Publications
80 results found
Cho H-H, Congrave DG, Gillett AJ, et al., 2024, Suppression of Dexter transfer by covalent encapsulation for efficient matrix-free narrowband deep blue hyperfluorescent OLEDs., Nat Mater, Vol: 23, Pages: 519-526
Hyperfluorescence shows great promise for the next generation of commercially feasible blue organic light-emitting diodes, for which eliminating the Dexter transfer to terminal emitter triplet states is key to efficiency and stability. Current devices rely on high-gap matrices to prevent Dexter transfer, which unfortunately leads to overly complex devices from a fabrication standpoint. Here we introduce a molecular design where ultranarrowband blue emitters are covalently encapsulated by insulating alkylene straps. Organic light-emitting diodes with simple emissive layers consisting of pristine thermally activated delayed fluorescence hosts doped with encapsulated terminal emitters exhibit negligible external quantum efficiency drops compared with non-doped devices, enabling a maximum external quantum efficiency of 21.5%. To explain the high efficiency in the absence of high-gap matrices, we turn to transient absorption spectroscopy. It is directly observed that Dexter transfer from a pristine thermally activated delayed fluorescence sensitizer host can be substantially reduced by an encapsulated terminal emitter, opening the door to highly efficient 'matrix-free' blue hyperfluorescence.
Hou X, Coker JF, Yan J, et al., 2024, Structure–Property Relationships for the Electronic Applications of Bis-Adduct Isomers of Phenyl-C₆₁ Butyric Acid Methyl Ester, Chemistry of Materials, Vol: 36, Pages: 425-438, ISSN: 0897-4756
Higher adducts of a fullerene, such as the bis-adduct of PCBM (bis-PCBM), can be used to achieve shallower molecular orbital energy levels than, for example, PCBM or C60. Substituting the bis-adduct for the parent fullerene is useful to increase the open-circuit voltage of organic solar cells or achieve better energy alignment as electron transport layers in, for example, perovskite solar cells. However, bis-PCBM is usually synthesized as a mixture of structural isomers, which can lead to both energetic and morphological disorder, negatively affecting device performance. Here, we present a comprehensive study on the molecular properties of 19 pure bis-isomers of PCBM using a variety of characterization methods, including ultraviolet photoelectron spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, single crystal structure, and (time-dependent) density functional theory calculation. We find that the lowest unoccupied molecular orbital of such bis-isomers can be tuned to be up to 170 meV shallower than PCBM and up to 100 meV shallower than the mixture of unseparated isomers. The isolated bis-isomers also show an electron mobility in organic field-effect transistors of up to 4.5 × 10⁻² cm²/(Vs), which is an order of magnitude higher than that of the mixture of bis-isomers. These properties enable the fabrication of the highest performing bis-PCBM organic solar cell to date, with the best device showing a power conversion efficiency of 7.2%. Interestingly, we find that the crystallinity of bis-isomers correlates negatively with electron mobility and organic solar cell device performance, which we relate to their molecular symmetry, with a lower symmetry leading to more amorphous bis-isomers, less energetic disorder, and higher dimensional electron transport. This work demonstrates the potential of side chain engineering for optimizing the performance of fullerene-based organic electronic devices.
Siemons N, 2023, Controlling swelling in mixed transport polymers through alkyl side-chain physical cross-linking, Proceedings of the National Academy of Sciences of USA, Vol: 120, ISSN: 0027-8424
Semiconducting conjugated polymers bearing glycol side chains can simultaneously transport both electronic and ionic charges with high charge mobilities, making them ideal electrode materials for a range of bioelectronic devices. However, heavily glycolated conjugated polymer films have been observed to swell irreversibly when subjected to an electrochemical bias in an aqueous electrolyte. The excessive swelling can lead to the degradation of their microstructure, and subsequently reduced device performance. An effective strategy to control polymer film swelling is to copolymerize glycolated repeat units with a fraction of monomers bearing alkyl side chains, although the microscopic mechanism that constrains swelling is unknown. Here we investigate, experimentally and computationally, a series of archetypal mixed transporting copolymers with varying ratios of glycolated and alkylated repeat units. Experimentally we observe that exchanging 10% of the glycol side chains for alkyl leads to significantly reduced film swelling and an increase in electrochemical stability. Through molecular dynamics simulation of the amorphous phase of the materials, we observe the formation of polymer networks mediated by alkyl side-chain interactions. When in the presence of water, the network becomes increasingly connected, counteracting the volumetric expansion of the polymer film.
Carwithen BP, Hopper TR, Ge Z, et al., 2023, Confinement and exciton binding energy effects on hot carrier cooling in lead halide perovskite nanomaterials, ACS Nano, Vol: 17, Pages: 6638-6648, ISSN: 1936-0851
The relaxation of the above-gap (“hot”) carriers in lead halide perovskites (LHPs) is important for applications in photovoltaics and offers insights into carrier–carrier and carrier–phonon interactions. However, the role of quantum confinement in the hot carrier dynamics of nanosystems is still disputed. Here, we devise a single approach, ultrafast pump–push–probe spectroscopy, to study carrier cooling in six different size-controlled LHP nanomaterials. In cuboidal nanocrystals, we observe only a weak size effect on the cooling dynamics. In contrast, two-dimensional systems show suppression of the hot phonon bottleneck effect common in bulk perovskites. The proposed kinetic model describes the intrinsic and density-dependent cooling times accurately in all studied perovskite systems using only carrier–carrier, carrier–phonon, and excitonic coupling constants. This highlights the impact of exciton formation on carrier cooling and promotes dimensional confinement as a tool for engineering carrier–phonon and carrier–carrier interactions in LHP optoelectronic materials.
Martin BAA, Frost JM, 2023, Multiple phonon modes in Feynman path-integral variational polaron mobility, Physical Review B, Vol: 107, ISSN: 2469-9950
The Feynman path-integral variational approach to the polaron problem, along with the associated Feynman-Hellwarth-Iddings-Platzman (FHIP) linear-response mobility theory, provides a computationally amenable method to predict the frequency-resolved temperature-dependent charge-carrier mobility, and other experimental observables in polar semiconductors. We show that the FHIP mobility theory predicts non-Drude transport behavior, and shows remarkably good agreement with the recent diagrammatic Monte Carlo mobility simulations of Mishchenko et al. [Phys. Rev. Lett. 123, 076601 (2019)10.1103/PhysRevLett.123.076601] for the abstract Fröhlich Hamiltonian. We extend this method to multiple phonon modes in the Fröhlich model action. This enables a slightly better variational solution, as inferred from the resulting energy. We carry forward this extra complexity into the mobility theory, which shows a richer structure in the frequency and temperature-dependent mobility, due to the different phonon modes activating at different energies. The method provides a computationally efficient and fully quantitative method of predicting polaron mobility and response in real materials.
Zhang H, Debroye E, Vina-Bausa B, et al., 2023, Stable Mott Polaron State Limits the Charge Density in Lead Halide Perovskites, ACS ENERGY LETTERS, Vol: 8, Pages: 420-428, ISSN: 2380-8195
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- Citations: 1
Moro S, Siemons N, Drury O, et al., 2022, The Effect of Glycol Side Chains on the Assembly and Microstructure of Conjugated Polymers, ACS NANO, Vol: 16, Pages: 21303-21314, ISSN: 1936-0851
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- Citations: 5
Siemons N, Pearce D, Cendra C, et 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.
Tan E, Kim J, Stewart K, et 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.
Rakowski R, Fisher W, Calbo J, et 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
Guster B, Melo P, Martin BAA, et 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.
Guster B, Melo P, Martin BAA, et 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
Zheng X, Hopper TR, Gorodetsky A, et 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.
Whalley LD, van Gerwen P, Frost JM, et 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
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- Citations: 25
Muscarella LA, Hutter EM, Frost JM, et al., 2021, Accelerated Hot-Carrier Cooling in MAPbI<sub>3</sub> Perovskite by Pressure-Induced Lattice Compression, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, Vol: 12, Pages: 4118-4124, ISSN: 1948-7185
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- Citations: 6
Davies DW, Savory CN, Frost JM, et al., 2020, Descriptors for electron and hole charge carriers in metal oxides, Journal of Physical Chemistry Letters, Vol: 11, Pages: 438-444, ISSN: 1948-7185
Metal oxides can act as insulators, semiconductors, or metals depending on their chemical composition and crystal structure. Metal oxide semiconductors, which support equilibrium populations of electron and hole charge carriers, have widespread applications including batteries, solar cells, and display technologies. It is often difficult to predict in advance whether these materials will exhibit localized or delocalized charge carriers upon oxidation or reduction. We combine data from first-principles calculations of the electronic structure and dielectric response of 214 metal oxides to predict the energetic driving force for carrier localization and transport. We assess descriptors based on the carrier effective mass, static polaron binding energy, and Fröhlich electron–phonon coupling. Numerical analysis allows us to assign p- and n-type transport of a metal oxide to three classes: (i) band transport with high mobility; (ii) small polaron transport with low mobility; and (iii) intermediate behavior. The results of this classification agree with observations regarding carrier dynamics and lifetimes and are used to predict 10 candidate p-type oxides.
Yang RX, Skelton JM, da Silva EL, et al., 2020, Assessment of dynamic structural instabilities across 24 cubic inorganic halide perovskites, Journal of Chemical Physics, Vol: 152, Pages: 024703-1-024703-9, ISSN: 0021-9606
Metal halide perovskites are promising candidates for next-generation photovoltaic and optoelectronic applications. The flexible nature of the octahedral network introduces complexity when understanding their physical behavior. It has been shown that these materials are prone to decomposition and phase competition, and the local crystal structure often deviates from the average space group symmetry. To make stable phase-pure perovskites, understanding their structure–composition relations is of central importance. We demonstrate, from lattice dynamics calculations, that the 24 inorganic perovskites ABX3 (A = Cs, Rb; B = Ge, Sn, Pb; X = F, Cl, Br, I) exhibit instabilities in their cubic phase. These instabilities include cation displacements, octahedral tilting, and Jahn-Teller distortions. The magnitudes of the instabilities vary depending on the chemical identity and ionic radii of the composition. The tilting instabilities are energetically dominant and reduce as the tolerance factor increases, whereas cation displacements and Jahn-Teller type distortions depend on the interactions between the constituent ions. We further considered representative tetragonal, orthorhombic, and monoclinic perovskite phases to obtain phonon-stable structures for each composition. This work provides insights into the thermodynamic driving force of the instabilities and will help guide computer simulations and experimental synthesis in material screening.
Davies D, Savory C, Frost JM, et al., 2019, Descriptors for Electron and Hole Charge Carriers in Metal Oxides
<jats:p><div> <div> <div> <p>Metal oxides can act as insulators, semiconductors or metals depending on theirchemical composition and crystal structure. Metal oxide semiconductors, which support equilibrium populations of electron and hole charge carriers, have widespreadapplications including batteries, solar cells, and display technologies. It is often difficult to predict in advance whether these materials will exhibit localized or delocalizedcharge carriers upon oxidation or reduction. We combine data from first-principlescalculations of the electronic structure and dielectric response of 214 metal oxides topredict the energetic driving force for carrier localization and transport. We assess descriptors based on the carrier effective mass, static polaron binding energy, and Frohlichelectron–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) smallpolaron transport with low mobility; and (iii) intermediate behaviour. The results ofthis classification agree with observations regarding carrier dynamics and lifetimes andare used to predict 10 candidate p-type oxides.</p> </div> </div> </div></jats:p>
Davies D, Savory C, Frost JM, et al., 2019, Descriptors for Electron and Hole Charge Carriers in Metal Oxides
<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>
Yang RX, Skelton J, Da Silva EL, et al., 2019, Assessment of Dynamic Structural Instabilities Across 24 Cubic Inorganic Halide Perovskites
<jats:p><div><div>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<sub>3</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. </div></div></jats:p>
Yang RX, Skelton J, Da Silva EL, et al., 2019, Assessment of Dynamic Structural Instabilities Across 24 Cubic Inorganic Halide Perovskites
<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>
Shi X, Nádaždy V, Perevedentsev A, et 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
Whalley LD, Frost JM, Morgan BJ, et al., 2019, Impact of nonparabolic electronic band structure on the optical and transport properties of photovoltaic materials, Physical review B: Condensed matter and materials physics, Vol: 99, ISSN: 1098-0121
The effective mass approximation (EMA) models the response to an external perturbation of an electron in a periodic potential as the response of a free electron with a renormalized mass. For semiconductors used in photovoltaic devices, the EMA allows calculation of important material properties from first-principles calculations, including optical properties (e.g., exciton binding energies), defect properties (e.g., donor and acceptor levels), and transport properties (e.g., polaron radii and carrier mobilities). The conduction and valence bands of semiconductors are commonly approximated as parabolic around their extrema, which gives a simple theoretical description but ignores the complexity of real materials. In this work, we use density functional theory to assess the impact of band nonparabolicity on four common thin-film photovoltaic materials—GaAs, CdTe, Cu2ZnSnS4 and CH3NH3PbI3—at temperatures and carrier densities relevant for real-world applications. First, we calculate the effective mass at the band edges. We compare finite-difference, unweighted least-squares and thermally weighted least-squares approaches. We find that the thermally weighted least-squares method reduces sensitivity to the choice of sampling density. Second, we employ a Kane quasilinear dispersion to quantify the extent of nonparabolicity and compare results from different electronic structure theories to consider the effect of spin-orbit coupling and electron exchange. Finally, we focus on the halide perovskite CH3NH3PbI3 as a model system to assess the impact of nonparabolicity on calculated electron transport and optical properties at high carrier concentrations. We find that at a concentration of 1020cm−3 the optical effective mass increases by a factor of two relative to the low carrier-concentration value, and the polaron mobility decreases by a factor of three. Our work suggests that similar adjustments should be made to the predicted optical and transport proper
Wilson JN, Frost JM, Wallace SK, et al., 2019, Dielectric and ferroic properties of metal halide perovskites, APL Materials, Vol: 7, ISSN: 2166-532X
Halide perovskite semiconductors and solar cells respond to electric fields in a way that varies across time and length scales. We discuss the microscopic processes that give rise to the macroscopic polarization of these materials, ranging from the optical and vibrational response to the transport of ions and electrons. The strong frequency dependence of the dielectric permittivity can be understood by separating the static dielectric constant into its constituents, including the orientational polarization due to rotating dipoles, which connects theory with experimental observations. The controversial issue of ferroelectricity is addressed, where we highlight recent progress in materials and domain characterization but emphasize the challenge associated with isolating spontaneous lattice polarization from other processes such as charged defect formation and transport. We conclude that CH3NH3PbI3 exhibits many features characteristic of a ferroelastic electret, where a spontaneous lattice strain is coupled to long-lived metastable polarization states.
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.
Gold-Parker A, Gehring PM, Skelton JM, et al., 2018, Acoustic phonon lifetimes limit thermal transport in methylammonium lead iodide, Proceedings of the National Academy of Sciences of the United States of America, Vol: 115, Pages: 11905-11910, ISSN: 0027-8424
Hybrid organic–inorganic perovskites (HOIPs) have become an important class of semiconductors for solar cells and other optoelectronic applications. Electron–phonon coupling plays a critical role in all optoelectronic devices, and although the lattice dynamics and phonon frequencies of HOIPs have been well studied, little attention has been given to phonon lifetimes. We report high-precision momentum-resolved measurements of acoustic phonon lifetimes in the hybrid perovskite methylammonium lead iodide (MAPI), using inelastic neutron spectroscopy to provide high-energy resolution and fully deuterated single crystals to reduce incoherent scattering from hydrogen. Our measurements reveal extremely short lifetimes on the order of picoseconds, corresponding to nanometer mean free paths and demonstrating that acoustic phonons are unable to dissipate heat efficiently. Lattice-dynamics calculations using ab initio third-order perturbation theory indicate that the short lifetimes stem from strong three-phonon interactions and a high density of low-energy optical phonon modes related to the degrees of freedom of the organic cation. Such short lifetimes have significant implications for electron–phonon coupling in MAPI and other HOIPs, with direct impacts on optoelectronic devices both in the cooling of hot carriers and in the transport and recombination of band edge carriers. These findings illustrate a fundamental difference between HOIPs and conventional photovoltaic semiconductors and demonstrate the importance of understanding lattice dynamics in the effort to develop metal halide perovskite optoelectronic devices.
Rice B, Guilbert AAY, Frost JM, et 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.
Gallop NP, Selig O, Giubertoni G, et 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.
Hopper T, Gorodetsky A, Frost JM, et 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.
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
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