Imperial College London

Dr Jarvist Moore Frost

Faculty of Natural SciencesDepartment of Chemistry

Royal Society URF (Lecturer)
 
 
 
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Contact

 

jarvist.frost Website

 
 
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Location

 

601FMolecular Sciences Research HubWhite City Campus

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Summary

 

Publications

Publication Type
Year
to

79 results found

Siemons N, Pearce D, Cendra C, Yu H, Tuladhar SM, Hallani RK, Sheelamanthula R, LeCroy GS, Siemons L, White AJP, Mcculloch I, Salleo A, Frost JM, Giovannitti A, Nelson Jet al., 2022, Impact of side chain hydrophilicity on packing, swelling and ion interactions in oxy-bithiophene semiconductors., Advanced Materials, Vol: 34, ISSN: 0935-9648

Exchanging hydrophobic alkyl-based side chains to hydrophilic glycol-based side chains is a widely adopted method for improving mixed-transport device performance, despite the impact on solid state packing and polymer-electrolyte interactions being poorly understood. Presented here is a Molecular Dynamics (MD) force field for modelling alkoxylated and glycolated polythiophenes. The force field is validated against known packing motifs for their monomer crystals. MD simulations, coupled with X-ray Diffraction (XRD), show that alkoxylated polythiophenes will pack with a 'tilted stack' and straight interdigitating side chains, whilst their glycolated counterpart will pack with a 'deflected stack' and an s-bend side chain configuration. MD simulations reveal water penetration pathways into the alkoxylated and glycolated crystals - through the π-stack and through the lamellar stack respectively. Finally, the two distinct ways tri-ethylene glycol polymers can bind to cations are revealed, showing the formation of a meta-stable single bound state, or an energetically deep double bound state, both with a strong side chain length dependance. The minimum energy pathways for the formation of the chelates are identified, showing the physical process through which cations can bind to one or two side chains of a glycolated polythiophene, with consequences for ion transport in bithiophene semiconductors. This article is protected by copyright. All rights reserved.

Journal article

Tan E, Kim J, Stewart K, Pitsalidis C, Kwon S, Siemons N, Kim J, Jiang Y, Frost JM, Pearce D, Tyrrell JE, Nelson J, Owens RM, Kim Y-H, Kim J-Set al., 2022, The role of long-alkyl-group spacers in glycolated copolymers for high performance organic electrochemical transistors, Advanced Materials, Vol: 34, ISSN: 0935-9648

Semiconducting polymers with oligoethylene glycol sidechains have attracted strong research interest for organic electrochemical transistor (OECT) applications. However, key molecular design rules for high-performance OECTs via efficient mixed electronic/ionic charge transport are still unclear. Herein, we synthesize and characterize new glycolated copolymers (gDPP-TTT and gDPP-TTVTT) with diketopyrrolopyrrole (DPP) acceptor and thiophene-based (TTT or TTVTT) donor units for accumulation mode OECTs, where a long-alkyl-group (C12 ) attached to DPP unit acts as a spacer distancing the oligoethylene glycol from the polymer backbone. gDPP-TTVTT shows the highest OECT transconductance (61.9 S cm-1 ) and high operational stability, compared to gDPP-TTT and their alkylated counterparts. Surprisingly, gDPP-TTVTT also shows high electronic charge mobility in field-effect transistor, suggesting efficient ion injection/diffusion without hindering its efficient electronic charge transport. The elongated donor unit (TTVTT) facilitates the hole polaron formation more localized to the donor unit, leading to faster and easier polaron formation with less impact on polymer structure during OECT operation, as opposed to the TTT unit. This is supported by molecular dynamics (MD) simulation. We conclude that these simultaneously high electronic and ionic charge transport properties are achieved due to the long-alkyl-group spacer in amphipathic sidechains, providing an important molecular design rule for glycolated copolymers. This article is protected by copyright. All rights reserved.

Journal article

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

Journal article

Guster B, Melo P, Martin BAA, Brousseau-Couture V, De Abreu JC, Miglio A, Giantomassi M, Côté M, Frost JM, Verstraete MJ, Gonze Xet al., 2022, Erratum: Fröhlich polaron effective mass and localization length in cubic materials: Degenerate and anisotropic electronic bands (Physical Review B (2021) 104 (235123) DOI: 10.1103/PhysRevB.104.235123), Physical Review B, Vol: 105, ISSN: 2469-9950

Polarons, that is, charge carriers correlated with lattice deformations, are ubiquitous quasiparticles in semiconductors, and play an important role in electrical conductivity. To date most theoretical studies of so-called large polarons, in which the lattice can be considered as a continuum, have focused on the original Fröhlich model: a simple (non-degenerate) parabolic isotropic electronic band coupled to one dispersionless longitudinal optical phonon branch. The Fröhlich model allows one to understand characteristics such as polaron formation energy, radius, effective mass and mobility. Real cubic materials, instead, have electronic band extrema that are often degenerate or anisotropic and present several phonon modes. In the present work, we address such issues. We keep the continuum hypothesis inherent to the large polaron Fröhlich model, but waive the isotropic and non-degeneracy hypotheses, and also include multiple phonon branches. For polaron effective masses, working at the lowest order of perturbation theory, we provide analytical results for the case of anisotropic electronic energy dispersion, with two distinct effective masses (uniaxial) and numerical simulations for the degenerate 3-band case, typical of III-V and II-VI semiconductor valence bands. We also deal with the strong-coupling limit, using a variational treatment: we propose trial wavefunctions for the above-mentioned cases, providing polaron radii and energies. Then, we evaluate the polaron formation energies, effective masses and localisation lengths using parameters representative of a dozen II-VI, III-V and oxide semiconductors, for both electron and hole polarons...In the non-degenerate case, we compare the perturbative approach with the Feynman path integral approach in characterisizing polarons in the weak coupling limit.

Journal article

Guster B, Melo P, Martin BAA, Brousseau-Couture V, de Abreu JC, Miglio A, Giantomassi M, Côté M, Frost JM, Verstraete MJ, Gonze Xet al., 2021, Fröhlich polaron effective mass and localization length in cubic materials: Degenerate and anisotropic electronic bands, Physical Review B, Vol: 104, ISSN: 2469-9950

Polarons, that is, charge carriers correlated with lattice deformations, are ubiquitous quasiparticles in semiconductors, and play an important role in electrical conductivity. To date most theoretical studies of so-called large polarons, in which the lattice can be considered as a continuum, have focused on the original Fröhlich model: a simple (nondegenerate) parabolic isotropic electronic band coupled to one dispersionless longitudinal optical phonon branch. The Fröhlich model allows one to understand characteristics such as polaron formation energy, radius, effective mass, and mobility. Real cubic materials, instead, have electronic band extrema that are often degenerate (e.g., threefold degeneracy of the valence band), or anisotropic (e.g., conduction bands at X or L), and present several phonon modes. In the present paper, we address such issues. We keep the continuum hypothesis inherent to the large polaron Fröhlich model, but waive the isotropic and nondegeneracy hypotheses, and also include multiple phonon branches. For polaron effective masses, working at the lowest order of perturbation theory, we provide analytical results for the case of anisotropic electronic energy dispersion, with two distinct effective masses (uniaxial) and numerical simulations for the degenerate three-band case, typical of III-V and II-VI semiconductor valence bands. We also deal with the strong-coupling limit, using a variational treatment: we propose trial wave functions for the above-mentioned cases, providing polaron radii and energies. Then, we evaluate the polaron formation energies, effective masses, and localization lengths using parameters representative of a dozen II-VI, III-V, and oxide semiconductors, for both electron and hole polarons. We show that for some cases perturbation theory (the weak-coupling approach) breaks down. In some other cases, the strong-coupling approach reveals that the large polaron hypothesis is not valid, which is another distinc

Journal article

Zheng X, Hopper TR, Gorodetsky A, Maimaris M, Xu W, Martin BAA, Frost JM, Bakulin AAet al., 2021, Multi-pulse terahertz spectroscopy unveils hot polaron photoconductivity dynamics in metal-halide perovskites, Journal of Physical Chemistry Letters, Vol: 12, Pages: 8732-8739, ISSN: 1948-7185

The behavior of hot carriers in metal-halide perovskites (MHPs) present avaluable foundation for understanding the details of carrier-phonon coupling inthe materials as well as the prospective development of highly efficient hotcarrier and carrier multiplication solar cells. Whilst the carrier populationdynamics during cooling have been intensely studied, the evolution of the hotcarrier properties, namely the hot carrier mobility, remain largely unexplored.To address this, we introduce a novel ultrafast visible pump - infrared push -terahertz probe spectroscopy (PPP-THz) to monitor the real-time conductivitydynamics of cooling carriers in methylammonium lead iodide. We find a decreasein mobility upon optically depositing energy into the carriers, which istypical of band-transport. Surprisingly, the conductivity recovery dynamics areincommensurate with the intraband relaxation measured by an analogousexperiment with an infrared probe (PPP- IR), and exhibit a negligibledependence on the density of hot carriers. These results and the kineticmodelling reveal the importance of highly-localized lattice heating on themobility of the hot electronic states. This collective polaron-latticephenomenon may contribute to the unusual photophysics observed in MHPs andshould be accounted for in devices that utilize hot carriers.

Journal article

Whalley LD, van Gerwen P, Frost JM, Kim S, Hood SN, Walsh Aet al., 2021, Giant Huang-Rhys Factor for Electron Capture by the Iodine Intersitial in Perovskite Solar Cells, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol: 143, Pages: 9123-9128, ISSN: 0002-7863

Journal article

Muscarella LA, Hutter EM, Frost JM, Grimaldi GG, Versluis J, Bakker HJ, Ehrler Bet al., 2021, Accelerated Hot-Carrier Cooling in MAPbI(3) Perovskite by Pressure-Induced Lattice Compression, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, Vol: 12, Pages: 4118-4124, ISSN: 1948-7185

Journal article

Davies DW, Savory CN, Frost JM, Scanlon DO, Morgan BJ, Walsh Aet al., 2020, Descriptors for electron and hole charge carriers in metal oxides, Journal of Physical Chemistry Letters, Vol: 11, Pages: 438-444, ISSN: 1948-7185

Metal oxides can act as insulators, semiconductors, or metals depending on their chemical composition and crystal structure. Metal oxide semiconductors, which support equilibrium populations of electron and hole charge carriers, have widespread applications including batteries, solar cells, and display technologies. It is often difficult to predict in advance whether these materials will exhibit localized or delocalized charge carriers upon oxidation or reduction. We combine data from first-principles calculations of the electronic structure and dielectric response of 214 metal oxides to predict the energetic driving force for carrier localization and transport. We assess descriptors based on the carrier effective mass, static polaron binding energy, and Fröhlich electron–phonon coupling. Numerical analysis allows us to assign p- and n-type transport of a metal oxide to three classes: (i) band transport with high mobility; (ii) small polaron transport with low mobility; and (iii) intermediate behavior. The results of this classification agree with observations regarding carrier dynamics and lifetimes and are used to predict 10 candidate p-type oxides.

Journal article

Yang RX, Skelton JM, da Silva EL, Frost JM, Walsh Aet al., 2020, Assessment of dynamic structural instabilities across 24 cubic inorganic halide perovskites, Journal of Chemical Physics, Vol: 152, Pages: 024703-1-024703-9, ISSN: 0021-9606

Metal halide perovskites are promising candidates for next-generation photovoltaic and optoelectronic applications. The flexible nature of the octahedral network introduces complexity when understanding their physical behavior. It has been shown that these materials are prone to decomposition and phase competition, and the local crystal structure often deviates from the average space group symmetry. To make stable phase-pure perovskites, understanding their structure–composition relations is of central importance. We demonstrate, from lattice dynamics calculations, that the 24 inorganic perovskites ABX3 (A = Cs, Rb; B = Ge, Sn, Pb; X = F, Cl, Br, I) exhibit instabilities in their cubic phase. These instabilities include cation displacements, octahedral tilting, and Jahn-Teller distortions. The magnitudes of the instabilities vary depending on the chemical identity and ionic radii of the composition. The tilting instabilities are energetically dominant and reduce as the tolerance factor increases, whereas cation displacements and Jahn-Teller type distortions depend on the interactions between the constituent ions. We further considered representative tetragonal, orthorhombic, and monoclinic perovskite phases to obtain phonon-stable structures for each composition. This work provides insights into the thermodynamic driving force of the instabilities and will help guide computer simulations and experimental synthesis in material screening.

Journal article

Davies D, Savory C, Frost JM, Scanlon D, Morgan B, Walsh Aet al., 2019, Descriptors for Electron and Hole Charge Carriers in Metal Oxides, Publisher: American Chemical Society (ACS)

<jats:p>Metal oxides can act as insulators, semiconductors or metals depending on their chemical composition and crystal structure. Metal oxide semiconductors, which support equilibrium populations of electron and hole charge carriers, have widespread applications including batteries, solar cells, and display technologies. It is often difficult to predict in advance whether these materials will exhibit localized or delocalized charge carriers upon oxidation or reduction. We combine data from first-principles calculations of the electronic structure and dielectric response of 214 metal oxides to predict the energetic driving force for carrier localization and transport. We assess descriptors based on the carrier effective mass, static polaron binding energy, and Frohlich electron–phonon coupling. Numerical analysis allows us to assign p and n type transport of a metal oxide to three classes: (i) band transport with high mobility; (ii) small polaron transport with low mobility; and (iii) intermediate behaviour. The results of this classification agree with observations regarding carrier dynamics and lifetimes and are used to predict 10 candidate p-type oxides.</jats:p>

Working paper

Yang RX, Skelton J, Da Silva EL, Frost JM, Walsh Aet al., 2019, Assessment of Dynamic Structural Instabilities Across 24 Cubic Inorganic Halide Perovskites, Publisher: American Chemical Society (ACS)

<jats:p>Metal halide perovskites are promising candidates for next-generation photovoltaic and optoelectronic applications. The flexible nature of the octahedral network introduces complexity when understanding their physical behavior. It has been shown that these materials are prone to decomposition, phase competition, and the local crystal structure often deviates from the average space group symmetry. To make stable phase-pure perovskites, understanding their structure-composition relations is of central importance. We demonstrate, from lattice dynamics calculations, that the 24 inorganic perovskites ABX<jats:sub>3</jats:sub> (A = Cs, Rb; B = Ge, Sn, Pb; X = F, Cl, Br, I) exhibit instabilities in their cubic phase. These instabilities include cation displacements, octahedral tilting, and Jahn-Teller distortions. The magnitudes of the instabilities vary depending on the chemical identity and ionic radii of the composition. The tilting instabilities are energetically dominant, and reduce as the tolerance factor increases, whereas cation displacements and Jahn-Teller type distortions depend on the interactions between the constituent ions. We further considered representative tetragonal, orthorhombic and monoclinic perovskites phases to obtain phonon-stable phases for each composition. This work provides insights into the thermodynamic driving force of the instabilities and will help guide synthesis in material screening.</jats:p>

Working paper

Shi X, Nádaždy V, Perevedentsev A, Frost J, Wang X, von Hauff E, Mackenzie R, Nelson Jet al., 2019, Relating Chain Conformation to the Density of States and Charge Transport in Conjugated Polymers: The Role of the β-phase in Poly(9,9-dioctylfluorene), Physical Review X, Vol: 9, ISSN: 2160-3308

Charge transport in π-conjugated polymers is characterised by a strong degree of disorder in both the energy of conjugated segments and the electronic coupling between adjacent sites. This disorder arises from variations in the structure and conformation of molecular units, as well as the weak inter-molecular binding interactions. Although disorder in molecular conformation can be expected to influence the density of states (DoS) distribution, and hence optoelectronic properties of the material, until now, there has been no direct study of the relationship between a distinct conformational defect and the charge transport properties of a conjugated polymer. Here, we investigate the impact of introducing an extended, planarised chain geometry, known as the ‘β-phase’, on hole transport through otherwise amorphous films of poly(9,9-dioctylfluorene) (PFO). We show that whilst β-phase introduces a striking ~hundredfold drop in time-of-flight (ToF) hole mobility (μh) at room temperature, it reduces the steady-state μh measured from hole-only devices by a factor of less than ~5. In order to reconcile these observations, we combine high-dynamic-range ToF photocurrent spectroscopy and energy-resolved electrochemical impedance spectroscopy to extract the hole DoS of the conjugated polymer. Both methods show that the effect of the β-phase content is to introduce a sharp sub-bandgap feature into the DoS of glassy PFO lying ~0.3 eV above the highest occupied molecular orbital. The observed energy of the conformational trap is consistent with electronic structure calculations using a tight-binding approach. Using the obtained DoS with a drift-diffusion model capable of resolving charge carriers in both time and energy, we show how the seemingly contradictory transport phenomena obtained via the time-resolved, frequency-resolved, and steady-state methods are reconciled. The results highlight the significance of energetic redistribut

Journal article

Whalley LD, Frost JM, Morgan BJ, Walsh Aet 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

Journal article

Wilson JN, Frost JM, Wallace SK, Walsh Aet 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.

Journal article

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.

Journal article

Gold-Parker A, Gehring PM, Skelton JM, Smith IC, Parshall D, Frost JM, Karunadasa HI, Walsh A, Toney MFet 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.

Journal article

Rice B, Guilbert AAY, Frost JM, Nelson Jet al., 2018, Polaron states in fullerene adducts modeled by coarse-grained molecular dynamics and tight binding, Journal of Physical Chemistry Letters, Vol: 9, Pages: 6616-6623, ISSN: 1948-7185

Strong electron–phonon coupling leads to polaron localization in molecular semiconductor materials and influences charge transport, but it is expensive to calculate atomistically. Here, we propose a simple and efficient model to determine the energy and spatial extent of polaron states within a coarse-grained representation of a disordered molecular film. We calculate the electronic structure of the molecular assembly using a tight-binding Hamiltonian and determine the polaron state self-consistently by perturbing the site energies by the dielectric response of the surrounding medium to the charge. When applied to fullerene derivatives, the method shows that polarons extend over multiple molecules in C60 but localize on single molecules in higher adducts of phenyl-C61-butyric-acid-methyl-ester (PCBM) because of packing disorder and the polar side chains. In PCBM, polarons localize on single molecules only when energetic disorder is included or when the fullerene is dispersed in a blend. The method helps to establish the conditions under which a hopping transport model is justified.

Journal article

Gallop NP, Selig O, Giubertoni G, Bakker HJ, Rezus YLA, Frost JM, Jansen TLC, Lovrincic R, Bakulin AAet al., 2018, Rotational cation dynamics in metal halide perovskites: Effect on phonons and material properties, Journal of Physical Chemistry Letters, Vol: 9, Pages: 5987-5997, ISSN: 1948-7185

The dynamics of organic cations in metal halide hybrid perovskites (MHPs) have been investigated using numerous experimental and computational techniques because of their suspected effects on the properties of MHPs. In this Perspective, we summarize and reconcile key findings and present new data to synthesize a unified understanding of the dynamics of the cations. We conclude that theory and experiment collectively paint a relatively complete picture of rotational dynamics within MHPs. This picture is then used to discuss the consequences of structural dynamics for electron–phonon interactions and their effect on material properties by providing a brief account of key studies that correlate cation dynamics with the dynamics of the inorganic sublattice and overall device properties.

Journal article

Hopper T, Gorodetsky A, Frost JM, Müller C, Lovrincic R, Bakulin Aet al., 2018, Ultrafast Intraband Spectroscopy of Hot-Carrier Cooling in Lead-Halide Perovskites, ACS Energy Letters, Vol: 3, Pages: 2199-2205, ISSN: 2380-8195

The rapid relaxation of above-band-gap “hot” carriers (HCs) imposes the key efficiency limit in lead-halide perovskite (LHP) solar cells. Recent studies have indicated that HC cooling in these systems may be sensitive to materials composition, as well as the energy and density of excited states. However, the key parameters underpinning the cooling mechanism are currently under debate. Here we use a sequence of ultrafast optical pulses (visible pump–infrared push–infrared probe) to directly compare the intraband cooling dynamics in five common LHPs: FAPbI3, FAPbBr3, MAPbI3, MAPbBr3, and CsPbBr3. We observe ∼100–900 fs cooling times, with slower cooling at higher HC densities. This effect is strongest in the all-inorganic Cs-based system, compared to the hybrid analogues with organic cations. These observations, together with band structure calculations, allow us to quantify the origin of the “hot-phonon bottleneck” in LHPs and assert the thermodynamic contribution of a symmetry-breaking organic cation toward rapid HC cooling.

Journal article

Mckechnie S, Frost JM, Pashov D, Azarhoosh P, Walsh A, van Schilfgaarde Met al., 2018, Dynamic symmetry breaking and spin splitting in metal halide perovskites, PHYSICAL REVIEW B, Vol: 98, ISSN: 2469-9950

Journal article

Yang Y, Rice B, Shi X, Brandt JR, da Costa RC, Hedley GJ, Smilgies D-M, Frost JM, Samuel IDW, Otero-de-la-Roza A, Johnson ER, Jelfs KE, Nelson J, Campbell AJ, Fuchter MJet 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.

Journal article

Leventis A, Royakkers J, Rapidis AG, Goodeal N, Corpinot MK, Frost JM, Bucar D-K, Blunt MO, Cacialli F, Bronstein Het 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

Journal article

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.

Journal article

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.

Journal article

Xi R, Skelton JM, da Silva EL, Frost JM, Walsh Aet 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.

Journal article

Freeman DME, Musser AJ, Fros JM, Stern HL, Forster AK, Fallon KJ, Rapidis AG, Cacialli F, McCulloch I, Clarke TM, Friend RH, Bronstein Het 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

Journal article

Whalley LD, Frost JM, Jung YK, Walsh Aet 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.

Journal article

Selig O, Sadhanala A, Müller C, Lovrincic R, Chen Z, Rezus YL, Frost JM, Jansen TL, Bakulin Aet 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.

Journal article

Whalley LD, Skelton JM, Frost JM, Walsh Aet 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.

Journal article

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