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  • Journal article
    Few S, Djapic P, Strbac G, Nelson J, Candelise Cet al., 2020,

    Assessing local costs and impacts of distributed solar PV using high resolution data from across Great Britain

    , Renewable Energy, Vol: 162, Pages: 1140-1150, ISSN: 0960-1481

    Highly spatially resolved data from across Great Britain (GB) are combined with a distribution network modelling tool to assess impacts of distributed photovoltaic (PV) deployment up to 2050 on local networks, the costs of avoiding these impacts, and how these depend upon context. Present-day deployment of distributed PV, meter density, and network infrastructure across GB are found to be highly dependent on rurality, and data on these are used to build up three representative contexts: cities, towns, and villages. For each context, distribution networks are simulated, and impacts on these networks associated with PV deployment and growth in peak load up to 2050 calculated. Present-day higher levels of PV deployment in rural areas are maintained in future scenarios, necessitating upgrades in ambitious PV scenarios in towns and villages from around 2040, but not before 2050 in cities. Impacts of load growth are more severe than those of PV deployment, potentially necessitating upgrades in cities, towns, and villages from 2030. These are most extensive in cities and towns, where long feeders connect more customers, making networks particularly susceptible to impacts. Storage and demand side response are effective in reducing upgrade costs, particularly in cities and towns.

  • Journal article
    Lubert-Perquel D, Szumska AA, Azzouzi M, Salvadori E, Ruloff S, Kay CMW, Nelson J, Heutz Set al., 2020,

    Structure dependence of kinetic and thermodynamic parameters in singlet fission processes.

    , Journal of Physical Chemistry Letters, Vol: 11, Pages: 9557-9565, ISSN: 1948-7185

    Singlet fission-whereby one absorbed photon generates two coupled triplet excitons-is a key process for increasing the efficiency of optoelectronic devices by overcoming the Shockley-Queisser limit. A crucial parameter is the rate of dissociation of the coupled triplets, as this limits the number of free triplets subsequently available for harvesting and ultimately the overall efficiency of the device. Here we present an analysis of the thermodynamic and kinetic parameters for this process in parallel and herringbone dimers measured by electron paramagnetic resonance spectroscopy in coevaporated films of pentacene in p-terphenyl. The rate of dissociation is higher for parallel dimers than for their herringbone counterparts, as is the rate of recombination to the ground state. DFT calculations, which provide the magnitude of the electronic coupling as well as the distribution of molecular orbitals for each geometry, suggest that weaker triplet coupling in the parallel dimer is the driving force for faster dissociation. Conversely, localization of the molecular orbitals and a stronger triplet-triplet interaction result in slower dissociation and recombination. The identification and understanding of how the intermolecular geometry promotes efficient triplet dissociation provide the basis for control of triplet coupling and thereby the optimization of one important parameter of device performance.

  • Journal article
    He Q, Eisner FD, Pearce D, Hodsden T, Rezasoltani E, Medranda D, Fei Z, Nelson J, Heeney Met al., 2020,

    Ring fusion in tetrathienylethene cored perylene diimide tetramers affords acceptors with strong and broad absorption in the near-UV to visible region

    , Journal of Materials Chemistry C, Vol: 8, Pages: 17237-17244, ISSN: 2050-7526

    In this work, we designed and synthesized two novel perylene diimide (PDI) tetramers based on a tetrathienylethene core, named TTE-PDI4 and FTTE-PDI4, and investigated their application as non-fullerene acceptors for organic photovoltaics. The free rotation of PDIs and adjacent thiophene units renders TTE-PDI4 with a highly twisted molecular geometry. The ring fusion of TTE-PDI4 yields FTTE-PDI4, a more rigid molecule with increased intramolecular stacking. Interestingly, TTE-PDI4 and FTTE-PDI4 possess similar energy levels but very different UV-Vis absorptions, with the latter showing strong broad-band absorption with multiple sharp peaks in the 300–600 nm region. Through time-dependent density functional theory (TD-DFT) calculations, we show that this broad absorption spectrum in FTTE-PDI4 arises from the combination of multiple bright transitions in the visible region with a strong vibronic progression, tentatively assigned to the dominant C[double bond, length as m-dash]C stretching mode. TTE-PDI4, despite having a lower energy absorption onset, shows weaker absorption at long wavelengths. Due to its higher absorption as well as its increased rigidity, FTTE-PDI4 shows a higher photocurrent and hence a higher power conversion efficiency (PCE), of 6.6%, when blended with the polymer donor PFBDB-T than TTE-PDI4 based blends (PCE of 3.8%). The greater rigidity of FTTE-PDI4 is likely to contribute to the good fill factor of the blend devices. Potential for further improvement through reducing voltage losses is identified.

  • Journal article
    Cong S, Creamer A, Fei Z, Hillman SAJ, Rapley C, Nelson J, Heeney Met al., 2020,

    Tunable control of the hydrophilicity and wettability of conjugated polymers by a postpolymerization modification approach.

    , Macromolecular Bioscience, Vol: 20, Pages: 1-8, ISSN: 1616-5187

    A facile method to prepare hydrophilic polymers by a postpolymerization nucleophillic aromatic substitution reaction of fluoride on an emissive conjugated polymer (CP) backbone is reported. Quantitative functionalization by a series of monofunctionalized ethylene glycol oligomers, from dimer to hexamer, as well as with high molecular weight polyethylene glycol is demonstrated. The length of the ethylene glycol sidechains is shown to have a direct impact on the surface wettability of the polymer, as well as its solubility in polar solvents. However, the energetics and band gap of the CPs remain essentially constant. This method therefore allows an easy way to modulate the wettability and solubility of CP materials for a diverse series of applications.

  • Journal article
    Rezasoltani E, Guilbert AAY, Yan J, Rodríguez-Martínez X, Azzouzi M, Eisner F, Tuladhar SM, Hamid Z, Wadsworth A, McCulloch I, Campoy-Quiles M, Nelson Jet al., 2020,

    Correlating the phase behavior with the device performance in binary poly-3-hexylthiophene: nonfullerene acceptor blend using optical probes of the microstructure

    , Chemistry of Materials, Vol: 32, Pages: 8294-8305, ISSN: 0897-4756

    The performance of photovoltaic devices based on blends of conjugated polymers with nonfullerene acceptors depends on the phase behavior and microstructure of the binary, which in turn depends on the chemical structures of the molecular components and the blend composition. We investigate the correlation between the molecular structure, composition, phase behavior, and device performance of a model system consisting of semicrystalline poly-3-hexylthiophene (P3HT) as the donor polymer and three nonfullerene acceptors, two of which (O-IDTBR/EH-IDTBR) have a planar core with different side chains and one (O-IDFBR) of which has a twisted core. We combine differential scanning calorimetry with optical measurements including UV–Vis spectroscopy, photoluminescence, spectroscopic ellipsometry, and Raman spectroscopy and photovoltaic device performance measurements, all at varying blend composition. For P3HT:IDTBR blends, the crystallinity of polymer and acceptor is preserved over a wide composition range and the blend displays a eutectic phase behavior, with the optimum solar cell composition lying close to the eutectic composition. For P3HT:IDFBR blends, increasing acceptor content disrupts the polymer crystallinity, and the optimum device composition appears to be limited by polymer connectivity rather than being linked to the eutectic composition. The optical probes allow us to probe both the crystalline and amorphous phases, clearly revealing the compositions at which component mixing disrupts crystallinity.

  • Journal article
    Babacan O, De Causmaecker S, Gambhir A, Fajardy M, Rutherford AW, Fantuzzi A, Nelson Jet al., 2020,

    Assessing the feasibility of carbon dioxide mitigation options in terms of energy usage

    , Nature Energy, Vol: 5, Pages: 720-728, ISSN: 2058-7546

    Measures to mitigate the emissions of carbon dioxide (CO2) can vary substantially in terms of the energy required. Some proposed CO2 mitigation options involve energy-intensive processes that compromise their viability as routes to mitigation, especially if deployed at a global scale. Here we provide an assessment of different mitigation options in terms of their energy usage. We assess the relative effectiveness of several CO2 mitigation routes by calculating the energy cost of carbon abatement (kilowatt-hour spent per kilogram CO2-equivalent, or kWh kgCO2e–1) mitigated. We consider energy efficiency measures, decarbonizing electricity, heat, chemicals and fuels, and also capturing CO2 from air. Among the routes considered, switching to renewable energy technologies (0.05–0.53 kWh kgCO2e–1 mitigated) offer more energy-effective mitigation than carbon embedding or carbon removal approaches, which are more energy intensive (0.99–10.03 kWh kgCO2e–1 and 0.78–2.93 kWh kgCO2e–1 mitigated, respectively), whereas energy efficiency measures, such as improving building lighting, can offer the most energy-effective mitigation.

  • Journal article
    Xiao B, Calado P, Mackenzie R, Kirchartz T, Yan J, Nelson Jet al., 2020,

    Relationship between fill factor and light intensity in solar cells based on organic disordered semiconductors: The role of tail states

    , Physical Review Applied, Vol: 14, Pages: 024034 – 1-024034 – 17, ISSN: 2331-7019

    The origin of the relationship between fill factor (FF) and light intensity (I) in organic disordered-semiconductor-based solar cells is studied. An analytical model describing the balance between transport and recombination of charge carriers, parameterized with a factor, Γm, is introduced to understand the FF-I relation, where higher values of Γm correlate to larger FFs. Comparing the effects of direct and tail-state-mediated recombination on the FF-I plot, we find that, for low-mobility systems, direct recombination with constant transport mobility can deliver only a negative dependence of Γm,dir on light intensity. By contrast, tail-state-mediated recombination with trapping and detrapping processes can produce a positive Γm,t versus sun dependency. The analytical model is validated by numerical drift-diffusion simulations. To further validate our model, two material systems that show opposite FF-I behavior are studied: poly{4,8-bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-[4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl]} (PTB7-Th):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) devices show a negative FF-I relation, while PTB7-Th:(5Z,5′Z)-5,5′-{[7,7′ -(4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl)]bis(methanylylidene)}bis(3-ethyl-2-thioxothiazolidin-4-one) (O-IDTBR) devices show a positive correlation. Optoelectronic measurements show that the O-IDTBR device presents a higher ideality factor, stronger trapping and detrapping behavior, and a higher density of trap states, relative to the PC71BM device, supporting the theoretical model. This work provides a comprehensive understanding of the correlation between FF and light intensity for disordered-semiconductor-based solar cells.

  • Journal article
    Woods DJ, Hillman S, Pearce D, Wilbraham L, Flagg L, Duffy W, Mcculloch I, Durrant J, Guilbert A, Zwijnenburg M, Sprick RS, Nelson J, Cooper Aet al., 2020,

    Side-chain tuning in conjugated polymer photocatalysts for improved hydrogen production from water

    , Energy & Environmental Science, Vol: 13, Pages: 1843-1855, ISSN: 1754-5692

    Structure–property–activity relationships in solution processable polymer photocatalysts for hydrogen production from water were probed by varying the chemical structure of both the polymer side-chains and the polymer backbone. In both cases, the photocatalytic performance depends strongly on the inclusion of more polar groups, such as dibenzo[b,d]thiophene sulfone backbone units or oligo(ethylene glycol) side-chains. We used optical, spectroscopic, and structural characterisation techniques to understand the different catalytic activities of these systems. We find that although polar groups improve the wettability of the material with water in all cases, backbone and side-chain modifications affect photocatalytic performance in different ways: the inclusion of dibenzo[b,d]thiophene sulfone backbone units improves the thermodynamic driving force for hole transfer to the sacrificial donor, while the inclusion of oligo ethylene glycol side-chains aids the degree of polymer swelling and also extends the electron polaron lifetime. The best performing material, FS-TEG, exhibits a HER of 72.5 μmol h−1 for 25 mg photocatalyst (2.9 mmol g−1 h−1) when dispersed in the presence of a sacrificial donor and illuminated with λ > 420 nm light, corresponding to a hydrogen evolution EQE of 10% at 420 nm. When cast as a thin film, this HER was further boosted to 13.9 mmol g−1 h−1 (3.0 mmol m−2 h−1), which is among the highest rates in this field.

  • Journal article
    Azzouzi M, Calado P, Telford A, Eisner F, Hou X, Kirchartz T, Barnes P, Nelson Jet al., 2020,

    Overcoming the limitations of transient photovoltage measurements for studying recombination in organic solar cells

    , Solar Rrl, Vol: 4, ISSN: 2367-198X

    Transient photovoltage (TPV) measurements are frequently used to study recombination processes in thin-film solar cells by probing the decay of a small optically-induced voltage perturbation to infer the charge carrier dynamics of devices at open circuit. However, the validity of this method to probe organic semiconductors has recently come into doubt due to large discrepancies in reported carrier lifetime values for the same systems, and the reporting of unrealistic reaction order values. In this work, we explore the validity of TPV to extract reliable charge carrier lifetimes in thin-film solar cells through the use of time-dependent drift diffusion simulations and measurements. We find that in low mobility materials, TPV serves primarily as a probe of charge carrier redistribution in the bulk rather than bulk recombination dynamics, and that the extracted time constant is highly mobility dependent. To address this shortcoming, we develop Transient Photo-Charge (TPQ) a new technique to measure the charge carrier density during the photo-voltage decay and apply it to study the recombination dynamics in a series of (fullerene and non-fullerene) organic solar cell systems. We show that using this technique the charge carrier recombination lifetime in the active layer can be more accurately determined.

  • Journal article
    Kaienburg P, Krückemeier L, Lübke D, Nelson J, Rau U, Kirchartz Tet al., 2020,

    How solar cell efficiency is governed by the αμτ product

    , Physical Review Research, Vol: 2, Pages: 023109 – 1-023109 – 15, ISSN: 2643-1564

    The interplay of light absorption, charge-carrier transport, and charge-carrier recombination determines the performance of a photovoltaic absorber material. Here we analyze the influence on the solar-cell efficiency of the absorber material properties absorption coefficient α, charge-carrier mobility μ, and charge-carrier lifetime τ, for different scenarios. We combine analytical calculations with numerical drift-diffusion simulations to understand the relative importance of these three quantities. Whenever charge collection is a limiting factor, the αμτ product is a good figure of merit (FOM) to predict solar-cell efficiency, while for sufficiently high mobilities, the relevant FOM is reduced to the ατ product. We find no fundamental difference between simulations based on monomolecular or bimolecular recombination, but strong surface-recombination affects the maximum efficiency in the high-mobility limit. In the limiting case of high μ and high surface-recombination velocity S, the α/S ratio is the relevant FOM. Subsequently, we apply our findings to organic solar cells which tend to suffer from inefficient charge-carrier collection and whose absorptivity is influenced by interference effects. We estimate that a modest increase in absorption strength by a factor of 1.5 leads to a relative efficiency increase of more than 10% for state-of-the-art organic solar cells.

  • Journal article
    Giovannitti A, Rashid RB, Thiburce Q, Paulsen BD, Cendra C, Thorley K, Moia D, Mefford JT, Hanifi D, Weiyuan D, Moser M, Salleo A, Nelson J, McCulloch I, Rivnay Jet al., 2020,

    Energetic control of redox-active polymers toward safe organic Bioelectronic materials

    , Advanced Materials, Vol: 32, ISSN: 0935-9648

    Avoiding faradaic side reactions during the operation of electrochemical devices is important to enhance the device stability, to achieve low power consumption, and to prevent the formation of reactive side‐products. This is particularly important for bioelectronic devices, which are designed to operate in biological systems. While redox‐active materials based on conducting and semiconducting polymers represent an exciting class of materials for bioelectronic devices, they are susceptible to electrochemical side‐reactions with molecular oxygen during device operation. Here, electrochemical side reactions with molecular oxygen are shown to occur during organic electrochemical transistor (OECT) operation using high‐performance, state‐of‐the‐art OECT materials. Depending on the choice of the active material, such reactions yield hydrogen peroxide (H2O2), a reactive side‐product, which may be harmful to the local biological environment and may also accelerate device degradation. A design strategy is reported for the development of redox‐active organic semiconductors based on donor–acceptor copolymers that prevents the formation of H2O2 during device operation. This study elucidates the previously overlooked side‐reactions between redox‐active conjugated polymers and molecular oxygen in electrochemical devices for bioelectronics, which is critical for the operation of electrolyte‐gated devices in application‐relevant environments.

  • Journal article
    Moia D, Abe M, Wagner P, Saguchi H, Koumura N, Nelson J, Barnes PRF, Mori Set al., 2020,

    The effect of the dielectric environment on electron transfer reactions at the interfaces of molecular sensitized semiconductors in electrolytes

    , The Journal of Physical Chemistry C: Energy Conversion and Storage, Optical and Electronic Devices, Interfaces, Nanomaterials, and Hard Matter, Vol: 124, Pages: 6979-6992, ISSN: 1932-7447

    Electron transfer theories predict that rates of charge transfer vary with the dielectric properties of the environment where the reaction occurs. An appropriate description of this relation for molecular sensitized semiconductors in electrolytes must account for the restricted geometry of these systems compared to “free” molecules in solution. Here, we explore the extent to which dielectric properties of the surrounding medium can explain the rates of charge transfer processes, measured using transient absorption spectroscopy, involving photo-oxidized thiophene–carbazole-based molecules on oxide semiconductors in inert or redox-active electrolytes. We observe no clear correlation between the activation energy of hole hopping between molecules on oxide surfaces or the recombination rate between photogenerated electrons in the oxide and holes on the adsorbed molecules and the dielectric properties of the surrounding solvent. The activation energy of hole hopping tends to increase with time following initial photogeneration of the holes, which we attribute to energetic disorder in the molecular monolayer. The recombination rate in different solvents scales with the hole hopping rate. It can also be varied by adding inert salts in the electrolyte and by controlling the access of cations in solution to the oxide surface. Finally, we show that fast electron transfer from cobalt complexes to photo-oxidized molecules in solvents with low polarity is verified, but the kinetics are limited by the ionic dissociation. Our study highlights the importance of electronic coupling between the redox-active components and their solvation, besides the reorganization energy and the driving force, in the determination of electron transfer rates at molecular sensitized interfaces in electrolytes.

  • Journal article
    Hamid Z, Wadsworth A, Rezasoltani E, Holliday S, Azzouzi M, Neophytou M, Guilbert AAY, Dong Y, Little MS, Mukherjee S, Herzing AA, Bristow H, Kline RJ, DeLongchamp DM, Bakulin AA, Durrant JR, Nelson J, McCulloch Iet al., 2020,

    Influence of Polymer Aggregation and Liquid Immiscibility on Morphology Tuning by Varying Composition in PffBT4T-2DT/Nonfullerene Organic Solar Cells

    , ADVANCED ENERGY MATERIALS, Vol: 10, ISSN: 1614-6832
  • Journal article
    Guilbert AAY, Zbiri M, Finn PA, Jenart M, Fouquet P, Cristiglio V, Frick B, Nelson J, Nielsen CBet al., 2019,

    Mapping Microstructural Dynamics up to the Nanosecond of the Conjugated Polymer P3HT in the Solid State

    , CHEMISTRY OF MATERIALS, Vol: 31, Pages: 9635-9651, ISSN: 0897-4756
  • Journal article
    Warren R, Privitera A, Kaienburg P, Lauritzen AE, Thimm O, Nelson J, Riede MKet al., 2019,

    Controlling energy levels and Fermi level en route to fully tailored energetics in organic semiconductors

    , NATURE COMMUNICATIONS, Vol: 10, ISSN: 2041-1723
  • Journal article
    Szumska AA, Sirringhaus H, Nelson J, 2019,

    Symmetry based molecular design for triplet excitation and optical spin injection

    , Physical Chemistry Chemical Physics, Vol: 21, Pages: 19521-19528, ISSN: 1463-9076

    Spintronics, as a relatively new scientific field, is developing rapidly together with our understanding of spin related phenomena and spin manipulation. One of the challenges in the field is spin injection, which has been achieved optically in inorganic crystalline semiconductors, but not yet in organic semiconductors. Here, we introduce an approach whereby we apply group theory and computational methods to design molecular materials in which spin can be injected optically via circularly polarized light (CPL). Our approach is based on the use of group theory and double group theory to identify families of molecules whose symmetry satisfies design rules for optical excitation of triplets of particular properties. Employing such screening prior to detailed calculation can accelerate design by first identifying any structures that fail some criterion on grounds of symmetry. Here, we show using group theory and computational methods that particular families of molecules possess a low lying triplet state that can be excited with circularly polarized light causing spin polarization of an excited electron. Such structures are of potential interest for organic or molecular spintronics. We present an efficient procedure to identify candidate point groups and determine the excited state symmetry using group theory, before full calculation of excited states using relativistic quantum chemistry.

  • Journal article
    Vezie MS, Azzouzi M, Telford AM, Hopper TR, Sieval AB, Hummelen JC, Fallon K, Bronstein H, Kirchartz T, Bakulin AA, Clarke TM, Nelson Jet al., 2019,

    Impact of marginal exciton–charge-transfer state offset on charge generation and recombination in polymer:fullerene solar cells

    , ACS Energy Letters, Vol: 4, Pages: 2096-2103, ISSN: 2380-8195

    The energetic offset between the initial photoexcited state and charge-transfer (CT) state in organic heterojunction solar cells influences both charge generation and open-circuit voltage (Voc). Here, we use time-resolved spectroscopy and voltage loss measurements to analyze the effect of the exciton–CT state offset on charge transfer, separation, and recombination processes in blends of a low-band-gap polymer (INDT-S) with fullerene derivatives of different electron affinity (PCBM and KL). For the lower exciton–CT state offset blend (INDT-S:PCBM), both photocurrent generation and nonradiative voltage losses are lower. The INDT-S:PCBM blend shows different excited-state dynamics depending on whether the donor or acceptor is photoexcited. Surprisingly, the charge recombination dynamics in INDT-S:PCBM are distinctly faster than those in INDT-S:KL upon excitation of the donor. We reconcile these observations using a kinetic model and by considering hybridization between the lowest excitonic and CT states. The modeling results show that this hybridization can significantly reduce Voc losses while still allowing reasonable charge generation efficiency.

  • Journal article
    Cheetham NJ, Ortiz M, Pereyedentsev A, Dion-Bertrand L-I, Greetham GM, Sazanoyich I, Towrie M, Parker AW, Nelson J, Silva C, Bradley DDC, Hayes SC, Stavrinou PNet al., 2019,

    The Importance of Microstructure in Determining Polaron Generation Yield in Poly(9,9-dioctylfluorene)

    , CHEMISTRY OF MATERIALS, Vol: 31, Pages: 6787-6797, ISSN: 0897-4756
  • Journal article
    Yang W, Godin R, Kasap H, Moss B, Dong Y, Hillman SAJ, Steier L, Reisner E, Durrant JRet al., 2019,

    Electron accumulation induces efficiency bottleneck for hydrogen production in carbon nitride photocatalysts

    , Journal of the American Chemical Society, Vol: 141, Pages: 11219-11229, ISSN: 1520-5126

    This study addresses the light intensity dependence of charge accumulation in a photocatalyst suspension, and its impact on both charge recombination kinetics and steady-state H2 evolution efficiency. Cyanamide surface functionalized melon-type carbon nitride (NCNCNx) has been selected as an example of emerging carbon nitrides photocatalysts because of its excellent charge storage ability. Transient spectroscopic studies (from ps to s) show that the bimolecular recombination of photogenerated electrons and holes in NCNCNx can be well described by a random walk model. Remarkably, the addition of hole scavengers such as 4-methylbenzyl alcohol can lead to ∼400-fold faster recombination kinetics (lifetime shortening to ∼10 ps). We show that this acceleration is not the direct result of ultrafast hole extraction by the scavenger, but is rather caused by long-lived electron accumulation in NCNCNx after hole extraction. The dispersive pseudo-first order recombination kinetics become controlled by the density of accumulated electrons. H2 production and steady-state spectroscopic measurements indicate that the accelerated recombination caused by electron accumulation limits the H2 generation efficiency. The addition of a reversible electron acceptor and mediator, methyl viologen (MV2+), accelerates the extraction of electrons from the NCNCNx and increases the H2 production efficiency under one sun irradiation by more than 30%. These results demonstrate quantitatively that while long-lived electrons are essential to drive photoinduced H2 generation in many photocatalysts, excessive electron accumulation may result in accelerated recombination losses and lower performance, and thus highlight the importance of efficient electron and hole extraction in enabling efficient water splitting photocatalysts.

  • Journal article
    Islam MS, Bruce PG, Catlow CRA, Nelson Jet al., 2019,

    Energy materials for a low carbon future

  • Journal article
    Azzouzi M, Cabas-Vidani A, Haass SG, Rohr JA, Romanyuk YE, Tiwari AN, Nelson Jet al., 2019,

    Analysis of the voltage losses in CZTSSe solar cells of varying Sn content

    , Journal of Physical Chemistry Letters, Vol: 10, Pages: 2829-2835, ISSN: 1948-7185

    The performance of kesterite (Cu2ZnSn(S,Se)4, CZTSSe) solar cells is hindered by low open circuit voltage (Voc). The commonly used metric for Voc-deficit, namely, the difference between the absorber band gap and qVoc, is not well-defined for compositionally complex absorbers like kesterite where the bandgap is hard to determine. Here, nonradiative voltage losses are analyzed by measuring the radiative limit of Voc, using external quantum efficiency (EQE) and electroluminescence (EL) spectra, without relying on precise knowledge of the bandgap. The method is applied to a series of Cu2ZnSn(S,Se)4 devices with Sn content variation from 27.6 to 32.9 at. % and a corresponding Voc range from 423 to 465 mV. Surprisingly, the lowest nonradiative loss, and hence the highest external luminescence efficiency (QELED), were obtained for the device with the lowest Voc. The trend is assigned to better interface quality between absorber and CdS buffer layer at lower Sn content.

  • Journal article
    Shi X, Nádaždy V, Perevedentsev A, Frost J, Wang X, von Hauff E, Mackenzie R, Nelson Jet al., 2019,

    Relating chain conformation to the density of states and charge transport in conjugated polymers: The role of the β-phase in poly(9,9-dioctylfluorene)

    , Physical Review X, Vol: 9, ISSN: 2160-3308

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

  • Journal article
    Eisner F, Azzouzi M, Fei Z, Hou X, Anthopoulos T, Dennis TJ, Heeney M, Nelson Jet al., 2019,

    Hybridization of local exciton and charge-transfer states reduces non-radiative voltage losses in organic solar cells

    , Journal of the American Chemical Society, Vol: 141, Pages: 6362-6374, ISSN: 1520-5126

    A number of recent studies have shown that the nonradiative voltage losses in organic solar cells can be suppressed in systems with low energetic offsets between donor and acceptor molecular states, but the physical reasons underpinning this remain unclear. Here, we present a systematic study of 18 different donor/acceptor blends to determine the effect that energetic offset has on both radiative and nonradiative recombination of the charge-transfer (CT) state. We find that, for certain blends, low offsets result in hybridization between charge-transfer and lowest donor or acceptor exciton states, which leads to a strong suppression in the nonradiative voltage loss to values as low as 0.23 V associated with an increase in the luminescence of the CT state. Further, we extend a two-state CT-state recombination model to include the interaction between CT and first excited states, which allows us to explain the low nonradiative voltage losses as an increase in the effective CT to ground state oscillator strength due to the intensity borrowing mechanism. We show that low nonradiative voltage losses can be achieved in material combinations with a strong electronic coupling between CT and first excited states and where the lower band gap material has a high oscillator strength for transitions from the excited state to the ground state. Finally, from our model we propose that achieving very low nonradiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems.

  • Journal article
    Eisner FD, Azzouzi M, Fei Z, Hou X, Anthopoulos TD, Dennis TJS, Heeney M, Nelson Jet al., 2019,

    Hybridization of Local Exciton and Charge-Transfer States Reduces Nonradiative Voltage Losses in Organic Solar Cells.

    , J Am Chem Soc

    A number of recent studies have shown that the nonradiative voltage losses in organic solar cells can be suppressed in systems with low energetic offsets between donor and acceptor molecular states, but the physical reasons underpinning this remain unclear. Here, we present a systematic study of 18 different donor/acceptor blends to determine the effect that energetic offset has on both radiative and nonradiative recombination of the charge-transfer (CT) state. We find that, for certain blends, low offsets result in hybridization between charge-transfer and lowest donor or acceptor exciton states, which leads to a strong suppression in the nonradiative voltage loss to values as low as 0.23 V associated with an increase in the luminescence of the CT state. Further, we extend a two-state CT-state recombination model to include the interaction between CT and first excited states, which allows us to explain the low nonradiative voltage losses as an increase in the effective CT to ground state oscillator strength due to the intensity borrowing mechanism. We show that low nonradiative voltage losses can be achieved in material combinations with a strong electronic coupling between CT and first excited states and where the lower band gap material has a high oscillator strength for transitions from the excited state to the ground state. Finally, from our model we propose that achieving very low nonradiative voltage losses may come at a cost of higher overall recombination rates, which may help to explain the generally lower FF and EQE of highly hybridized systems.

  • Journal article
    Azzouzi M, Kirchartz T, Nelson J, 2019,

    Factors Controlling open-circuit voltage losses in organic solar cells

    , Trends in Chemistry, Vol: 1, Pages: 49-62, ISSN: 2589-5974

    The performance of solar cells based on molecular electronic materials is limited by relatively low open-circuit voltage (Voc) relative to the absorption threshold. These voltage losses must be reduced to achieve competitive power-conversion efficiencies. Voltage losses are assigned to the molecular heterojunction required to dissociate photogenerated excitons and to relatively fast electron–hole recombination. Recent studies using luminescence have helped quantify these losses and understand their molecular origin. Recently, higher voltages and lower losses have been achieved using new molecular acceptors in place of traditional fullerenes, suggesting that optimizing chemical structure could enable improved device performance. This mini-review combines a device-physics perspective with a body of experimental observations to explore the practical and theoretical limits to Voc.

  • Journal article
    Moia D, Gelmetti I, Calado P, Fisher W, Stringer M, Game O, Hu Y, Docampo P, Lidzey D, Palomares E, Nelson J, Barnes PRFet al., 2019,

    Ionic-to-electronic current amplification in hybrid perovskite solar cells: ionically gated transistor-interface circuit model explains hysteresis and impedance of mixed conducting devices

    , Energy and Environmental Science, Vol: 12, Pages: 1296-1308, ISSN: 1754-5692

    Mobile ions in hybrid perovskite semiconductors introduce a new degree of freedom to electronic devices suggesting applications beyond photovoltaics. An intuitive device model describing the interplay between ionic and electronic charge transfer is needed to unlock the full potential of the technology. We describe the perovskite-contact interfaces as transistors which couple ionic charge redistribution to energetic barriers controlling electronic injection and recombination. This reveals an amplification factor between the out of phase electronic current and the ionic current. Our findings suggest a strategy to design thin film electronic components with large, tuneable, capacitor-like and inductor-like characteristics. The resulting simple equivalent circuit model, which we verified with time-dependent drift-diffusion simulations of measured impedance spectra, allows a general description and interpretation of perovskite solar cell behaviour.

  • Journal article
    Moia D, Giovannitti A, Szumska AA, Maria IP, Rezasoltani E, Sachs M, Schnurr M, Barnes PRF, McCulloch I, Nelson Jet al., 2019,

    Design and evaluation of conjugated polymers with polar side chains as electrode materials for electrochemical energy storage in aqueous electrolytes

    , Energy & Environmental Science, Vol: 12, Pages: 1349-1357, ISSN: 1754-5692

    We report the development of redox-active conjugated polymers that have potential applications in electrochemical energy storage. Side chain engineering enables processing of the polymer electrodes from solution, stability in aqueous electrolytes and efficient transport of ionic and electronic charge carriers. We synthesized a 3,3′-dialkoxybithiophene homo-polymer (p-type polymer) with glycol side chains and prepared naphthalene-1,4,5,8-tetracarboxylic-diimide-dialkoxybithiophene (NDI-gT2) copolymers (n-type polymer) with either a glycol or zwitterionic side chain on the NDI unit. For the latter, we developed a post-functionalization synthesis to attach the polar zwitterion side chains to the polymer backbone to avoid challenges of purifying polar intermediates. We demonstrate fast and reversible charging of solution processed electrodes for both the p- and n-type polymers in aqueous electrolytes, without using additives or porous scaffolds and for films up to micrometers thick. We apply spectroelectrochemistry as an in operando technique to probe the state of charge of the electrodes. This reveals that thin films of the p-type polymer and zwitterion n-type polymer can be charged reversibly with up to two electronic charges per repeat unit (bipolaron formation). We combine thin films of these polymers in a two-electrode cell and demonstrate output voltages of up to 1.4 V with high redox-stability. Our findings demonstrate the potential of functionalizing conjugated polymers with appropriate polar side chains to improve the accessible capacity, and to improve reversibility and rate capabilities of polymer electrodes in aqueous electrolytes.

  • Journal article
    Calado P, Burkitt D, Yao J, Troughton J, Watson TM, Carnie MJ, Telford AM, O'Regan BC, Nelson J, Barnes PRet al., 2019,

    Identifying dominant recombination mechanisms in perovskite solar cells by measuring the transient ideality factor

    , Physical Review Applied, Vol: 11, ISSN: 2331-7019

    The light ideality factor determined by measuring the open circuit voltage (VOC) as function of light intensity is often used to identify the dominant recombination mechanism in solar cells. Applying this ‘Suns-VOC’ technique to perovskite cells is problematic since the VOC evolves with time in a way which depends on the previously applied bias (Vpre), bias light intensity, and device architecture/processing. Here we show that the dominant recombination mechanism in two structurally similar CH3NH3PbI3 devices containing either mesoporous Al2O3 or TiO2 layers can be identified from the signature of the transient ideality factor following application of a forward bias, Vpre, to the device in the dark. The transient ideality factor, is measured by monitoring the evolution of VOC as a function of time at different light intensities. The initial values of ideality found using this technique were consistent with estimates of ideality factor obtained from measurements of photoluminescence vs light intensity and electroluminescence vs current density. Time-dependent simulations of the measurement on modelled devices, which include the effects of mobile ionic charge, reveal that this initial value can be correlated to an existing zero-dimensional model while steady-state values must be analysed taking into account the homogeneity of carrier populations throughout the absorber layer. The analysis shows that Shockley Read Hall (SRH) recombination through deep traps at the charge collection interfaces is dominant in both architectures of measured device. Using transient photovoltage measurements directly following illumination on bifacial devices we further show that the perovskite/electron transport layer interface extends throughout the mesoporous TiO2 layer, consistent with a transient ideality signature corresponding to SRH recombination in the bulk of the film. This method will be useful for identifying performance bottlenecks in new variants of perovskite and

  • Journal article
    Dimitrov SD, Azzouzi M, Wu J, Yao J, Dong Y, Tuladhar PS, Schroeder BC, Bittner ER, McCulloch I, Nelson J, Durrant JRet al., 2019,

    Spectroscopic investigation of the effect of microstructure and energetic offset on the nature of interfacial charge transfer states in polymer: fullerene blends

    , Journal of the American Chemical Society, Vol: 141, Pages: 4634-4643, ISSN: 0002-7863

    Despite performance improvements of organic photovoltaics, the mechanism of photoinduced electron-hole separation at organic donor-acceptor interfaces remains poorly understood. Inconclusive experimental and theoretical results have produced contradictory models for electron-hole separation in which the role of interfacial charge-transfer (CT) states is unclear, with one model identifying them as limiting separation and another as readily dissociating. Here, polymer-fullerene blends with contrasting photocurrent properties and enthalpic offsets driving separation were studied. By modifying composition, film structures were varied from consisting of molecularly mixed polymer-fullerene domains to consisting of both molecularly mixed and fullerene domains. Transient absorption spectroscopy revealed that CT state dissociation generating separated electron-hole pairs is only efficient in the high energy offset blend with fullerene domains. In all other blends (with low offset or predominantly molecularly mixed domains), nanosecond geminate electron-hole recombination is observed revealing the importance of spatially localized electron-hole pairs (bound CT states) in the electron-hole dynamics. A two-dimensional lattice exciton model was used to simulate the excited state spectrum of a model system as a function of microstructure and energy offset. The results could reproduce the main features of experimental electroluminescence spectra indicating that electron-hole pairs become less bound and more spatially separated upon increasing energy offset and fullerene domain density. Differences between electroluminescence and photoluminescence spectra could be explained by CT photoluminescence being dominated by more-bound states, reflecting geminate recombination processes, while CT electroluminescence preferentially probes less-bound CT states that escape geminate recombination. These results suggest that apparently contradictory studies on electron-hole separation can be exp

  • Journal article
    Salerno F, Rice B, Schmidt JA, Fuchter MJ, Nelson J, Jelfs KEet al., 2019,

    The influence of nitrogen position on charge carrier mobility in enantiopure aza[6]helicene crystals

    , Physical Chemistry Chemical Physics, Vol: 21, Pages: 5059-5067, ISSN: 1463-9076

    The properties of an organic semiconductor are dependent on both the chemical structure of the molecule involved, and how it is arranged in the solid-state. It is challenging to extract the influence of each individual factor, as small changes in the molecular structure often dramatically change the crystal packing and hence solid-state structure. Here, we use calculations to explore the influence of the nitrogen position on the charge mobility of a chiral organic molecule when the crystal packing is kept constant. The transfer integrals for a series of enantiopure aza[6]helicene crystals sharing the same packing were analysed in order to identify the best supramolecular motifs to promote charge carrier mobility. The regioisomers considered differ only in the positioning of the nitrogen atom in the aromatic scaffold. The simulations showed that even this small change in the chemical structure has a strong effect on the charge transport in the crystal, leading to differences in charge mobility of up to one order of magnitude. Some aza[6]helicene isomers that were packed interlocked with each other showed high HOMO-HOMO integrals (up to 70 meV), whilst molecules arranged with translational symmetry generally afforded the highest LUMO-LUMO integrals (40-70 meV). As many of the results are not intuitively obvious, a computational approach provides additional insight into the design of new semiconducting organic materials.

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