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  • 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
    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
    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
    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
    Rohr J, Nelson J, Kirchartz T, 2017,

    On the correct interpretation of the low voltage region in intrinsic single-carrier devices

    , Journal of Physics: Condensed Matter
  • 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
  • Journal article
    Kumar N, Zoladek-Lemanczyk A, Guilbert AAY, Su W, Tuladhar SM, Kirchartz T, Schroeder BC, McCulloch I, Nelson J, Roy D, Castro FAet al., 2017,

    Simultaneous topographical, electrical and optical microscopy of optoelectronic devices at the nanoscale

    , Nanoscale, Vol: 9, Pages: 2723-2731, ISSN: 2040-3364

    Novel optoelectronic devices rely on complex nanomaterial systems where the nanoscale morphology and local chemical composition are critical to performance. However, the lack of analytical techniques that can directly probe these structure–property relationships at the nanoscale presents a major obstacle to device development. In this work, we present a novel method for non-destructive, simultaneous mapping of the morphology, chemical composition and photoelectrical properties with <20 nm spatial resolution by combining plasmonic optical signal enhancement with electrical-mode scanning probe microscopy. We demonstrate that this combined approach offers subsurface sensitivity that can be exploited to provide molecular information with a nanoscale resolution in all three spatial dimensions. By applying the technique to an organic solar cell device, we show that the inferred surface and subsurface composition distribution correlates strongly with the local photocurrent generation and explains macroscopic device performance. For instance, the direct measurement of fullerene phase purity can distinguish between high purity aggregates that lead to poor performance and lower purity aggregates (fullerene intercalated with polymer) that result in strong photocurrent generation and collection. We show that the reliable determination of the structure–property relationship at the nanoscale can remove ambiguity from macroscopic device data and support the identification of the best routes for device optimisation. The multi-parameter measurement approach demonstrated herein is expected to play a significant role in guiding the rational design of nanomaterial-based optoelectronic devices, by opening a new realm of possibilities for advanced investigation via the combination of nanoscale optical spectroscopy with a whole range of scanning probe microscopy modes.

  • Journal article
    Calado P, Telford AM, Bryant D, Li X, Nelson J, Barnes PRFet al., 2016,

    Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis

    , Nature Communications, Vol: 7, ISSN: 2041-1723

    Ionic migration has been proposed as a possible cause of photovoltaic current-voltage hysteresis in hybrid perovskite solar cells. A major objection to this hypothesis is that hysteresis can be reduced by changing the interfacial contact materials; this is unlikely to significantly influence the behaviour of mobile ionic charge within the perovskite phase. Here we show that the primary effects of ionic migration can in fact be observed regardless of whether the contacts were changed to give devices with or without significant hysteresis. Transient optoelectronic measurements combined with device simulations indicate that electric-field screening, consistent with ionic migration, is similar in both high and low hysteresis CH3NH3PbI3 cells. Simulation of the photovoltage and photocurrent transients shows that hysteresis requires the combination of both mobile ionic charge and recombination near the perovskite-contact interfaces. Passivating contact recombination results in higher photogenerated charge concentrations at forward bias which screen the ionic charge, reducing hysteresis.

  • Journal article
    Barnes PRF, Vaissier V, Garcia Sakai V, Li X, cabral J, nelson Jet al., 2016,

    How mobile are dye adsorbates and acetonitrile molecules on the surface of TiO2 nanoparticles? A quasi-elastic neutron scattering study

    , Scientific Reports, Vol: 6, ISSN: 2045-2322

    Motions of molecules adsorbed to surfaces may control the rate of charge transport within monolayers in systems such as dye sensitized solar cells. We used quasi-elastic neutron scattering (QENS) to evaluate the possible dynamics of two small dye moieties, isonicotinic acid (INA) and bis-isonicotinic acid (BINA), attached to TiO2 nanoparticles via carboxylate groups. The scattering data indicate that moieties are immobile and do not rotate around the anchoring groups on timescales between around 10 ps and a few ns (corresponding to the instrumental range). This gives an upper limit for the rate at which conformational fluctuations can assist charge transport between anchored molecules. Our observations suggest that if the conformation of larger dye molecules varies with time, it does so on longer timescales and/or in parts of the molecule which are not directly connected to the anchoring group. The QENS measurements also indicate that several layers of acetonitrile solvent molecules are immobilized at the interface with the TiO2 on the measurement time scale, in reasonable agreement with recent classical molecular dynamics results.

  • Journal article
    Loheeswaran S, Thanihaichelvan M, Ravirajan P, Nelson Jet al., 2016,

    Controlling recombination kinetics of hybrid poly-3-hexylthiophene (P3HT)/titanium dioxide solar cells by self-assembled monolayers

    , JOURNAL OF MATERIALS SCIENCE-MATERIALS IN ELECTRONICS, Vol: 28, Pages: 4732-4737, ISSN: 0957-4522
  • Journal article
    Baran D, Ashraf RS, Hanifi DA, Abdelsamie M, Gasparini N, Röhr JA, Holliday S, Wadsworth A, Lockett S, Neophytou M, Emmott CJ, Nelson J, Brabec CJ, Amassian A, Salleo A, Kirchartz T, Durrant JR, McCulloch Iet al., 2016,

    Reducing the efficiency-stability-cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells.

    , Nature Materials, Vol: 16, Pages: 363-369, ISSN: 1476-4660

    Technological deployment of organic photovoltaic modules requires improvements in device light-conversion efficiency and stability while keeping material costs low. Here we demonstrate highly efficient and stable solar cells using a ternary approach, wherein two non-fullerene acceptors are combined with both a scalable and affordable donor polymer, poly(3-hexylthiophene) (P3HT), and a high-efficiency, low-bandgap polymer in a single-layer bulk-heterojunction device. The addition of a strongly absorbing small molecule acceptor into a P3HT-based non-fullerene blend increases the device efficiency up to 7.7 ± 0.1% without any solvent additives. The improvement is assigned to changes in microstructure that reduce charge recombination and increase the photovoltage, and to improved light harvesting across the visible region. The stability of P3HT-based devices in ambient conditions is also significantly improved relative to polymer:fullerene devices. Combined with a low-bandgap donor polymer (PBDTTT-EFT, also known as PCE10), the two mixed acceptors also lead to solar cells with 11.0 ± 0.4% efficiency and a high open-circuit voltage of 1.03 ± 0.01 V.

  • Journal article
    Fallon KJ, Wijeyasinghe N, Manley EF, Dimitrov SD, Yousaf SA, Ashraf RS, Duffy W, Guilbert AAY, Freeman DME, Al-Hashimi M, Nelson J, Durrant JR, Chen LX, McCulloch I, Marks TJ, Clarke TM, Anthopoulos TD, Bronstein Het al., 2016,

    Indolo-naphthyridine-6,13-dione Thiophene Building Block for Conjugated Polymer Electronics: Molecular Origin of Ultrahigh n-Type Mobility

    , CHEMISTRY OF MATERIALS, Vol: 28, Pages: 8366-8378, ISSN: 0897-4756
  • Journal article
    Sandwell P, Chambon C, Saraogi A, Chabenat A, Mazur M, Ekins-Daukes N, Nelson Jet al., 2016,

    Analysis of energy access and impact of modern energy sources in unelectrified villages in Uttar Pradesh

    , Energy for Sustainable Development, Vol: 35, Pages: 67-79, ISSN: 0973-0826

    Bringingaccesstomodernenergysourcestothepoorestinsocietyisakeygoalofmanypolicymakers,businessesandcharities,butinorder tobea success projects and schemesmust be foundedonaccuratedata. We undertooka survey of energy demand and usage patterns in households in unelectrified villages in Uttar Pradesh, India toassess access to and utilisation of energy sources for lighting and cooking. The times of usage were recordedand analysed and the effect on usage patterns of transitioning from traditional to modern energy sourcesis assessed. We quantify the cost and greenhouse gas emissions of current energy use in order to provide abenchmark of potential mitigation through the use of renewable energy technologies: a typical householdwith kerosene lamps only for lighting spends INR 3243 (US$50.67) and emits 381 kgCO2eqper year; householdswithmoderncookingenergyspend17%morethroughincreasedusage,butemit28%lessgreenhousegasescom-pared to those with traditional stoves only. Cell phone ownership was found to be 50% amongst adults. We usedemographic and utilisation data to construct an hourly demand profile of basic electricity demand extrapolatedto each month of the year, and present an example of aspirational demand assess the impact of desirable appli-ances. A Monte Carlo simulation is used to highlight the daily and seasonal variation in total energy and powerdemand. A hybrid system, with solar power and battery storage meeting daytime demand and higher-capacitydiesel- or biomass-powered generation meeting the remainder during evening peaks and winter months,would satisfy demand most effectively.

  • Journal article
    Moia D, Szumska A, Vaissier V, Planells M, Robertson N, O'Regan BC, Nelson J, Barnes PRet al., 2016,

    Interdye Hole Transport Accelerates Recombination in Dye Sensitized Mesoporous Films

    , Journal of the American Chemical Society, Vol: 138, Pages: 13197-13206, ISSN: 1520-5126

    Charge recombination between oxidized dyes attached to mesoporous TiO2 and electrons in the TiO2 was studied in inert electrolytes using transient absorption spectroscopy. Simultaneously, hole transport within the dye monolayers was monitored by transient absorption anisotropy. The rate of recombination decreased when hole transport was inhibited selectively, either by decreasing the dye surface coverage or by changing the electrolyte environment. From Monte Carlo simulations of electron and hole diffusion in a particle, modeled as a cubic structure, we identify the conditions under which hole lifetime depends on the hole diffusion coefficient for the case of normal (disorder free) diffusion. From simulations of transient absorption and transient absorption anisotropy, we find that the rate and the dispersive character of hole transport in the dye monolayer observed spectroscopically can be explained by incomplete coverage and disorder in the monolayer. We show that dispersive transport in the dye monolayer combined with inhomogeneity in the TiO2 surface reactivity can contribute to the observed stretched electron-hole recombination dynamics and electron density dependence of hole lifetimes. Our experimental and computational analysis of lateral processes at interfaces can be applied to investigate and optimize charge transport and recombination in solar energy conversion devices using electrodes functionalized with molecular light absorbers and catalysts.

  • Conference paper
    Ekins-Daukes NJ, Sandwell P, Nelson J, Johnson AD, Duggan G, Herniak Eet al., 2016,

    What does CPV need to achieve in order to succeed?

    , 12th International Conference on Concentrator Photovoltaic Systems (CPV), Publisher: American Institute of Physics, ISSN: 0094-243X

    The recent and dramatic reduction in flat-plate crystalline silicon (c-Si) technology has changed the competitive land-scape for concentrator PV (CPV) systems. Three system cost targets are considered, €1/Wp corresponding to the system cost of c-Si today, €0.75/Wp corresponding to the likely c-Si cost in 2020 and €0.5/Wp corresponding to a likely lower limit for c-Si in the long term. To compete successfully with c-Si, system efficiency needs to be raised from the present 30% to 40%, suggesting cell efficiencies of 50% and module efficiency of 44%. The module should be manufactured at an area cost below €275/m2 which implies a packaged cell cost of €3/cm2 and module + tracking cost €190/m2.

  • Journal article
    Vella E, Li H, Grégoire P, Tuladhar SM, Vezie MS, Few SPM, Bazán CM, Nelson J, Silva-Acuña C, Bittner ERet al., 2016,

    Ultrafast decoherence dynamics govern photocarrier generation efficiencies in polymer solar cells

    , Scientific Reports, Vol: 6, ISSN: 2045-2322

    All-organic-based photovoltaic solar cells have attracted considerable attention because of their lowcostprocessing and short energy payback time. In such systems the primary dissociation of an opticalexcitation into a pair of photocarriers has been recently shown to be extremely rapid and efficient,but the physical reason for this remains unclear. Here, two-dimensional photocurrent excitationspectroscopy, a novel non-linear optical spectroscopy, is used to probe the ultrafast coherent decayof photoexcitations into charge-producing states in a polymer:fullerene based solar cell. The twodimensionalphotocurrent spectra are interpreted by introducing a theoretical model for the descriptionof the coupling of the electronic states of the system to an external environment and to the appliedlaser fields. The experimental data show no cross-peaks in the twodimensional photocurrent spectra, aspredicted by the model for coherence times between the exciton and the photocurrent producing statesof 20fs or less.

  • Journal article
    Sandwell P, Duggan G, Nelson J, Ekins-Daukes Net al., 2016,

    The environmental impact of lightweight HCPV modules: efficient design and effective deployment

    , Progress in Photovoltaics, Vol: 24, Pages: 1458-1472, ISSN: 1099-159X

    We present a life cycle analysis of a lightweight design of high concentration photovoltaic module. The materials and processes used in construction are considered to assess the total environmental impact of the module construction in terms of the cumulative energy demand and embodied greenhouse gas emissions, which were found to be 355.3MJ and 27.9 kgCO2eq respectively. We consider six potential deployment locations and the system energy payback times are calculated to be 0.22–0.33 years whilst the greenhouse gas payback times are 0.29–0.88 years. The emission intensities over the life- times of the systems are found to be 6.5–9.8 g CO2eq/kWh, lower than those of other HCPV, PV and CSP technologies in similar locations.

  • Journal article
    Leguy AM, Goñi AR, Frost JM, Skelton J, Brivio F, Rodríguez-Martínez X, Weber OJ, Pallipurath A, Alonso MI, Campoy-Quiles M, Weller MT, Nelson J, Walsh A, Barnes PRet al., 2016,

    Dynamic disorder, phonon lifetimes, and the assignment of modes to the vibrational spectra of methylammonium lead halide perovskites

    , Physical Chemistry Chemical Physics, Vol: 18, Pages: 27051-27066, ISSN: 1463-9084

    We present Raman and terahertz absorbance spectra of methylammonium lead halide single crystals (MAPbX3, X = I, Br, Cl) at temperatures between 80 and 370 K. These results show good agreement with density-functional-theory phonon calculations. Comparison of experimental spectra and calculated vibrational modes enables confident assignment of most of the vibrational features between 50 and 3500 cm(-1). Reorientation of the methylammonium cations, unlocked in their cavities at the orthorhombic-to-tetragonal phase transition, plays a key role in shaping the vibrational spectra of the different compounds. Calculations show that these dynamic effects split Raman peaks and create more structure than predicted from the independent harmonic modes. This explains the presence of extra peaks in the experimental spectra that have been a source of confusion in earlier studies. We discuss singular features, in particular the torsional vibration of the C-N axis, which is the only molecular mode that is strongly influenced by the size of the lattice. From analysis of the spectral linewidths, we find that MAPbI3 shows exceptionally short phonon lifetimes, which can be linked to low lattice thermal conductivity. We show that optical rather than acoustic phonon scattering is likely to prevail at room temperature in these materials.

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