyecation/neutral dye pair to the conventional DFT calculationson the isolated molecules. We quantify the error made in thereorganization energy of hole exchange between dye molecules(λi). We choose three indolene-based organic dyes with application to dye-sensitized solar cells, namely, D149, D102, and D131,for which experimental values of λ are available. We find that, although highly system dependent, the intermolecular interactionbetween the charge donor and acceptor can lead to a 0.25 eV change in λi, illustrating the limitations of the widely used originalmethod in predicting the rate of charge transfer.
Brivio F, Frost JM, Skelton JM, et al., 2015, Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal, and cubic phases of methylammonium lead iodide, PHYSICAL REVIEW B, Vol: 92, ISSN: 1098-0121
Bakulin AA, Selig O, Bakker HJ, et al., 2015, Real-Time Observation of Organic Cation Reorientation in Methylammonium Lead Iodide Perovskites, Journal of Physical Chemistry Letters, Vol: 6, Pages: 3663-3669, ISSN: 1948-7185
The introduction of a mobile and polarized organic moiety as a cation in 3D lead-iodide perovskites brings fascinating optoelectronic properties to these materials. The extent and the time scales of the orientational mobility of the organic cation and the molecular mechanism behind its motion remain unclear, with different experimental and computational approaches providing very different qualitative and quantitative description of the molecular dynamics. Here we use ultrafast 2D vibrational spectroscopy of methylammonium (MA) lead iodide to directly resolve the rotation of the organic cations within the MAPbI3 lattice. Our results reveal two characteristic time constants of motion. Using ab initio molecular dynamics simulations, we identify these as a fast (∼300 fs) “wobbling-in-a-cone” motion around the crystal axis and a relatively slow (∼3 ps) jump-like reorientation of the molecular dipole with respect to the iodide lattice. The observed dynamics are essential for understanding the electronic properties of perovskite materials.
Grancini G, Kandada ARS, Frost JM, et al., 2015, Role of microstructure in the electron-hole interaction of hybrid lead halide perovskites, Nature Photonics, Vol: 9, Pages: 695-701, ISSN: 1749-4885
Organic–inorganic metal halide perovskites have demonstrated high power conversion efficiencies in solar cells and promising performance in a wide range of optoelectronic devices. The existence and stability of bound electron–hole pairs in these materials and their role in the operation of devices with different architectures remains a controversial issue. Here we demonstrate, through a combination of optical spectroscopy and multiscale modelling as a function of the degree of polycrystallinity and temperature, that the electron–hole interaction is sensitive to the microstructure of the material. The long-range order is disrupted by polycrystalline disorder and the variations in electrostatic potential found for smaller crystals suppress exciton formation, while larger crystals of the same composition demonstrate an unambiguous excitonic state. We conclude that fabrication procedures and morphology strongly influence perovskite behaviour, with both free carrier and excitonic regimes possible, with strong implications for optoelectronic devices.
Weller MT, Weber OJ, Frost JM, et al., 2015, Cubic Perovskite Structure of Black Formamidinium Lead Iodide, alpha-[HC(NH2)(2)]PbI3, at 298 K, Journal of Physical Chemistry Letters, Vol: 6, Pages: 3209-3212, ISSN: 1948-7185
Leguy AMA, Frost JM, McMahon AP, et al., 2015, Corrigendum: The dynamics of methylammonium ions in hybrid organic–inorganic perovskite solar cells, Nature Communications, Vol: 6, ISSN: 2041-1723
Eames C, Frost JM, Barnes PRF, et al., 2015, Ionic transport in hybrid lead iodide perovskite solar cells, Nature Communications, Vol: 6, ISSN: 2041-1723
Solar cells based on organic–inorganic halide perovskites have recently shown rapidly rising power conversion efficiencies, but exhibit unusual behaviour such as current–voltage hysteresis and a low-frequency giant dielectric response. Ionic transport has been suggested to be an important factor contributing to these effects; however, the chemical origin of this transport and the mobile species are unclear. Here, the activation energies for ionic migration in methylammonium lead iodide (CH3NH3PbI3) are derived from first principles, and are compared with kinetic data extracted from the current–voltage response of a perovskite-based solar cell. We identify the microscopic transport mechanisms, and find facile vacancy-assisted migration of iodide ions with an activation energy of 0.6 eV, in good agreement with the kinetic measurements. The results of this combined computational and experimental study suggest that hybrid halide perovskites are mixed ionic–electronic conductors, a finding that has major implications for solar cell device architectures.
Leguy AMA, Frost JM, McMahon AP, et al., 2015, The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells, Nature Communications, Vol: 6, ISSN: 2041-1723
Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH3NH3+ ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH3NH3PbI3 crystals with a room temperature residence time of ~14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1–200-ps time window. Monte Carlo simulations of interacting CH3NH3+ dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH3NH3+ in either antiferroelectric or ferroelectric domains. Collective realignment of CH3NH3+ to screen a device’s built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ~0.1–1 ms, faster than most observed hysteresis.
Murray AT, Frost JM, Hendon CH, et al., 2015, Modular design of SPIRO-OMeTAD analogues as hole transport materials in solar cells, Chemical Communications, Vol: 51, Pages: 8935-8938, ISSN: 1364-548X
We predict the ionisation potentials of the hole-conducting material SPIRO-OMeTAD and twelve methoxy isomers and polymethoxy derivatives. Based on electronic and economic factors, we identify the optimal compounds for application as p-type hole-selective contacts in hybrid halide perovskite solar cells.
Dimitrov SD, Wheeler S, Niedzialek D, et al., 2015, Polaron pair mediated triplet generation in polymer/fullerene blends, Nature Communications, Vol: 6, ISSN: 2041-1723
Electron spin is a key consideration for the function of organic semiconductors in light-emitting diodes and solar cells, as well as spintronic applications relying on organic magnetoresistance. A mechanism for triplet excited state generation in such systems is by recombination of electron-hole pairs. However, the exact charge recombination mechanism, whether geminate or nongeminate and whether it involves spin-state mixing is not well understood. In this work, the dynamics of free charge separation competing with recombination to polymer triplet states is studied in two closely related polymer-fullerene blends with differing polymer fluorination and photovoltaic performance. Using time-resolved laser spectroscopic techniques and quantum chemical calculations, we show that lower charge separation in the fluorinated system is associated with the formation of bound electron-hole pairs, which undergo spin-state mixing on the nanosecond timescale and subsequent geminate recombination to triplet excitons. We find that these bound electron-hole pairs can be dissociated by electric fields.
Few S, Frost JM, Nelson J, 2015, Models of charge pair generation in organic solar cells, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 17, Pages: 2311-2325, ISSN: 1463-9076
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Steiner F, Foster S, Losquin A, et al., 2015, Distinguishing the influence of structural and energetic disorder on electron transport in fullerene multi-adducts, MATERIALS HORIZONS, Vol: 2, Pages: 113-119, ISSN: 2051-6347
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- Citations: 43
Manke F, Frost JM, Vaissier V, et al., 2015, Influence of a nearby substrate on the reorganization energy of hole exchange between dye molecules, PHYSICAL CHEMISTRY CHEMICAL PHYSICS, Vol: 17, Pages: 7345-7354, ISSN: 1463-9076
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Butler KT, Frost JM, Walsh A, 2014, Ferroelectric materials for solar energy conversion: photoferroics revisited, Energy & Environmental Science, Vol: 8, Pages: 838-848, ISSN: 1754-5706
The application of ferroelectric materials (i.e. solids that exhibit spontaneous electric polarisation) in solar cells has a long and controversial history. This includes the first observations of the anomalous photovoltaic effect (APE) and the bulk photovoltaic effect (BPE). The recent successful application of inorganic and hybrid perovskite structured materials (e.g. BiFeO3, CsSnI3, CH3NH3PbI3) in solar cells emphasises that polar semiconductors can be used in conventional photovoltaic architectures. We review developments in this field, with a particular emphasis on the materials known to display the APE/BPE (e.g. ZnS, CdTe, SbSI), and the theoretical explanation. Critical analysis is complemented with first-principles calculation of the underlying electronic structure. In addition to discussing the implications of a ferroelectric absorber layer, and the solid state theory of polarisation (Berry phase analysis), design principles and opportunities for high-efficiency ferroelectric photovoltaics are presented.
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