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  • Journal article
    Warren PR, Hardigree JFM, Lauritzen AE, Nelson J, Riede Met al., 2019,

    Tuning the ambipolar behaviour of organic field effect transistors via band engineering

    , AIP ADVANCES, Vol: 9
  • Journal article
    Alberi K, Nardelli MB, Zakutayev A, Mitas L, Curtarolo S, Jain A, Fornari M, Marzari N, Takeuchi I, Green ML, Kanatzidis M, Toney MF, Butenko S, Meredig B, Lany S, Kattner U, Davydov A, Toberer ES, Stevanovic V, Walsh A, Park N-G, Aspuru-Guzik A, Tabor DP, Nelson J, Murphy J, Setlur A, Gregoire J, Li H, Xiao R, Ludwig A, Martin LW, Rappe AM, Wei S-H, Perkins Jet al., 2019,

    The 2019 materials by design roadmap

    , Journal of Physics D: Applied Physics, Vol: 52, ISSN: 0022-3727

    Advances in renewable and sustainable energy technologies critically depend on our ability to design and realize materials with optimal properties. Materials discovery and design efforts ideally involve close coupling between materials prediction, synthesis and characterization. The increased use of computational tools, the generation of materials databases, and advances in experimental methods have substantially accelerated these activities. It is therefore an opportune time to consider future prospects for materials by design approaches. The purpose of this Roadmap is to present an overview of the current state of computational materials prediction, synthesis and characterization approaches, materials design needs for various technologies, and future challenges and opportunities that must be addressed. The various perspectives cover topics on computational techniques, validation, materials databases, materials informatics, high-throughput combinatorial methods, advanced characterization approaches, and materials design issues in thermoelectrics, photovoltaics, solid state lighting, catalysts, batteries, metal alloys, complex oxides and transparent conducting materials. It is our hope that this Roadmap will guide researchers and funding agencies in identifying new prospects for materials design.

  • Journal article
    Nightingale J, Wade J, Moia D, Nelson J, Kim J-Set al., 2018,

    Impact of molecular order on polaron formation in conjugated polymers

    , The Journal of Physical Chemistry C, Vol: 122, Pages: 29129-29140, ISSN: 1932-7447

    The nature of polaron formation has profound implications on the transport of charge carriers in conjugated polymers, but still remains poorly understood. Here we develop in situ electrochemical resonant Raman spectroscopy, a powerful structural probe that allows direct observation of polaron formation. We report that polaron formation in ordered poly(3-hexyl)thiophene (P3HT) polymer domains (crystalline phase) results in less pronounced changes in molecular conformation, indicating smaller lattice relaxation, compared to polarons generated in disordered polymer domains (amorphous phase) for which we observe large molecular conformational changes. These conformational changes are directly related to the effective conjugation length of the polymer. Furthermore, we elucidate how blending the P3HT polymer with phenyl C-61 butyric acid methyl ester (PCBM) affects polaron formation in the polymer. We find that blending disturbs polymer crystallinity, reducing the density of polarons that can form upon charge injection at the same potential, whilst the lost capacity is partly restored during post-deposition thermal annealing. Our study provides direct spectroscopic evidence for a lower degree of lattice reorganisation in crystalline (and therefore more planarised) polymers than in conformationally disordered polymers. This observation is consistent with higher charge carrier mobility and better device performance commonly found in crystalline polymer materials.

  • Journal article
    Sachs M, Sprick RS, Pearce D, Hillman SAJ, Monti A, Guilbert AAY, Brownbill NJ, Dimitrov S, Shi X, Blanc F, Zwijnenburg MA, Nelson J, Durrant JR, Cooper AIet al., 2018,

    Understanding structure-activity relationships in linear polymer photocatalysts for hydrogen evolution

    , Nature Communications, Vol: 9, ISSN: 2041-1723

    Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. However, beyond basic considerations such as spectral absorption, the factors that dictate their photocatalytic activity are poorly understood. Here we investigate a series of linear conjugated polymers with external quantum efficiencies for hydrogen production between 0.4 and 11.6%. We monitor the generation of the photoactive species from femtoseconds to seconds after light absorption using transient spectroscopy and correlate their yield with the measured photocatalytic activity. Experiments coupled with modeling suggest that the localization of water around the polymer chain due to the incorporation of sulfone groups into an otherwise hydrophobic backbone is crucial for charge generation. Calculations of solution redox potentials and charge transfer free energies demonstrate that electron transfer from the sacrificial donor becomes thermodynamically favored as a result of the more polar local environment, leading to the production of long-lived electrons in these amphiphilic polymers.

  • 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
    Azzouzi M, Yan J, Kirchartz T, Liu K, Wang J, Wu H, Nelson Jet al., 2018,

    Non-radiative energy losses in bulk-heterojunction organic photovoltaics

    , Physical Review X, Vol: 8, ISSN: 2160-3308

    The performance of solar cells based on molecular electronic materials is limited by relatively high nonradiative voltage losses. The primary pathway for nonradiative recombination in organic donor-acceptor heterojunction devices is believed to be the decay of a charge-transfer (CT) excited state to the ground state via energy transfer to vibrational modes. Recently, nonradiative voltage losses have been related to properties of the charge-transfer state such as the Franck-Condon factor describing the overlap of the CT and ground-state vibrational states and, therefore, to the energy of the CT state. However, experimental data do not always follow the trends suggested by the simple model. Here, we extend this recombination model to include other factors that influence the nonradiative decay-rate constant, and therefore the open-circuit voltage, but have not yet been explored in detail. We use the extended model to understand the observed behavior of series of small molecules:fullerene blend devices, where open-circuit voltage appears insensitive to nonradiative loss. The trend could be explained only in terms of a microstructure-dependent CT-state oscillator strength, showing that parameters other than CT-state energy can control nonradiative recombination. We present design rules for improving open-circuit voltage via the control of material parameters and propose a realistic limit to the power-conversion efficiency of organic solar cells.

  • Journal article
    Rice B, LeBlanc LM, Otero-de-la-Roza A, Fuchter MJ, Johnson ER, Nelson J, Jelfs KEet al., 2018,

    A computational exploration of the crystal energy and charge-carrier mobility landscapes of the chiral [6]helicene molecule (vol 10, pg 1865, 2018)

    , NANOSCALE, Vol: 10, Pages: 9410-9410, ISSN: 2040-3364
  • Journal article
    Giovannitti A, Maria I, Hanifi D, Donahue M, Bryant D, Barth K, Makdah B, Savva A, Moia D, Zetek M, Barnes P, Reid O, Inal S, Rumbles G, Malliaras G, Nelson J, Rivnay J, McCulloch Iet al., 2018,

    The role of the side chain on the performance of n-type conjugated polymers in aqueous electrolytes

    , Chemistry of Materials, Vol: 30, Pages: 2945-2953, ISSN: 0897-4756

    We report a design strategy that allows the preparation of solution processable n type materials from low boiling point solvents for organic electrochemical transistors (OECTs). The polymer backbone is based on NDI-T2 copolymers where a branched alkyl side chain is gradually exchanged for a linear ethylene glycol based side chain. A series of random copolymers are prepared with glycol side chain percentages of 0, 10, 25, 50, 75, 90 and 100 with respect to the alkyl side chains. These are characterized in order to study the influence of the polar side chains on interaction with aqueous electrolytes, their electrochemical redox reactions and performance in OECTs when operated in aqueous electrolytes. We observe that glycol side chain percentages of >50 % are required to achieve volumetric charging while lower glycol chain percentages show a mixed operation with high required voltages to allow for bulk charging of the organic semiconductor. A strong dependence of the electron mobility on the fraction of glycol chains was found for copolymers based on NDI-T2, with a significant drop as alkyl side chains are replaced by glycol side chains.

  • Journal article
    Rohr J, Shi X, Haque S, Kirchartz T, Nelson Jet al., 2018,

    Charge transport in Spiro-OMeTAD investigated through space-charge-limited current measurements

    , Physical Review Applied, Vol: 9, ISSN: 2331-7019

    Extracting charge-carrier mobilities for organic semiconductors from space-charge-limited conduction measurements is complicated in practice by nonideal factors such as trapping in defects and injection barriers. Here, we show that by allowing the bandlike charge-carrier mobility, trap characteristics, injection barrier heights, and the shunt resistance to vary in a multiple-trapping drift-diffusion model, a numerical fit can be obtained to the entire current density–voltage curve from experimental space-charge-limited current measurements on both symmetric and asymmetric 2,2′,7,7′-tetrakis(N,N-di-4-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) single-carrier devices. This approach yields a bandlike mobility that is more than an order of magnitude higher than the effective mobility obtained using analytical approximations, such as the Mott-Gurney law and the moving-electrode equation. It is also shown that where these analytical approximations require a temperature-dependent effective mobility to achieve fits, the numerical model can yield a temperature-, electric-field-, and charge-carrier-density-independent mobility. Finally, we present an analytical model describing trap-limited current flow through a semiconductor in a symmetric single-carrier device. We compare the obtained charge-carrier mobility and trap characteristics from this analytical model to the results from the numerical model, showing excellent agreement. This work shows the importance of accounting for traps and injection barriers explicitly when analyzing current density–voltage curves from space-charge-limited current measurements.

  • Journal article
    Fei Z, Eisner FD, Jiao X, Azzouzi M, Rohr JA, Han Y, Shahid M, Chesman ASR, Easton CD, McNeill CR, Anthopoulos TD, Nelson J, Heeney Met al., 2018,

    Correction: An alkylated indacenodithieno[3,2-b] thiophene-based nonfullerene acceptor with high crystallinity exhibiting single junction solar cell efficiencies greater than 13% with low voltage losses (vol 30, 2018)

    , Advanced Materials, Vol: 30, ISSN: 0935-9648
  • Journal article
    Lee HKH, Telford AM, Rohr JA, Wyatt MF, Rice B, Wu J, Maciel ADC, Tuladhar SM, Speller E, McGettrick J, Searle JR, Pont S, Watson T, Kirchartz T, Durrant JR, Tsoi WC, Nelson J, Li Zet al., 2018,

    The role of fullerenes in the environmental stability of polymer: fullerene solar cells

    , Energy and Environmental Science, Vol: 11, Pages: 417-428, ISSN: 1754-5692

    Environmental stability is a common challenge for the commercialisation of low cost, encapsulation-free organic opto-electronic devices. Understanding the role of materials degradation is the key to address this challenge, but most such studies have been limited to conjugated polymers. Here we quantitatively study the role of the common fullerene derivative PCBM in limiting the stability of benchmark organic solar cells, showing that a minor fraction (<1%) of photo-oxidised PCBM, induced by short exposure to either solar or ambient laboratory lighting conditions in air, consistent with typical processing and operating conditions, is sufficient to compromise device performance severely. We identify the effects of photo-oxidation of PCBM on its chemical structure, and connect this to specific changes in its electronic structure, which significantly alter the electron transport and recombination kinetics. The effect of photo-oxidation on device current–voltage characteristics, electron mobility and density of states could all be explained with the same model of photoinduced defects acting as trap states. Our results demonstrate that the photochemical instability of PCBM and chemically similar fullerenes remains a barrier for the commercialisation of organic opto-electronic devices.

  • Journal article
    Rohr JA, Moia D, Haque SA, Kirchartz T, Nelson Jet al., 2018,

    Exploring the validity and limitations of the Mott-Gurney law for charge-carrier mobility determination of semiconducting thin-films

    , Journal of Physics: Condensed Matter, Vol: 30, ISSN: 0953-8984

    Using drift-diffusion simulations, we investigate the voltage dependence of the dark current in single carrier devices, typically used to determine charge-carrier mobilities. For both low and high voltages, the current increases linearly with the applied voltage. Whereas the linear current at low voltages is mainly due to space charge in the middle of the device, the linear current at high voltage is caused by charge-carrier saturation due to a high degree of injection. As a consequence, the current density at these voltages does not follow the classical square law derived by Mott and Gurney, and we show that for trap-free devices, only for intermediate voltages, a space-charge-limited drift current can be observed with a slope that approaches two. We show that, depending on the thickness of the semiconductor layer and the size of the injection barriers, the two linear current-voltage regimes can dominate the whole voltage range, and the intermediate Mott-Gurney regime can shrink or disappear. In this case, which will especially occur for thicknesses and injection barriers typical for single-carrier devices used to probe organic semiconductors, a meaningful analysis using the Mott-Gurney law will become unachievable, because a square-law fit can no longer be achieved, resulting in the mobility being substantially underestimated. General criteria for when to expect deviations from the Mott-Gurney law when used for analysis of intrinsic semiconductors are discussed.

  • Journal article
    Fei Z, Eisner FD, Jiao X, Azzouzi M, Röhr JA, Han Y, Shahid M, Chesman ASR, Easton CD, McNeill CR, Anthopoulos TD, Nelson J, Heeney M, Heeney MJ, Fei Z, Jiao X, Eisner F, Azzouzi M, Rohr J, Han Y, Shahid M, Chesman A, Easton C, McNeill C, Nelson J, Anthopoulos Tet al., 2018,

    An alkylated indacenodithieno[3,2-b]thiophene-based nonfullerene acceptor with high crystallinity exhibiting single junction solar cell efficiencies greater than 13% with low voltage losses

    , Advanced Materials, Vol: 30, Pages: 1-7, ISSN: 0935-9648

    A new synthetic route, to prepare an alkylated indacenodithieno[3,2-b]thiophene-based nonfullerene acceptor (C8-ITIC), is reported. Compared to the reported ITIC with phenylalkyl side chains, the new acceptor C8-ITIC exhibits a reduction in the optical band gap, higher absorptivity, and an increased propensity to crystallize. Accordingly, blends with the donor polymer PBDB-T exhibit a power conversion efficiency (PCE) up to 12.4%. Further improvements in efficiency are found upon backbone fluorination of the donor polymer to afford the novel material PFBDB-T. The resulting blend with C8-ITIC shows an impressive PCE up to 13.2% as a result of the higher open-circuit voltage. Electroluminescence studies demonstrate that backbone fluorination reduces the energy loss of the blends, with PFBDB-T/C8-ITIC-based cells exhibiting a small energy loss of 0.6 eV combined with a high JSCof 19.6 mA cm-2.

  • Journal article
    Rice B, LeBlanc LM, Otero-de-la-Roza A, Fuchter MJ, Johnson ER, Nelson J, Jelfs KEet al., 2018,

    A computational exploration of the crystal energy and charge-carrier mobility landscapes of the chiral [6]helicene molecule

    , Nanoscale, Vol: 10, Pages: 1865-1876, ISSN: 2040-3364

    The potential of a given π-conjugated organic molecule in an organic semiconductor device is highly dependent on molecular packing, as it strongly influences the charge-carrier mobility of the material. Such solid-state packing is sensitive to subtle differences in their intermolecular interactions and is challenging to predict. Chirality of the organic molecule adds an additional element of complexity to intuitive packing prediction. Here we use crystal structure prediction to explore the lattice-energy landscape of a potential chiral organic semiconductor, [6]helicene. We reproduce the experimentally observed enantiopure crystal structure and explain the absence of an experimentally observed racemate structure. By exploring how the hole and electron-mobility varies across the energy–structure–function landscape for [6]helicene, we find that an energetically favourable and frequently occurring packing motif is particularly promising for electron-mobility, with a highest calculated mobility of 2.9 cm2 V−1 s−1 (assuming a reorganization energy of 0.46 eV). We also calculate relatively high hole-mobility in some structures, with a highest calculated mobility of 2.0 cm2 V−1 s−1 found for chains of helicenes packed in a herringbone fashion. Neither the energetically favourable nor high charge-carrier mobility packing motifs are intuitively obvious, and this demonstrates the utility of our approach to computationally explore the energy–structure–function landscape for organic semiconductors. Our work demonstrates a route for the use of computational simulations to aid in the design of new molecules for organic electronics, through the a priori prediction of their likely solid-state form and properties.

  • Journal article
    Few SPM, Schmidt O, Offer GJ, Brandon N, Nelson J, Gambhir Aet al., 2018,

    Prospective improvements in cost and cycle life of off-grid lithium-ion battery packs: An analysis informed by expert elicitations

    , Energy Policy, Vol: 114, Pages: 578-590, ISSN: 0301-4215

    This paper presents probabilistic estimates of the 2020 and 2030 cost and cycle life of lithium-ion battery (LiB) packs for off-grid stationary electricity storage made by leading battery experts from academia and industry, and insights on the role of public research and development (R&D) funding and other drivers in determining these. By 2020, experts expect developments to arise chiefly through engineering, manufacturing and incremental chemistry changes, and expect additional R&D funding to have little impact on cost. By 2030, experts indicate that more fundamental chemistry changes are possible, particularly under higher R&D funding scenarios, but are not inevitable. Experts suggest that significant improvements in cycle life (eg. doubling or greater) are more achievable than in cost, particularly by 2020, and that R&D could play a greater role in driving these. Experts expressed some concern, but had relatively little knowledge, of the environmental impact of LiBs. Analysis is conducted of the implications of prospective LiB improvements for the competitiveness of solar photovoltaic + LiB systems for off-grid electrification.

  • Journal article
    Wadsworth A, Hamid Z, Bidwell M, Ashraf RS, Khan JI, Anjum DH, Cendra C, Yan J, Rezasoltani E, Guilbert AAY, Azzouzi M, Gasparini N, Bannock JH, Baran D, Wu H, de Mello JC, Brabec CJ, Salleo A, Nelson J, Laquai F, McCulloch Iet al., 2018,

    Progress in Poly (3-Hexylthiophene) Organic Solar Cells and the Influence of Its Molecular Weight on Device Performance

    , ADVANCED ENERGY MATERIALS, Vol: 8, ISSN: 1614-6832
  • Journal article
    Prashanthan K, Thivakarasarma T, Ravirajan P, Planells M, Robertson N, Nelson Jet al., 2017,

    Enhancement of hole mobility in hybrid titanium dioxide/poly(3-hexylthiophene) nanocomposites by employing an oligothiophene dye as an interface modifier

    , JOURNAL OF MATERIALS CHEMISTRY C, Vol: 5, Pages: 11758-11762, ISSN: 2050-7526
  • Journal article
    Schmidt O, Gambhir A, Staffell IL, Hawkes A, Nelson J, Few Set al., 2017,

    Future cost and performance of water electrolysis: An expert elicitation study

    , International Journal of Hydrogen Energy, Vol: 42, Pages: 30470-30492, ISSN: 0360-3199

    The need for energy storage to balance intermittent and inflexible electricity supply with demand is driving interest in conversion of renewable electricity via electrolysis into a storable gas. But, high capital cost and uncertainty regarding future cost and performance improvements are barriers to investment in water electrolysis. Expert elicitations can support decision-making when data are sparse and their future development uncertain. Therefore, this study presents expert views on future capital cost, lifetime and efficiency for three electrolysis technologies: alkaline (AEC), proton exchange membrane (PEMEC) and solid oxide electrolysis cell (SOEC). Experts estimate that increased R&D funding can reduce capital costs by 0–24%, while production scale-up alone has an impact of 17–30%. System lifetimes may converge at around 60,000–90,000 h and efficiency improvements will be negligible. In addition to innovations on the cell-level, experts highlight improved production methods to automate manufacturing and produce higher quality components. Research into SOECs with lower electrode polarisation resistance or zero-gap AECs could undermine the projected dominance of PEMEC systems. This study thereby reduces barriers to investment in water electrolysis and shows how expert elicitations can help guide near-term investment, policy and research efforts to support the development of electrolysis for low-carbon energy systems.

  • Journal article
    Miller TS, Suter TM, Telford AM, Picco L, Payton OD, Russell-Pavier F, Cullen PL, Sella A, Shaffer MSP, Nelson J, Tileli V, McMillan PF, Howard CAet al., 2017,

    Single crystal, luminescent carbon nitride nanosheets formed by spontaneous dissolution

    , Nano Letters, Vol: 17, Pages: 5891-5896, ISSN: 1530-6984

    A primary method for the production of 2D nanosheets is liquid-phase delamination from their 3D layered bulk analogues. Most strategies currently achieve this objective by significant mechanical energy input or chemical modification but these processes are detrimental to the structure and properties of the resulting 2D nanomaterials. Bulk poly(triazine imide) (PTI)-based carbon nitrides are layered materials with a high degree of crystalline order. Here, we demonstrate that these semiconductors are spontaneously soluble in select polar aprotic solvents, that is, without any chemical or physical intervention. In contrast to more aggressive exfoliation strategies, this thermodynamically driven dissolution process perfectly maintains the crystallographic form of the starting material, yielding solutions of defect-free, hexagonal 2D nanosheets with a well-defined size distribution. This pristine nanosheet structure results in narrow, excitation-wavelength-independent photoluminescence emission spectra. Furthermore, by controlling the aggregation state of the nanosheets, we demonstrate that the emission wavelengths can be tuned from narrow UV to broad-band white. This has potential applicability to a range of optoelectronic devices.

  • Journal article
    Nelson J, 2017,

    A map to find winners

    , NATURE MATERIALS, Vol: 16, Pages: 969-970, ISSN: 1476-1122
  • Conference paper
    Sandwell P, Ekins-Daukes N, Nelson J, 2017,

    What are the greatest opportunities for PV to contribute to rural development?

    , SNEC 11th International Photovoltaic Power Generation Conference and Exhibition (SNEC), Publisher: Elsevier Science BV, Pages: 139-146, ISSN: 1876-6102

    Minigrid systems powered by solar photovoltaics and battery storage are being deployed around the world to provide basic energy access and facilitate economic development. We use a minigrid simulation and optimisation tool that we have developed to assess various minigrid options in meeting the growing electricity demand of a community in rural Uttar Pradesh, India, in terms of the reliability of the service they provide, the cost of electricity, and total greenhouse gas emissions. We assess the breakeven distance at which off-grid minigrids are favourable in comparison to extending an unreliable grid network with a minigrid backup system, both with and without a carbon price. We suggest that policy recommendations that would encourage the use of minigrids for sustainable rural development, for example allowing subsidies to be available for system expansions and minimum service reliability requirements.

  • Journal article
    Guilbert AAY, Zbiri M, Dunbar ADF, Nelson Jet al., 2017,

    Quantitative Analysis of the Molecular Dynamics of P3HT:PCBM Bulk Heterojunction

    , Journal of Physical Chemistry B, Vol: 121, Pages: 9073-9080, ISSN: 1520-5207

    The optoelectronic properties of blends of conjugated polymers and small molecules are likely to be affected by the molecular dynamics of the active layer components. We study the dynamics of regioregular poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) blends using molecular dynamics (MD) simulation on time scales up to 50 ns and in a temperature range of 250–360 K. First, we compare the MD results with quasi-elastic neutron-scattering (QENS) measurements. Experiment and simulation give evidence of the vitrification of P3HT upon blending and the plasticization of PCBM by P3HT. Second, we reconstruct the QENS signal based on the independent simulations of the three phases constituting the complex microstructure of such blends. Finally, we found that P3HT chains tend to wrap around PCBM molecules in the amorphous mixture of P3HT and PCBM; this molecular interaction between P3HT and PCBM is likely to be responsible for the observed frustration of P3HT, the plasticization of PCBM, and the partial miscibility of P3HT and PCBM.

  • Journal article
    Rodriguez-Martinez X, Vezie MS, Shi X, McCulloch I, Nelson J, Goni AR, Campoy-Quiles Met al., 2017,

    Quantifying local thickness and composition in thin films of organic photovoltaic blends by Raman scattering

    , Journal of Materials Chemistry C, Vol: 5, Pages: 7270-7282, ISSN: 2050-7526

    We report a methodology based on Raman spectroscopy that enables the non-invasive and fast quantitative determination of local thickness and composition in thin films (from a few monolayers to hundreds of nm) of one or more components. We apply our methodology to blends of organic conjugated materials relevant in the field of organic photovoltaics. As a first step, we exploit the transfer-matrix formalism to describe the Raman process in thin films including reabsorption and interference effects of the incoming and scattered electric fields. This allows determining the effective solid-state Raman cross-section of each material by studying the dependence of the Raman intensity on film thickness. These effective cross sections are then used to estimate the local thickness and composition in a series of polymer:fullerene blends. We find that the model is accurate within ±10 nm in thickness and ±5 vol% in composition provided that (i) the film thickness is kept below the thickness corresponding to the first maximum of the calculated Raman intensity oscillation; (ii) the materials making up the blend show close enough effective Raman cross-sections; and (iii) the degree of order attained by the conjugated polymer in the blend is similar to that achieved when cast alone. Our methodology opens the possibility of making quantitative maps of composition and thickness over large areas (from microns to centimetres squared) with diffraction-limited resolution and in any multi-component system based thin film technology.

  • Journal article
    Yang Y, Rice B, Shi X, Brandt JR, Correa da Costa R, 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., 2017,

    Emergent Properties of an Organic Semiconductor Driven by its Molecular Chirality

    , ACS Nano, Vol: 11, Pages: 8329-8338, 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
    Speller EM, McGettrick JD, Rice B, Telford AM, Lee HKH, Tan C-H, De Castro CS, Davies ML, Watson TM, Nelson J, Durrant JR, Li Z, Tsoi WCet al., 2017,

    Impact of Aggregation on the Photochemistry of Fullerene Films: Correlating Stability to Triplet Exciton Kinetics

    , ACS APPLIED MATERIALS & INTERFACES, Vol: 9, Pages: 22739-22747, ISSN: 1944-8244
  • Journal article
    Few SPM, Chia C, Teo D, Kirkpatrick J, Nelson Jet al., 2017,

    The impact of chemical structure and molecular packing on the electronic polarisation of fullerene arrays

    , Physical Chemistry Chemical Physics, Vol: 19, Pages: 18709-18720, ISSN: 1463-9084

    Electronic polarisation contributes to the electronic landscape as seen by separating charges in organic materials. The nature of electronic polarisation depends on the polarisability, density, and arrangement of polarisable molecules. In this paper, we introduce a microscopic, coarse-grained model in which we treat each molecule as a polarisable site, and use an array of such polarisable dipoles to calculate the electric field and associated energy of any arrangement of charges in the medium. The model incorporates chemical structure via the molecular polarisability and molecular packing patterns via the structure of the array. We use this model to calculate energies of charge pairs undergoing separation in finite fullerene lattices of different chemical and crystal structures. The effective dielectric constants that we estimate from this approach are in good quantitative agreement with those measured experimentally in C60 and phenyl-C61-butyric acid methyl ester (PCBM) films, but we find significant differences in dielectric constant depending on packing and on direction of separation, which we rationalise in terms of density of polarisable fullerene cages in regions of high field. In general, we find lattices containing molecules of more isotropic polarisability tensors exhibit higher dielectric constants. By exploring several model systems we conclude that differences in molecular polarisability (and therefore, chemical structure) appear to be less important than differences in molecular packing and separation direction in determining the energetic landscape for charge separation. We note that the results are relevant for finite lattices, but not necessarily for infinite systems. We propose that the model could be used to design molecular systems for effective electronic screening.

  • Journal article
    Wheeler SGM, Bryant D, Troughton J, Kirchartz T, Watson T, Nelson J, Durrant Jet al., 2017,

    Transient optoelectronic analysis of the impact of material energetics and recombination kinetics on the open-circuit voltage of hybrid perovskite solar cells

    , Journal of Physical Chemistry C, Vol: 121, Pages: 13496-13506, ISSN: 1932-7455

    Transient optoelectronic measurements were used to evaluate the factors determining the open-circuit voltage of a series of planar photovoltaic devices based on hybrid perovskite layers with varying iodine/bromine ratios. Employing differential charging and transient photovoltage measurements, we used a simple device model based on the charge-carrier-density dependence of nongeminate recombination to re-create correctly not only the measured device open-circuit voltage (VOC) as a function of light intensity but also its dependence on bromine substitution. The 173 (±7) mV increase in device voltage observed with 20% bromine substitution is shown to result from a 227 (±8) mV increase in effective electronic band gap, which was offset in part by a 56 (±5) mV voltage loss due to faster carrier recombination. The faster recombination following 20% bromine substitution can be avoided by indene–C60 bisadduct (ICBA) substitution into the [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) electron-collection layer, resulting in a further 73 (±7) mV increase in device VOC. These results are consistent with surface recombination losses at the perovskite/fullerene interface being the primary limitation on the VOC output of bromine-substituted devices. This study thus presents, and experimentally validates, a simple model for the device physics underlying voltage generation in such perovskite-based solar cells and demonstrates that this approach can provide key insights into factors limiting this voltage output as a function of material energetics.

  • Journal article
    Steiner F, Poelking C, Niedzialek D, Andrienko D, Nelson Jet al., 2017,

    Influence of orientation mismatch on charge transport across grain boundaries in tri-isopropylsilylethynyl (TIPS) pentacene thin films

    , Physical Chemistry Chemical Physics, Vol: 19, Pages: 10854-10862, ISSN: 1463-9076

    We present a multi-scale model for charge transport across grain boundaries in molecular electronic materials that incorporates packing disorder, electrostatic and polarisation effects. We choose quasi two-dimensional films of tri-isopropylsilylethynyl pentacene (TIPS-P) as a model system representative of technologically relevant crystalline organic semiconductors. We use atomistic molecular dynamics, with a force-field specific for TIPS-P, to generate and equilibrate polycrystalline two-dimensional thin films. The energy landscape is obtained by calculating contributions from electrostatic interactions and polarization. The variation in these contributions leads to energetic barriers between grains. Subsequently, charge transport is simulated using a kinetic Monte-Carlo algorithm. Two-grain systems with varied mutual orientation are studied. We find relatively little effect of long grain boundaries due to the presence of low impedance pathways. However, effects could be more pronounced for systems with limited inter-grain contact areas. Furthermore, we present a lattice model to generalize the model for small molecular systems. In the general case, depending on molecular architecture and packing, grain boundaries can result in interfacial energy barriers, traps or a combination of both with qualitatively different effects on charge transport.

  • Journal article
    Baran D, Tuladhar S, Economopoulos SP, Neophytou M, Savva A, Itskos G, Othonos A, Bradley DDC, Brabec CJ, Nelson J, Choulis SAet al., 2017,

    Photovoltaic limitations of BODIPY:fullerene based bulk heterojunction solar cells

    , SYNTHETIC METALS, Vol: 226, Pages: 25-30, ISSN: 0379-6779
  • Journal article
    Hermerschmidt F, Savva A, Georgiou E, Tuladhar SM, Durrant JR, McCulloch I, Bradley DDC, Brabec CJ, Nelson J, Choulis SAet al., 2017,

    Influence of the Hole Transporting Layer on the Thermal Stability of Inverted Organic Photovoltaics Using Accelerated-Heat Lifetime Protocols

    , ACS APPLIED MATERIALS & INTERFACES, Vol: 9, Pages: 14136-14144, ISSN: 1944-8244

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